What Can We Expect in September?

Before talking about September, it’s worth reflecting on August’s personality.  Last month featured 23 days at/above 90 degrees at National Airport which broke the 1980 record of 22.  August 2016 also finished with an average temperature of 82.7 degrees (just behind the 1980 record of 82.8 degrees).  Washingtonians also experienced their second highest average daily high temperature of 91.7 degrees for the month of August – narrowly behind the August 1980 record of 92 degrees.  In Washington, D.C., the highest number of August 100 degree days since 1930 occurred last month with three (August 13-15).

Now that we’ve turned the calendar to September, the start of meteorological fall, many are eager for the cooler weather associated with that.  Average high and low temperatures in Washington, D.C. drop significantly from 84/67 degrees on September 1, to 74/57 on September 30.  The hottest September temperature on record in Washington, D.C. is 104 degrees (September 7, 1881), while September’s coolest temperature is 36 degrees (September 23, 1904).

As the Mid-Atlantic Region has gotten more developed in recent decades, no record low temperatures have been set in Washington, D.C. in September since 1963.  I recently wrote about the effect increased urbanization has had on keeping warmer overnight low temperatures.  That can be especially significant when forecasting winter weather if temperatures downtown remain in the mid to upper 30s, while temperatures in the suburbs cool to at or below freezing.

National Weather Service data indicates there have been recent September extremes in Washington, D.C.  For example, there was a record high of 99 degrees on September 24, 2010; while September 2011 was significantly wetter than average (3.72”) with 8.84” of rain.  Some will recall that most of the rain that occurred in September 2011 was from the remnants of Tropical Storm Lee.  September is traditionally the busiest month of the Atlantic hurricane season and a number of tropical systems have impacted the Mid-Atlantic Region, including Hurricane Floyd in 1999, Hurricane Isabel in 2003, and the remnants of Tropical Storm Lee.

Tropical systems can be tricky to forecast since their tracks can change often and unlike thunderstorms, tropical systems are large in diameter and can effect areas hundreds of miles from their centers.  That’s what made forecasting Hurricane Hermine so tricky.  While the forecast tracks given by the computer models have vacillated a number of times in recent days, “Hermine” will make landfall in the Florida panhandle late tonight.  Hermine is also significant for being the first hurricane in the Gulf of Mexico since Ingrid in 2013, a record stretch of 1,081 days.  My colleagues and I on the WUSA9 weather team will keep you posted.