September Wrap: A Tale of Two Climates

Shenandoah National Park, Virginia  (Courtesy: Mallory Hughes)

September has come to a close on a fall-like note with temperatures cooler than average and highs only near 70° across the DC Metro Area.  This month will finish as the fifth consecutive drier than average and eighth consecutive warmer than average September in Washington, D.C.

This month’s warmest temperature in Washington, D.C. was 92° on September 24.  That was also significant since it was DC’s first 90°/+ temperature since August 22.  The coolest temperature experienced this month was on September 10 (53°).  This month’s weather can actually be broken up in two distinct halves in the Nation’s Capital.

The first half of the month was cooler and wetter than average for much of the Mid-Atlantic Region.  Twelve of the first thirteen days of September featured cooler than average high temperatures.  There was also a significant amount of rainfall over the first week of the month, with 1.39” at National Airport through September 6.  More than half fell on September 2 (0.77”), when the high temperature was only 65°.  That was the first time since September 6, 2011 that a daily high temperature during the first half of September remained below 70°.

The Mid-Atlantic Region has had a very warm and dry second half of September.  Since 1993, Washington, D.C. has averaged nearly four days of 90° heat and one day at 95°/+ in September.  Washingtonians were near average in that respect this year with three days of 90°/+ this month, which didn’t occur until this past week.  That puts this month in a tie for the second highest amount of 90° heat during the final week of September, behind only September 1881 (6).  According to NOAA, DC’s average monthly temperature (combining daily high and low temperatures) for September finished 1.5° above average.

Washingtonians have had no measurable rain during the second half of September and that will ensure a monthly total of 1.43” (less than half the monthly average of 3.72”).  The National Weather Service said this is the first time since 1941 that no measureable rainfall occurred during the second half of September in Washington, D.C.  NOAA expects a warmer and drier than average start to October.  That means no significant changes should be expected in the short-term from the warm and dry conditions of late September in the DC Metro Area.