Drought vs. Rainfall Deficit

Maryland Drought Monitor Index (May 16, 2017)

May 2017 has officially become DC’s first wetter than average month since last May following yesterday’s rainfall that brought the monthly total to 4.18”.  Over the first three weeks of May, there have been 12 days with rain.  More rain in the forecast could help make this the first cooler than average month of 2017.

Prior to this month, Washington, D.C. has had only three wetter than average months since August 2015.  That helped create a rainfall deficit of 13.75” as of April 30, 2017.  Over the same period of time, the Nation’s Capital has also experienced a significant amount of warmer than average temperatures.  That includes three of DC’s warmest months on record (December 2015, February 2016 and April 2016).  The combination of above average temperatures and below average precipitation over a sustained period of time led to moderate drought conditions for much of the Mid-Atlantic Region.

As recently as the March 28, 2017 Drought Monitor Index, a portion of the DC Metro Area was under “severe drought” conditions.  However, near average April rainfall helped eliminate the severe drought conditions.  Since April 20, 5.04” of rain has occurred at National Airport, more rain than Washingtonians got from September 1 through November 30, 2016.  Although that hasn’t made a significant dent in the area’s rainfall deficit, it did help eliminate the drought conditions according to the most recent Drought Monitor Index.  

Now while there aren’t any current drought conditions in the Mid-Atlantic Region, there is still a rainfall deficit dating back to 2015.  It is possible for an area to have a long-term rainfall deficit and drought conditions.  However, if there are a wet few weeks that eliminate drought conditions, the deficit could remain depending on how large it is.  It’s important to point out that if an area has a sustained period of below average rainfall than the entire deficit cannot be effectively eliminated in a short period of time.

The Mid-Atlantic Region has had a short-term rainfall deficit on several occasions in recent decades.  For example, Washingtonians experienced rainfall deficits at different stages of the hot 1995, 1999 and 2011 summers.  However, those rainfall deficits were quickly made up when the remnants of decaying tropical systems produced a significant amount rainfall.

Just How Hot Was this Week in the Nation's Capital?

Keeping Cool in Great Falls, MD

Washingtonians got their first real taste of summer this week with temperatures and relative humidity more characteristic of Independence Day.  Although downtown Washington, D.C. didn’t meet the official criteria of a heat wave (at least 3 consecutive days at or above 90°), Dulles and BWI Airports both did.  

In fact, Dulles and BWI Airports experienced some record heat this week while National Airport fell short.  This week’s summer-like temperatures represented a major shift in the weather pattern compared to the cooler than average first half of the month.  It was also the warmest May weather in the Nation’s Capital since 2015.  It’s also important to point out that May 2015 was the warmest May on record in Washington, D.C.

The hottest temperature this week at National Airport was 93°.  Although not a record high, yesterday’s temperature was DC’s hottest May temperature since May 31, 2011 (98°).  It was also the third 90° day in the Nation’s Capital of the year.  The third 90° day didn’t occur last year until June 19.  However, most DC residents will remember last summer for the very hot weather that occurred in July, August and September instead of the cooler than average May 2016. 

Although average high temperatures are in the 70s for most of May in Washington, D.C., warmer temperatures are fairly common.  Since 1993, the Nation’s Capital has averaged approximately four May days with temperatures of at least 85° and a little over one day with 90° temperatures.  Over the last two and a half decades, Washingtonians have seen May temperatures of 95° or greater roughly every four years.  According to the National Weather Service, Washington, D.C.’s earliest 100° temperature was on June 5, 1925.

While the first two weeks of this month were more than 2° below average in Washington, D.C., this week’s heat helped push May’s average temperature to being 0.3° above average through May 19.  However, the weather pattern has shifted again and NOAA expects below average temperatures into early June.  While a warmer than average day or two is entirely possible between now and then, Washingtonians could end up with their first cooler than average month since last May.