Is it Too Early to Think About Snow ?

The DC Area has had its fair share of memorable winter storms with four of DC’s Top Ten snowstorms over the last 15 years.  According to NOAA’s recently released winter weather outlook “…La Nina has a 55- to 65-percent chance of developing before winter sets in.”  That’s not necessarily good news for area snow lovers.

Episodes of La Nina tend not to create favorable conditions for above average snowfall in the Mid-Atlantic Region.  However, should the right set of circumstances develop, than a large snowstorm can develop regardless of La Nina.  The Nation’s Capital has had record snowstorms in what were otherwise relatively snow-free winters.  Such was the case during the 2015-2016 winter season when DC residents saw only 4.4” of snow outside the city’s fourth largest snowstorm on record (on January 22-23, 2016).

Forecasting snow is especially challenging in the DC Metro Area because of the local geography.  The Potomac River and Chesapeake Bay offer moderating influences on temperature that can make a big difference in precipitation type.  The “urban heat island” effect of DC Metro Area can also impact precipitation type and amount during winter storms.  Areas north and west often see considerable more snow than areas downtown and south and east of town.

That begs the question of what “average” snowfall is in the Nation’s Capital.  According to the National Weather Service, Washington, D.C. averages 15.4” of snow annually (measured at National Airport), while the more rural Dulles Airport away from the Potomac River averages 22”.  There can be a difference of several inches in snow totals over a small distance, like between National Airport and areas downtown.

Storm track is of paramount importance when forecasting winter precipitation and accumulations.  If the storm tracks west of the DC Metro than Washingtonians are on the warm side of the storm and see primarily rain while if the storm tracks to the east of town than snow can occur.  However, if the storm tracks too far east than little, if any, precipitation occurs.  There have also been rare winters where more snow has occurred at National Airport than at Dulles such as in 2001-2002 (3.2” vs. 2.6”) and 1980-1981 (4.5” vs. 4.4”).  This should serve as another illustration of the nuance involved in winter weather in Nation’s Capital.  

Meteorologists look at large scale features such as La Nina or El Nino and the NAO or North Atlantic Oscillation when issuing winter weather outlooks.  But each season is different.  There have been very strong El Nino winters where Washingtonians experienced almost no snow as in 1997-1998 (0.1” at DCA).  Meanwhile, the 2015-2016 winter also featured a very strong El Nino event but was a snowier than average winter season.

For those people interested in how much snow may occur in the DC Metro Area during the 2017-2018 winter, you can check out the WUSA9 Winter Weather Outlook my colleagues and I released this past Friday.