Exceptionally Cool September Weather

 

DC’s coolest September weather in quite some time has come as a shock to the system to many.  That’s because September’s have typically been unusually warm in the Nation’s Capital over the last decade.  Not since 2009 have Washingtonians had a cooler than average September.  As cool as the last few days have been, temperatures will rebound to more seasonable levels later this week and that will allow monthly temperatures to return to near average. 

DC’s average monthly temperature for the first twenty days of September is nearly a full degree (0.8°) below average.  Only seven of the first eighteen days of September 2020 have been warmer than average.  By comparison, there were only three cooler than average days in the Nation’s Capital during all of September 2019. 

My colleagues and I on the WUSA9 Weather Team expect high temperatures to remain the 60s in the Nation’s Capital this afternoon.  That would be the third consecutive day with highs in the 60s.  That hasn’t happened during the month of September since September 16 – September 18, 2011.  Having high temperatures of less than 70° during the month of September has been rare in the DC Metro Area in recent years, with a total of only five such days during the last five years.

Low temperatures have been quite cool, also.  Frost and freeze advisories have been in effect for areas west of the immediate DC Metro Area the last two mornings, with lows falling to the 30s and 40s.  Baltimore tied its September 20 record low of 42° that occurred twice previously in the 1950s.  The coolest observed temperature the last few days at Dulles Airport was 41° on September 20 – just 1° from tying the record low of 40° set in 1979.  DC’s coolest temperature measured at National Airport was 46° on September 20.  However, the Nation’s Capital hasn’t set a record low temperature during the month of September since 1988.  That can be largely attributed to the significant increase in the intensity of the urban heat island effect in that time.  Today was the third consecutive September morning low temperatures fell to the 40s at National Airport. 

The last time daily low temperatures fell to the 40s in the Nation’s Capital on consecutive days during September was in 2000.  Low temperatures were in the 40s in the Nation’s Capital on four out of five days from September 26 – September 30, 2000.  So, it would be accurate to say this is the coolest it’s been in September in 20 years.

Large temperature swings are common in the DC Metro Area during the fall when a dry air mass is in place.  Such will be the case this week when high temperatures reach the 70s again tomorrow after three consecutive days in the 60s, with highs around 80° again on Wednesday.