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Mesoscale Discussion 1464
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   MESOSCALE DISCUSSION 1464
   NWS STORM PREDICTION CENTER NORMAN OK
   1231 PM CDT SAT JUL 18 2015

   AREAS AFFECTED...EASTERN IOWA...NORTHERN ILLINOIS...AND SOUTHERN
   WISCONSIN

   CONCERNING...SEVERE POTENTIAL...WATCH POSSIBLE 

   VALID 181731Z - 181930Z

   PROBABILITY OF WATCH ISSUANCE...60 PERCENT

   SUMMARY...THUNDERSTORMS ARE EXPECTED TO DEVELOP/INTENSIFY THIS
   AFTERNOON. VERY LARGE HAIL AND DAMAGING THUNDERSTORM WIND GUSTS WILL
   BE THE MAIN THREATS...BUT ONE OR TWO TORNADOES CANNOT BE RULED OUT.

   DISCUSSION...THUNDERSTORMS ARE EXPECTED TO DEVELOP OVER THE NEXT
   SEVERAL HOURS AS ASCENT ASSOCIATED WITH A MID-LEVEL VORTICITY
   MAXIMUM /CURRENTLY ACROSS NORTH-CENTRAL IOWA/ OVERSPREADS THE AREA.
   INITIAL THUNDERSTORMS SHOULD DEVELOP ALONG OR SOUTH OF A DECAYING
   OUTFLOW BOUNDARY LEFT OVER FROM OVERNIGHT CONVECTION...WHERE THE
   ATMOSPHERE HAS BECOME MODERATELY UNSTABLE AND IS WEAKLY CAPPED.
   EFFECTIVE DEEP-LAYER SHEAR IS SUFFICIENT FOR ORGANIZED MULTI-CELL
   CLUSTERS AND/OR SUPERCELLS...ALTHOUGH THE LACK OF WIDESPREAD
   CONVECTIVE INHIBITION SUGGESTS THUNDERSTORMS SHOULD EVENTUALLY GROW
   UPSCALE. THUNDERSTORMS SOUTH OF THE OUTFLOW BOUNDARY WILL HAVE THE
   POTENTIAL TO PRODUCE LARGE HAIL AND DAMAGING THUNDERSTORM
   WINDS...AND A TORNADO OR TWO CANNOT BE RULED OUT...ESPECIALLY IN THE
   VICINITY OF THE REMNANT OUTFLOW BOUNDARY AND/OR LAKE BREEZE...WHERE
   LOW-LEVEL VORTICITY MAY BE LOCALLY AUGMENTED. NORTH OF THE OUTFLOW
   BOUNDARY...THUNDERSTORMS MAY EXHIBIT CHARACTERISTICS OF ELEVATED
   CONVECTION OWING TO THE WORKED-OVER AIRMASS. HERE...THE MAIN THREAT
   SHOULD BE LARGE HAIL...ALTHOUGH GIVEN THE STRONGER DEEP-LAYER SHEAR
   AND LOWER CLOUD BASES...ISOLATED DAMAGING WIND GUSTS AND A TORNADO
   OR TWO WILL BE POSSIBLE.

   ONE COMPLICATING FACTOR WILL BE THE EVOLUTION OF THE CLOUD SHIELD
   DEVELOPING ACROSS NORTHERN MISSOURI AND NORTHWESTERN ILLINOIS.
   SHOULD THIS DEVELOP FARTHER NORTHWARD...OVERSPREADING NORTHERN
   ILLINOIS AND SOUTHERN WISCONSIN...THUNDERSTORM DEVELOPMENT MAY END
   UP BEING MORE ISOLATED IN NATURE...DELAYING POTENTIAL UPSCALE
   GROWTH.

   ..MARSH/CORFIDI.. 07/18/2015


   ATTN...WFO...GRB...LOT...ILX...MKX...DVN...ARX...DMX...

   LAT...LON   41489297 42669244 43869038 44178849 43758740 42268754
               41358799 41048894 40688998 40689197 41489297 

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Page last modified: July 18, 2015
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