Severe Weather Forecast Discussion
Wednesday March 30, 2005 - Prepared by Andrew Revering
 
This discussion is NOT intended for navigational purpose, but rather to help spotters better prepare for a potential severe weather event.

Summary
Forecasts are done using model guidance in the form of maps and forecast soundings available at http://www.f5data.com/.
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Discussion
 
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I'm probably going to fail miserably with this forecast because I'm going so far against the SPC's forecast, so I should preface this by suggesting you also consult the SPC forecast for today.
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Wednesday's setup looks almost completely like a cold-core low setup. I don't see much reason to believe traditional severe storm parameters will work well for forecasting on Wednesday, primarily due to such a lack of deep moisture in the warm sector ahead of the cold front.
 
However, when applying the teachings of Jon Davies, one could see a small area of severe storms and non-supercellular tornadoes forming in Iowa, particularly in North-Central and Eastern Iowa in the early afternoon hours. Keep in mind that in this setup, Davies says that it is quite possible to have tornadoes out of low-topped thunderstorms which do not appear severe on radar, and do not have mesocyclones. In some cases these tornadoes form from cells barely visible on radar. These are more of a landspout type tornado, which are typically weak however, occasionally do produce an F2 or F3 if enough storm relative helicity and deep layer shear can get involved. At this time it does not appear as if any tornadoes (if they do form) would be of any significant size. However, Wednesday's setup does indicate 30-40 knots of 6km vector shear which may be sufficient for some of these cells to possess supercellular characteristics.
 
Since I'm using Jon Davies name and supposedly applying his tactics here, I should refer you to his paper regarding the topic, and suggest that you read his findings yourself rather than taking my interpretation of it for face value... just in case I'm mis-representing his work.
http://ams.confex.com/ams/pdfpapers/81563.pdf
 
First off, lets look at the mid-day cold core low setup for north-central Iowa.
 
500mb closed low, moving northeast: 500mblow.png
Surface low and triple point within 200 miles of the 500mb low: sfc.png
Surface dew points in the low 50s in north-central Iowa: sfcdew.png
Vorticity Advection: vort.png
500mb Dry Intrusion: 500hum.png
700mb Dry Intrusion:    700hum.png
500mb Temperatures of about -21c: 500mbtemps.png
Surface thermal nose pointing into area: sfctemp.png
 
Additionally, we have some low-level, 3km MLCAPE: 3kmmlcape.png
and some 3km low level lapse rates: 3kmlapse.png
 
Given this information, I would say much of Iowa should keep an eye out for the placement of potentially a tight cluster of severe storms and perhaps tornadoes in the early afternoon. Currently I'm targeting Webster City, IA as the epicenter for greatest risk.
 
In the late afternoon the cold core low severe/tornado threat diminishes as the dry intrusion gets choked off, and the low and triple point become farther displaced ahead of the mid level low. This will be the transition from the unusual cold core low setup to the more typical, widely accepted method of forecasting for severe storms.
 
Later in the afternoon the focus for severe weather may be along the draping cold front from the low in Northeast Iowa down the Mississippi River to Arkansas. Just head of and along this boundary a SLIGHT risk for severe weather may exist. I know the Storm Prediction Center has a large MODERATE risk out for this area, but I have a hard time believing that will verify given the minimal moisture depth. At 850mb the dew points will be 5-8 degrees C at best. The best deep moisture will be pooling along and just ahead of the surface boundary. It's speed and orientation will likely mean primarily linear severe thunderstorms with a minimal tornado risk in this area. Primary threat would be high winds and large hail.
 
Therefore, I have to go with a slight risk for severe storms in the late afternoon from Southern Wisconsin and Eastern Iowa into all of Illinois, Southeast Missouri and Eastern and Southern Arkansas.
 
A smaller area of greater severe potential may be embedded in this region in the late afternoon where the deep moisture is maximized and shear is better. This would likely set up in or near East-Central or Southeast Arkansas. Therefore, I have a secondary severe weather target of De Witt, AR in the late afternoon.
 
Severe risk at Noon central: svr18z.png
Tornado risk at Noon central: tor18z.png
 
Severe risk at 6pm central: svr00z.png
Tornado risk at 6pm central: tor00z.png
 
 
Live, operational computer model graphics, forecast soundings and more [F5Data] are available at: http://www.f5data.com/
 

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