A Cool and Wet September

 

September 2020 is poised to finish both cooler and wetter than average in the Nation’s Capital for the first time since 2006.  Through yesterday, DC’s average monthly temperature was 1.1° below average.  While temperatures today will be warmer than average with highs in the low 80s in the DC Metro Area, a major shift in the weather pattern is on tap as the week progresses.

The first half of September was warmer than average with only five cooler than average days through September 15.  But, the second half of the month couldn’t have been more different.  Temperatures were below average on eight consecutive days through September 22 and yesterday was DC’s first warmer than average day since September 11.  NOAA’s Climate Prediction Center expects much of the next two weeks – through October 11 – to be cooler than average in the DC Metro Area.

That’s in stark contrast to last fall when Washingtonians not only experienced the third warmest September on record, but also the warmest October temperature. The high of 98° on October 2, 2019 was DC’s hottest overall October temperature on record.  While September 2019 had nine days of 90° heat, there have been only two such days this month.  That would be DC’s fewest number of 90° September days since there weren’t any in 2011.

September has also been quite wet in the DC Metro Area, with measurable rainfall on 10 of the first 27 days of the month for a total of 5.22”.  Washingtonians have experienced some weather whiplash during the month of September in recent years.  September 2018 finished as DC’s fifth wettest with 9.73”, while last September was DC’s fourth driest with only 0.25”.

Rainfall in the late summer and fall in the Nation’s Capital can vary quite a bit depending on whether any tropical systems are present.  Some DC residents may recall the record rainfall in late October 2012 due to former Hurricane Sandy.  That helps illustrate that significant rainfall can occur well into the fall season.  Fortunately, nothing is currently brewing in the tropics during what has been a very active season. 

September 10 was DC’s rainiest day of the month with a daily record of 2.88”, while this month’s hottest temperature (91°) occurred on both September 3-4.  September’s coolest temperature of 46° occurred on September 20.