D.C.’s Streaky September Weather

 

Avenel Park, Potomac, Maryland

Residents of the D.C. Metro Area are in store for a prolonged stretch of mostly sunny and stellar fall weather.  That follows yesterday morning’s heavy rain that’s becoming an increasingly distant memory for Washingtonians.  Rainfall totals of 1”-2” were common across the Nation’s Capital and nearby suburbs with higher totals further north and west of town.  That rain has helped guarantee that September 2021 will finish as a wetter than average month in Washington, D.C.

D.C.’s two-day rainfall total for September 22-23 of 1.63” helped push D.C.’s monthly total to 4.03”.  NOAA updated the temperature and precipitation averages for the United States earlier this year.  NOAA’s weather “averages” through last year were based on the 30-year average for the period of 1981 – 2010.  However, they now reflect the period of 1991 – 2020.  The updated weather averages in the Nation’s Capital are generally warmer and wetter because the 2010’s were a largely warmer and wetter decade than the 1980s.

Washington, D.C.’s new September rainfall average of 3.93” is greater than the previous average of 3.72”.  That’s somewhat contingent on the influence of rainfall from decaying tropical systems as was observed earlier this month.  Washingtonians saw widespread 1”-2” rainfall totals with the remnants of Hurricane Ida on September 1.

Following the wet start to September, D.C. residents had a relatively dry two-week period with a combined rainfall total of only 0.26” from September 2 – September 15.  Temperatures were also cooler than average on 7 of those 14 days.  That was followed by a streak of 11 consecutive warmer than average days from September 12 – September 22.  High temperatures reached 90° on two of those days (September 13, September 15).  This month’s first 90° temperature on September 13 made for D.C.’s latest first September 90° temperature since 2017 (September 24). 

With no more 90° heat expected during the next week, Washingtonians would have consecutive Septembers with two or fewer 90° days for the first time since 2003-2004.  Last September was a cooler than average month that also had only two 90° days and an average monthly temperature (combining daily high and low temperatures) of 70.2°.  However, September 2020 was only 0.6° cooler than average using D.C.’s average September temperature of 71° for the period of 1981-2010.  It wasn’t until this year that D.C.’s average September temperature increased to 72.4° with NOAA’s updated climate data. 

This month has been 1.3° warmer than average in the Nation’s Capital through September 23.  However, monthly temperatures will finisher closer to average with several days of below average temperatures expected for the D.C. Metro Area during the final week of September.