From the ABC 7 Weather team

July Weather in D.C.: Hot, Humid & Stormy

July 6, 2014 - 06:09 PM
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While the nation’s capital couldn’t have seen a more perfect July 4th for fireworks viewing with low humidity and comfortable temperatures, the dog days of summer return next week. The hot spell will also bring a greater thunderstorm risk but this is a typical July pattern.

The mercury will soar into the 90s while heat indices climb close to 100 degrees early next week. In the meantime, a cold front will likely ram into this heat and humidity, triggering midweek thunderstorms once again.

Downpours and gusty winds are likely with the first round that breaks the heat spell either late Tuesday evening or Wednesday afternoon. This pattern is all too common across the Washington metro area.

Analyzing past weather trends since 1980, July tops the list for the most combined hail, wind and tornado events in the region (see table below).

July Climatology

The biggest threat from any single thunderstorm in July is wind gusts. In order to qualify as a severe thunderstorm, gusts must at least reach 58 mph. July also ranks #1 for the most tornadoes area-wide. On average, there are 6 tornadoes in the region (the western Maryland panhandle into the Shenandoah Valley, northern Virginia, the District and all the way to the Chesapeake Bay) during the month of July.

Notice the breakdown of severe weather reports per hour on any given day. Hail, gusty winds and tornadoes are most common at 6 p.m. (22 UTC = 6 p.m. between daylight saving time (in early March) and standard time (in early November). This makes sense given the atmosphere is generally most unstable late on a summer's day after the sun has been warming the ground all day.

Hourly Severe Climatology

A more in-depth analysis of tornado climatology for the region shows a peak in the average number of tornadoes per day in July and then again in September. Part of the reason for the September high is that the ninth month of the year coincides with the peak in the Atlantic Hurricane Season. The District is vulnerable to the effects of a tropical system that moves north and east from the Gulf of Mexico or inland from the Atlantic.

Peak Tornado Season

Most tornadoes that occur in this region are ranked the lowest on the Fujita Scale (now the Enhanced Fujita Scale). F0 (now EF0) has winds of 40-72 mph. Most tornadoes track from southwest to northeast (denoted by the diagonal lines on the map). A few have been known to track west to east or northwest to southeast as well. The longer the line, the longer the tornado track but most tornadoes track only brief distances.

Average Tornado Strength

Keep ahead of storms this summer with the ABC7 weather team. For your latest forecast, click here.


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