Weather Quiz

True or False.  May 2018 has become D.C.’s first May that ranks among the Top Ten warmest and Top Ten wettest on record.

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Answer to May 19 Weather Quiz question.

True.  May’s lowest record high temperature is 91°.  Interesting to note that record high was set earlier this month on May 2.

An Exceptional Weather Month Comes to a Close

May 2018 will finish as one of the warmest and wettest on record in the Nation’s Capital.  That’s somewhat unusual because months with ten or more rainy days tend to be cooler than average.  This month has had 11 days with measurable rainfall in Washington, D.C. and could finish with 12.

However, Washingtonians didn’t see the first measurable rainfall of the month until May 12.  As fate would have it, May 12 would be the first of eight consecutive days of rain in Washington, D.C.  That was D.C.’s longest streak since measurable rainfall occurred on 15 consecutive days ending May 11, 2016.  A total of 7.41” of rain has occurred so far this month in the Nation’s Capital.  Despite above average rainfall this month, temperatures were above average on 27 of May’s first 30 days in Washington, D.C. 

Washington, D.C. has a monthly average temperature of 72.2° through May 30 and that ranks as the third warmest May on record.  This month got off to a warm start with above average temperatures on ten of the first 11 days of the month.  That included D.C.’s first heat wave of 2018 with high temperatures in the low 90s from May 2 through May 4.  By comparison, Washingtonians didn’t experience the first heat wave of 2017 until June 11 – 14.

Severe weather also occurred in the D.C. Metro Area on May 14.  That was also D.C.’s wettest day of the month with a daily record 1.49” of rain at National Airport.  Aside from May’s warm temperatures and record rainfall, this month’s other major weather headline in the D.C. Metro Area is the flooding that occurred in Ellicott City, MD on May 27.  Many consider this week’s flooding in Ellicott City to be comparable to, if not worse than, the epic July 2016 flooding.  In both cases, the flooding resulted from the combination of a slow moving cluster of thunderstorms that trained, or moved over the same area over and over again, combined with favorable topography for flooding.

If May were to end yesterday, this month would finish as the third warmest and eighth wettest May on record in Washington, D.C.  However, with additional rainfall possible this afternoon, it is possible for this month to become D.C.’s seventh wettest May on record. 

D.C.’s Wettest May’s on Record (National Airport - "DCA")

1.  10.69" (1953, 1889)
3.  10.66" (2008)
4.  10.60" (1886)
5.   8.87" (1948)
6.   8.05" (2009)
7.   7.77" (1989)
8.   7.06" (2003)
9.   6.99" (1946)
10.  6.97" (1897)
Average: 3.99”

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D.C.’s Warmest May’s (DCA - in Degrees Fahrenheit)

1.  73.2° (2015)
2.  73.0° (1991)
3.  71.8° (2004)
4.  71.6° (1944)
5.  71.4° (2012)
6.  70.5° (1880)
7.  70.1° (1953)
8.  70.0° (1911)
9.  69.8° (1959)
10.  69.7° (1975)
Average: 66.0°

"Alberto" and the start of the 2018 Atlantic Hurricane Season

NOAA's Projected Track for "Alberto"  (Source: National Hurricane Center)

The 2018 Atlantic hurricane season has gotten another head start with a named storm prior to the June 1 start of the season for the fourth time in the past six years.  The National Hurricane Center determined that “Alberto” developed into a subtropical storm on May 25 near the Yucatan Peninsula.  The National Hurricane Center has recognized “subtropical storms” since 1972 and they have characteristics of both a tropical and extratropical storm.

Alberto is impacting parts of the U.S. Gulf Coast and the state of Florida with heavy rain and gusty winds along with some coastal flooding and isolated severe weather.  The National Hurricane Center doesn't expect Alberto to reach hurricane strength before it makes landfall tomorrow.  That means heavy rain and the potential for flooding will likely be the greatest dangers associated with Alberto.  

NOAA recently issued its seasonal outlook for the 2018 Atlantic hurricane season.  It calls for 10 to 16 tropical storms, of which 5 to 9 become hurricanes with 1 to 4 of those becoming major hurricanes.  A tropical cyclone becomes a tropical storm when sustained winds around the center reach 39 mph, while a tropical storm is upgraded to hurricane status when sustained winds reach at least 74 mph.  A hurricane is considered “major” when it reaches Category 3 status on the Saffir-Simpson Scale with sustained winds of at least 111 mph. 

The private weather forecasting company, AccuWeather, released a similar Atlantic hurricane forecast last month for 12 to 15 tropical storms, 6 to 8 hurricanes with 3 to 5 becoming major hurricanes expected in 2018.  Colorado State University (CSU) scientists also released a seasonal hurricane forecast last month that called for 14 tropical storms in the Atlantic Ocean, with 7 hurricanes and 3 major hurricanes.  CSU will update its forecast this week.  

An average Atlantic hurricane season has 12 tropical storms, 6 hurricanes and 3 major hurricanes.  By comparison, the record-setting 2017 Atlantic Basin hurricane season (that includes the Caribbean Sea and Gulf of Mexico) saw 17 tropical storms of which 10 became hurricanes.  Six of the 10 hurricanes that developed last season intensified into major hurricanes. According to the National Hurricane Center, last season saw the highest number of Atlantic tropical storms since 2012, as well as the most major hurricanes since 2005.  

NOAA is watching the possibility of a weak episode of El Nino developing during the hurricane season (June 1 – November 30) and, therefore, doesn’t expect this season to be as busy as last year.  El Nino refers to the periodic warming of the tropical east Pacific Ocean.  That helps create less favorable conditions for tropical storm formation in the tropical Atlantic, namely an increased amount of wind shear and slightly cooler sea surface temperatures.  Tropical storms and hurricanes rely on warm ocean water (commonly at least 80° Fahrenheit to a depth of at least 200’) as a fuel source.

People should remember that the overall number of tropical storms and hurricanes is less important than where they track.  The Atlantic Ocean can have a very active hurricane season like in 2010 when a total of 19 tropical storms including 12 hurricanes developed, but none had significant impacts in the United States.  Meanwhile, a quiet hurricane season, like in 1992 had just one major hurricane.  However, Hurricane Andrew remains only the third and most recent Category 5 hurricane to make landfall in the United States.

How Unusual Has D.C.’s Weather Been during May 2018 ?

May Rainfall has its benefits

Wetter than average months with lots of rainy days tend to be cooler than average months as well.  Such has been the case more often than not during Washington, D.C.'s ten wettest May’s.  However, that’s exactly what makes this May so unusual in the Nation’s Capital.  Washington, D.C. has had 7.37" of rain since May 1 and that's almost double the May average of 3.99". Meanwhile, D.C’s average May temperature (combining daily high and low temperatures) is 71.4° through May 23 (6.6° above average).

In addition to already being a unique weather month given D.C.’s warm and wet conditions, May 2018 has also featured some very streaky weather.  Ten of the first 11 days of May were warmer than average in Washington, D.C.  That includes four 90° days at National Airport (D.C.'s official weather reporting site).  By comparison, D.C.'s fourth day of 90° heat didn’t occur until June 11 last year.

D.C.'s first measurable rainfall this month didn't occur until May 12.  Washingtonians then had measurable rainfall on nine of the following 11 days with multiple rainfall records set in the process.  However, six of the 11 days from May 12 through May 22 were warmer than average in Washington, D.C. despite the rainy weather. That, combined with the very warm start to the month, has put May 2018 in a strong position to rank among D.C.’s Top Five warmest May’s on record.

According to the National Weather Service, eight of the ten warmest May's in Washington, D.C. were drier than average (the only exceptions were in 1953 and 1975). For example, while May 1953 tied for D.C. wettest, it was also the seventh warmest on record in the Nation's Capital.

A similar pattern exists at Dulles Airport where seven of its ten warmest May’s on record were also drier than average.  Dulles Airport has had 6.5" of rain (above the May average of 4.55") with an average monthly temperature of 68.2° through May 23. That would put May 2018 among the ten wettest and warmest on record at Dulles Airport. 

D.C.’s Wettest May’s on Record (National Airport - "DCA")

1.  10.69" (1953, 1889)
3.  10.66" (2008)
4.  10.60" (1886)
5.   8.87" (1948)
6.   8.05" (2009)
7.   7.77" (1989)
8.   7.06" (2003)
9.   6.99" (1946)
10.  6.97" (1897)
3.99” - Average

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D.C.’s Warmest May’s (DCA - in Degrees Fahrenheit)

1.  73.2° (2015)
2.  73.0° (1991)
3.  71.8° (2004)
4.  71.6° (1944)
5.  71.4° (2012)
6.  70.5° (1880)
7.  70.1° (1953)
8.  70.0° (1911)
9.  69.8° (1959)
10.  69.7° (1975)
66.0° - Average

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Dulles Airport's Top Ten Wettest May’s (IAD)

1.  10.26" (2009, 1988)

3.  9.38" (2008)
4.  8.71" (2003)
5.  8.49" (2017)
6.  8.47" (1971)
7.  7.71" (1989)
8.  7.07" (1996)
9.  6.34" (2014)
10.  6.08" (2016)
4.55” - Average

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Dulles Airport's Top Ten Warmest May’s

69.8° (2004)
69.3° (1991)
68.9° (2015)
68.5° (2012)
67.0° (2010)
66.9° (1965)
66.8° (1982)
66.3° (2011)
66.1° (2007, 1960)

63.2° - Average