Notable May Severe Weather Outbreaks

 

Photo Credit - hail: Kristen Leitch

May is the height of severe weather season in the United States.  While some outbreaks have impacted the DC Metro Area, the strongest tornadoes tend to form in the Midwest and the South.  Some significant May outbreaks that have occurred in recent years are.

2019:  Three tornadoes touched down in central Maryland on May 30, producing two EF-0’s and an EF-1 in Frederick and Howard Counties.  Wind damage was more widespread across the DC Area, with a wind gust as high as 66 mph reported at Davison Army Airfield in Fairfax County, Virginia. 

Just a week earlier, even more severe weather impacted the D.C. Metro Area.  An EF-1 tornado was confirmed in Columbia, Maryland that caused some structural damage.  There were many severe wind gusts from the thunderstorms that developed.  Wind gusted as high as 66 mph at National Airport, 63 mph at Andrews AFB in Maryland, and 67 mph in Gaithersburg, Maryland.  Damage was reported that ranged from downed trees and power lines to minor structural damage of homes and businesses.

2016:  Large hail sometimes occurs in the Mid-Atlantic Region.  On May 2, Rockville, Maryland, had hail 2.5” – 2.75” in diameter, ranging in size from tennis ball to baseball.  Baseball-sized hail was also reported in Charles County, Maryland, while golf ball-size hail occurred in Huntingtown, MD.  There were other reports of hail in the D.C. Metro Area that were smaller.

2013:  The last EF-5 tornado in the United States occurred on May 20 in Moore, Oklahoma.  It had a long and destructive track as part of a larger, three-day severe weather outbreak.  The strongest tornadoes often develop from something called a “supercell thunderstorm,” which is a long lasting thunderstorm that has a rotating updraft. 

One of the supercell thunderstorms that developed during this outbreak spawned the Moore, OK tornado.  Although not at EF-5 intensity during its entire track, it reached peak intensity near the town of Moore.  According to NOAA, it caused two dozen fatalities and remains the third costliest tornado on record having caused $2 billion in damage (unadjusted for inflation). 

2011:  The town of Joplin, Missouri experienced a devastating EF-5 tornado on May 22.  According to NOAA, the Joplin EF-5 remains the costliest tornado on record having caused $2.8 billion in damages (unadjusted for inflation).  With a total of 181 fatalities, it was also the single deadliest U.S. tornado since 1947.  The spring of 2011 was an unusually active severe weather season in the United States with a total of six EF-5 tornadoes.  That was the highest annual total since 1974.

 

DC’s May Weather Extremes

 

After a warm start to May, there has been a significant shift in DC’s weather pattern with several cooler than average days ahead.  That’s not uncommon as May is a transitional weather month in the Nation’s Capital.

Temperatures have been warmer than average on four of the first six days of the month, including highs in the 80s on three days.  By comparison, DC’s first 80° day last year didn’t occur until May 15.  The first half of May, in particular, can feature a wide range of weather.  For example, DC’s first heat wave of 2018 occurred from May 2 – May 4 when high temperatures reached the 90s.  A “heat wave” is defined as a minimum of three consecutive days with highs in the 90s. 

By comparison, last May not only didn’t have any 90° heat, but was also DC’s coolest May since 2005 with three days of highs in the 50s combined with two days of lows in the 30s.  While the Nation’s Capital has averaged roughly one May day with highs in the 50s dating back to the early 1990s, DC has averaged only one May day with lows in the 30s per decade in the last 30 years.

DC residents can expect an average of one to two May days in the 90s.  May 2015 was DC’s warmest May on record with a total of seven days of 90° heat, including two heat waves.  That was followed by May 2016, which had a combined 16 days with below-average highs in the 50s and 60s. 

Being DC’s wettest month of the year on average, May can either have a lot of rainy days or a few days with heavier amounts of rain.  For example, May 2018 finished with four days of 1” or more of rain.  Meanwhile, May 2019 had 13 days of measurable rainfall in the Nation’s Capital without any exceptionally wet (1”/+) days.  May 2021 has gotten off to a similar start with three days of light rain and several more days of occasional light rain in the forecast during the next week. 

May also tends to be an active severe weather month across much of the United States.   Some communities in the DC Metro Area can see very heavy rainfall depending on where thunderstorms occur.  Such was the case this past Monday, May 3, when severe weather, including a tornado, occurred in Frederick County, Maryland.  While that damaging weather event occurred north of the immediate DC Metro Area, only 0.06” of rain was officially measured at National Airport.  This underscores the need to pay close attention to the latest weather forecasts during the spring and summer months when showers and thunderstorms often threaten.

Feeling More Like Summer

 

As the final month of meteorological spring, May often features some very diverse weather in the Nation’s Capital.  Unlike March or April that can still have chilly temperatures, summer-like heat become more common during May.  Temperatures continue to rise throughout the month as both the amount and intensity of the daily sunlight increases with the rising sun angle.

Temperatures in the Nation’s Capital range from an average high/low of 71°/52° on May 1 to 80°/61° on May 31.  DC’s hottest May temperature on record is 99° that occurred on May 31, 1991.  By comparison, the coldest May temperature was 33° on May 11, 1906.  Seven of the last 10 May’s through 2020 were warmer than average, while seven of the 10 from 2001 – 2010 were cooler than average. 

With a monthly average temperature of 63.8°, last May was DC’s coolest since 2005.  However, four of DC’s 10 warmest May’s have occurred in just the last decade.  There have been exceptions as May 2016 finished more than 2° cooler than average and had five days with March-like highs in the 50s.  Conversely, May 2018 got off to a hot start with highs in the 90s on May 2 – May 4, but finished as DC’s third warmest May. 

May has the highest average monthly rainfall in the Nation’s Capital with 3.99” for the 30-year period of 1981 – 2010.  Like with temperatures, there has been a near 50/50 split with 11 of the last 20 May’s being drier than average.  However, there have been exceptions as four of DC’s 10 wettest May’s occurred in the last 20 years. 

May is an active severe weather month in the DC Metro Area.  Looking at DC’s May climate records, one can see that on several dates a trace (enough to accumulate, but not enough to measure) of snowfall has occurred.  That wasn’t actually snow, but hail.  For example, on May 2, 2016 a severe weather outbreak produced a lot of hail in the DC Metro Area.  Small accumulations of hail were reported in spots such as at National Airport where a “trace” of frozen precipitation was observed.