After a predominantly mild and wet December, it may be tempting to give up hope for the return of winter weather in Washington. As the image above (courtesy NOAA) indicates, temperatures across much of the eastern two-thirds of the United States have been running well above average the past thirty days. So what’s in store for the Washington Region over the next several weeks? Specifically, is a return to a colder weather pattern on the horizon? Meteorologists will look at variety of “teleconnections” or relationships between large-scale features in the mid to upper levels of the atmosphere that offer insights into the medium-range forecast. Four key teleconnections indicate that a significant pattern change may be in store for the eastern United States as we head into mid and late January.
1. The Madden-Julian Oscillation (MJO)
The Madden-Julian oscillation is a significant driver of the weather pattern and consists of eight phases, each impacting global weather by shifting the location of tropical convection in a relatively short period of time, generally during the course of several weeks. During the winter months, various phases of the MJO can signal heavy rainfall events along the west coast of the United States or even be a precursor to predicting arctic outbreaks east of the Rockies. The image below (courtesy NOAA) shows typical temperature anomalies (departure from average) across the United States associated with each phase of the MJO in the month of January.
Forecasting the Madden-Julian Oscillation (MJO)
The image below (courtesy NOAA) shows the latest forecast of the MJO from the European (ECMF) forecast model. Current indications show the MJO shifting out of phase 7 and into phase 8 the second week in January 2019 signaling an end to the unseasonably mild conditions and a return to below average temperatures across the eastern United States.
Three Additional Key Teleconnections that Impact Global Weather Patterns
In addition to the MJO the latest forecast models indicate that three additional closely-watched teleconnections may be shifting into phases that are conducive for arctic air to begin intruding the eastern United States from mid January into February 2019.
2. The Arctic Oscillation (AO)
The Arctic Oscillation (AO) is a variation in strength of the polar jet stream across the high-latitudes of the northern hemisphere. The positive phase of the AO favors a strong polar jet stream or strong westerlies over the high-latitudes, conducive for maintaining a strongPolar Vortex and keeping arctic air locked up near the arctic circle. Conversely, the negative phase of the AO favors a weak Polar Vortex allowing arctic air to spill southward infiltrating the middle latitudes.
The following images (courtesy MAD Teleconnections) compare the surface temperature anomalies (departure from average) from January to April across the United States during both the positive and negative phases of the AO.
The latest European forecast model (image below, courtesy WxBell) indicates the AO may shift to its negative phase in mid January allowing arctic air to infiltrate the lower 48 from Canada.
3. The North Atlantic Oscillation (NAO)
The North Atlantic Oscillation (NAO) is a variation in the upper-level pattern across the north Atlantic ocean that has a significant impact on the weather conditions across western Europe and eastern North America especially during the later half of winter. The positive phase of the NAO favors above average temperatures across the eastern United States while a negative phase of the NAO favors below average temperatures. The image below (courtesy frontierweather.com) shows the two distinct differences in atmospheric height anomalies (departure from average) over the North Atlantic associated with both the negative and positive phase of the NAO.
The NAO remained neutral for the majority of December 2018, however the latest forecast from the European model (image below courtesy WxBell) indicates this teleconnection may shift to its negative phase in mid to late January, favoring below-average temperatures across the Northeast United States.
4. The Eastern Pacific Oscillation (EPO)
The EPO is a teleconnection pattern that influences the behavior of the jet stream pattern over the eastern Pacific ocean. The EPO has persistently remained in its positive phase (images below courtesy DaculaWeather.com) throughout the second half of December 2018 allowing for maritime (mild pacific air) to infiltrate the lower 48. During the negative phase of the EPO the pacific jet stream weakens and is shifted northward allowing for cross-polar flow from Siberia to infiltrate portions of the lower 48, typically east of the Rockies.
The latest European model (image below courtesy WxBell) forecasts the EPO to shift into its negative phase in late January and would be yet another sign that the upper-level pattern is becoming more favorable for well below average temperatures to impact the eastern two-thirds of the United States.
U.S. Climate Forecast System (CFSv2)
The image below (courtesy TropicalTidbits.com) shows the latest forecast for Geopotential Height Anomalies across the northern Hemisphere through early February. Atmospheric “heights” refer to the thickness of the atmosphere that varies greatly across the globe. The blue colors indicate lower (thinner) atmospheric heights as compared to average and the red colors indicate higher (thicker) atmospheric heights. The behavior of the jet stream is greatly impacted by these Geopotential “height anomalies” therefore accurately forecasting where these ridges and dips (in the jet stream) set up is key to forecasting the weather beyond the 5 days.
This latest U.S. climate model is finally showing a significant teleconnection pattern change to include a ridge of high pressure forming over Greenland (-NAO); a ridge of high pressure developing across the eastern Pacific ocean (-EPO) and building heights (ridge) across the Arctic indicative of a weakening polar vortex (-AO).
These four teleconnections indicate that a major pattern change is in store for the Eastern United States as we head from mid-January into February.