Late-Season 90° Heat

Great Falls, MD is a great place to cool off !

The frequency of 90° temperatures in Washington, D.C. drops precipitously in September.  Average high temperatures fall from their highest level (89°) on July 22 to 84° by September 1.  As the transition from summer to fall gets underway in late-August and early-September, Washingtonians can expect to see less and less 90° heat.

While 90° becomes less common as August gives way to September in the Nation’s Capital, the last several Septembers have featured a fair amount of such heat.  Last August and September were the second warmest on record. Washingtonians experienced 23 days of 90°/+ in August 2016, while September had 7 days of 90°/+.  According to the National Weather Service, the last 90° day to occur last year was on September 23 (90°).

Having 90° well into September has been a fairly regular occurrence over the last decade.  For example, it was 99° on September 24, 2010 at National Airport.  That was not only a record high for the date, but remains Washington, D.C.’s hottest observed temperature on or after September 9.  Longtime residents may recall that record high temperatures occurred in the DC Metro Area on October 8 (91°) and October 9 (94°) in 2007.  The Nation’s Capital has had 90° as late in the year as October 11, but that’s the exception and not the norm.

The last August to be both cooler and wetter than average was in 2014.  That’s unusual, though, since 14 of the last 20 August’s have been warmer than average in Washington, D.C.  Meanwhile, the last September to be both cooler and wetter than average in the Nation’s Capital was in 2006.  In the Mid-Atlantic Region, impacts from decaying tropical systems can contribute to above average rainfall.  Although this summer has been wetter than average, that hasn’t been due to tropical systems. 

June and July were both warmer than average, but this month is poised to become only the fifth cooler than average August since 2001.  Yesterday’s high temperature of 92° at National Airport was the hottest day in the Nation’s Capital since July 22 (96°).  My colleagues and I on the WUSA9 weather team are expecting cooler than average temperatures over the final week of August.

Catastrophic August Hurricanes

Hurricane Charley in August 2004  (Source: NOAA)

Some of the most significant hurricanes in U.S. history have occurred during the month of August.  These four hurricanes are infamous for being both damaging and deadly.

Katrina (2005):  A powerful Category 5 hurricane over the Gulf of Mexico less than 48 hours before making landfall, many feared Katrina was the doomsday scenario for the U.S. Gulf Coast.  Fortunately, Katrina weakened to a high end Category 3 hurricane before making landfall just east of New Orleans.  However, because Katrina was such a large and intense hurricane, the storm surge was still commensurate with a stronger hurricane.  The fact that Katrina weakened slightly and tracked east of the city, means it could have been even worse.  

The worst-case scenario for a hurricane in New Orleans would be if a Category 4 or 5 storm were to make landfall just west of the city.  That’s because the strongest winds and most dangerous conditions are on the right or eastern side of the hurricane.  According to NOAA, Katrina caused approximately $108 billion in damage and had a death toll of more than 1,200.

Charley (2004):  Just a year earlier, the west coast of Florida saw another very powerful hurricane.  After making landfall in Cuba as a Category 3 hurricane, Charley entered the Gulf of Mexico where it became even stronger.  Initially, it was feared that Charley would make landfall in the major urban area of Tampa-St Petersburg.  However, its path shifted shortly before making landfall and it came ashore near the town of Punta Gorda (south of Tampa) as a Category 4 hurricane with sustained winds of 150 mph.  Charley was the strongest hurricane to make landfall in the United States since Andrew in 1992.  Charley left roughly $15.8 billion in damage with an estimated death toll of 15.

Andrew (1992):  Unlike Katrina and Charley which occurred during very active hurricane seasons, Andrew was the only major hurricane of the 1992 season.  Conditions were ideal for Andrew to reach Category 5 intensity, as it passed through the Bahamas on its way to South Florida.  Andrew was relatively small in diameter and was a fast-moving storm.  Those facts combined with its track approximately 30 miles south of downtown Miami, helped to prevent a higher death toll.  Still, Andrew was the costliest natural disaster in U.S. history, causing approximately $26 billion in damage (unadjusted for inflation).  Andrew held that distinction until Katrina, 13 years later.  However, Andrew remains the last Category 5 hurricane to make landfall in the United States.

Camille (1969):  The 1969 season was an active hurricane season in the Atlantic similar to 2004-2005.  However, Camille stood out among all the other storms.  It was the second Category 5 hurricane to make landfall in the United States (and first since 1935) when it came ashore near the town of Waveland, Mississippi along the U.S. Gulf Coast.  Camille brought catastrophic damage and a death toll of more than 200 occurred.

These four hurricanes all rank among the Top 20 costliest hurricanes on record in the United States.  Fortunately, there are no active tropical storms or hurricanes anywhere in the Atlantic Ocean Basin today.

The First Half of August

July’s wet weather has continued over the first half of August.  With 12.17” of rain since July 1, Washington, D.C. has had an exceptionally wet summer despite the third driest June on record.  The amount of rain since July 1 makes the last six weeks the wettest period (12.17”) in Washington, D.C. since June-July 2015 (19.95”).  Washingtonians got 3.31” of rain on July 28 for their largest daily rainfall total since 2012.

Rain events in the Mid-Atlantic Region during the summer and fall can often be attributed to current or former tropical systems such as Floyd in 1999 or Irene in 2011.  While there have been eight tropical storms and two hurricanes in the Atlantic Ocean so far this season, none have directly impacted the United States.  But mid-August is when the hurricane season tends to get more active.  Ironically, the last six weeks have been exceptionally wet in the DC Metro Region without any tropical systems.  The wetter than average conditions have reduced Washington, D.C.’s ongoing rainfall deficit.

From August 1, 2015 through April 30, 2017, Washington, D.C. amassed a rainfall deficit of 13.75”, with 17 of those 20 months being drier than average.  However, both May and July were significantly wetter than average and that trend has continued during the first half of August.  That’s offset some otherwise drier than average months earlier in the year to give Washingtonians a rainfall surplus of 2.94” for the year.

The above average rainfall has also contributed to a cooler than average start to August.  Unlike last year, temperatures this month have been nearly a full degree cooler than average.  There have been five days of temperatures at 90°/+ over the first 17 days of August, compared to 13 such days last year.

A shift in the weather pattern will bring more summer-like temperatures to the Mid-Atlantic Region over the next week.  NOAA indicates a higher than average potential for cooler than average temperatures for the final week of August.  Scattered showers and thunderstorms are possible across the DC Metro Area on several days over the next week, so odds are that the trend of above average rainfall will continue as the end of meteorological summer approaches on August 31.