DC’s September Weather Extremes

 

Denver, Colorado just went from temperatures in the 90s to seeing accumulating snowfall in less than 48 hours.  While nothing that dramatic has happened in the DC Metro Area, there have been some notable September weather extremes. 

During September DC averages 3.72” of rain, the fourth highest monthly average of the year.  That amount can fluctuate considerably depending on whether or not a tropical system impacts the DC Metro Area.  For example, Washingtonians saw 10.27” of rain in September 1999, largely the result of remnant moisture from Hurricane Dennis and record rainfall from Hurricane Floyd.  Also, the remnants of Tropical Storm Lee helped make September 2011 DC’s wettest October (8.84”) since 1999. 

DC’s fifth wettest September occurred just two years ago with 9.73”.  Part of that was the result of the remnants of Hurricane Florence, which brought heavy rain and severe weather to the DC Metro Area on September 17.  Last September was the opposite extreme in the nation’s capital because it finished as DC’s fourth driest.

DC’s warmest daily average high temperature during September is 84° on September 1, but falls to 74° by month’s end.  Despite the downward trend in daily average temperatures, DC still averages between three and four days of 90° heat in September dating back to the early 1990s.  In that time there have been as many as 10 days in the 90s twice in 1998 and 2010.  Each of those Septembers rank among DC’s 10 warmest. 

Some may wonder if having a wetter than average September could prevent it from ranking among DC’s warmest.  The short answer is no as was seen in September 2018.  That month not only ranked as DC’s fifth wettest with 9.73” of rain, but finished tied for DC’s sixth warmest.  Having nearly 60% of the month’s total rainfall on just three days helped allow for much of September 2018 to be warmer than average.  In fact, a total of only five days in all of September 2018 were cooler than average in the nation’s capital.

DC’s signature weather event so far this month was the severe weather event on September 3.  There were multiple reports of thunderstorm wind damage with an EF-1 tornado in the town of Edgewater, Maryland, near Annapolis.  Fortunately, the few days since then have been quiet on the weather front in the DC Metro Area.  However, September is also the busiest month of the Atlantic hurricane season so it’s important to keep an eye on all things weather as the month progresses.