November 12, 1965 Severe Weather Event

[Hoosier]

This one has always kinda interested me.  It wasn't huge by any means, but the environment it occurred in was a bit unusual.  Coming up on 50 years so why not post a little about it.

 

Here's a 500 mb loop with sfc pressure overlaid.  As you can see, we have a wave that eventually attains a negative tilt, with the surface low deepening fairly rapidly:

 

 

 

The end result was several tornadoes in IL/northwest IN, including some strong ones, with some scattered wind/hail reports. 

 

 

(Should note that Grazulis is a bit different and has the northwest Indiana F3 as an F2, which is probably more appropriate given the damage description, and has the long track F2 just west of that in Grundy/Will/Cook counties as an F3)

 

 

Stuff like that happens in Fall, but take a look at the surface map just prior to the start:

 

 

 

 

Midway Airport observation around the time of the tornadoes:

 

 

 

METAR KMDW 122100Z 20020KT 4SM FU BKN/// OVC/// 14/11 A//// RMK SLP008 T01440106

 

That's a temperature/dewpoint of 58/51.  You might be able to tack on a degree or two to each to get a representative environment for the tornadoes that occurred just south of Chicago.

 

Temps in the 50s with dewpoints in the low/mid 50s really doesn't scream tornado environment, especially one that would produce strong tornadoes, but that's exactly what happened.  Why?

 

Although temperatures/dewpoints at the surface were fairly low, temperatures aloft were colder than what you would normally see for a severe weather event.  Below I've attached some 850 mb, 700 mb, and 500 mb temperature/geopotential height maps, which shows how this was an unusually cold system aloft.  Based on interpolation, 850 mb temps may have only been in the +3-5C range, 700 mb temps may have been -3C to -5C, and 500 mb temps may have been about -20C to -22C during the time that severe weather was occurring (the freezing level may have been like 7000 feet!).  All of this suggests that mid-level lapse rates were good for the time of year and this probably enabled there to be just enough surface based instability for tornadoes.   

 

 

 

 

 

 

 

 

 

 

 

 

 

 

850 mb winds were pretty strong as you might expect, but not much moisture aloft as shown by pretty low 850 mb dewpoints.

 

 

 

 

Thermodynamically, this setup was almost more like a cold-core setup with the very cold temperatures aloft, but it wasn't one in the classic sense.  Interesting nonetheless.

[andyhb]

Didn't realize Meteocentre had reanalysis maps.

[Hoosier]

Didn't realize Meteocentre had reanalysis maps.

 

 

Indeed, comes in handy!

[andyhb]

Yes, yes it will for me now.