For Tim’s  2011-2012 Washington DC Area Winter Outlook please click here.

Nobody will forget “Snowmaggedon” or the “Snowpocalypse” that crippled the National Capital region during the winter of 2009 – 2010.  The pattern that created the ‘parade of blizzards’ across the Mid-Atlantic region from Washington to Philadelphia was unprecedented. A southern storm track was expected given the strengthening El Nino in the Pacific ocean but the consistent feed of cold air spilling into the region out of Canada was quite impressive.  The North Atlantic Oscillation and Arctic Oscillation remained extremely negative for much of the winter months creating the perfect set up for a parade of Nor’easters to ride up the Mid-Atlantic coast.

NASA’s GOES project put together an amazing satellite time-lapse of the unprecedented pattern which left the Nation’s Capital, Baltimore and Philadelphia buried in snow. Click here to see NASA’s time-lapse depicting the very active southern branch of the jet stream (high clouds moving west to east from Baja California to the Gulf coast) responsible for the active storm track cutting across the Gulf coast and up the Mid-Atlantic coastline.

Source: NOAA Climate Data 1887 – 2010

After the snowiest winter on record in Washington followed by the hottest summer since 1980 what will the Winter 2010 -2011 season bring?  La Nina conditions have replaced the strong El Nino pattern of last winter.

According to NOAA, a moderate to strong La Nina will persist through much of Winter 2010 – 2011 (Image below depicts the cooler sea surface waters along the equatorial pacific, circled in red).

La Nina conditions have developed and are expected to persist through Winter 2010 – 2011

La Nina winters are typically mild with below average snowfall. However…The La Nina winter of 1995 -1996 was one of the snowiest winters of the past 20 years when the Blizzard of 1996 (January 7, 1996) crippled the Washington Area with 17 to 30″ of snow.  Another La Nina winter in 1999-2000 brought a poorly forecast snowstorm out of North Carolina dumping 2 feet of snow on Raleigh and over 18″ of snow in Washington’s southern and eastern suburbs.

The storm track this winter will vary and will be highly dependent on the cyclical behaviors of the Pacific North American (PNA) ridge, North Atlantic Oscillation (NAO) and Arctic Oscillation (AO). The graphic above shows why a positive North Atlantic Oscillation (NAO) or “progressive/zonal flow” between the Azores and Greenland keeps the arctic air across Canada and the storm track to the north and west of Washington (along the northern branch of the jet stream). If you enjoy mild, snow-less winters you had better hope that the NAO remains positive.

• Summer 2010 featured a pattern of “blocking” as stubborn ridges of high pressure kept much of the United States (southern plains, southeast and Mid-Atlantic) and eastern Europe (Russia) baking in triple digit heat.

• If high latitude ‘blocking’ occurs during the upcoming winter, one Nor’easter could bring a decent snowfall to the Washington Area. The jet stream that travels around the globe is like a fast-moving stream of water. Think of “blocking” as  large rock thrown into the middle of the stream.  The jet stream is diverted around the ‘block’ creating a highly ‘amplified’ pattern that you see in my graphic below.

• The Greenland high or “Greenland Block” causes the jet stream to ‘buckle’ or drop drastically far south unleashing arctic air into the continental United States.  The trough, or ‘buckle’ in the jet stream (U-shaped dip) causes cold air to clash with the warmer Gulf Stream off the southeast US coast.  These ingredients along with disturbances riding the jet stream (river of fast-moving air) produce the powerful Nor’easters that give the Washington Area its snow storms.

Bottom Line: A negative NAO can throw a wrench into an overall mild pattern as it did a week before the Blizzard of 1996 when temperatures in the Washington Area were in the 70s in early January!

My Prediction for 2010 – 2011

I spent a great deal of time digging through DC climate data and found similarities between the current pattern (hot, dry summer), active hurricane season, negative PDO, positive AMO, SST anomalies/ENSO cycle/strength and here is what I think:

Temperatures:

• Based on the analogs I used temperatures may run 2 degrees above average (December, January and February).  However, before you pack away your winter coats, La Nina winters feature a jet stream which can vary greatly… there will likely be several arctic outbreaks.

Snowfall:

• On average, the storm track will stay north and west of Washington for much of the winter as storms enter the Pacific northwest and traverse the mid-west before exiting off the New England coast.  However, throw in some blocking (negative NAO, positive PNA, negative AO) and a Nor’easter could deliver a decent snowfall to the area. Within the Northeast Megalopolis – I think New England has the greatest chance of receiving above average snowfall .

Bottom Line: I expect slightly below average to near normal snowfall in the Washington Area.

Average Seasonal Snowfall for the Washington DC Region: