Weather Quiz

Which occasion has the warmest average temperatures in Washington, D.C. ?

A.  St. Patrick’s Day

B.  Columbus Day

C.  Halloween

D.  Veteran’s Day

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Answer to October 19 Weather Quiz question.

False.  The Nation’s Capital averages a trace of snow in October.  That’s not enough to measure.  The last time Washingtonians experienced a trace of October snow occurred was on October 29, 2011.

What is typical Halloween Weather in the D.C. Metro?

Washingtonians can expect some pretty nice weather on Halloween.  Washington, D.C.’s average high and low temperatures on October 31 are 64°/46° respectively.  Temperatures were near average last Halloween in the Nation’s Capital (62°/48°).  Today’s high temperatures have been near average in the upper 50s and low 60s.

The warmest Halloween on record in the Nation’s Capital occurred in 1950 when it was 85°.  For perspective, 85° is Washington, D.C.’s average high temperature on August 26.  Meanwhile, D.C.’s coolest Halloween came in 1925 when the high temperature was only 41°.  By comparison, D.C.’s coldest average high temperature is 43° from January 2 – 22.  According to NOAA, the last time measureable rainfall occurred in Washington, D.C. on Halloween was in 2009.

The warmest temperature this month (86°) occurred twice on October 9 and 10.  Meanwhile, Washington, D.C.’s coolest October temperature occurred on October 27 (43°).  By comparison, last October’s warmest/coolest temperatures in the Nation’s Capital were 87° (October 19, 2016) and 41° (October 26, 2016).  This month had six consecutive days with high temperatures in the 80s (October 5 – 10).  Record high low temperatures also occurred on four consecutive days (October 8 – 11) in Washington, D.C.  This month also had high temperatures in the 70s on 17 days.  By comparison, high temperatures remained in the 50s and 60s on a combined 8 days. 

However, October has concluded with more seasonal temperatures over the final week in the Nation’s Capital.  Through October 30, Washington, D.C.’s average monthly temperature is 65.3° (combining daily high and low temperatures).  Once today’s temperatures are factored in, this October should finish as the third warmest on record in Washington, D.C.  Meanwhile, DC’s warmest October on record occurred in 2007 when temperatures reached the 90s three times.  Washingtonians also saw 11 other days in the 80s in October 2007, compared to six this month.

This month will finish as D.C.’s third consecutive drier than average (3.40”) October with 2.02” of rain.  The current streak of three straight drier than average October’s follows six consecutive wetter than average Octobers in the Nation’s Capital.  NOAA expects near average temperatures and precipitation for the Mid-Atlantic Region in November.

Washington, D.C.’s Warmest Octobers

1.  67.1° (2007)
2.  65.2° (1984)
3.  64.7° (1971)
4.  64.5° (1947)
5.  64.4° (1941)
6.  63.4° (1949)
7.  63.3° (1973)
8.  63.2° (1975, 1919)
10.  63.1° (2016)

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.

Rainfall Deficit Continues to Grow

Washingtonians have been fortunate to have warm and dry conditions for much of the fall to this point.  However, gardeners and allergy sufferers have a heightened awareness of just how dry it has been.

The drier than average fall in the Nation’s Capital continues the trend that began in August 2005. Since then, 21 of the last 27 months (including this month) have been drier than average.  Consequently, the Nation’s Capital has a rainfall deficit of 11.72” for that period (through October 24).  Washingtonians have a rainfall deficit of 6.05” just since October 1, 2016.  

This year has featured some moderately improved conditions, with May, July and August all being significantly wetter than average in Washington, D.C.  In fact, July 2017 was DC’s wettest since 1969 with 9.15”.  Consequently, Washington, D.C. actually has a rainfall surplus since January 1, 2017 of 0.7”.  However, that total is down significantly from September 1, 2017, when DC residents had an annual rainfall surplus of 3.25”.  

This turn of events can be directly attributed to how dry this fall has been in the Mid-Atlantic Region.  The dry spell has coincided with warmer than average temperatures as well.  Through October 24, Washington, D.C. has had six days this month with high temperatures in the 80s, the highest such total since 2007 (11).  

NOAA recently issued a winter weather outlook which calls for a “55- to 65-percent chance of La Nina developing before winter sets in.”  The opposite of El Nino, La Nina typically favors warmer than average conditions in the Mid-Atlantic Region during the winter.  So, snow-lovers tend not to like La Nina winters in the DC Metro Area.  

This warm October, combined with the expected La Nina, may or may not be a good predictor of winter.  Of the 10 warmest Octobers on record, six of the following winters saw at least a weak La Nina.  Seven of the following 10 winters saw below average snowfall in the Nation’s Capital.  Since this October is well-positioned to finish among the warmest on record, it will be interesting to see if the 2017-2018 winter is also warmer with less snow than average.  

However, a significant snowstorm can form under the right conditions in an otherwise warmer and drier than average winter.  DC residents can think back to January 2016 when the fourth largest snowstorm on record occurred.  The 2015-2016 winter was a warmer than average winter and outside of that record storm, DC residents only saw 4.4” of snow (less than a third of the 15.4” seasonal average according to NOAA).