September severe weather: the month of tropical tornadoes

Track of Hurricane Ivan, which spawned 40 tornadoes in the region in 2004. (Wikipedia)

Track of Hurricane Ivan, which spawned 34 tornadoes in the region in September, 2004. (Wikipedia)

After a quiet August month, the Washington, D.C. area experiences a spike in severe weather during the month of September.

Interestingly enough, this increase in severe weather is not with the usual culprits of hail and damaging wind, but instead in the form of tornadoes. Hail and damaging wind reports continue the decreasing trend as the fall season commences, but tornado reports increase.

September has the third highest number of tornadoes by month sitting behind only May and July.  So what is behind the unique nature of September severe weather?  Hint: It has everything to do with the fact that September is the peak month of hurricane season.

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

All September 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:

85 confirmed tornadoes

94 hail reports

264 damaging wind reports

September tornado climatology

With 85 confirmed tornadoes, the month of September experiences the third highest number of tornadoes during the convective season behind May and July.  This may come as a bit of a surprise given September is often seen as a transitional month moving out of summer and into autumn.  (Remember, meteorological fall begins on September 1.) 

The seasonal transition out of convective season and into the fall season accounts for the decrease in overall damaging wind and hail events, but does not account for the uptick in tornadoes.  Summer and fall aside, hurricane season (the season that bridges the gap between the previous two) is the primary catalyst for September tornadoes across the D.C. region, with one particular tropical event to blame: Hurricane Ivan.

The remnants of Hurricane Ivan made landfall near Gulf Shores, Alabama on September 16, 2004 as a Category 3 hurricane.  After making landfall, it weakened to a tropical depression as it moved north and east along the spine of the Appalachian Mountains impacting the D.C. region on September 17, 2004.  When the remnants of Ivan hit Virginia, what ensued was the greatest tornado outbreak day in Virginia history, with 40 total tornadoes.

Confirmed tornadoes from the remnants of Hurricane Ivan. Storm data courtesy the Storm Prediction Center. Map by Kathryn Prociv

Confirmed tornadoes from the remnants of Hurricane Ivan. Storm data courtesy the Storm Prediction Center. Map by Kathryn Prociv

Out of all the September tornadoes from 1950-2011, 34 of the total 85 tornadoes are from September 17, 2004; in other words, 40% of September tornadoes between 1950-2011 occurred on that one day alone.

(CWG’s Ian Livingston and I will be writing a Hurricane Ivan anniversary post which will go into even more detail regarding the tornado outbreak across the D.C. area from that event.)

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

F/EF0: 25

F/EF1: 36

F/EF2: 17

F/EF3: 6

F/EF4: 1

The F4 tornado occurred on September 24, 2001 just north of Culpeper, Virginia.

The second greatest number of tornadoes on a single September day occurred on September 5, 1979 with 12 tornadoes produced by the remnants of Hurricane David.  Finally, a third prolific tornado day across the D.C. region experienced 10 tornadoes on September 8, 2004 from the remnants of Hurricane Frances.

The Sterling National Weather Service provides a nice list of tornadoes caused by hurricanes, with records going back to 1915 here.

Due to this climatological propensity for tornadoes caused by tropical systems, it seems suitable to coin September as “the month of tropical tornadoes.”

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

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

September hail climatology

Where September is a month for tornadoes, it is not a month for hail.  With just over 90 hail reports September is the month with the second fewest hail reports behind March.

Taking into consideration that much of September’s severe weather results from tropical systems, it is not surprising that hail is uncommon.  Tropical systems are often warm-core systems meaning the entire temperature profile is warm from the surface to higher up in the atmosphere.  This lack of cold air aloft is a major contributing factor for the infrequent occurrences of hail producing thunderstorms.

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

Less than 1 inch: 43 percent (below severe criteria)

1 to 2 inches: 53 percent

Greater than 2 inches: 1 percent

According to the Storm Prediction Center database, there has never been a hailstone reported greater than 3” in diameter during the month of September.  The largest report on the map was 2.75” in diameter and occurred on September 3, 1993 near Hagerstown, MD.

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

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

September wind climatology

While September continues the theme of damaging winds as the most common type of severe weather event, the total number drops off dramatically compared to previous months.  With roughly 260 wind events September has the second fewest number of damaging wind reports behind, you guessed it, March.

The sharp decline in damaging wind events reflects the overall decrease in convective activity during the month of September as the atmosphere cools down and transitions into a more autumnal pattern.  Nonetheless, September still experiences its fair share of thunderstorms, and like all the succeeding months damaging winds is the most likely severe weather threat with any thunderstorm that develops.

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

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

Conclusion

While September may experience an overall downswing in the frequency of severe weather due to the transition into autumn, it is extremely important to recognize that tornadoes can pose a real threat this month.  This tornado threat is especially potent with any tropical systems that affect the D.C. area.

As the D.C. region begins winding down out of the hot summer months and convective storm season and into the cooler autumn the severe season wanes to make way for winter.  For thunderstorms enthusiasts, it is a melancholy time of year as the days get shorter and discussions of winter weather dominate the conversation.

Nonetheless, true weather buffs appreciate all weather equally, so bring on the forecast discussions and winter weather outlooks for the next several months, all the while casually looking ahead toward spring when thunderstorms shall return, again.

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