Why was last year so snowy? Part I
Winter outlook preview
By Wes Junker, Winter Weather Expert
(From the editor: Please welcome Wes Junker, who will serve as Capital Weather Gang's winter weather expert moving forward. Wes worked for 30 years an operational meteorologist at the National Weather Service and has written many scholarly articles on forecasting snow and extreme precipitation. Read Wes's full bio.)
(Over the next several days, Wes and other CWG contributors will recap last winter and put it into context. On Thursday, CWG will release its 2010-2011 Winter Outlook)
Last winter was the snowiest on record for much of the mid-Atlantic region from Washington D.C. to Philadelphia, Pa. Baltimore shattered its old record with 77" compared to the 62.5" from the 1995-1996 season. Washington received 56.1" of snow which broke the snowfall record of 54.4" that was previously set in 1899.
Three of the heaviest 10 snowstorms on record for Baltimore and two of the top 10 for Washington DC occurred last winter. In retrospect, a few readers probably have wondered why last year was so snowy compared to other recent years.
This two part article will attempt to address that question.
Several atmospheric oscillations were aligned in a manner to increase the chances of receiving more snow that normal. The El Nino Southern Oscillation (ENSO) is the most recognizable of the group.
ENSO has two phases (El Nino and La Nina) typically taking two to four years to transition from one phase to the other. During the two phases, there are large scale differences in the atmospheric circulation patterns across the tropical Pacific that have a marked impact on trade winds.
During the cold phase of ENSO (La Nina), enhanced easterly trade winds across the Eastern Pacific lead to upwelling off the coast of South America and the winds blow the surface water away from the land mass. Cooler waters from below the surface rise and replace the warm surface waters. This colder water gradually shifts westward with time which helps keep the eastern Pacific water temps cold, the warmest waters in the equatorial Pacific are then located in the far western Pacific as is most of the convection.
However, during an El Nino event, the trade winds weaken and at times are replaced by westerly winds which lead to less upwelling and warmer than normal waters along the equator from South America westward to at least the date line. The changes in the atmospheric circulation pattern and warmer water temperatures across the eastern and central equatorial Pacific help produce more convection than normal across the region (see diagram below for how the location of the convection shifts between phases).
The differences in the location of the convection along the equator force changes in the circulation patterns at mid-latitudes, where we live. Below is a schematic representation of the differences in flow patterns and how they affect North America.
During an El Nino there often is a split in the jet stream with a very strong subtropical Pacific Jet that extends from the Pacific across the southern U.S. Waves in the atmosphere along this southern jet stream help to keep the storm track across the south leading to wetter and colder weather across the south into the southern mid-Atlantic region. Such a storm track increases the chances for a storm to take the perfect snow storm track for the Baltimore/Washington area. It takes a low across the south and then off the North Carolina coast near or just inland of Cape Hatteras.
However, El Nino years often lock the polar jet stream across Canada which prevents arctic air from penetrating into the U.S. resulting in warmer than normal temperatures across the northern tier of states.
Because of the favorable storm track, El Nino storms are more likely to be significant snowstorms than storms during a La Nina winter. Therefore, El Nino winters tend to average more total seasonal snowfall than normal across the south and mid-Atlantic (see figure to right) even though the frequency of days when Washington area receives measurable snow is not that different from an average winter.
Despite on average having more snow than other winters, El Nino winters can have a lack of Arctic air, especially during strong El Nino years when the equatorial Pacific is unusually warm and relatively mild Pacific air floods the country.
For example, during two strong El Nino winters (1972-1973 and 1997-1998) Washington recorded only 0.1" of snow, the lowest seasonal totals of any years since records have been kept starting in 1888. While El Nino raises the potential for snow, the phase is no guarantee that a season will be snowy. Cooperation with other more northerly atmospheric weather patterns or oscillations and a little serendipity is needed to get an exceptionally snowy winter.
Tomorrow, I'll discuss how these northern oscillations set up to support an exceptionally snowy winter last year.
| November 1, 2010; 10:45 AM ET
Categories: Capital Weather Gang, Snowmageddon, Winter Storms
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