It’s the battery that keeps recharging — an energizer bunny type pattern with regards to the warmth. In the Northeastern part of the United States, the 2010-2012 period has been the warmest on record for many cities. For the NYC local area, this past cold season, November through March, was the mildest on record. April will finish slightly warmer than normal as expected (probably +1 to +2 range in NYC), and not as anomalously mild as the previous months. The past several days have seen the longest stretch of negative departures since mid January, and before that, since October. However, it’s not saying much as this current stretch of moderately below normal temperatures should end by early next week. Prior to that time, late season frosts and freezes are likely in the suburbs of I-95 cities tonight and again late this weekend into Monday/Tuesday mornings.

Looking ahead to May, it doesn’t appear that we’ll break the seemingly eternal stretch of consecutive warmer than normal months. The weak-moderate La Nina event of 2011-2012 is slowly fading away, although SOI daily values have been positive in recent weeks, suggesting that we’re unlikely to see a rapid onset of an El Nino regime. Latest SSTA in ENSO region 3.4 indicates a reading of -0.17c, which is cold-neutral, and thus we’re not receiving any strong influence from ENSO right now. MJO tropical forcing patterns tend to be more noticable in times of weaker ENSO, and that will be spiraling toward phase 8 over the next couple weeks, a warm signal for much of the United States at this time of year.

NCEP model guidance projects a phase 8 MJO, while ECMWF based guidance essentially remains in the circle of death over the coming weeks. Either way, if the MJO does attain low amplitude phase 8, the signal produced for the eastern US will be a warm one.

Both the NAO and AO have remained fairly stable over the past several months, with predominately positive modalities for both indices. Although the AO has briefly turned negative this week (and we see the cool weather in response over the Northeast), models are in strong consensus that the AO will shoot slightly positive in early May. Meanwhile PNA forecast guidance suggests a slightly negative to near neutral look in the medium range.

When we combine the above factors, we get a May with near neutral ENSO conditions, a slightly positive NAO, AO, and near neutral PNA. MJO signalling will be weak, possibly low amplitude phase 8 in a couple weeks if anything.

The resultant analog package is as follows: Not many changes from the list of years that comprised the April forecast (and those years worked very well by the way with both temperature and precipitation departures in the US):

1951

1976

1986

2001

2009

The analog forecast May 500mb pattern:

Predicted May temperature anomalies:

Projected May precipitation anomalies:

Canadian temperature anomalies over the next 10 days — near to slightly above normal for the most part:

Conclusion:

1.) May should feature slightly warmer than normal temperature departures in the Northeast US, with above normal temps stretching westward into the Great Lakes and Mid-west.
2.) Precipitation should increase over the southern Plains and Southeastern US
3.) Precipitation anomalies in the Northeast should be near normal (maybe slightly below avg), which is a step in the right direction after a very dry Jan-April period around here.
4.) May will start cool in the Northeast but trend milder. Bursts of 80s and summery weather is likely on a number of occasions.
5.) Summer outlook will be posted in late May. Early indications are that May “may” be the last month of the consecutive warmer than normal month stretch here in the Northeast. Our first “cooler than normal” temp month in awhile is likely to occur in either June or July.

March has entered the record books as the second warmest March on record in NYC, after the warmest February on record, second warmest winter (DJF) on record, and warmest DJFM period on record. Truth be told, it’s been about as mild as it can get in our area for the cool season. Since November, we’ve seen countless days in the 50s and 60s, with very few days featuring sub freezing or even below normal temperatures. The growing season is off to a remarkably early start, about 3-4 weeks ahead of schedule for most in the tristate area, thanks to the record warmth of recent months. But this can’t last forever, or can it?

Looking ahead into April, it’s safe to say we can close the books on the winter 2011-12 dismal snowfall season for NYC and the coast, barring a freak mid/late spring storm (chances for that happening are slim to none). The weak/moderate La Nina is finally weakening, as evidenced by recent Southern Oscillation Index (SOI) values. When the index is in a predominately positive state, it indicates enhanced tropical easterly trade winds, more upwelling to the west of South America, and cooler waters (hence La Nina). When the SOI is in a negative state, generally we see weakened easterly trade winds, which allow the warm water in the west Pacific to gradually slide eastward and pile up (the development of an El Nino). The latter is what should occur over the next several months. Statistical and dynamic models are in fairly good consensus that we’ll reach warm neutral, potentially weak El Nino status, sometime this summer. Daily SOI values have been in the tank lately, with readings as low as -26 to -28. This means the environment is becoming rapidly less favorable for the maintenance of the La Nina.

Atmospheric angular momentum (AAM) is another good way to measure ENSO state, and unlike the SOI, when the AAM is in a positive state, it generally means the atmosphere is more El Nino-like. Over the past few months we’ve seen a predominately negative AAM, but over the past couple weeks, values have surged to near neutral. This is yet another indication of the dying La Nina.

A third factor to examine is the most obvious one — sea surface temperature anomalies (SSTA). Tropical Pacific SST’s have warmed quite significantly since late winter, and crucial ENSO region 3.4 is nearing neutral — now at -0.22c as of the latest weekly update. This suggests a cold neutral ENSO regime currently, a major rebound from the values down around moderate La Nina criteria a couple months ago. These SST’s will continue to warm, slowly but surely, over the coming weeks and months.

So what will April have in store? Here’s what the US looks like temp wise in the first three days of the month. Note a continuation of the big heat in the Mid-west, and a touch of coolness on the West and East Coasts.

While I don’t expect the month of April to finish with temperature departures as warm as they are right now in the Central US, I am anticipating another fairly warm month nationwide, with the West Coast and New England likely being the coolest spots.

ENSO region 3.4 will be near neutral in the .weeks, and looking at the other important teleconnections, both AO and PNA values should bounce around near 0 / slightly positive, while the NAO is the most negative its been since last October. However, the NAO should not tank significantly negative, and may actually rebound to neutral to slightly above by late April. What all this means is the atmosphere should be in a state that does not support the maintenance of well below normal temperature regimes or well above normal temp regimes. Rather, temperatures in the Northeast should average out close to normal in the next couple weeks.

Easter week (4/8 – 4/14) may end up slightly cooler than normal in the Northeast as a storm cuts off near Newfoundland, keeping mid levels rather chilly into the middle of the month. However, thereafter, that chilly pattern should breakdown even in the Northeast, giving way to more warm shots by the final 10 days of April. I’m most confident about the region from the Rockies to the Ohio Valley (most of the country) finishing warmer than normal for April. I’m a bit less confident but still believe the Northeastern part of the United States should end up warmer than normal for April, and also drier than average.

Given the expected ENSO and teleconnection pattern, I came up with 4 analogs for the next month: 1976, 1986, 2001, and 2009. All of them were warm from the Rockies to the Northeast coast, with a cool pattern on the West Coast. All of them had less precipitation than normal in the Eastern US, with a precip maximum in the central Plains.

H5 pattern for April based on the analogs:

Surface temp anomaly:

Precipitation anomaly:

Now that we’re near neutral ENSO, the MJO will have a greater influence on our pattern (it has had a significant influence this past winter). Tropical forcing can be helpful in determining mid latitude ridge/trough position and general temp/precip patterns. MJO will be swinging through phase 8 and phase 1 in the next couple weeks, which is a cool signal in the East. Thereafter, things become less certain, but MJO may enter the circle of death. As noted earlier, the cool period for Easter week seems reasonable, with warming temps thereafter.

MJO phase 1 map for the US in April — note the coolness in the East, but it should not be as impressive or expansive as this.

Conclusion:

– Near/slightly above normal the rest of this week, then a slightly cooler than normal week for 4/8-4/14 in the Northeast
– The Mid-west should continue well above normal temperatures
– West Coast cooler than average throughout most of April
– Northeast temps will begin to warm after mid month, with the possibility that some of the torch in the Central US shoots eastward by the end of the month
– Final temperature departures for April should be slightly above normal in the Northeast, but not as warm relative to averages as the past several motnhs (i.e., closer to “normal” than the past 4 months).
– Precip departures below average in the East, wetter in the Plains

It’s been a struggle just to get the flakes flying this winter, nevermind a significant snowstorm. With the next 7-14 days likely to continue the above normal temperature regime, it’s highly unlikely the NYC metro area receives an accumulating snow event. Global teleconnectors remain unfavorable, with a similar pattern to what we saw most of this winter — low heights over the arctic and north atlantic (cold) and higher than normal heights (warm) across North America. Since the next couple weeks look shot, I thought I’d look beyond March 15th, at past weather patterns, to examine synoptic set-ups that yielded a snow event in the metro area.

March 2004 featured some moderate snows in the latter part of the month; below is the 500mb composite for that period:

We had a positive NAO, AO, but the positive PNA (western US ridging) was a major help, as well as good cold air supply in southern Canada. 850mb temperatures were well below 0c, and surface high pressure was well positioned in Quebec.

Surface air temperature anomaly for that same period – note the chill in Eastern Canada and the Northeast US:

The year prior to this, the metro area witnessed a very late season snowstorm on April 7th, 2003. Below is the 500mb pattern for that day:

Mid level riding in the Northeast and SE Canada, and check out the surface temp anomaly map – talk about some extreme negative departures:

March 15th, 1999, unlike the previous cases, had a strongly negative AO in March, and the event featured a nice 500mb block to the west of Greenland – near ideal orientation for a bomb of negative heights in the Eastern US.

The low pressure center was fairly intense, about 996mb on the mid atlantic coast, with plentiful surface cold in place; high pressure was present but not overly strong.

March 31st-April 1st 1997 was a minor event for NYC, but suburbia NJ recorded several inches to upwards of 10″ of snow in some locations. This was a major to historic snow for portions of southern New England.

Similar to our previous cases, the H5 pattern is far from the ideal that one would picture for the winter months of DJF. Positive heights in the northern latitudes, and a trough in the PAC NW. But shorter wavelengths in the spring allow an East Coast trough to be possible even w/ no Western ridge. What we did have – yet again – was mid level ridging to the N/W and thus plentiful surface high pressure, low level cold drainage into the storm system.

Surface temp anomalies — one word: Wow

April 9th-10th 1996 — a significant snow for NJ, to close out the snowiest winter on record for most. Once again, high pressure (1025mb approximately) situated to our west, plentiful cold air (850mb temps of -5c to -7c), and in this case, the most high latitude blocking we’ve seen of our cases thus far:

You’ve got a negative NAO, AO and positive PNA, negative EPO (Western US ridge, Alaska ridge) also known as the teleconnectors lined up perfectly for a significant late seaon snow event.

The surface temp anomaly – coincides nicely with the negative heights at 500mb – very cold:

March 18t, 1994, a 3-6″ snow event, had a set-up at H5 like this:

Big neg height anomalies from the PAC NW (again – generally unfavorable in winter, but not early spring) and a mid level ridge to the west of Greenland (conducive for east coast cyclogenesis)

I thought I’d examine one more, infamous Northeast late season snowstorm — April 12th, 1982 — the most significant of all late season events for NYC, with about 10″ (over a foot accumulated in NW suburbs).

The 500mb pattern, featured a PAC NW trough (low heights in the gulf of Alaska), a positive AO, and NAO probably negative. Nothing in the global teleconnectors would scream major snowfall – but in early spring, a strong West Coast trough actually teleconnects to the trough on the East Coast. Furthermore, we’ve got mid level ridging in the central US, plentiful surface cold air, and probably strong high pressure as well:

So the common denominator to all these snow events — unseasonably cold air, with mid level ridging (and surface high pressure) preferrably to the north/west of the storm. Some events had northern latitude blockings, others didn’t. Many featured a PAC NW trough and Central US ridge.

One thing we’ve seen very little of this winter in North America is unseasonably cold air – in fact we’ve had the opposite. Here’s the NAEFS week 2 forecast for the country (through March 18th):

Not a pretty look – very mild across Canada and the US, basically a repeat of the DJF pattern. If this verifies, which it should, an above normal temp regime does not bode well for late season snow events. Maybe we can see some cooling after March 20th, but I wouldn’t bet on it. Either way, the snow enthusiast can hold out hope until early-mid April for a pattern similar to one of the cases presented here. I’d say March will probably go snowless (or very close to it) in NYC. Once we make our way to the end of March, chances for accum snow fall off a cliff. But, we shall how the next several weeks play out.

Much of this winter we’ve seen a significant lack in major storm systems in the United States. A simple look at the NOAA watch/warning map for the US throughout the past few months reveals that weather events have been MIA in North America. This is largely because intense mid latitude cyclones feed off of baroclinicity and thermal gradients — we have NOT seen that most of the winter. Why? Temperature anomalies in North America tell the story:

Below are the temperature anomalies for January 2012 — if one were to produce a December 2011 or February 2012 map, it would look similar, as the same general temp pattern has prevailed.

Note the large expanse of warmth virtually everywhere – very little in the way of thermal gradients/temp contrasts. As a result, short waves have been relatively weak, and unable to produce much snow in the Great Lakes/Northeast. Furthermore, without strong low pressure systems, cold air cannot pour southeastward, and Lake Effect snow has been absent for the most part b/c of this. No polar outbreaks –> no lake effect snow, and no strong cyclones –> no arctic outbreaks in the Northeast. It’s been a benign, boring, winter from a meteorological stand-point in North America.

Not the case for Asia and Europe – that’s for sure. The AO transition in mid Jan yielded the development of a blocking feature on the Asian side of the north pole, resulting in record breaking cold and snows across Eurasia, down to Rome, Italy. As we move into the future, that pattern is changing, with a rise in AO values into the positive territory, meaning a return to lower than normal heights in the northern latitudes. What it also means is arctic air will FINALLY enter our side of the globe, with Canada becoming drastically chillier over the coming week.

Note below, the 10 day temp anomaly forecast for Canada. A major change from the map posted earlier. Believe it or not – COLDER THAN NORMAL AIR will overspread most of Canada in the week 1 to week 2 period. Unfortunately for the Northeast, a positive AO and NAO also means the Southeast ridge is robust, and one can see by the warmer anomalies in SE Canada, the resistance to this arctic air.

The above image is the 8-10 day ECMWF and GFS model forecast. Notice the strong SE ridge, typical of a La Nina, but also the big time negative height anomalies dominating in Canada, and down into the West/Plains.

The PNA will be transitioning negative over the coming week, another aspect typical for La Ninas. It seems we’re entering a pattern which is more normal for LA NINA event, one which many of us thought would be present the bulk of this winter.

Although we’re going to see temp anomalies reverse drastically in North America, as already mentioned, it’ll be difficult (once again) to get sustained cold into the Northeast. So in a local sense, the pattern for the I-95 BOS-DCA corridor may not change much at all due to the resistance from a strong SE ridge. However, the larger scale North American regime is undergoing that change, which will finally get an intense baroclinic zone going. We’re likely to see numerous, strong low pressure systems propagating SW-NE from the Plains into the Great Lakes/Interior Northeast, which could put down snows in the Plains, Lakes, and maybe interior New England. The inland storm tracks still means no snow for the big cities of the Northeast.

Surface temperature anomalies for Marches which featured a weak La Nina in place look like this — note the similarity to the above forecasts, big cold in Canada, into the West, and across the northern tier of the United States. Ridging in the South/East.

When we examine Marches with a slightly positive or greater AO values, this is what we get:

A similar picture, with cold in the north, warm in the South.

The one wildcard is the MJO — tropical forcing now in favorable phases for cold in Northeast, but since other teleconnectors are unfavorable — positive AO, positive NAO, negative PNA, the arctic air is likely to just “bounce” in and out of the region, if we get any at all. More probably is moderated shots of cold air behind storm systems, nowhere near as cold as what the Mid-west/West may see.

In closing, the pattern for the next few weeks into the month of March:

1.) Major reversal in North American temp anomalies, with arctic air building in Canada, should yield a strengthening baroclinic zone with powerful lows moving SW to NE across the United States, dropping snow in the MID-WEST, LAKES, and INTERIOR NORTHEAST. Lakes effect snow may get initiated as well.

2.) Continued warm pattern for I-95/East Coast due to the SE ridge and other unfavorable teleconnectors. We’ll see more cold shots due to the stronger lows moving to our NW, but the poor tracks means continued no snow.

3.) Early March regime of cold West/northern Plains/Lakes should prevail. Warmth in the southern plains, southeast and Northeast coastal plain.

4.) Nothing can be ruled out in weather, but chances are against a snowstorm in the I-95 corridor the next few weeks. If it does happen, it’ll be a thread the needle situation given there’s no downstream NAO blocking, or strong ridging in the Western US. Not much reason to forecast any change in the terrible snow pattern of winter 2011-2012. Will NYC see a 6″+ snow event this winter? It doesn’t look good for the next few weeks, but on the bright side, some more exciting times ahead in terms of tracking weather. Strong lows means big winds, severe weather in the Southeast US, snows to the NW of the storm systems.

5.) At this juncture, March should come in like a lion in the North and West, and like a lamb along the East coast.

As we near the end of January, winter 2011-12 will go in the books as a very warm winter thus far across much of the nation. December finished with a temperature departure of +5.8 in NYC, and January should finish solidly above normal (at least +2), given the next week will not feature below normal temperatures.

The monstrous +AO vortex which has been dominating the weather scene over the arctic has finally met its demise, after 60-70 days of torturing snow lovers in the Great Lakes and Northeast. However, we’re not exactly seeing a reversal in AO modality. While the stratospheric warming and lower solar values have played a role in weakening the vortex, there’s nothing that indicates the AO and NAO will plummet into the tank (negative) ala 2009-10 or 2010-11. Instead, NAO/AO values will probably remain near neutral over the coming weeks, with a slight edge toward the negative side. This is certainly a drastic improvement from December and January, but it’s important to say that we’re impoving from one of the warmest patterns imaginable in the Northeast. The PNA looks to continue with a near neutral regime overall – low amplitude ridges moving through the Western US. These ridges have not done much to cool down the Northeast US so far this winter, largely b/c of the strongly positive NAO and AO. But now that these two indices have neutralized, it will be much easier to get some polar air into the Northeast. Notice I said polar airmasses — the neutral AO/NAO also suggests that major, long lasting arctic air will not attack the Northeast over the next few weeks.

The MJO tropical forcing has been unfavorable for Eastern cold much of this winter, circulating through the typical Nina octants of 4-6, and avoiding octants 7-8-1 like the plague. If we take a look at January temp departures so far in the US, and compare that to the current phase of the MJO, we’ll notice a pretty apparent similarity.

US Jan 1-21 temps departures:

MJO phase 4-5-6 for January, note the blowtorch across the nation, particularly the Plains east:

MJO forecasts over the next 10-20 days indicate more of the same — octants 5-6.

There is a hint of phase 7 in the later period by some ensemble members, but we’ve seen this trick many a time over the course of the winter. Forecasts for MJO phase 7 gradually shift to the circle of death, and more unfavorable phases as the time frame approaches. Hence why I’m not confident for conducive tropical forcing at least through day 15. Maybe we’ll see some help from the tropics after February 15th, but that remains to be seen.

So what we have going forward is MJO forcing similar to what we’ve seen all winter – but then wouldn’t February blowtorch in the East as well? La Nina climatology tends toward a warmer the normal temp pattern in the East during February, particularly if we have a positive modality of the NAO and AO. As mentioned earlier, model consensus is fairly strong for a neutral to slightly negative AO over the next few weeks, and an NAO near neutral. Some model runs have attempted to initiate a west based, Greenland block, then other runs have taken it away. Most winters with a very powerful +AO/vortex in the front part of the winter don’t turn out well in the Northeast corridor. This is evidenced by snowfall totals in NYC — virtually every December since 1950 which featured less than 3″ of snow in NYC, was a below normal snowfall season. This winter, NYC had nothing in December, which bodes well for a below avg snow season. Keep in mind this does not mean a total clunker of a winter; I could see a couple events bringing us up into the 10s or even 20″+, depending upon how March plays out. The one exception I found was the winter of 1977-78, where NYC saw less than 2″ through Jan 1st, yet finished the winter with 50″+. The ENSO state was completely different that year, with a weak nino (which tend to bring snowy mid to late winters). The point remains — while obviously its not guaranteed, the chances of NYC seeing 28″+ for the winter season are very, very low.

What this means for the sensible weather pattern in the Northeast:

6) Since the strong AO vortex event has reached its demise, its also unlikely that March will feature a strongly positive NAO or AO. That being said, I believe March will provide more opportunities for winter weather than both December and January in the Northeast.

The autumn of 2011 has entered the record books as one of the warmest Nov/Dec couplets on record for the Northeastern part of the United States, along with much of the Great Lakes and Eastern US. The unfortunate aspect about this excessive warm period is the fact the snow totals have been extremely low, to nil, across much of the Northeast corridor. This is not a coincidence. The global regime in place for Nov-Dec was one which favored a low height field surrounding the north pole, and plentiful high/positive heights in the mid latitudes. While Alaska has been getting pummeled with snowstorm after snowstorm, the United States has been experiencing quiet, tranquil, undisturbed weather overall, with very few major storm systems to talk about.

Now that this period is largely behind us, I think it’s important to examine potential reasons for why this occurred. Why did the Arctic Oscillation (AO) shoot way positive after two consecutive years of very negative values? Why was December the polar opposite of most weak/mdt La Nina Decembers? Rather than cold dominating the Plains, Lakes, and New England, we saw an extended autumn, with birds chirping, grass growing, and flowers blooming, up through Christmas this year.

It’s my opinion that the chain of reactions/interactions that led to the pattern of Nov/Dec 2011 can ultimately be traced back to the behaviour of the Sun. In the years of 2009-2011, we continued to see a deep minimum in terms of solar values – geomagnetic activity (i.e, Ap index), solar flux, sun spot number (SSN) were all very low. However, during autumn 2011, solar flux values spiked upward, with numbers surging all the way from 25-30 up to as high as 180 on the dalies by early November. Geomagnetic activity, while low overall, also saw a relative spike during this time frame.

Note on the following graph, courtesy of climatedata.info, the transition from very low solar flux to much higher values later in the year 2011.

I believe solar behaviour can modulate the wind patterns and anomalies of the thermosphere, mesosphere, stratosphere, and sometimes all the way down through the troposphere. There are time frames where the conditions are more “ripe” for solar patterns to impact troposphereic circulation patterns.

In this case, higher solar flux values (and heightened solar activity in general) began to affect the wind anomalies of the upper stratosphere in the equatorial regions. These wind anomalies interact with planetary Rossby waves (waves which form from air flowing over continents/land masses, and often propagate from the troposphere up through the stratosphere), and are drawn north toward the pole, eventually propagating downward in the stratosphere. There, they can enhance/strengthen the stratospheric vortex, or weaken it, which then, often fortells the behaviour of tropospheric patterns like the AO and NAO.

The intensity of the stratospheric vortex has a fairly high correlation with the tropospheric vortex (which modulates AO values). When the strat vortex is powerful, the troposphere tends to follow suit, and vice versa.

The Northern Annular Mode (NAM) is an index utilized to measure the intensity of the vortex in the stratosphere; there is also a surface NAM, measuring the tropospheric vortex strength (this is very closely related to the AO). The important thing to take away is that a strengthening vortex, or POSITVE NAM in the stratosphere, often precedes a positive NAM and strong vortex in the troposphere. The same can be said for the opposite case – weakening vortex in the stratosphere corresponds to weakening vortex in the troposphere. These strong and weak vortex events tend to persist about 2 months, which we’ve experienced this autumn. The strongly positive NAM / vortex event initiated in early November, and is likely finally weakening now that we’ve surpassed the 60 day mark.

Note the following graph of strong/weak vortex events (weak vortex events coinciding with stratospheric warming, by the way).

In the following graph, one can see the predictability of the stratospheric NAM for the surface NAM.

As is always the case, meteorology is extremely complex, yet everything is interconnected. When we thrown the Quasi-Biennial Oscillation in to the mix (QBO), it gets more interesting. The QBO involves wind patterns in the stratosphere of the equatorial regions. As mentioned before, since this is a stratospheric oscillation, the QBO is probably modulated by spikes and declines in solar activity. The QBO has two distinct phases – the westerly (positive) phase, is a more regular, undisturbed wind pattern, whereas, the easterly (negative) phase is more irregular, and tends to disturb the stratospheric vortex, causing it to weaken. However – West and East QBO’s can have very different effects depending upon the time of the winter season.

The solar–>stratosphere–> troposphere connection tends to be enhanced in early winter during West QBO events, and that same connection is enhanced in late winter during east QBO events.

Note in the following graph, that in times of westerly QBO and low solar flux, the NAM tends to be more negative in the early winter. When the QBO is westerly with high solar flux, the NAM tends to be neutral or positive in early winter. Then in the mid to late winter, when we have low solar flux and easterly QBO, the NAM tends to be lower/more negative. When the solar flux is high with an easterly QBO, the NAM is more positive.

Relating this back to what we’ve seen over the past couple months — 1) A Westerly/Positive QBO, 2) Higher Solar flux values
Thus, this combination in early winter is quite conducive for a positive NAM, which translates to a positive AO and stronger than normal vortex in the troposphere. We’ve seen the effects of this w/ the resultant weather – very cold over the northern latitudes and but warmer in the mid latitudes.

The bottom line to all this — the forcing responsible for the Eastern US’s miserable late fall/early winter thus far possibly originates with higher solar activity (especially solar flux) which was able to influence wind anomalies in the stratosphere, interact with the westerly QBO, in turn strengthening the stratospheric polar vortex, meaning a positive NAM, and a strong tropospheric vortex. The West QBO allowed the heightened solar activity to propagate from the equatorial stratosphere to the pole and downward through the troposphere, influencing circulation patterns such as the AO and NAO.

The QBO is now reversing negative, and solar flux values are lower than they were the past couple months. The strong vortex event has already lasted more than 60 days, so it’s unlikely that a strongly positive AO will continue the rest of the winter. Whether a negative AO will initiate is another question entirely. This post’s purpose is meant to examine potential reasoning behind the strong vortex event of autumn 2011/Dec 2011, which consequently led to the blowtorch warmth and snowless conditons in the Eastern US.

Referrences for this post:
1) Baldwin and Dunkerton 2004: The solar cycle and stratosphere–troposphere
dynamical coupling
2) Alexander Ruzmaikin and Joan Feynman 2002: Solar influence on a major mode of atmospheric variability

As we make our way into the 3rd week of December, temperature departures so far this month look like this:

The colder than normal December is verifying for the majority of the country; however, many in the Northeast and Mid-atlantic are beginning to wonder, will winter ever show up this year? While we have seen some transient shots of cold air – one such airmass overhead right now – there hasn’t been any sustained, well below normal air over the Northeastern part of the US. The pattern thus far as been conducive for the development of Western/Central US troughs, via the very low height field encompassing the Arctic and North Atlantic. The slightly favorable North Pacific regime – a +PNA/-EPO – has allowed for some jet bucking in the East, but for the most part it’s been a “dangle the carrot tease” type of pattern. The bulk of the polar/arctic air has remained locked up over the northern plains and Canada.

As long as we continue to see a low height field in the Arctic/north atlantic (+NAO/+AO), it will be difficult to achieve sustained colder than normal temps and/or snowstorm threats in the Northeast corridor. The slightly positive PNA will persist for the next couple weeks, so that will at least force some cold air into the northern tier, creating a tight west to east baroclinic zone from the Plains into the Northeast, aka a gradient pattern. Good examples of gradient patterns are December 2007 and December 2008 – both Nina years – and both featuring many southwest flow events, that is, storms moving SW-NE along the tight thermal boundary separating the relative warmth over the Southeastern US from the relative chill over New England. The NYC metro area will find itself in this battleground zone for the next couple weeks, and with any luck, that zone will shift far enough south to perhaps provide a white Christmas for parts of the area. I’m more confident on areas NW of NYC seeing accumulting snow before Christmas – as far as the coast – I don’t believe the global regime shift will have responded to impact sensible weather just yet. Is it possible to get a white Christmas on the coast? Yes, but we need to hope for a temporary shift south in the Canadian Polar vortex, to force that baroclinic zone safely south of our region.

An illustration of our pattern through Christmas. This is the ECMWF H5 map for days 8-10.

So for the NYC area and the coastal corridor from DCA-BOS, anticipate a fairly typical Nina pattern through week 2, including rain events followed by shots of polar air, averaging out near normal temp wise overall.

Now, beyond that time frame – there is controversy as to whether blocking will ever show up this winter. As I’ve stated a few times since my winter outlook this fall, the question for me is not IF the blocking will show up, but WHEN. Initially, a few months ago, I thought it’d be December into the first half of January, but now I believe it will be the end of December through most of January, and I’m going to explain why (I think) the original time frame likely failed.

Note in the following image, the Solar cycle Ap index progression with the purple dots denoting monthly values. Geomagnetic activity has generally been below 10 since 2007, but in the Sep-Oct period, we saw a surge in activity up near 15 which likely aided in the development of a strongly positive +AO/NAO regime for the month of November into early December. However, notice how last year, we had a drop off in values just prior to December, preceding the major NAO blocking event. This year we have the same, but geomag activity has been much higher this autumn compared to last autumn. With that said, considering values have now been consistently below 5 over the past month, the tropospheric response should occur sooner rather than later.

Furthermore, solar flux was high in the Oct-early Nov period, relative to recent years, which added fuel to the fire in terms of +AO/NAO maintenance.

Solar flux is now falling down from its peak near 180 to around 140 now, and sunspot number (less important for blocking) is also falling.

While this has delayed the blocking thus far, as noted above, current values on both the geomagnetic and solar flux data suggest an initiation of negative AO and NAO fairly shortly (by the end of this month).

Stratospheric conditions will also be changing over the next few weeks, with a slow but steady warming of the 10mb-50mb layers initiating in Eastern Asia/north pacific and eventually propagating northward towards Alaska and into the Arctic Circle. I’m utiziling the potential warming event as another factor for the blocking, but if a major warming does not happen, I don’t believe that’s necessary for the NAO and AO to turn negative by the end of this month into January.

Now – 50hPa:

240hrs later – 50hPa:

Note the brighter oranges developing near Alaska, indicative of a warming occurring. At this point, the warming has not spread over the entire Arctic, but it is a definite step in the right direction, which should take another 1-2 weeks to fully realize (meaning the full warming probably taking at least 3 weeks to materialize).

So how will our sensible weather be impacted?

The main point to underline is the next 1-2 weeks are a continuation of the Nina regimes (back and forth), week 2 into 3 is a transition period (Christmas), and much more interesting pattern should unfold thereafter.

Heading into mid November, we have a global pattern in place which argues for the maintenance of ridging into the Southeast/Eastern US, at least for the next couple weeks. The three “big ones” in terms of important teleconnections, the PNA, AO, and NAO, are all in an unfavorable phase for the time being, in terms of East Coast troughiness. Low height anomalies dominate the arctic, north atlantic, and Western US, lending support to the persistence of high heights (warmth) in the Eastern US. The question now becomes – when/will this pattern change? There have been some rumblings lately about December possibly being significantly more tame than expected a few weeks ago. While the timing may be slightly later than anticipated, my confidence remains high on the development of the much talked about negative NAO block. Thus, as the title says, I don’t believe it’s a matter of IF a pattern change will occur, it’s WHEN. Let’s dive into some of the specifics…

One of the primary foundations to my negative NAO forecast this winter is the maintenance of very low levels of geomagnetic activity, which, in my view, has the ability to significantly influence circulation patterns in the stratosphere and troposphere. Last winter lends evidence to the theory, given a negative NAO/Greenland blocking feature dominated the scene in the face of a poor Pacific (raging La Nina) and positive QBO (unfavorable stratospheric wind field).

While sunspot activity has picked up somewhat in the past couple years, note the low geomagnetic indicators. Recent days suggest barely detectable values for both the K index and Ap index.

K index – values 3 and under are low; values under 2 are very low.

A indices – values under 10 are low; values under 5 are very low.

Moving to tropical forcing, which I also utilized in my winter outlook, the MJO will be propagating through octants which are not conducive to the initiation of a -NAO block or sustained troughiness in the Eastern US, for 3 weeks at minimum. Most models are in consensus on a phase 1/2 approach by the end of November, and the Euro ensembles continue to push the MJO wave through phases 3-4 for the first 10 days of December. Phase 5 will likely by reached by about December 10th.

All of the above MJO phases mentioned, 2 through 6, but especially 3-5, are blowtorch city for much of the United States. Given the negative PNA/+EPO signal already in place (Gulf of Alaska/Pac NW US trough) coupled with the MJO signalling, the first 10 days of December could be warm to very warm across much of the country. This however is NOT to say the month will continue in this manner.

The MJO should continue to progress through octant 5-6, reaching a more favorable phase for Eastern troughiness by mid December. When we combine the future tropical forcing, state of geomagnetic activity, and second year Nina analogs for December, my confidence remains high that December will be a colder than normal month for the Great Lakes/Northeast. In fact, this delay in NAO block initiation may work out in our favor, considering December 1st-15th generally isn’t a snow period anyway, as climatology is quite poor for the I-95 prior to December 15th in terms of large snowfalls. If the pattern change sets in as I expect it to by mid December, it will last longer into January than I originally thought, putting the thaw period later in January rather than taking up a large chunk of the month.

Bitterly cold air will undoubtedly build over the snow covered areas of Western Canada over the next 2 weeks; ensembles agree that Canada will turn colder than normal by week 2 – so it’s not a matter of IF the cold air is avaiable. It’s a matter of CAN we get the forcing mechanism to transport this cold air into the Lakes/Northeast in a sustained fashion.

The answer to that question should be yes, and it should occur approximately 4 weeks from now. We’ll be in a largely back/forth cold/mild pattern until then – Pacific origin cool air swinging through the northern tier. If December starts off torching, don’t fret, because that’s not likely to be the case for the entire month.

Introduction and Methodology:

Last winter, most of the Northeast corridor experienced their second consecutive winter with frequent strong nor’easters, heavy snowfalls, and well above normal snowfall totals for the season. I employed solar activity for the first time, as a basis for my NAO forecast, a factor highly important in terms of sensible weather in both the United States and Western Europe. The results were good; the NAO averaged negative (actually very negative) in response to the unprecedented geomagnetic minimum we entered a few years ago. The tropospheric response to the onset of the geomagnetic minimum, with extreme blocking in the northern hemispheric winter, took about 1-1.5 years, which is fairly typical for the NAO-geomagnetic correlation. The question now becomes – does the upcoming winter feature a similar level of north Atlantic blocking to the past couple winters? There are some different external drivers on the playing field this winter, which will be examined. The final outlook for the US, specifically the NYC area and the Northeast, will utilize such variables as analogs, tropical forcing, statistical correlations, solar forcing, pattern persistence, and ENSO climatology, among others.

Solar pattern and the NAO:

Solar cycle 24 has proven to be the weakest in centuries, with sunspot numbers comparable to both the Dalton and Maunder minima. Sunspot number has increased over the past year; however, the Sun’s geomagnetic field remains very quiet. This disconnect could very well suggest that sunspot number will not rise much more than we’re seeing currently. Many forecasts for solar cycle 24 have been revised to portray a much weaker cycle overall.

In a paper by Boberg and Lunstedt, 2003, they discussed the solar wind electric field modulation of the NAO. Their findings were quite fascinating. For brief background, the solar wind is essentially a stream of charged particles ejected from the Sun’s atmosphere; these particles carry protons and electrons through space, and sometimes when conditions are favorable, can enter Earth’s magnetosphere and ionosphere.

The electric field strength of the solar wind is depicted by “E”, the first panel in the graph above. If one compares that to the second panel, depicting NAO data from 1974 to 2001, one can see a fairly impressive correlation between the two variables. In their study, the linear correlation between the “E” index and the NAO was found to be 0.65.

However, it is worth noting that the solar-NAO connection is significantly more pronounced in the winter months, November-March. This is likely because the interaction between Earth’s atmospheric layers – ionosphere down to the stratosphere and finally troposphere – is more pronounced during this time frame. The correlation between “E index” and the atmospheric pressure at various levels, from 20 hPa to sea level , was examined. The results were a fairly evenly distributed correlation of about 0.6 to 0.7 around 20N and 20S, from the upper stratosphere down to sea level. Thus, the NAO appeared to be affected quite significantly by the solar wind during the winter months, via downward propagating circulations.

The geomagnetic indicators continue to remain weak this autumn. The Kp index has been hovering between 1-2, and the ap index has generally been between 3-7. Values lower than 3 for the Kp index and values lower than 10 for the ap index are considered to be weak/very weak.

When we couple the weaker than normal electric field strength of the solar wind, quiet geomagnetic activity (Kp, ap indices, etc) we end up with an atmosphere that continues to favor the development of positive height anomalies in the north atlantic. The Southern Hemispheric winter featured a strongly negative AAO (basically our version of the AO) possibly foretelling the northern hemispheric pressure pattern. Given that the entire Earth has been subjected to very low levels of solar, geomagnetic activity, it is not unreasonable to suggest that a similar pattern will occur in the northern hemispheric winter as we saw in the southern hemisphere the past several months (that is, a regime conducive for strong blocking).

Last winter saw extreme blocking in the face of a strong La Nina and +QBO, both unfavorable conditions for the presence of blocks. However, the weakened geomagnetic environment was sufficient to override said factors. La Nina will not be a strong this upcoming winter, but the solar pattern is not as impressive as it was last autumn (not as weak). Regardless, the solar regime going into this winter will be more than enough to promote a predominately negative north atlantic oscillation (NAO). Since there is a high statistical correlation between the NAO and AO, I also anticipate a negative AO.

Another factor to examine is the autumn NAO pattern, that is, the index’s modality in the months of September and October. Positive values correlate well (about 0.65-0.7) to a negative NAO winter, and vice versa. This autumn has featured a slightly positive NAO thus far, and the remainder of October looks to be neutral to slightly positive. The AO has seen similar values. In turn, this strengthens my argument for the third consecutive negative NAO/AO winter, one which should feature at least one month of strongly negative NAO/AO values. My confidence is high on a winter average, DJFM, moderately negative NAO/AO. Given this expectation, it significantly aids my sensible temp outlook for the United States.

Pacific Pattern

Given that the Pacific Ocean covers a large percentage of the world, it’s not surprising that its oceanic patterns need to be examined for a long range outlook. Last winter featured a strong La Nina event; what about this year?

Latest data suggests ENSO region 3.4 has fallen to -0.78c, a value within the “weak La Nina” classification. Looking back over the past 60 years, if we were to have a strong La Nina, ENSO region 3.4 should already be < -1.0c. The probability of the current event strengthening into a strong La Nina is very low, given the current readings, the fact that this is a second year event (second year La Nina’s are almost always weaker in intensity than the first year), and climate forecast models.

ENSO forecast models are somewhat in disagreement on the peak intensity of the ensuing event. The CFS projects a strong La Nina, the European projects a low-end moderate La Nina, and most statistical models indicate a peak anywhere from -0.7c to -1.2c.

Given the fact that we are already near -0.8c in region 3.4, and most Nina’s peak in the Nov-Dec time frame, I anticipate a peak around -1.0c to -1.3c, which is within the “moderate” strength criteria.

As far as the Pacific Decadal Oscillation (PDO), whether a La Nina or El Nino is present in the tropical Pacific has a large bearing on the modality of the PDO. In addition, this past summer featured PDO values that were moderate to strongly negative:

June: -0.69 

July: -1.86

August: -1.76

In almost all cases, summers that featured moderately to strongly negative PDO’s also had a negative PDO during the winter.

Similar to the high correlation between the NAO and AO, the PDO is also correlated highly to Pacific North American Index (PNA) values. Winters with moderate La Nina’s and negative PDO’s virtually always mean the PNA will be mainly negative as well. There may be transient periods of positive values; however, the winter, DJFM, should be negative.

Below is the current SSTA map of the world. One can see the negative (blues) in the eastern and tropical Pacific, indicating the presence of the negative PDO and La Nina. In the Atlantic, we see a much warmer SSTA regime, suggesting that the Atlantic Multidecadal Oscillation (AMO) is warm/positive. This should persist through the winter, making this year quite similar to many winters in the 1950s. The decade of the 1950s had a predominately negative PDO/positive AMO regime, similar to the present. Furthermore, the decadal NAO cycle was heading negative as it is now. Thus, one will note that most of my analogs for this winter are from that time period.

Tropical Forcing

Yet another factor to throw into the mix is tropical forcing, which has been shown to have distinct, fairly predictable impacts on downstream height anomalies, making it a viable tool for long term outlooks. The Madden-Julian Oscillation (MJO) propagates through 8 octants (suitable name), each phase producing a certain pattern across the United States, depending upon the time of year. For example, a phase 2 MJO could mean a cold pattern in the Northeast in one month, yet that same phase could mean warmth in another month.

The MJO will be progressing through phases 1-2 over the next two weeks, which at this time of year, promotes large scale troughiness in the Eastern US. The pattern will also turn quite a bit wetter in the next couple weeks, as the MJO also has effects on precipitation anomalies. As the wave of upward motion propagates W-E over our longitude, both temp and precip patterns are impacted.

As an example, below are the temperature composites for the various phases of the MJO, centered on the month of October.

Putting it all together

The dominant factors at play for the winter of 2011-12 should be a weak geomagnetic field, weak electric field strength of the solar wind, a negative NAO, AO, PNA, PDO, and ENSO (moderate La Nina). In addition, a factor not mentioned in the discussion is the QBO, which measures variations in the equatorial stratospheric wind, will be near neutral to slightly negative. Data for winters with this modality in conjunction with the solar external forcing noted already support negative NAO winters with plentiful blocking. Notice that all the indices mentioned above will be in a negative state, suggesting that the middle latitudes of the northern hemisphere are in for a colder than normal winter overall. However, that does not mean everyone will be cold. Notable pockets of warmer than normal temperature departures will exist this winter, one such location over much of the Southern United States. Coupling the conditions expected with analogs (years with similar regimes to this one) suggest an overall warm winter (DJF) from the Southwestern US eastward through the central/southern Plains, and the Southeastern US. The DJF departure map for the Northeast of slightly below average temperatures is a bit deceiving, as I believe there will be periods of extreme cold and extreme warmth spattering the winter in the Northeast. The northern Rockies into the Mid-western US and Great Lakes should have a more predominately colder than average winter.

My analogs for this winter: 1954-55, 1955-56, and 2008-09.

A temperature composite of those winters portrays the following result for Dec/Jan/Feb:

Looking at the winter more closely, I believe the front part of the winter will feature the coldest temperatures relative to normal in the Eastern US. December 1^st-January 10^th should feature generally colder than normal temperatures in the Northeast. Expect a thaw to occur from about January 10^th to January 25^th with warmer than normal temperatures in the East. Then winter will return with another bout of cold temperatures in the East from near the end of January through the middle of February, followed by rapid, strong warming for the second half of February. I believe the time frames most favorable for significant Northeast/Mid-Atlantic snowstorms are the December 15^th-23^rd period and the January 25^th-February 10^th period, +/- 5 days or so. The most wintry part of the season for the Northeast/East should be the 40 day period Dec 1^st through January 10^th, although other notable winter events are possible late January through early February for the I-95 corridor. The interior Northeast, most of New England, and the Great Lakes should have snow events spattered throughout the entire DJF period. This is where the greatest snowfall totals with respect to normal should occur – the lower Lakes into the interior Northeast. With that said, the I-95 corridor from RIC to BOS will also see above normal snowfall in conjunction with slightly colder than normal temps for the winter season.

Forecast Temperature departures for NYC:
December: -2.5 to -3.5

January: +1.5 to +2.5

February: -0.5 to +0.5

DJF target departure: -0.33

Precipitation: Above normal

Snowfall: Above normal; target snowfall range is 35-40”

*Keep in mind snowfall has the least predictability of any other factor in the long range.

Select snowfall for other Northeast cities:

Boston: 60-65”

Philadelphia: 25-30”

Washington DC: 13-18”

Richmond: One winter event possible

It’s time to look forward to the warm season, after a long, snowy winter and a cloudy spring. Will we see a repeat of the scorcher summer of last year? Or will it be more like the cool, rainy, “year without a summer” of 2009? Or maybe it will be something in between? In order to answer these questions about the future, it’s prudent to examine both the past and present to look for clues.

The spring pattern this year has been the polar opposite of 2010: colder temperature anomalies relative to normal across the northern US, and warmer anomalies in the Southern US. This temperature anomaly orientation has enhanced the north to south thermal gradient, consequently aiding the massive severe weather / flooding season we’ve witnessed. Large temperature gradients, with cool in the Mid-west, heat in the South, and high humidity in the Southeast, are the perfect ingredients for large scale convective outbreaks. Contrast that with 2010’s spring pattern: the inverse temp pattern (warmer north, colder south relative to averages) inhibits T-storm and severe weather outbreaks.

Zooming in on our local region, we’ll likely fall within a relatively short list of 12 years (since 1970) which featured both a warmer than normal April AND May temperature wise. It may be difficult to believe as April was a very cloudy, gloomy month, but temperature departures have been above normal for the better part of this spring.

April Temperature Anomalies:

May Temperature Anomalies (so far):

With the last two weeks of May likely to feature increasing warmth in the Eastern US, it’s likely that we’ll finish with a warmer than normal May.

Below are the years which had both a warm April and May:
1980, 1985, 1986, 1977, 1998, 2004, 2001, 2006, 2009, 1987, 1991, 1999

The ensuing summers: (7) warmer than normal, and (5) cooler than normal
Warm: 1980, 1998, 1987, 1991, 1999, 2001, 2006
Cool: 1985, 1986, 1977, 2004, 2009

However, if we throw the Atlantic Multi-decadal Oscillation (AMO) into the mix, we see that 4/5 of those cool summers featured a negative (cool) AMO signal, and 6/7 of the warm summers featured a positive (warm) AMO signal. A negative AMO implies the sea surface temperatures (SST’s) in the Atlantic are cooler than normal, while a positive AMO means SST’s are warmer than average overall.

We’re currently within a positive (warm) AMO regime (AMO value for April was slightly warmer than normal) lending more credence to the warm analog years noted above: 1980, 1998, 1987, 1991, 1999, 2001, 2006.

Below is a composite temp anomaly map of summers with a negative AMO since 1970:

In contrast, here is the composite temp anomaly map of summers with a positive AMO since 1970:

The correlation is clear b/t AMO modality and temperature departures during the summer season (particularly in the Eastern US).

Another important factor to consider is ENSO. Over the past two years, we’ve seen ENSO vacillate from strong El Nino (2009-10), to strong La Nina (2010-11), and now weakening to near neutral values. It’s been awhile since we’ve held a near-neutral ENSO status, and this regime looks to continue for the summer and probably autumn. What will next winter feature? Considering the transition to negative PDO base state (colder Pacific), and the statistics regarding post strong La Nina event winters, there’s a high probability of remaining in the La Nina state. My guess right now would be weak La Nina, but that’s for another discussion.

The latest value for ENSO region 3.4 is -0.38c, within the neutral territory, and as per most modeling, we should stay there throughout the summer.

The Multivariate ENSO index (MEI) is another method of diagnosing ENSO state, and right now we have values around -1.5, which suggests the atmosphere is still in a moderate to strong La Nina state, even though the ENSO region SST’s are telling us we’re near neutral.

Looking at years that were similar to 2011 in terms of ENSO progression, the best fits are: 1971, 1974, 1985, 1989, 1999, and 2008.

However, when we include the AMO state, 1971, 1974, and 1985 are eliminated due to a cooler than normal Atlantic being present.

When we include the Pacific Decadal Oscillation (PDO; like the AMO except for the Pacific), we should eliminate 1989, because a positive (warm) PDO was present.
This leaves us with 1999 and 2008 being the top 2 overall analogs for the upcoming summer: -PDO, +AMO, and similar ENSO progression. 1999 also has the similar spring temp anomaly progression for April/May, while 2008 has the most compatible solar environment to the current year.

The North Atlantic Oscillation (NAO), while more applicable for the winter season, still has some influence in the summer months. Below is a comparison between positive NAO and negative NAO summer temp departures (since 1980).

Positive NAO Summers:

Negative NAO Summers:

I anticipate the summer of 2011 to have a neutral/negative NAO, based upon the current Atlantic tripole configuration (warm north, cool central, warm south), a continued low solar environment, post strong Nina statistics, and NAO trends over the past few years. Since 2008 featured a summertime –NAO, I see that year as the best analog for the upcoming summer (due to all the various factors mentioned).

Conclusion:
Summer 2011 should be warmer than normal in the Northeast, with the biggest heat occurring in the front part of the summer, and the coolest temperatures in August. Precipitation will be greater than normal for the majority of the Northeast for June-July-August. In addition, I anticipate this summer to be more humid than is considered “normal” due in large part to the already very warm/moist Gulf and Atlantic. An overall warm, humid, wet summer should be on the way.

NYC projected temp departures
June: +2 to +3
July: 0 to +1
August: -1 to -2
Overall Summer departures: 0 to +1
Precipitation: Above to well above normal

The idea that the front half of the summer would be much warmer than the second worked out well, along with the forecast of above to well above normal rainfall.

Actual / Forecast temp departures for NYC:

June: +1.1 / +2.5
July: +3.7 / +0.5
August: +0.1 / -1.5
Overall: +1.6 / 0 to +1

Grades:
June – B
July – D
August – B
Monthly grade – C+
Precip grade: A
Overall JJA Grade: B

Final Summer 2011 grade: B