Topic: Derecho

[Jason(pato)]

Hi guys,

Not sure if this was the right place to put this but here goes anyway. I read with some interest on the U.S site Tornadovideos.net about a phenomenon called a derecho. My question to some of the people on here who have chased in the States is, what is this phenomenon, have any of you experienced it and what are the conditions which spawn such an event. My thoughts were it was something similar to the squall lines we get here in Australia or something like a "southerly buster". I saw something on the weather channel too where this type of system whipped up winds of around 100mph, levelling lots of trees in the process. Anyway I hope someone can shed some light on this because I found it quite interesting.

Thanks in advance

Jason

[Jimmy Deguara]

Jason,

In our first trip to the United States 2001, David and I experienced one of the most impressive derecho event. The supercell complex stretched from horizon to horizon - there was no escape. The long lived event chased us for the 3 hours as we headed south into Oklahoma towards southern Oklahoma. As we headed east towards Lawton, the Dercho hit and almost blasted us off the road as we drove. The winds recorded by storm chasers of up to 106mph (at least 160km/h). The storm structure though was something spectacular - check out the images and videos of this amazing event. The lightning was also something incredible!

Sunday 27 May 2001 HP Supercell monster Derecho Event - Woodward to Lawton, Oklahoma

http://www.australiasevereweather.com/storm_news/2001/docs/200105-04.htm



Videos:

http://www.australiasevereweather.com/video/movies/2001/0527jd01.wmv

http://www.australiasevereweather.com/video/movies/2001/0527jd01.wmv

http://www.australiasevereweather.com/video/movies/2001/0527jd02.wmv

http://www.australiasevereweather.com/video/movies/2001/0527jd03.wmv

http://www.australiasevereweather.com/video/movies/2001/0527jd04.wmv

http://www.australiasevereweather.com/video/movies/2001/0527jd05.wmv

http://www.australiasevereweather.com/video/movies/2001/0527jd06.wmv

http://www.australiasevereweather.com/video/movies/2001/0527jd07.wmv

http://www.australiasevereweather.com/video/movies/2001/0527jd08.wmv

http://www.australiasevereweather.com/video/movies/2001/0527jd09.wmv

http://www.australiasevereweather.com/video/movies/2001/0527jd10.wmv

http://www.australiasevereweather.com/video/movies/2001/0527jd11.wmv

http://www.australiasevereweather.com/video/movies/2001/0527jd12.wmv

http://www.australiasevereweather.com/video/movies/2001/0527jd13.wmv

http://www.australiasevereweather.com/video/movies/2001/0527jd14.wmv

http://www.australiasevereweather.com/video/movies/2001/0527jd15.wmv


Regards,

Jimmy Deguara

[Mike]

Fantastic video snippets Jimmy.  Just mind boggling the size of that supercell - where did it end!

How long did the Derecho last that you experienced?  Was the supercell part of an MCS or squall line that you know of?

[Jimmy Deguara]

Mike,

Thanks for your curiosity. Actually, in my opinion, the derecho preceeded the development of what is likely a major MCS or MCC. The complex covered almost the entire two states of Oklahoma and Kansas - check the satellite imagery:

7:15pm CDT
8:02pm CDT

radar image at 7:15pm CDT

You can see the line clearly in the radar imagery and the TVS signatures indicating tornadic circulations.

Now to attend to your queries, the system began its career in southern Kansas, traversed Oklahoma and entered into Texas and would have travelled with damaging winds overnight throughout parts of Texas. This is typical of violent Derecho events in upper level north westerly airflow and high cape situations (>5000 J/kg) with good moisture source and good inflow - they are long lived.

The derecho develops as part of a merged cluster of supercells that can form a squall line or may also have a supercell developing on one end. They attain different characeristics throughout their life cycle and severe straight line winds eventually dominate in the system as they merge into a squall line structure. Tornadic spin ups can also form in some regions within the system. There have been some good documented case studies of derecho events including this event and also an event that occurred in the last couple of years. Well worth looking up.

Regards,

Jimmy Deguara

Regards,

Jimmy Deguara

[Mike]

Thanks, Jimmy. Interesting satpics with massive storm cluster profiles.  Your explanation answers my question...but...in terms of supercell width they seem to be massive on the images,  is this the largest you've seen and if so why?

[Jason(pato)]

Thanks Jimmy. The satpic beggars belief. Never have I seen something so vast. From your explanation I would deduce that nothing like this has occured in Australia because our setups wouldn't favour an event so extreme. I haven't had the time to view your vids yet but the pics speak volumes of the shear immensity of this type of system. Certainly a fascinating subject which I will be looking into some more to satisfy my curiosities.

[Mike]

Given the immense size of the storm system across the state lines it just is beyond logic!  One can only view the videos to appreciate the size of this system and the storms within - i still can't believe what i see until viewing the footage!  The storm system covers three or four states and they are by no means 'close'!  The atmospheric conditions for that period were indeed over the top with CAPE over 5000j'kg - we get those numbers in Darwin but what a contrast in set ups and  what explodes afterward - fascinating.