Severe Weather Forecast Discussion
Monday July 14, 2003 - Prepared by Andrew Revering
 
 
THIS DISCUSSION BROUGHT TO YOU BY STORMTOURS.COM,
PROVIDING WEEK LONG STORM CHASE VACATIONS IN TORNADO ALLEY.

 
 
Summary
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Discussion
 
An incredible severe weather event may be on tap for Monday in Minnesota. Although the computer models are unreliable at best this time of year, it does appear that there may be some truth to this scenario.
 
A frontal boundary will extend through the length of Minnesota by mid-afternoon. Ongoing convection in the northern part of the state may inhibit significant severe storms by mid-afternoon, but farther south (lets say near New Ulm, MN) I anticipate a supercell to develop around 4 p.m. in an unstable airmass along an outflow boundary. At this point in time, the lifted index is expected to be near -5, MLCAPE near 1500. Surface dew point in the low 70s and 850mb dew point near 60 should be more than enough instability and moisture for fuel.
 
A significant shortwave coming through the area by mid-afternoon should be sufficient to lift the boundary layer. That will be the match to start the fire. Convective Inhibition or negative buoyancy is projected to erode in said area due to strong mechanical forcing. This will be along an area of increasing convergence on the surface boundary at the nose of the moisture axis.
 
A bit of a warm front should be laying just to the south, somewhere along the MN/IA border. This warm front may be prevented from traveling too far to the north due to cloud cover caused from overnight convection. This will strengthen the warm front and become a focal point for tornadic supercells as the boundary creates backed winds at the surface and increases convergence in the area of the triple point.
 
A low level jet (850mb) from northwest Iowa into southeast Minnesota will intensify mechanical forcing. These winds from the SW working together with the backed winds at the surface should help to generate incredibly high levels of 0-1km Helicity, or low level corkscrew turning in the atmosphere... key for tornadic development.
 
The 850mb and 700mb temperatures along with CINH values suggest that capping inversion will not be a problem. Incredible amounts of 500mb dry intrusion are anticipated by late afternoon. As this dry intrusion moves into the area, central and southern Minnesota should have explosive development of severe storms. Associated with this dry intrusion will be increasing mid level winds which will increase the amounts of speed and directional shear, supportive of supercells and tornadoes. Given the extent of area effected and the strong 500mb winds expected, a segmented bowing line of severe storms are likely to develop later in the period. The area most likely affected by the bowing line of severe storms would be East-Central MN into Western Wisconsin and far Southeast Minnesota. Farther south and west of that area is where the greater tornadic potential would be as cells would have better opportunity to become isolated supercells, especially early in their development stage.
 
As the storms that develop near 4 p.m. in Southwest/South-Central Minnesota get going, they will travel east-southeast along the warm front to near Mankato and Albert Lea. The atmosphere in this area expected to be more than favorable for long-lasting significant tornadoes.
 
The Energy-Helicity Index (EHI) is forecasted to be over a 4, 500mb to Surface wind differential of about 40 kts, 0-1km Helicity of around 200, VGP of near 0.50, LCL of about 875mb, LFC of about 800mb and CINH less than abs(-50). All of these parameters indicate significant tornado potential, with potentially tornadoes producing F2 to F5 damage.
 
My target for the day is Mankato, MN.
 
Now... with all of that said, the threat of inhibited storms from overnight convection is real. However, I do believe that most of the blow-off will remain to the north and east of the target area. Primarily north of I-94. The remaining outflow boundaries may help to align the warm front parallel to the mean storm vector for supporting long-lived supercells along that boundary.
 
The other questionmark is whether or not model data will let me down again. If it does, then I'm helpless. However, latest model data seems to indicate the above scenario... in my opinion. The SPC at the hour I'm writing this puts their target much farther south into Iowa. We'll see what happens.
 
 

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