Rainy Fridays and Key Weather Trends

NOAA's Severe Weather Outlook for April 26, 2019

It shouldn’t come as a surprise to Washingtonians that rain is in the forecast today on another Friday.  Since the start of the year, the Nation’s Capital has had measurable rain on 11 of 17 Friday’s.  If you count a trace of rain (rainfall that occurs but isn’t enough to measure), that number increases to 14 out of 17.  The wettest of those Friday’s came on March 1 (0.64”), while DC’s last dry Friday was March 29.  

Today’s expected rainfall can go a long way toward making April 2019 another wetter than average month in Washington, D.C.  Through April 25, DC has a monthly rainfall deficit of 0.92”.  If this month finishes drier than average, then April 2019 would become only the second drier than average month dating back to last April.

The first three months of 2019 were each wetter than average and continued the exceptionally wet weather pattern that made last year DC’s wettest on record (66.28”).  From April 1, 2018 through March 31, 2019, Washington, D.C. had 69.45” of rain.  That means DC residents had a rainfall surplus of 29.71” – making it DC’s wettest 12-month period on record.  By comparison, the calendar year of 2018 had a slightly lower rainfall total because January 2018 and March 2018 were each drier than average.  For the 30-year period from 1981-2010, Washington, D.C. has averaged 39.74” of annual rainfall.

One of the key weather trends so far this year has been the frequency rain has fallen on Friday’s.  One of last year’s significant trends was the record number of days (24) that one inch or more of rain occurred in Washington, D.C.  During the first four months of 2018, the Nation’s Capital had four such days, but only two so far this year (through April 25).  

Making last year’s record rainfall seem more extreme is how the copious rainfall occurred after a prolonged stretch of dry weather in the Mid-Atlantic Region.  The Nation’s Capital saw 25 drier than average months in the 32-month stretch from August 2015 through March 2018 when DC accrued a rainfall deficit of 16.42”.  By comparison, the Nation’s Capital experienced a net swing of 46.13” – going from a deficit of 16.42” to a surplus of 29.71” for the 12-month period ending March 31, 2019.  This helps illustrate that while trends can dominate weather headlines, new trends can supplant old ones relatively quickly.

NOAA’s Storm Prediction Center placed a significant portion of the Mid-Atlantic Region under an “enhanced risk” for severe weather today.  That would make this the second consecutive Friday and third day since April 14 that a significant amount of severe weather has occurred in the DC Metro Area.