Feb 21, 2014

February 20 Tornadoes and Forecast Verification

The storm reports graphic from SPC shows that most of the tornado reports were along the northern edge of the area I outlined yesterday. This was an area with limited instability, but intense lifting and helicity. A rapid refresh sounding for central Illinois near Springfield has only around 700 MLCAPE, but helicity over 800 m2/s2:
If it had been realized in the near storm environment, it may have been significant that SBCAPE was more than double the MLCAPE value:
The only tornado report in my moderate risk area was from a storm in far southern Illinois that hit Carbondale with 1 inch hail, and later went across I-64. This storm was actually the most interesting on radar, which may suggest it was producing tornadoes that nobody observed:

Further south, limiting factors for tornado development included clouds and rain during peak heating hours, and winds that were backing with height in the 850 to 600 mb layer, as seen on the Nashville sounding, but not so much at Little Rock:

Winds backing with height tends to reduce storm relative helicity encourage lines instead of discrete cells.

I made a 9 day forecast for this event which was one day late, and around 400 miles to the southwest.

Dew points were generally mid 50s in central Illinois to around 60 in southern Illinois.Here are some rapid refresh analysis charts from 12Z:

The same storm that produced tornadoes also prompted blizzard warnings from southern Minnesota to central Iowa. 10.5" of snow fell in north-central Iowa.

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