Feeling Like Summer

Fall officially arrived this past Friday with the autumnal equinox.  However, the second half of September has felt more like August in the Nation’s Capital.  That’s following a cooler than average start to the month.  September is typically a transitional weather month in the Mid-Atlantic Region with summer-like temperatures possible, but cooler temperatures more and more frequent as the month wears on.

This month has featured two distinct weather patterns.  From September 1 – 13, all but one day saw cooler than average temperatures in Washington, D.C.  In fact, the high and low temperature of 65°/57° on September 2 was DC’s coolest weather during the first half of September since September 6, 2011 (67°/64°).  The coolest temperature so far this month occurred on September 10 (53°). 

September also got off to a wet start with 1.39” of rain through September 6.  Since then, only 0.04” of rain has fell at National Airport.  As active as the tropics have been the last few weeks, the DC Metro Area hasn’t seen significant precipitation from former tropical systems.  However, hurricane season runs through the end of November and Hurricane Maria is still east of the Bahamas and bears watching.  Given how wet July and August were in the Mid-Atlantic Region, Washington, D.C. has a rainfall surplus of more than 1.5” since January 1.

Nine of the last 10 days have been warmer than average in the Nation’s Capital.  Despite these above average temperatures, DC’s average temperature since September 1 (combining daily high and low temperatures) remains 0.4° below average.  That’s poised to change with above average temperatures on tap for much of the upcoming week.  The warmest temperature so far this month occurred yesterday when it was 89° at National Airport.  Since 1993, Washington, D.C. has averaged nearly four days of 90° heat and one day of 95°/+ temperatures during the month of September. 

Washingtonians sweat through seven days at 90°/+ in September 2016, while DC’s hottest September temperature last year was 98° on September 8.  There have been as many as 10 days of 90°/+ heat in the Nation’s Capital during the month of September twice in the last 25 years (1998, 2010).  The record high for this date in Washington, D.C. is 99° set in 2010.  By comparison, it hasn’t been 99° in the Nation’s Capital since August 15, 2016 (100°). 

Despite the recent above average temperatures, Washingtonians haven’t experienced any 90° heat since August 22 (92°).  That could change today as high temperatures climb to the upper 80s across the DC Metro Area, with 90° temperatures possible in spots.  According to NOAA, DC’s last September without 90° heat came in 2009.  That was also the last time Washingtonians experienced a cooler than average September.

2017 Hurricane Season Breaks Records

Hurricane Maria's forecast track   Source: NOAA

September is typically the busiest month of the hurricane season for the Atlantic Ocean Basin, which includes the Gulf of Mexico and Caribbean Sea.  This season has been no exception with an historic stretch of devastating hurricanes over the last few weeks. 

There have been two Category 5 hurricanes in the Atlantic Ocean (Irma, Maria) for only the sixth time in recorded history and first time since 2007 (Dean, Felix).  The record for most Category 5 hurricanes during an Atlantic hurricane season (June 1 – November 30) is 4 (Emily, Katrina, Rita, Wilma) in 2005.  One Category 5 hurricane is rare enough, but more than one is exceptionally rare.  For example, prior to 2005, the last time at least two Category 5 hurricanes occurred during the same season was in 1961 (Carla, Hattie). 

It’s interesting to note that before weather satellites began being used in the 1960s, it was very difficult to determine if a tropical storm or hurricane existed if it remained over the open ocean.  There could have been additional seasons with Category 5 hurricanes, but there was no reliable way to know they were there.  Not only were there no weather satellites during the 1932 and 1933 hurricanes seasons (when there were also two Category 5 hurricanes) but that was also before they were named.

Hurricane Irma broke multiple records during its 75 hours as a Category 5 hurricane over four days (September 5 – 9).  It had the highest sustained winds (185 mph) of any Atlantic hurricane outside of the Gulf of Mexico or Caribbean Sea.  In fact, Irma’s peak winds of 185 mph tie it with Gilbert, Wilma and the unnamed 1935 Florida Keys hurricane as second highest on record behind only Hurricane Allen (190 mph) in 1980.  Moreover, Irma’s 75 hours at Category 5 intensity also ranks as the second highest length of time at Category 5 intensity, narrowly behind the 78 hours a November 1932 hurricane in the Caribbean Sea.

At peak intensity, Maria was slightly more intense than Irma.  Although Maria’s maximum sustained winds were 175 mph, its minimum central air pressure was 908 millibars (mb).  Scientists consider air pressure to be a more accurate measure of hurricane intensity than wind speed.  That’s because lag time can exist in between changes in the air pressure and wind speed of strengthening or weakening hurricanes.  On account of pressure, Hurricane Maria ranks as the tenth strongest hurricane on record in the Atlantic Ocean.  Maria’s lowest air pressure (908 mb) was not only lower than Irma’s (914 mb), but was the lowest observed pressure of any Atlantic hurricane since Category 5 Dean (905 mb) in 2007. 

2016-2017 are the first consecutive years since 2004-2005 to feature a Category 5 hurricane in the Atlantic Ocean, following last season’s Hurricane Matthew.  If the hurricane season were to end today with 13 tropical storms and 7 hurricanes, including 4 major ones, it would be a touch busier than the seasonal average of 12, 6 and 3.  However, hurricane season runs through November 30, so those totals can only increase.

Hurricane Maria Keeps Rare Company

Maria's forecast Track from the National Hurricane Center

Hurricane Maria became the second Category 5 storm of the 2017 Atlantic hurricane season on Monday, September 18.  That puts this season in rare territory with multiple Category 5 hurricanes.  Check out this story I wrote for the WUSA9 website for more details on the significance of Hurricane Maria.

What’s Memorable about Summer 2017?

Great Falls, Maryland

Meteorological summer is comprised of the months of June, July and August.  These three months couldn’t have been more different in the Nation’s Capital this year.  To begin, the summer of 2017 was warmer than average, with an average temperature of 78.8° (as compared to 77.7°).

Washington, D.C.’s hottest temperature of the summer was 98° on July 20, 2017.  By comparison, last year’s hottest temperature was 101° on August 13.  The longest heat wave of 2017 was six days – from July 17 – 22.  A “heat wave” is defined as a minimum of three consecutive days with temperatures 90°/+.  Washington, D.C. averages 36 days of 90° heat in an entire year.  There were 37 days of 90°/+ temperatures at National Airport (DCA) just this summer.  This year’s first day of 90° heat came on April 29, while last year’s first 90° day didn’t occur at DCA until June 11. 

This summer’s coolest temperature was 56° on June 8.  Unlike 2016, when September featured seven days of 90°/+, it hasn’t been 90°/+ in Washington, D.C. since August 22 (92°).  Part of that can be attributed to just how wet the summer was.  While June 2017 was the third driest on record in the Nation’s Capital, the proverbial floodgates opened in July.  The most rain fell at National Airport in July (9.15”) since 1969.  August 2017 was also wetter than average and that led to a seasonal total of 14.86” from June 1 through August 31 (4.42” wetter than average).

In fact, five of the last six summers have been wetter than average in Washington, D.C.  Sometimes the remnants of tropical systems bring appreciable rainfall to the Nation’s Capital.  However, in the last six years only the summer of 2011 saw any rainfall from tropical systems in the Mid-Atlantic Region.  That’s when Hurricane Irene made landfall in North Carolina and brought the DC Metro Area some much-needed rainfall.

Does a warmer and wetter than average summer foreshadow a colder and snowier than average winter?  No correlation can be drawn between the two.  There have been instances of hot and dry summers (1995) followed by snowier than average winters (1995-1996).  Conversely, Washingtonians experienced their second hottest summer on record in 2011 (that was also wetter than average), but the 2011-2012 winter saw less than 15% of the average annual snowfall (15.4”).  September remains the most active month of the hurricane season which continues through November, so we still have some time to go before thinking about winter.