August severe weather: Violent storms often take a time out

From a severe weather perspective, August can be one boring month in D.C., Maryland and Virginia.  The absence of wild weather can be a disappointment for storm lovers, but of course a good thing as far as reduced damage to life and property.

While August can still be a stormy month characterized by afternoon pop-ups, when it comes to large hail, damaging winds and tornadoes, it is the second quietest of the spring and summer months (March-September).

All August severe weather reports (tornado, wind, and hail) from 1950-2011. Storm data courtesy of the Storm Prediction Center. Map by Kathryn Prociv

All August severe weather reports (tornado, wind and hail) from 1950-2011. Storm data courtesy the Storm Prediction Center. Map by Kathryn Prociv

Examining the map of historical August severe weather reports (1950-2011), we get the following breakdown of damaging storm impacts for areas west of the Chesapeake Bay:

50 confirmed tornadoes

313 hail reports

1074 damaging wind reports

August tornado climatology

With only 50 confirmed tornadoes, it ranks as the month with the third fewest tornadoes of the spring and summer.

The fact that August is a quiet tornado month is not surprising given the atmosphere’s tendency toward high pressure ridges and zonal regimes, which reduce the temperature contrasts required for explosive storms and are unfavorable for tornado development.  What may be surprising, though, is the sharp deviation from July, which features the second highest number of tornadoes per month behind May with twice the number of tornadoes as August.

Tornadoes by Fujita/Enhanced Fujita (EF) category during the month of August:

F/EF0: 20

F/EF1: 18

F/EF2: 7

Unknown: 4

There have been no August tornadoes documented at a strength higher than F/EF2.

The most prolific tornado day for the D.C. area during the month of August was on Aug. 30, 2005, when 7 confirmed tornadoes touched down just east of the Blue Ridge Mountains near Culpeper and Marshall, Va.

All August tornado reports from 1950-2011. Storm data courtesy of the Storm Prediction Center. Map by Kathryn Prociv

All August tornado reports from 1950-2011. Storm data courtesy of the Storm Prediction Center. Map by Kathryn Prociv

August hail climatology

Similar to August tornado climatology, the hail climatology is also paltry in terms of the number of hail reports and hail size.  With approximately 300 hail reports in the 1950-2011 record, August has had fewer hail reports than April, May, June and July.  The only severe weather season month with fewer hail reports is March.

During the month of August the thermal profile of the atmosphere is hot basically from the surface to high altitudes. The absence of large quantities of cold air aloft is the primary reason why hail is relatively rare during this month.

Here is the breakdown of hail reports by size (diameter in inches) for the month of August:

Less than 1 inch: 49 percent (below severe criteria)

1 to 2 inches: 50 percent

Greater than 2 inches: <2 percent

According to the Storm Prediction Center database, only one 3 inch hailstone has ever been reported during the month of August, and that occurred in Virginia (coordinates unknown) on August 11, 2004.

All August hail reports from 1950-2011. Storm data courtesy of the Storm Prediction Center. Map by Kathryn Prociv

All August hail reports from 1950-2011. Storm data courtesy of the Storm Prediction Center. Map by Kathryn Prociv

August wind climatology

August wind climatology follows two trends already discussed both in this article and previous articles.  First, damaging winds, yet again, are the most common severe weather type during the month of August  (this is consistent with all the previous months, and thus damaging winds are the most common severe weather event March through August).  Anyone want to take a wild guess at what the most common severe weather event will be in September?…

Second, we find a decreasing number in overall damaging wind reports during this month compared to other months.  August has had less damaging wind events since 1950 than May, June and July.

The month of August does hold one impressive wind statistic, and that is the occurrence of two 78 mph wind gusts.  The first occurred on Aug. 5, 2010, on the west side of the District, and the second occurred in Gaithersburg, Md., on Aug. 12, 2010.  So while damaging winds may occur slightly less often with thunderstorms in August than during previous convective months, it can certainly pack a big punch when it does happen.

All August wind reports from 1950-2011. Storm data courtesy of the Storm Prediction Center. Map by Kathryn Prociv

All August wind reports from 1950-2011. Storm data courtesy of the Storm Prediction Center. Map by Kathryn Prociv

Conclusion

For those who are not fans of wild and severe weather, August is a month when you can breathe a cautious sigh of relief.  While thunderstorms are still prevalent, the likelihood of them producing severe weather in the way of damaging winds, large hail, and tornadoes is less than the previous several months.

Of course, just because severe weather is less common this month, never let your guard down as even an ordinary thunderstorm can produce dangerous conditions.  Remember, “when thunder roars go indoors,” and when you come across a flooded roadway, “turn around, don’t drown.”

Stay tuned for the September severe weather climatology article which will be the final monthly round up for this convective season!

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