Forecast Verification Update

13 new tornado reports have been added to the SPC storm reports for February 20th. Several more tornado reports have been added along the northern border of my slight risk area, several have been added in the moderate risk zone, three were added near the southern edge of the slight risk, and three were added to the northeast of the risk area:

Overall, this indicates that my forecast was more accurate than it had seemed before.

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.

Tornado Threat Today

Expect thunderstorms to develop in eastern Missouri and eastern Arkansas by 20Z (2 PM) today. Primary hazards should be damaging wind and tornadoes. CAPE will be over 1,000 j/kg in Arkansas and Mississippi, with 0-1 km helicity above 200. In southern Illinois, CAPE will only be in the 500 - 1,000 range, but with 0-1 km helicity above 450.

Tornado Outlook Feb 19

Shifted slightly northeastward from the previous forecast.

Tornado Outlook Feb 18

There is a possibility of tornadoes during the afternoon and overnight on Thursday.

Tornado Outlook Feb 17

The tornado threat has been moved up one day to Thursday, Feb 20. The severity of the threat is reduced because of weaker instability.