The 2017 Summer

Great Falls, Maryland 

A week of temperatures more characteristic of early June than early August is on tap for Washingtonians, despite starting the month with four consecutive days at 90°/+.  No 90° heat is on the seven-day forecast in the Nation’s Capital.  That’s a far cry from the above average temperatures that occurred for much of June and July.

A cooler than average first half of August would also be considerably different than August 2016.  Last August was the second hottest on record in Washington, D.C. with 23 days at 90°/+ including three consecutive days at 100°/+.  That was the first time Washingtonians experienced two or more consecutive days of triple-digit heat during the month of August since 1997.  

The hottest temperature so far this summer occurred on July 20 (98°).  Last month was the fifth wettest July on record, but was only the twelfth warmest.  Aside from the lack of triple-digit heat this summer, another key difference compared to last year can be found among the frequency of 90° days in Washington, D.C.

Although last year saw an unusually high number of 90° days (58), the first didn’t occur until June 11, 2016.  By comparison, the first day of 90° heat this year occurred on April 29 (91° at National Airport).  According to the National Weather Service, Washington, D.C. averages 36 days at 90°/+ in a given year.  2017 currently stands at 34 such days.  Average high temperatures in Washington, D.C. fall from 88° on August 1 to 84° on August 31.

While four of the last five Augusts have been drier than average in the Nation’s Capital, today’s rainfall could go a long way toward making this month wetter than average.  Triple-digit heat is relatively rare in the Mid-Atlantic Region.  While it’s been 100° as late in the year as September 8 (1939) in Washington, D.C., triple-digit heat gets less and less likely as August wears on.    

Hurricane Season 2017

Tropical Storm Arlene, April 2017  Source: NOAA

The busiest time of the six-month hurricane season is from August through September.  A busier than average hurricane season is forecasted by NOAA and the team of scientists at Colorado State University.  An average Atlantic hurricane season features 12 tropical storms, 6 hurricanes and 3 major hurricanes (Category 3 or higher on the Saffir-Simpson Hurricane Intensity Scale).  There have been 5 tropical storms so far this season.

The first and strongest named storm so far in 2017 was Tropical Storm Arlene, which had 60 mph sustained winds at peak intensity.  Arlene was the first Atlantic tropical storm to form during the month of April since 2003.  This past week, Emily was a short-lived tropical storm that brought appreciable rainfall to parts of central Florida.

The fact that no hurricanes have formed yet this season is not significant.  For example, 7 tropical storms formed before the first hurricane of the 2011 season (Irene) developed on August 22.  Some may recall that Irene was one of 7 hurricanes that formed during the very active 2011 season and it impacted the Mid-Atlantic Region.

All tropical storms and hurricanes are significant regardless of their intensity when they make landfall.  No major hurricane has made landfall anywhere in the United States since Wilma in October 2005.  While, several destructive and deadly hurricanes have impacted the United States since then (Ike in 2008, Irene in 2011 and Isaac in 2012), none were at Category 3/+ intensity when they made landfall.  Tropical Storm Allison in 2001 became the first and only tropical storm to have its name retired for being destructive and deadly. 

Colorado State University released its updated seasonal hurricane forecast this morning and they expect a total of 16 tropical storms, 8 hurricanes and 3 major hurricanes in the Atlantic Ocean.  NOAA will issue its early August update very soon.  One factor that scientists look at when issuing seasonal hurricane forecasts is the presence of El Nino or La Nina, which can influence sea surface temperatures and the amount of wind shear in the tropics.  

A busier than average hurricane season doesn’t necessarily mean the United States will be impacted.  The 2010 Atlantic hurricane season was one of the busiest on record with 19 tropical storms, 12 hurricanes and 5 major hurricanes, but the United States only saw minimal impacts.  Meanwhile, the 1992 hurricane season was quieter than average with only 7 named storms in the Atlantic Ocean.  However, Hurricane Andrew developed in 1992 and remains the last Category 5 hurricane to make landfall in the United States.

Final Month of Meteorological Summer

This summer has been quite unique in the Nation’s Capital.  June was the third driest (1.13”) since 1871 and was the eighth consecutive warmer than average June.  Washingtonians, then, experienced their seventh wettest July (9.15”), and wettest since 1969.  At least 0.65” of rain occurred at National Airport on five separate July days, while an inch or more fell on two other days.

The 3.31” rainfall on July 28 not only set a daily rainfall record for July 28 in Washington, D.C., but was also the first time at least 3” of rain occurred in a single day in nearly five years.  Some may recall that 4.84” of rain fell from October 28 – 30, 2012 when the remnants of Hurricane Sandy affected the Mid-Atlantic Region.  Most of that total occurred on October 29 when 3.85” fell at National Airport.

That helps illustrate how rare 3” rainfall totals are in the DC Metro Area, especially outside of current or former tropical systems.  Dulles Airport saw its wettest July on record (8.80”) last month, dating back to 1963.  The wetter than average final week of July also contributed to cooler than average temperatures.  That caused July to go from being the third warmest on record July 25 (83.3° average monthly temperature), to twelfth warmest (81.7°).  

August is the third and final month of “meteorological” summer that spans from June 1 through August 31.  Average high and low temperatures in Washington, D.C. range from 88°/71° on August 1 to 84°/68° on August 31.  The National Weather Service says Washingtonians can expect an average of 2.93” of rain this month.  That’s DC’s first monthly average under 3” since February (2.62”).  A significant change between July and August, though, is that the tropics tend to heat up quite a bit during the month of August, so any rainfall in the Mid-Atlantic Region could be influenced by tropical systems.

August 2016 was the second warmest on record in the Nation’s Capital.  Although two of the last three months have been wetter than average, the DC Metro Area has been significantly drier than average, overall, dating back to August 1, 2015.  However, NOAA expects near average temperatures and precipitation in August for much of the eastern United States.