What a turn around this winter has been able to pull off over the past week or two. Record-breaking warmth in December gave way to arctic blasts and a historic, crippling blizzard this month. We also saw an atypical fantastic performance prior to the blizzard by the NAM model for this region. We warned the winter could return with a vengeance in a public article we wrote shortly after Christmas. Pattern signals and ensemble guidance were strongly hinting at opportunities for more cold and snow.
While many aspects of our Winter Forecast published in November have worked out, there have been setbacks and surprises as well. This post will serve as an update and stepping stone for the several weeks of winter still left to come.
Review of the Season So Far
December averaged more than 10 degrees above normal and was the warmest on record, for New York City and Philadelphia metro areas. We had forecasted the month to be warmer than average (+1 to +3). This was heavily based on moderate to strong El Nino with +QBO winter analogs, showing the Gulf of Alaska low flooding the CONUS with Pacific air. A very strong stratospheric polar vortex also kept much of the cold bottled up near the arctic. Tropical forcing in Indian Ocean and Eastern Pacific also supported the MJO in phases 2/3/4 for much of the month. Both climate phenomena support a positive trend in AO/NAO/EPO modalities and enhanced a stronger, higher anomalous ridge over Eastern US for the month. As a result, very little snowfall was observed.
This all changed with the development of the record-breaking anomalous ridge over the Kara Sea in late December and early January. The MJO also propagated into phases 7/8/1. We saw more convection and tropical forcing near the dateline. This led to splitting of the tropospheric polar vortex and more high-latitude blocking by the middle of this month. The AO in particular crashed into very negative territory. We saw a couple winter storm threats that didn’t produce during the middle of January. But after the high-latitude blocking pattern relaxed, we saw the blizzard produce historic snowfall totals on January 24th. This storm brought most of the area the first significant snowfall of the season — and brought most climate reporting sites to near or above average seasonal snowfall.
While there have been few things that gone precisely as we anticipated, a gradual transition to a colder second half of winter had occurred this month. Artic blasts arrived, but remained generally transitional — as we anticipated. The high-latitude blocking that occurred earlier prior to blizzard, has regressed for at least the time being. The current pattern will support temperatures slightly above average to end this month. This January will likely still wind up close to +1 to +2 –slightly above average– with temperatures as we forecasted.
Model and ensemble guidance
As we look ahead, model guidance shows deep trough digging and amplifying over Western and Central US. This will cause a strong ridge to build over Eastern US. Low pressure will develop over the Southern Plains then intensify and track into the Great Lakes. Ahead of this storm, above normal temperatures are likely for the local region for the early and middle part of next week. A cold front associated with this low will be moving through with heavy rainfall and gusty winds possible on Wednesday. Behind this front will be is another cold airmass that will bring temperatures back down to near to below normal.
Longer range ensemble guidance for the following week indicates a trough near the Aleutian Islands, inducing a -EPO/+PNA pattern and an artic outbreak for the United States. A ridge developing over parts of the Northeast Pacific and West Coast, working in tandem with a trough expanding from the Central to Eastern United States, will allow arctic air to invite the Midwest and Central Plains. Temperatures in the Eastern United States should gradually trend toward below normal. The GFS and ECMWF ensembles are indicating a number of systems moving through during this pattern, which could evolve into winter storm threats for our region as time goes on.
What is the El Nino, PDO, MJO really doing going forward?
Let’s start with this fact: The El Nino has passed its peak. SST departures have continued to very slowly decline since November. With that said, the El Nino remains very strong and a dominant feature in the global pattern. Last week the SST departure for Region 3.4 was at 2.5. Earlier CFS model projection indicated that Region 3.4 would weaken 2.0 in January. The latest projection show a slower decline in this region through early Spring. Another indicator that El Nino influence remains very strong, is the Southern Oscillation Index (SOI). 30-Day Moving SOI values have declined over the past 15 days. This latest value, at -22.70-, has leveled off recently. A lower negative value indicates below normal pressures over Tahiti and above normal air pressure at Darwin. That supports stronger El Nino conditions over the Tropical Pacific. The El Nino has not weakened as rapidly as we anticipated.
However, warm SST anomalies have weakened over Eastern Pacific and strengthened over the Central Pacific this month. This is likely due weaker trade winds allow more upwelling of cooler subsurface waters, after the last Kelvin wave propagated east. The past 30 days has featured tropical forcing on average between 180W and 150W. This has a supported a trough/low further southwest over the Gulf of Alaska or Northeast Pacific. This is extremely dissimilar to the pattern in January of 1998 — the much discussed analog year –, which had tropical forcing closer 120W, supporting a Gulf of Alaska trough/low crashing into the West Coast. This year’s tropical forcing is also similar to other basin-wide El Nino forcing events, such as January 1958. That winter featured a colder than normal February with high-latitude blocking and above normal snowfall for New York City.
MEI and ONI values currently place this El Nino event in “Super” status close to 1997-98, which was a very mild and snowless winter for our area. However, there have been major differences in the 500mb mean pattern for January between those years. January 2016 has so far featured a trough near the Aleutians, a stout ridge over Western Canada and Greenland blocking and Polar vortex that extended from Hudson Bay into Southeast Canada. The atmospheric pattern during January of 1998 was highly unfavorable for our area compared to the pattern which has evolved in our area, and is expected to evolve over the next few weeks.
The PDO value from JISAO was +1.01 in for December. Warm sea-surface temperature anomalies have weakened and diminished in coverage across the Northeast Pacific, since Autumn. But +PDO greater than 1 is still remain for the rest of winter. This could still enhance -EPO/+PNA patterns for the next month.
The MJO will likely have significant influence on the atmospheric pattern for February. The ECMWF weeklies forecast the MJO to propagate through phases 4 and 5 during the first week of February. These phases, during Nino years, support the AO, NAO, or EPO in positive modality and ridge over East Coast with above average temperatures during the first week February. Then the MJO is forecast to propagate faster through phase 6,7, and then into 8 during the last two weeks of February. This supports a gradual trend towards a positive PNA modality and a negative AO, NAO, or EPO with more high-latitude blocking. Thus favoring more colder temperatures and storminess for the Eastern US.
Will there be a Sudden Stratospheric Warming Event?
A wave-1 stratospheric polar vortex displacement has been taking place with a minor stratospheric warming event this week. 10 hPA zonal winds are forecast to decrease significantly to near average, but then rise again, early next week. However, towards the second week of February, the model and ensemble guidance shows even stronger wave-1 polar vortex displacement. This causes the stratospheric polar vortex at various levels to elongate more north to south over Greenland and Eastern Northern America. A strong ridge forms over the Aleutian Islands or Bering Sea. 10 hPA temperatures start rising again with big spike in heat flux. Zonal winds at 10 hPA also start to decrease again. This might be first indications of a Sudden Stratospheric Warming event.
The polar vortex will be very disrupted, but remains resilient on the long-range model and ensemble forecasts. A wave-2 stratospheric polar vortex split is not being forecast yet. Overall, we aren’t anticipating a complete breakdown of the stratospheric polar vortex for February. However, this is not necessary for more wintry weather in our region. We have seen high-latitude blocking, artic blasts, and a historic blizzard this past month, without one. If it one occurs, it only aids in prolonging -AO/NAO blocking patterns for several weeks. Wave-breaking events–related to tropical forcing or mountain torque activity–will likely continue to cause more disruptions to the polar vortex in various levels of troposphere and stratosphere. Therefore more episodes of high-latitude blocking are likely, during middle to late February.
The QBO continues to be moderately strong westerly phase. The monthly value for December at 30mb was +11.39. The QBO peaked in October with a value at 30mb of +13.38. The zonal wind chart shows the QBO still +10.00 are above. A 30mb QBO monthly value +10.00 or greater typically supports a stronger stratospheric vortex and positive NAO during winter. However, we still observed a disturbed stratosphere polar vortex and a negative -AO/NAO this month. Our moderate to strong El Nino with +QBO analogs support a trough closer to the Aleutian Islands and high latitude blocking for February.
Conclusion and Expectations
In conclusion, our winter forecast for February and snowfall has not changed. We are currently anticipating a gradual transition to colder and stormier pattern again with a trend towards a positive PNA and negative AO/NAO/EPO modalities as we go into the second and third weeks of the month. The subtropical jet will also be active with more storm threats with wintry precipitation. As synoptic features in the pattern transition are in constant flux, these threats may evolve differently on each run of the models or ensembles. However, the potential for another major snowfall event will be increasing during the middle and later part of month.
This pattern progression is actually similar to what we recently observed in January. While it will play out differently, for sure, the parallels can be drawn from the pattern progression — with multiple chances for snow and colder than normal temperatures beginning around mid month and continuing through the end of February.