Ranking July 2017

U.S. Capital, Washington, D.C.   Source: National Park Service

To paraphrase, Charles Dickens, the legendary author this month has been a tale of two seasons in Washington, D.C.  The first three and a half weeks of July ranked among the hottest on record in the Nation’s Capital.  However, there has been a dramatic shift in the weather pattern across the Mid-Atlantic Region over the last week.  That’s helped produce one of the wettest Julys on record for Washingtonians with some fall-like temperatures.

Through July 29, Washington, D.C. has a total of 9.15” of rainfall at National Airport.  That makes this the seventh wettest July on record since 1871, and the third wettest since 1941 (the year National Airport opened).  That’s largely due to the 3.78” that’s fallen just since Friday.  Washington, D.C. averages 3.73” of rain during the entire month of July.  

What’s more, Friday, July 28 was the first time that more than 3” of rain fell in a single day (3.31”) in the Nation’s Capital since October 29, 2012 (3.85”).  The significantly wetter than average weather this month has gone a large way toward mitigating the ongoing rainfall deficit.  Dating back to August 1, 2015, Washington, D.C.’s rainfall deficit on June 30, 2017 stood at 14.84”, but that number has since fallen to 9.18”.

The rainy final week of the month also had a big influence on temperatures.  On July 25, Washington, D.C’s average temperature (combining daily high and low temperatures) was 83.3° (3.5° warmer than average).  That was largely because 19 of the first 24 days of the month had high temperatures of at least 90°.  The high temperatures at National Airport on July 28 and 29, respectively, were 81° and 77°, more characteristic of mid-to-late September than July.

Consequently, this month’s average temperature has fallen to 82.1°.  Eight of the last ten July’s have been warmer than average (79.8°) in the Nation’s Capital.  With 9.15” of rain this month, July 2017 has also been DC’s wettest month in more than two years since June 2015 (11.94”).  NOAA’s outlook for August in the Mid-Atlantic Region is for warmer than average temperatures and near average precipitation.

Washington, D.C.’s Wettest July's  Source: NOAA

1.  11.06” (1945)

2.  10.63” (1886)

3.  9.95” (1905)

4.  9.59” (1922)

5.  9.44” (1969)

6.  9.41” (1917)

7.  9.15” (2017)

8.  8.40” (1891)

9.  8.37” (1878)

10.  8.13” (1889)

Significant July Severe Weather

Tornado Statistics for July   Source: NOAA

There have been several days with severe weather in the Mid-Atlantic Region so far this month.  Following the third driest June on record in the Nation’s Capital, the new rainfall brought by these thunderstorms has been beneficial.  However, the severe weather has been destructive, too.  Here are some significant severe weather events in the DC Metro Area. 

July 22-24, 2017

Several rounds of thunderstorms brought severe weather and flash flooding to parts of the DC Metro Area.  The most significant occurred in Queen Anne’s County in Maryland on Sunday night.  An EF-2 tornado was on the ground from 1:29 am to 1:33 am, with peak winds of 125 mph.  The tornado caused appreciable damage to the towns of Bay City and Stephenville, Maryland.  Fortunately, no fatalities occurred.  At National Airport, a total of 2.37” of rain occurred in this span, which is more rain than occurred in the entire month of June (1.13”).

July 1, 2013

The month got off to a stormy start in the DC Metro Region with 0.6” of rain at National Airport on July 1.  Severe thunderstorms produced an EF-0 tornado near the town of Brandywine in Prince Georges County, Maryland.  It caused no fatalities and only minor damage.

July 25, 2010

A significant heat wave occurred during the later portion of the month that was capped off on July 25 with temperatures near 100° and an unstable atmosphere.  That’s when a squall line developed west of town and came through the Nation’s Capital with multiple severe wind gusts, resulting in a slew of downed trees and power lines.  Widespread power outages ensued that caused hardship to many during the heat wave.

July 27, 1994

An outbreak occurred on July 27, 1994, when a total of 21 tornadoes developed in Maryland, Virginia and Delaware.  According to data from NOAA, the states of Maryland and Virginia average three tornadoes during the month of July.

A thunderstorm becomes “severe” when it contains one or more of the following: wind gusts of at least 58 mph, hail 1” in diameter or greater, or a tornado.  While heavy rain and lightning don’t make a thunderstorm “severe” per se, all thunderstorms are potentially dangerous and should be taken seriously.

Putting a Dent in DC’s Rainfall Deficit

Warmer than Average weather Expect in August 2017  Source: NOAA

Washingtonians are on the verge of reaching a significant milestone as we enter the final week of this month.  August 1 will be the second anniversary of what has been a very warm and dry period in the Nation’s Capital.  As of this past Friday, Washington, D.C.’s rainfall deficit had swelled to 14.3” dating back to August 1, 2015.  That shortfall represents about one-third of the 39.74” of rain that DC averages in an entire year.

Prior to this month, only four months have been wetter than average in Washington, D.C. during the last two years.  In that time, the Nation’s Capital has set multiple record high and record high-low temperatures that contributed to the warmest December on record (2015) and a series of months finishing among the warmest on record.  Washingtonians also experienced the first 100° temperature in four years last summer, including a rare streak of three consecutive 100° days.  

The warmer than average temperatures combined with the below average rainfall contributed to moderate drought conditions for parts of the Mid-Atlantic Region earlier this year.  Fortunately, a wetter than average second half of April and month of May eliminated the drought conditions, but not the rainfall deficit.  This month has featured several days with significant rainfall in the DC Metro Area that have ensured this month will finish as only the fifth wetter than average month in the last two years.

July has also been a warmer than average month in the Nation’s Capital with 18 of the 22 days so far this month in the 90s.  High temperatures today and tomorrow are expected to reach the 90s, with several more chances during the upcoming week to pad that total.  The record for highest number of 90° days in Washington, D.C. during the month of July is 25 from 2011.  By comparison, following a cooler than average start, July 2016 finished with 23 of its 27 final days at 90°/+.  

There are several more rain chances the next few days that could provide some relief from the heat and make a dent in the ongoing rainfall deficit.  However, it will take several consecutive wetter than average months to make up the deficit entirely.  NOAA’s Climate Prediction Center is expecting near average precipitation for the Mid-Atlantic Region in August 2017.