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.
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