Current Heat Wave Disappoints

Although this week’s heat wave resembles recent heat waves in the DC Metro Region, it falls way short of historical heat waves.  The National Weather Service defines a “heat wave” as a minimum of three consecutive days with temperatures of at least 90 degrees.  With 90-degree heat expected through Saturday, that would make our current heat wave five days long.  Two things in particular have contributed, though, to make this week’s heat wave memorable.

Prior to Washington, D.C.’s high temperature of 91 degrees on Tuesday, July 5, the last time it had been at least 90 degrees in the Nation’s Capital was on June 21.  To add to that, the two days prior had been unseasonably cool with high temperatures of only 74 degrees at National Airport – more typical for early May or late September.  While the low 90s felt especially hot after high temperatures in the mid-70s, 91 degrees is only slightly warmer than the average high of 88 degrees in Washington, D.C. on July 5.

During the last three years, there were a total of four heat waves of at least five days, with the longest one being a 13-day stretch last summer from July 24 - August 5.  While the hottest temperature to occur during last summer’s event was 96 degrees, it’s already been 96 degrees this year in Washington, D.C. (on June 11) and temperatures will approach that again today and tomorrow.  However, no triple digit heat has occurred during any of these heat waves.

By comparison, the summers of 2010 – 2012 featured some of the hottest weather on record in the Mid-Atlantic Region.  There was a total of 67 days of 90-degree heat in 2010 in Washington, D.C. which tied 1980’s record for highest such number.  Next was July 2011 that was not only the hottest July in the Nation’s Capital, but the hottest month on record with 25 days at or above 90 degrees – including 3 days of triple digit heat.  The hottest part of the 2012 summer in Washington, D.C. was an eleven-day heat wave from June 28 – July 8.  That hot spell included five days of triple digit heat.  The record for longest “heat wave” in Washington, D.C. is 21 days that occurred twice during the hot summers of 1980 and 1988.

The fact that this week's heat wave won’t approach the hot weather benchmarks that were set during past summers should be a comfort to some.  All heat waves can be dangerous, though, so it’s important to remember to take precautions including minimizing your time outdoors, drinking extra of water and doing outdoor activities early in the day as much as possible when it’s cooler.  

Major Shift in the Weather Pattern

U.S. Capital  (July 4, 2016)

Although a few days late to arrive, more typical July-weather has begun to make its presence felt across the Mid-Atlantic Region.  On both July 3 and 4, the high temperatures were only 74 degrees at National Airport, the official weather reporting site for the Nation’s Capital.  That made this Independence Day the coolest since 1941 in Washington, D.C. according to the Capital Climate blog.  Baltimore, Maryland also had its coolest July 4 since 1979, with a high temperature of only 73 degrees.  Such temperatures are more characteristic of the first half of May rather than early July.

It was also a wet July 4^th holiday with 0.31” of rain officially at National Airport, though not as wet as past Independence Days.  The combination of cool and wet weather put a damper on many Independence Day plans.

However, a major shift in the weather pattern is bringing summer-like heat and humidity to the Mid-Atlantic Region, with the hottest weather of 2016 on tap the next few days.  Today’s high temperature of 91 degrees gave Washingtonians their first 90-degree day since June 21.  My colleagues and I on the WUSA9 weather team expect high temperatures to continue to be in the low to mid-90s every day through Sunday, with a few upper 90s possible in spots. 

Such a streak would easily meet the criteria for a “heat wave,” which is at least three consecutive days at or above 90 degrees.  The longest streak of 90 degree heat so far this year was a modest three day stretch from June 19 – 21.  National Weather Service data indicates that Washington, D.C. hasn’t had a streak of five or more consecutive days in the 90s since August 15 – 19, 2015.

Through July 5, 2015, Washingtonians had suffered through 19 days at or above 90 degrees.  However, for the same period this year, there have only been six-90 degree days.  Washington, D.C. averages 36 days of at least 90 degrees in a given year and there was a total of 52 such days last year so this streak of 90 degree temperatures will not only make beach-goers happy, but will also help the Nation’s Capital catch up.  Washington, D.C.'s hottest temperature so far this year was 96 degrees on June 11.

What Can Washingtonians Expect in July?

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

Washingtonians know from experience that July often features hazy, hot and humid conditions along with occasional showers and thunderstorms.  Although five of the last six July’s have been warmer than average in the Nation’s Capital, triple-digit heat has only occurred in three July’s since 2000.  July 2010 and July 2011 each featured three days of 100 degree heat, while July 2012 had seven such days – including a streak of four consecutive days above 100 degrees!  Consequently, the summers of 2010 to 2012 were three of the hottest on record. 

The hottest temperature to occur during those torrid summers was on July 7, 2012 – when a record high of 105 degrees was set in Washington, D.C.  That’s just a degree below the all-time record high of 106 degrees – that’s occurred twice in the Nation’s Capital.  July’s hottest temperature last year was 98 degrees on the 19^th.  While average high temperatures in Washington, D.C., are 88 degrees on both July 1 and July 31, it’s 89 degrees from July 7 to 22 – which is the hottest time of the year in the Nation’s Capital on average.  There hasn’t been any triple digit heat in Washington, D.C. since 2012.

Washington, D.C. averages 3.73” of rain in July – the third highest total of the year after May and June.  It’s interesting to note five of the last six July’s have been wetter than average in the Nation’s Capital.  Rainfall amounts can fluctuate depending on the location and frequency of showers and thunderstorms.  Tropical systems can sometimes impact the Mid-Atlantic Region in July and influence rainfall amounts.

In July 1996, Hurricane Bertha made landfall in North Carolina as a Category 2.  It brought significant rainfall to the Mid-Atlantic Region and contributed to making July 1996 almost 2” wetter than average in Washington, D.C.  More recently, Hurricane Arthur gave the Outer Banks of North Carolina a glancing blow in early July 2014, but didn’t impact to the Nation’s Capital.  Having two July hurricanes impact a similar area during the last twenty years shows it's possible – but infrequent.

NOAA’s Climate Prediction Center is expecting July 2016 to be a warmer than average month for the Mid-Atlantic Region.  However, temperatures will not get off to a blazing start with no 90-degree temperatures expected until after the Independence Day holiday.  That’s in stark contrast to July 2012 when the month began with 8 consecutive days of temperatures 95 degrees or hotter.  My colleagues and I on the WUSA9 weather team will continue to keep you apprised of the latest weather forecasts – both on-air and on the free WUSA9 app for your mobile devices.