Tornadoes occurred in N Illinois on the 24 August 2004

[mikegeukes]

Tornadoes occurred in Northern Illinois on the Evening of 24 August, 2004
http://www.crh.noaa.gov/lot/science/24Aug2...004summary.html

Mike

 

[David Draun]

One nado of F0 strength came within just a few miles of the NIU campus. Sirens were wailing here non-stop for 20 minutes. It took 10 minutes to complete sheltering procedures. That's too long. What if the storm jogged more to the north? The wind may not have destroyed much, but people still could have gotten hurt. Some people stayed out on the street as the sirens sounded. Plenty of sitting ducks. :?

 

[Mike Peregrine]

Interesting Mike - thanks for the post. That was one of the weirdest storms ...

Okay, so the narrative references this as a low-topped supe, not reaching more than 40,000 in height. Then it speaks about its interraction with a localized 'warm front' of sorts that prompted the storm to become tornadic. So I'm guessing that for one thing, this storm must have been sitting under a cold pool aloft, right? - Anyone notice what the UL temps were?

And also - on days with cold core supes ... my big question is, is it really an interraction with frontal boundaries that spark the storms to become tornadic as is hinted at here??? I have always thought (who knows if it's right or not - hard to keep up with this stuff) ... that it's more a case of wind dynamics on cold core days ... with these events you can end up with tornadoes all over the place if you aren't on your toes - more due to the relative storm position in relation to boundaries and low pressure further downstream. So I'm wondering if this is really a case of frontal interraction ... or wind profiles setting the stage for this event. Any ideas (and if my question makes absolutely no sense to you - I can't say that I blame you - just go on about your day) ...

 

[Mike Peregrine]

To make this question a little easier to understand - here's a map with a typical setup for mini supes ...

http://members.cox.net/jondavies1/tornado_...r_composite.gif

I guess I'm just curious if anyone can define the primary ingredients seen in the map against the setup on Monday that produced the storms in Illinois to see if they compare and what really served as the catalyst for these storms.

Funny how Illinois has been the epicenter for hard to explain weather events this year ... first Utica, now this one.

 

[Glen Romine]

Maybe I'll have to take a closer look at this event. I don't think this event fits the mini-supercell paradigm that Jon Davies describes - those events are more characterized by cold core lows and shallow tropospheres limiting the vertical extent - with large scale ascent leading to enhanced lapse rates despite the typical cool conditions beneath upper lows.

Here, if you look at the CAPE plot provided by Al P., you see a scrawny CAPE axis stretching up to the storm - but the surface parcel conditions are relatively uniform everywhere - so this seems to suggest at least the model thought there was a small cold pool there. Looking at the ILX 00Z sounding, there is a clear midlevel inversion, which probably caused the cell tops to be limited - not a shallow troposphere. But the DVN sounding showed somewhat similar characteristics, but with more lower tropospheric moisture. If I find time, maybe I'll look more closely later.

Glen