What Makes May 2020 So Unusual ?

Although May 2020 had a warm and dry final few days, it has been a cool and dry month overall.  Through yesterday, DC’s average temperature this month was more than 2° cooler than average.  It will also finish as a drier than average month, making this May DC’s first cooler and drier than average one since 2006.

DC’s warmest temperature this month was 87° on May 29, while the coolest temperature was 37° on May 9.  This will be DC’s first May since 2016 without any 90° heat.  That’s unusual since DC’s average first 90° temperature of the year occurs on or around May 18.  By comparison, DC’s warmest May in 2015 saw seven days of 90° heat.  The Nation’s Capital also had its first record low-high temperature during the month of May since 2003 when it was only 52° on May 9.  

Having a cooler than average May without any 90° temperatures isn’t a good predictor of what the upcoming summer will be like, however.  Such was the case in 2016 when DC residents didn’t see the first 90° of the year until June 11.  Yet, the summer of 2016 finished as DC’s third warmest on record overall.

This month has also been drier than average.  Some may find that surprising considering the many rainy days.  Rain occurred on 18 days this month, but only produced a total of 2.49”, which is 1.5” below DC’s May average.  More than a third of that rainfall total occurred on May 3, when Washingtonians had 0.93”.  Following a cool and rainy April, the Nation’s Capital is in good shape rainfall wise with a rain surplus for the year of 1.27”.  

May 2020 will finish as a fairly unique weather month in the Nation’s Capital without any 90° heat.  That’s combined with the fact it’s been DC’s first cooler and drier than average May in more than a decade.  While NOAA is expecting June to be warmer than average with near average precipitation in the DC Metro Area, its three-month forecast calls for wetter and warmer than average conditions. 

Severe Weather Can Strike in May

NOAA's Severe Weather Outlook for Today

May is one of the most active severe weather months of the year.  “Severe weather” is defined as a thunderstorm that produces any of the following: A) Wind gusts of at least 58 mph; B) hail 1” in diameter or greater; or C) a tornado.  The most dangerous thunderstorms (supercells) can have more than one type of severe weather as large hail often precedes tornadoes.  Locally, the DC Metro Area has had a number of memorable severe weather events in May.   

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 more severe weather impacted the DC 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 ranging from downed trees and power lines to minor structural damage of homes and businesses was reported.

2016:  Although relatively uncommon compared to other parts of the country, large hail does occur sometimes in the Mid-Atlantic region.  Such was the case on May 2 with several unusually large hail reports.   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 severe hail in the DC Metro Area that were smaller.

2008: Several tornadoes developed in north central Virginia on May 8.  A weak tornado also developed in Camp Springs, Maryland.  The strongest tornado in the DC Metro Area during this outbreak was an EF-2 in Stafford County, Virginia with peak winds of 120 mph.  A slew of severe wind and hail reports were noted as well in Maryland and Virginia. 

2002:  An F1 tornado was confirmed in Rockville, Maryland on May 13.  It was part of a larger outbreak that saw a number of severe wind and hail reports.  An F2 tornado occurred on May 2 north of Baltimore in Cecil County, Maryland with winds of 150 mph.  A weaker tornado was also reported in Rockville, Maryland.  There were also multiple reports of severe wind gusts and hail.

NOAA’s Storm Prediction Center placed part of the Mid-Atlantic region under a slight risk of severe weather today as a cold front approaches the East Coast.  My colleagues and I on the WUSA9 Weather Team will keep an eye on any weather watches and warnings that may be issued.

Looking Ahead to Summer

NOAA's Temperature Outlook for the 2020 Summer

The start of meteorological summer arrives on June 1 and is comprised of the three-month period of June, July and August.  NOAA’s three-month outlook issued on May 21 calls for warmer and wetter than average conditions in the Mid-Atlantic Region.  That’s in contrast to the long stretches of cooler and wetter than average weather in April and May.

During the six-week period from April 10 through May 21, 32 of the 42 days were cooler than average in the Nation’s Capital.  In that time, measurable rainfall occurred on exactly half of those days and added up to 7.29”.  Although a trace of rainfall is officially too little to measure, there were another 10 days since April 1 that DC had a trace of rain, which helped give the illusion of a lot of cloudy and damp weather. 

The first three weeks of May were quite cool in the Nation’s Capital with 16 cooler than average days.  Quite remarkably, there were more days (three) with highs in the 50s, than there were days in the 80s (two).  The final third of May 2020 will feature temperatures more characteristic of early June with highs in the 80s.  That will mitigate the impact of the cool start to the month in calculating the average monthly temperature.

Some may wonder if the dominant weather pattern of the spring is a reliable indicator of what the summer will be like.  The truth is not really.  One case in point was the spring of 2016.  May 2016 finished 2.1° cooler than average.  Moreover, Washingtonians didn’t experience the first 90° heat of 2016 until June 11.  That’s a few weeks after DC’s average first date of 90° highs.  But, the 2016 summer still finished as DC’s third hottest on record, behind only 2010 and 2011.
 

There hasn’t been any 90° heat so far this year.  NOAA’s seasonal forecasts look at a variety of factors such as the presence of El Nino or La Nina.  Following the predominantly cooler and wetter than average weather over the last couple of weeks, most DC Area residents would welcome warmer temperature.  Just not record-setting temperatures.