What type of weather do you think of for the month of July? If you’re thinking hot, humid, with afternoon storms you would be right on. Now what if I also told you July is also a peak tornado month for the D.C. area, with nearly as many tornadoes as the month of May?
Examining the map, a tally of all reports (1950-2011) west of the Chesapeake Bay during July includes:
91 confirmed tornadoes
348 hail reports
1679 damaging wind reports
July tornado climatology
July happens to be a top month for confirmed tornadoes right up there with the month of May. Are you surprised? I certainly am.
The fact that May holds the most monthly tornadoes per month is not surprising, but I would expect number two to be either April or June, rather than July which is a month firmly supplanted in the summer season. While it is interesting that July has the second most tornadoes, perhaps an even more staggering fact is the D.C. region has experienced DOUBLE the number of tornadoes in July (91) than June (45)!
Within the map extent and west of the Chesapeake Bay there have been 91 tornadoes during the month of July from 1950-2011 per the Storm Prediction Center tornado database.
Tornadoes by Fujita/Enhanced Fujita (EF) category during the month of June:
The F3 tornadoes occurred on July 25, 1985 near Rt. 29 on the border of Greene and Albemarle counties in Virginia and on July 19, 1996 in Carroll County, Maryland.
The most prolific tornado day for the D.C. area during the month of July was on July 14, 1994 when 14 confirmed tornadoes touched down across parts of Virginia and Maryland.
When breaking down D.C. area tornadoes by state, Maryland holds the gauntlet at 50.
July hail climatology
After several months of increasing hail reports, July is the month where the D.C. area experiences a lower frequency of hail-producing thunderstorms. The number of reports is noticeably less than June, and half that of May.
The reason for the lack of hail reports is due to a warm summer atmosphere that stretches from the surface to higher altitudes. Hail, and specifically large hail, is more likely to form with cold temperatures aloft that can support ice crystal formation and accretion into hailstones. The middle of the summer is not conducive to frequent cold intrusions aloft, so hail can be less prolific.
While hail may not be the most common of severe threats during the month of July, it still happens as noted above with roughly 350 hail reports around the D.C. area since 1950 during the month of July.
Here is the breakdown of hail reports by size (diameter in inches) for the month of June:
Less than 1 inch: 50 percent (below severe criteria)
1 to 2 inches: 46 percent
Greater than inches: 2 percent
The largest hailstone ever reported during the month of July was on July 2, 1968 with an insane diameter of 5 INCHES. This occurred on the Loudoun and Fairfax County border just north of Reston, Virginia. There have also been two reports of 4-inch diameter hail during the month of July: one on July 26, 1993 in Mineral County, West Virginia (panhandle) and then again on July 30, 1999 in Baltimore County, Maryland (just north of downtown Baltimore).
The bottom line is that while hail may not be the dominant severe threat for the D.C. area during the month of July, it is not impossible and even LARGE hail, though rare, is not out of the question!
July wind climatology
Like many of the previous months, wind damage is the primary type of severe weather across the D.C. area during the month of July. The 1,679 wind reports from 1950-2011 actually surpass the total wind damage reports for both May and June (including all the June wind reports from the June 2012 derecho).
For the D.C. region, there is an especially high density of wind reports across the urban cores of downtown D.C. and Baltimore. In fact, the highest wind gust ever reported during the month of July occurred on July 21, 1980 in downtown D.C.; a whopping 72 mph wind gust!
July may be known for its heat, humidity and long summer days but severe thunderstorms are certainly not out of the question. While many of us may let our guards down during the summer time, keep in mind it is a surprisingly active tornado month ranked highly with the month of May.
When planning those afternoon and evening pool parties, golf outings, or barbeques keep a weather eye on the sky as any summertime thunderstorms can quickly turn severe, capable of producing tornadoes, hail, and ESPECIALLY damaging winds.