How often are derechoes this strong

[STurner]

I dont hear of derechoes very often producing a number of strong tornadoes in a single day. It seems during the first week of May this was a pretty large event. On May 8th 9 strong tornadoes were spawned by mainly this derecho event which included 7 EF2's and 2 EF3's. Getting a couple EF3 tornadoes with discrete supercellular thunderstorms seems like a pretty rare event in itself but with a derecho. How often are events of this magnitude associated with a derecho for I dont remember any events associated with a derecho producing so many strong tornadoes?

 

[Robert Dewey]

Between bookend vorticies and the supercells that often develop ahead of the main line, I would say it's not all that uncommon. It also seems like most derecho events start out as a series or group of supercells... as was the case with the extreme derecho on 5/31/98.

http://www.spc.noaa.gov/misc/AbtDerechos/casepages/may30-311998page.htm

 

[Chris Hayes]

There are derecho events every year. Every once in a while you'll get an intense one like the one we just had a few days ago. I wouldnt say it's that often you'll have a derecho with winds over 100mph and multiple tornadoes. I could be wrong, but I'd say it occurs once every couple of years. However there are years where they occur more often. The one that we just had was mild compared to the one that went through Minnesotta, Wisconsin, and Michingan on May 31, 1998. That one had winds of over 130mph in places. The last big derecho I can rember having here (Canton Ohio) was on May 21, 2004. I'm not sure if that derecho started out as a cluster of supercells or not. I know it had it's origins in Illinois. It went across northern Indiana and southern Michingan then made a SE turn and turned into an intense bow echo and crossed NE Ohio. At the 2000 outlook we were put under a moderate risk and a PDS severe thunderstorm watch was issued. Using SeverPlot, the strongest measured gust of wind was 100kts, and it produces 2 F0 tornadoes, and the largest hail report was 2". Here is a picture of the derecho as it's approaching my house, and below it is a satelite image I saved as it was getting going over NC Ohio

 

[Mike Smith]

While I agree that derechoes happen every year, Friday's was an extraordinary event. Rarely do commercial television towers fall and rarely are winds actually clocked at speeds of higher than 100 mph.

The tornadoes with Friday's derecho were not extraordinary but the straight winds were.

 

[Robert Dewey]

Ah yes, didn't realize this was a double question. My answer applies only to the tornadic aspect.

 

[John Farley]

I'm not fully aware of the climatology, but my impression is that there were certain aspects of the strength/size of the tornadoes, and possibly even the number of them, that was unusual. NWS Springfield has 13 tornadoes on the map for their CWA, but Public Information Statements issued since the map was updated Saturday suggest a total of something like 15 or 16, and still counting. Five of them were EF2 or EF3, and three of them were a half-mile to 3/4 of a mile wide. Additional tornadoes occurred in the St. Louis, Paducah, and Louisville CWAs, and possibly others. It appears that there were bow-head tornadoes, tornadoes associated with mesovortices along the leading edge of the bow-echo, and tornadoes associated with embedded HP supercells. All of these happen with bow echoes and derechos, but the number, size, and intensity seems on the high side in this case, at least relative to other bow echo/derecho cases I'm familiar with.

 

[Brian Russ]

The Labor Day '98 Derecho in upstate NY started as a super cell though never
produced tornado type of damage. I also noted some of the recent Derecho
events lately as well. it's been a while now since we've had any here in NY.

http://www.spc.noaa.gov/misc/AbtDerechos/casepages/sep71998page.htm

Here's a a good read for more info.

http://www.spc.noaa.gov/misc/AbtDerechos/derechofacts.htm#historic

I have to admit since the first time experiencing a derecho in July '95 I've
been a bit obsessed learning about them. Back before the internet I was
on a quest to find info in any books I could find. The images of strobe lightning
and those winds are ones I'll never forget.