To start off my first long-range winter outlook, for the 2021-2022 season (Dec – Feb), I am going to start by looking at the SST anomalies, as well as the current state of the El Niño Southern Oscillation, and the role the current La Niña is having on our current and future weather patterns.
Currently we have a weak La Niña (Niño 3.4: -0.8°C) ongoing in the equatorial Pacific with relatively cool waters in the eastern and central portions of the equatorial Pacific. Another feature worth noting is the anomalous cold pool in the Gulf of Alaska with an anomalous warm blob stretching from Japan to the California coastline. This is associated with the negative phase of the Pacific Decadal Oscillation (PDO). Finally, I’d like to point out the anomalously warm waters off the Eastern U.S. coastline, especially in the Gulf of Maine.
That last point will allow for rapidly deepening coastal storms with the right storm track, as larger temperature gradients allow for greater baroclinic instability, leading to the development of mid-latitude cyclones and more rapidly-deepening storms. However, warmer-than-average SSTs off the northeast coastline could help draw in warmer air, especially along the coast.
Taking a look at the Tropical Tidbits SST Anomaly analogs for November, the years 2011, 2010, 2007, 1975, and 1973 all have similar SST anomalies through this period. These years will be noted in some of my Analogs below.
The first analogs I want to show are the temperature and precipitation anomalies for the above years (1973, 1975, 2007, 2010, and 2011). Using the 1981-2010 climate averages, there is a clear warm signal for much of the northeast, especially away from The Great Lakes. As for precipitation anomalies, there is not much of a signal across Pennsylvania, New Jersey, and New York. There is a slightly wetter-than-average signal across Connecticut, portions of Massachusetts, Vermont, New Hampshire, and Maine.
The next set of analogs I want to focus on relate to all the ENSO-cool, weak La Niña, and moderate La Niña winters (DJF). The top-left (or first) image is the 500mb Geopotential Height Anomalies for all ENSO-cool, weak La Niña, and moderate La Niña winters. The top-right (or second) image is the temperature anomalies for those years. The bottom-left (or third) image is the precipitation anomalies. Finally, the bottom-right (or fourth) image is the temperature anomalies for the last 9 back-to-back La Niña winters.
La Niña winters tend to focus colder air in northwest portions of the CONUS, but this winter may trend a little cooler as there is a cooler than average signal based on back-to-back La Ninas.
Going back to La Niña, another factor to consider is the region of coldest waters. Over the last several months, the region of coldest water has shifted eastward towards the eastern equatorial Pacific Ocean, away from the dateline (180°).
Below is the temperature anomalies for January – March years with an east-based La Niña and an easterly descending Quasi-Biennial Oscillation. Polar Vortex disruptions and Sudden Stratospheric Warming events are much more likely in an EQBO phase than in a WQBO phase. Latest model guidance is starting to see signs of more “attacks” on the Stratospheric Polar Vortex as we get into mid-to-late December, which could possibly bring colder conditions to the northern tiers of the U.S. by Late December / Early January, depending on where the PV gets disrupted and the magnitude of the disruption.
Storm tracks this winter appear to be mainly clippers and cutters with the potential for some Miller-B storm tracks.
Climate Forecast System (CFS) Temp/Precip forecasts for DJF
North American Multi-Model Ensemble (NMME) Temp/Precip forecasts for DJF
Climate Prediction Center DFJ Outlook
Keep in mind with the CPC’s outlook that these are probabilities. These do not tell you the expected departures from normal. They just tell you the likelihood of above-average, below-average, or equal-chances for either for temperature and precipitation.
Overall, I believe it is going to be difficult to get sustained cold throughout the winter with the ongoing -PDO and the resultant lack of blocking over Alaska (+EPO). Without the blocking over Alaska, mild Pacific air can flow into the United States, leading to warmer conditions especially across the Eastern U.S. Sustained blocking over Alaska (negative phase of the East Pacific Oscillation) would allow for more arctic air to be driven southward into the U.S. This does not appear to be dominant this winter, but with an easterly QBO and an east-based La Niña, this could favor slightly cooler conditions than last year. At least in the near term, I see shots of cold being replaced by milder temperatures. In terms of precipitation, I see western portions of Pennsylvania ending up slightly above-average with drier conditions towards eastern Pennsylvania. This is mainly due to a weaker subtropical jet and the decreased likelihood of coastal storms. Snow is likely going to be below-average as well across the Commonwealth of Pennsylvania. Storms will tend to cut across the northeast with some clipper-like systems diving down from Canada. There may be some Miller-B type storms, but I expect there to be more inland runners.
Winter weather enthusiasts will likely not be happy with the outlook for December. Below are model forecasts of the MJO (or the Madden-Julian Oscillation, as well as temperature composites for . The MJO is a tropical disturbance that propagates eastward around the global tropics with a cycle on the order of 30-60 days. The MJO has wide ranging impacts on the patterns of tropical and extratropical precipitation, atmospheric circulation, and surface temperature around the global tropics and subtropics. Phases 8, 1 and 2 are generally the better phases that support colder temperatures across the eastern U.S. The latest guidance through about mid-month depicts the MJO heading into phases 6 and 7, before returning to phase 6. Phases 4, 5, 6, and 7 tend to favor warmer conditions across the eastern U.S.
Another factor besides the MJO to monitor for December is the Stratospheric Polar Vortex. It is currently in a fairly strong state. The SPV at 10 hPa is fairly circular in nature, but slightly displaced from the North Pole. Looking at the GEFS 35-day Zonal Winds, it appears that the Polar Vortex is likely to remain strong with only one member with easterly winds. Zonal winds are positive when the winds are westerly, and the zonal winds are negative when the winds are easterly. A strong polar vortex is associated with strong westerly winds while a weak polar vortex is associated with weak westerly or any easterly winds. A weak polar vortex allows for the increased likelihood of arctic air outbreaks across North America, Europe, and/or Asia. At this time, it does not look like we will be seeing a weak polar vortex through at least mid-December. It is worth noting that disruptions to the SPV do take time to trickle down to the troposphere on the order of 1-2 weeks. So if the SPV gets disrupted by mid-December, it could take until the end of December to early January for its impacts to be felt at the surface.
Lastly, I want to show the GEFS Weekly Regimes, as it helps give an idea of the 500-mb height anomalies over the course of five weeks. For each week, ensemble members are placed into four different regimes (Arctic High, Arctic Low, Alaskan Ridge, and Pacific Trough). The regime with the most members indicates higher confidence for that 500-mb height anomaly configuration, but it is definitely not a forecast or a guarantee that it will happen, especially as you get farther out in time. Weeks 2-5 as of the latest model run are suggestive of the Arctic Low regime with blocking over the eastern U.S., which would result in a milder outlook with storms primarily tracking through the northeast rather than becoming coastal storms.