December 2015 is likely going to end up as the warmest December on record for numerous climate locations across the Northeast. Through the first 23 days of the month, the entire Northeast has not only been slightly above-average but WAY above-average. The graphic below depicts this very well:
Looking at the image and the color scale, you can see that temperatures through the first 23 days of the month have been running as much as 6-9F to 9-12F+ above-average!! These numbers are nothing short of impressive and come months end numerous records will be broken for warmest December on record. While winter only officially began just a few short days ago, the winter season has not gotten off to a great start for cold or snow lovers across the Northeast. In addition to the record warmth, numerous climate locations may also break or tie some records for least snowiest December on record (we will have to watch Monday/Tuesday (28th/29th) time frame for the potential of some winter precipitation across portions of New England).
The Northeast hasn't been the only region of the United States to experience above-average warmth this month, in fact much of the United States has seen December temperatures running near or above-average.
What was/were the causes for this anomalous December warmth and what does this mean for winter moving forward? These questions and more will be addressed below as we highlight how this winter may play out and what we can possibly expect.
EL-Nino Southern Oscillation (ENSO)
One of the biggest drivers of the global atmospheric weather pattern is that of the EL Nino-Southern Oscillation (ENSO). ENSO is broken down into three distinct phases; La Nina, EL Nino, and neutral ENSO conditions. Each state is defined based upon the sea-surface temperature anomalies (SSTAs) across the equatorial Pacific across a region from about 170E to 90W latitude and 5S to 5N in longitude. For visual reference see graphic below:
Since about mid-to-late winter/early spring the equatorial Pacific Ocean region has been suggestive of EL Nino conditions. EL Nino has been all over the news as of late but what exactly is EL Nino? An EL Nino episode occurs when SSTA's are running at least +0.5C above-normal for at least five consecutive trimonthly periods. (A few examples of trimonthly period would be December-January-February, January-February-March, February-March-April, etc.). La Nina and EL Nino episodes are also broken down into weak events, moderate events, and strong events. One method used to indicate the SSTA's across the ENSO regions and also indicate the strength of the ENSO event is called the Oceanic-Nino Index (ONI). For the purposes of EL Nino, a weak EL Nino occurs when the SSTA's are between +0.5C and +0.9C, a moderate EL Nino occurs when SSTA's are between +1.0 and +1.4, and a strong EL Nino occurs when SSTA's are at or greater than +1.5C. Below is ONI data dating back to the start of 2015. The red text indicates the beginning of the current EL Nino episode. With the latest ONI reading of +2.0, this indicates that this EL Nino episode is a pretty strong EL Nino. In fact, this will go down as one of the strongest EL Nino's on record when all said and done. (Note: The EL Nino was officially defined with the JJA reading).
Beyond ENSO classification and strength classification, ENSO can further broken down into where the ENSO event is 'based'. The classifications for this are west-based, east-based, and basin wide. Where each ENSO event is situated is just as, if not more important than the actual ENSO state itself. When determining where the ENSO event is based we can look at both the SSTA configuration across the equatorial Pacific and look at where the focus for tropical forcing is located (we will touch upon this aspect next). Below are the SSTA's as of December 24th, 2015:
Looking at the equatorial Pacific region (black highlighted circle) the waters are exceptionally warm across the region. Given how widespread the well above-average SST's are this is indicative of a basin wide EL Nino event.
Tropical Forcing
The next atmospheric variable we will discuss is that of tropical forcing. When talking about tropical forcing we are looking at convection (areas of showers and thunderstorms) which develop in and around the Indian Ocean and propagate eastward across the equatorial Pacific. When water vapor condenses to form clouds, this process releases what is called latent heat (latent meaning "hidden"). The more convection you have and the stronger the convection, the more heat that is released into the atmosphere. This is significant because this heat which is released (especially if there is a great deal of heat released) can have a major influence on the jet stream across North America. The strongest influences of the latent heat and subsequent ridging typically (not always) takes place anywhere from 30° to 50° east of where the tropical forcing is occurring.
When assessing tropical forcing, one thing we can look at are outgoing longwave radiation (OLR) anomalies across the Pacific Basin. Outgoing longwave radiation is a measure of the amount of solar radiation which is emitted back into space from the Earth's surface. The more convection and clouds, the less in the way of radiation emitted back into space. Assessing tropical forcing, however, can be a little tricky as over the course of time the forcing can shift across the Pacific. Looking at OLR anomalies from December 1st through the 22nd, we can see that the majority of the tropical forcing (enhanced convection has been located between about 170W-150W.
Stratosphere/Polar Vortex/Arctic Oscillation (AO)/ Quasi-Biennial Oscillation (QBO)
Outside of the strong basin-wide EL Nino other contributions to this anomalous warmth across the east has been due to the state of the Stratosphere, the strength of the polar vortex (which we can use the Arctic Oscillation (AO) to measure), and the phase of the Quasi-Biennial Oscillation (QBO). Through much of the fall and into the month of December, the AO has been quite positive. A positive AO is defined by below-average height anomalies in and around the Arctic region. These below-average height anomalies indicate a stronger than average Arctic low pressure center (polar vortex). This is enhanced by very strong westerly winds between about 30-50mb (lower stratosphere) which is indicated by the positive phase of the QBO. Below are graphics of the 500mb and 50mb height anomalies (left and center images respectively) along with the 50mb zonal wind anomalies (right image) from December 1st through the 22nd. (Note the below-average height anomalies are indicated by the darker blue/purple shadings and the positive wind anomalies (indicative of westerly winds are indicated by the brighter orange shadings).
Looking at the highlighted area on the below graphic, you can see the AO has been predominately positive with the exception of the brief period around the 15th of the month where the AO dipped slightly negative. However, since then the AO has been extremely positive.
Connecting the strong EL Nino, the placement of the strongest tropical forcing, and the strength of the polar vortex indicated by the very positive AO, the influences on the atmospheric pattern across the United States has been for a trough across Alaska, western Canada, and down into the western United States at times and for ridging across the eastern United States. We can reference the 500mb height anomaly graphic to see this.
With December coming to a close what can we expect for the duration of winter moving forward? Will a pattern favoring extreme warmth and no snow in the east continue or will winter finally show its true face? Below we will explore the signals which are in place which may be suggestive of where winter is headed moving forward.
We will begin by viewing the December 25th 12z 500mb height anomaly charts for the 8-10 day period from the American GFS forecast model (graphic on the left) and the European forecast model (graphic on the right). When looking at the run (and this has actually been developing over the past few runs) we notice some substantial differences within the pattern compared to what we saw for December.
The major differences here are the big ridge developing across the western-tier of the United States extending northward through western Canada and towards Alaska and the deep trough across the eastern United States. Below are observations and ensemble forecasts for both the AO as well as the Pacific-North American teleconnection pattern (PNA). The PNA can hold a strong influence on the atmospheric pattern across the Northern Hemisphere mid-latitudes. The state of the PNA is classified as being either positive, negative, or if there is no clear signal, neutral. The state of the PNA is measured by analyzing geopotential height anomalies at the 500mb level. The positive phase of the PNA is characterized by above-average height anomalies situated across the western-tier of Canada and the western-tier of the United States and below-average height anomalies situated across the southeastern United States. The negative phase of the PNA is a mere opposite of the positive phase with below-average heights situated across the western-tier of Canada and the western-tier of the United States and above-average height anomalies situated across the southeastern United States. With a positive phase, this leads to ridging out across the west and a trough at least across the southeast and with a negative phase, this leads to troughing across the west and ridging across the east.
While the ensemble spread is a bit more chaotic for the AO as opposed to the PNA, the consensus within the guidance is not only for the AO to drop but perhaps drop significantly. This would indicate a weakening polar vortex which favors a higher likelihood for colder air to spill southward into the central/eastern United States. The consensus for the PNA, however, is much stronger with the ensemble mean all favoring a rapid rise in the PNA. This would promote ridging developing into the western-tier of the United States northward into western Canada, Alaska and perhaps poking into portions of the Arctic. These signals all point to the likelihood that as we at least move through the final week of December and into the first week of the New Year, a major pattern change may take shape. However, will this new pattern change become a fixture for the duration of winter and what will come with the pattern change?
When looking even further down the road than just the next two weeks, we must continue to keep in mind how strong the EL Nino is along with the state of the Pacific as a whole along with the state of the lower stratosphere and the polar vortex. Throughout the month of December, the strong EL Nino has held its dominance throughout the Pacific and one of its fixtures has been the strong vortex in the Gulf of Alaska region. When you combine these with the strong polar vortex the result is for major warmth across much of the United States, especially in the east. In order to alter this pattern, you need the Gulf of Alaska vortex to weaken and retreat northward and the polar vortex to weaken. Unless the polar vortex continues to be attacked by wave propagating in the lower stratosphere/upper troposphere which would further induce weakening and unless the zonal westerlies weaken in the lower stratosphere, it will be very hard pressed to think the upcoming pattern change becomes a fixture through the remainder of winter. What this means is after a potential pattern change over the next 10-14 days, the pattern could once again begin a transition and we see more in the way of ridging once again building into the east, however, not likely to the extent we saw during the month of December.
The other major interest of note is the lack of colder air which will arrive if this pattern flip does indeed occur. For the most part, temperatures across much of Canada have been running above-average. We can see this from the image below.
Looking at the GFS ensembles from Friday 12/24/2015 0z run, the forecast for the 11-15 day period is continued warmth relative to average for much of Canada.
Unless the changes which are forecasted to happen within the lower stratosphere, the polar vortex, and Gulf of Alaska region with regards to the Gulf of Alaska vortex become a constant the pattern which looks to develop moving into January may not be a permanent fixture. This means we could see ridging become established once again across the east, however, this should not be to the degree which occurred in December unless the polar vortex were to once again restrengthen to the levels we've seen this month. Despite the potential for more troughing to occur across the east, the lack of cold in and across Canada should limit more in the way of cold working into the central and eastern United States. Below is my forecast for the month of January split into two parts; the first half of the month to reflect the potential upcoming pattern change and the second half to reflect the relaxation of that pattern change.
Moving forward into February if the changes discussed above headed into January don't become a constant it will be hard-pressed to find a source for major Arctic air to really work into the United States. In fact, it is very possible warmth (although not record warmth) may once again overspread a good portion of the United States. Below is my forecast for February.
This forecast does not mean we will not see periods of colder temperatures or even at time some really cold temperatures, however, when these episodes occur they should be rather brief, lasting no more than a couple of days at most.
I also provide this forecast with a low confidence rating. If the polar vortex does indeed continue weakening (this can be aided by a weakening of the westerly QBO) and the PNA can become positive, this would promote a higher likelihood for a pattern shift towards colder in the east and mild across the west.
As for snow lovers across the east, each scenario does not necessarily mean a poor snow winter (although once you get to New York City and points south things certainly don't look good), however, we would like to see some troughing become more present in the east and also see a more active pattern/storm track. Thus far, the strong ridging that has been in place across the east has worked to weaken storm systems moving through the jet stream to our west.
The winter of 2015-2016 is certainly not lost, however, big changes must occur. There are signs for these changes to take place but the million dollar question is, will they persist?
