DISC 8/19/09: Minneapolis MN surprise tornado

[Jon Davies]

I haven't seen any discussion about the tornado that came close to downtown Minneapolis on Wednesday. That was an interesting "surprise" case, very hard to pick out with small CAPE and lots of rain in the area. No watches or warnings in advance. I put this in a separate thread, so it doesn't get buried in the discussion focused on the KS/OK cap bust or the Illinois tornadoes.

In the Minneapolis event, the small tornadic cell was near a surface low and focus of boundaries beneath a sharp wave and vorticity center aloft, even though it occured within a socked-in area of clouds and precipitation. Diagnosis of the setting was seriously hindered by the presence of the extensive area of clouds and precipitation, and unimpressive total CAPE (only 300-400 J/kg).

FWIW, a short case study of the setting is on my blog:
http://davieswx.blogspot.com/

I'm always trying to learn...

Jon Davies

 

[Andrew Revering]

Oh I love that someone recognized that Minnesota gets tornadoes too. ;-)

Being from Minneapolis, I may have been watching more than most on this list just prior to when it occured... yet I didn't see it coming. As you said, total CAPE was quite minimal, but low level instability was actually pretty good. I think it was on the order of 50 joules in the lowest 3km. Also, the low level shear profile was pretty decent.

Now I'll go read your blog...

 

[Andrew Revering]

I also wanted to add that I think situations like this might not be as rare as we think. I was out after the first warning finally came out and I saw a brief white snake tornado in south-central Anoka County, associated with the same circulation that moved through the downtown area. This tornado did little damage, and I don't even think was rated... I did hear of someone losing shingles, but that was about it. It was in a heavily forested and lake area.

I feel like days like this could happen quite frequently, and even perhaps long before it came through the metro, but these very brief and weak F0s are just not noted for a couple reasons...

1) nobody chases a 300 joule MCS
2) they are almost undetectable on radar, so even if you did, you'd likely never see anything.
3) if its not going through a heavily populated area (like it did on Wednesday), chances of it doing notable damage to know a tornado came through is almost nil.

...and another note... this thing truely did go through downtown... it started in the south minneapolis area (city proper) and ended at the high-rises and metrodome area. The convection center which is right downtown had some minor damage I believe. ...and this was all from an F0... imagine if this was a much larger tornado.

 

[Andrew Revering]

I know normalized cape or NCAPE is typically large when you have a ton of instability, but the idea is that it tells you how 'thick' the cape profile is... considering we had tops of only on the order of 18000 feet, and 125 joules of about 300 were in the lowest 3km, does NCAPE show us anything about it's girth and if not, what about the distribution of CAPE in the low levels?

Considering the storms showed little rotation on radar (unless you had Level II TDWR) I can't imagine the rotation was very deep.

 

[John Farley]

The convection center which is right downtown had some minor damage I believe. ...and this was all from an F0... imagine if this was a much larger tornado.

I am guessing you mean "convention center" - but I guess that day it was also the "convection center." Sorry, couldn't resist.

 

[Jon Davies]

Andy,

The problem with NCAPE (normalized CAPE) is that it has no reference to the ground. It is a measure of CAPE "density" from LFC to EL, but says nothing about whether all the CAPE is below 500 mb and closer to the ground (as on Wed in MSP) or is located between 700 mb and 300 mb, or farther upward or whatever. When thinking about tornadoes (a surface phenomenon), some reference to where the CAPE is located in the vertical relative to the ground seems important regarding stretching near the ground. Unfortunately, 0-3 km CAPE doesn't suggest that much information regarding stretching, except maybe as to whether the environment is strongly surface based or not. Possibly 0-3 km CAPE as a percentage of the total CAPE or something like that might be useful in these really small CAPE settings.

Yeah, radar-wise you'd have to really really be watching like a hawk for rotation with Wednesday's MN cells to see it... I saw where Paul Douglas had an image of a weak couplet on his blog. But 25-30 or more miles away from the radar with such small shallow storms, just forget about it.

Jon

 

[Andrew Revering]

I am guessing you mean "convention center" - but I guess that day it was also the "convection center." Sorry, couldn't resist.

LOL... oh my gosh... yeah that shows you how often I type convection... yes I meant convention center.

 

[Dean Baron]

I was watching BV1 on the TWDR TMSP just before the tornado warning was issued and before the tornado reports came in. A relatively large but broad/weak couplet was evident moving north. Given the lack of big convection going on I dismissed the couplet as just a fluke from the radar site. But the tornado warning was issued shortly thereafter following the path of the couplet which was still evident on TDWR BV1. Not sure whether it was clear on KMPX BV1 (or SRV1) what was happening but TMSP had a pretty clear view of what was going on.

 

[Todd Krause]

Thanks, Jon. One of the interesting things about this storm is that it appears to have had no associated lightning strikes until some time after the tornado dissipated.

Todd