The image above (courtesy NOAA) shows sea surface temperature departure from average (anomalies) indicating weak La Nina conditions across the Equatorial Pacific. Notice the blue colors off the western coast of South America stretching westward along the equator indicative of cooler than average sea surface temperatures.
The graph below (courtesy CPC/IRI) shows a 70 to 80% chance that La Nina conditions will continue through Winter 2020-21.
La Nina Winters in Washington
During the six most recent weak La Nina events, the D.C. Region had slightly above average temperatures with near average to slightly below average precipitation during December, January and February.
Temperature Departure from Average
The previous six weak La Nina winters of 1995-96, 2005-06, 2007-08, 2010-11, and 2017-18 produced the following temperature anomalies across the United States. Keep in mind that this does not mean that the entire winter was milder in the east. The map below is a three month average (December, January and February).
Precipitation Departure from Average
The previous six weak La Nina winters of 1995-96, 2005-06, 2007-08, 2010-11, and 2017-18 produced the following precipitation anomalies across the United States. The map below is a three month average (December, January and February).
A strong northern branch of the jet stream is expected to bring some much needed rain and snow to portions of the Pacific Northwest and a storm track in the east should favor above average precipitation from the Ohio valley northeastward to the eastern Great Lakes.
Not all La Nina Winters are the Same & Other Factors to Consider
The strength of the La Nina has quite the impact on temperatures and there are other factors during the winter such as the behavior of the North Atlantic Oscillation (NAO) index as well as the Madden Julian Oscillation (MJO) that directly impact the behavior of the jet stream and storm track.
The images below show the variability of winter temperatures during recent strong, moderate and weak La Nina winters. Bottom Line: Winter Forecasts are rarely 100% accurate and if this La Nina event strengthens, the forecast will change.
Recent Weak La Nina Winters and Resulting Seasonal Snowfall totals at Washington National Airport
Most weak La Nina winters featured a storm track that cut through or just west of the Appalachian mountains, producing more wintry mix events in the D.C. Area. In these scenarios, cold air would become dammed up against the eastern slopes of the mountains allowing for snow to fall initially before the storm system passing to our west brings warmer air aloft changing snow over to the dreaded sleet and freezing rain.
Then there was also the famous winter of 1995-96 (also a weak La Nina winter) that produced the blizzard of January 1996.
Tim’s Winter 2020-21 Seasonal Snowfall Forecast:
The image below shows the percent of average snowfall that I expect across the Washington Region. For example at Washington National Airport, with a more western storm track (on average), I would suspect that DCA would receive only half (50%) of their average (15″) seasonal snowfall.
Average Snowfall Across the Washington Region during the Winter months:
Tim’s best guess for seasonal snowfall totals during the upcoming Winter across the Washington Metropolitan Area:
Accurately predicting snowfall more than 120 days out is honestly a long shot because there are simply too many variables in the teleconnection pattern that will ultimately dictate the behavior of the jet stream and resulting storm track.
For those of you that skipped much of my analysis and went straight to the map above, remember that the winter of 1995-96 had a very similar hurricane season to the one currently (lots of Atlantic storms!). In January of 1996 the Washington Area was basking in springlike temperatures in the middle 60s at Dulles, BWI and DCA. Several days later, the jet stream buckled and produced the Blizzard of 1996 which paralyzed the Mid-Atlantic.