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Mesoscale Discussion 1713
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   MESOSCALE DISCUSSION 1713
   NWS STORM PREDICTION CENTER NORMAN OK
   0800 PM CDT TUE AUG 18 2015

   AREAS AFFECTED...SOUTHERN WI AND NORTHEAST IL

   CONCERNING...TORNADO WATCH 494...

   VALID 190100Z - 190230Z

   THE SEVERE WEATHER THREAT FOR TORNADO WATCH 494 CONTINUES.

   SUMMARY...DAMAGING WINDS AND A TORNADO THREAT CONTINUE ACROSS
   NORTHEAST IL INTO PARTS OF SOUTHEAST WI...WITH THE POTENTIAL FOR
   ADDITIONAL WFO-LOT COUNTIES TO BE ADDED ACROSS FAR NORTHWEST
   INDIANA.  WEAKENING INSTABILITY AND SHEAR WITH EASTWARD EXTENT INTO
   NORTHERN INDIANA SUGGEST NEW WATCH ISSUANCE EAST OF THE WFO-LOT
   COUNTY WARNING AREA IS UNCERTAIN...THOUGH TRENDS WILL CONTINUE TO BE
   MONITORED.  MEANWHILE...THE COUNTIES IN THE WAKE OF THE QLCS PASSAGE
   CAN CONTINUE TO BE CANCELLED FROM THIS WATCH.

   DISCUSSION...AT 0040Z...MOSAIC RADAR IMAGERY SHOWED A QLCS
   CONTINUING TO ADVANCE TO THE EAST-NORTHEAST ACROSS NORTHEAST IL AND
   SOUTH-CENTRAL INTO SOUTHEAST WI.  THE ENVIRONMENT IMMEDIATELY
   DOWNSTREAM OF THESE STORMS HAS DESTABILIZED INTO THE EARLY EVENING
   WITH OBJECTIVE ANALYSES INDICATING MODERATE INSTABILITY EXTENDING
   FROM THE QLCS INTO WESTERN INDIANA AND THE FAR SOUTHERN EXTENT OF
   LAKE MICHIGAN.  AREA WSR-88D VWPS SHOWED A FAIRLY DEEP LAYER OF
   CONVERGENCE GENERALLY ATTENDANT TO THE QLCS...WHILE A S/SWLY LLJ
   EXTENDING INTO THE CHICAGO AREA HAD STRENGTHENED TO 40 KT.  THE
   STRONGER LLJ HAS BOOSTED SURFACE-1-KM SRH TO 225 M2/S2 SUGGESTING
   ANY DISCRETE STORMS AHEAD OF THE QLCS AND EMBEDDED CIRCULATIONS
   WITHIN THE QLCS WILL BE IN A KINEMATIC ENVIRONMENT FAVORABLE FOR A
   TORNADO THREAT THROUGH THE EARLY-MID EVENING.  FARTHER NORTH INTO
   SOUTHERN WI...THE ENVIRONMENT HAS BEEN STABILIZING WITH OBJECTIVE
   ANALYSES SHOWING SURFACE-BASED INHIBITION ALSO INCREASING ACROSS FAR
   SOUTHEAST WI.  THIS MAY TEND TO LIMIT THE SEVERE WEATHER THREAT
   ACROSS SOUTHEAST WI...THOUGH AN INCREASE IN SRH SUGGESTS A NON-ZERO
   SEVERE THREAT REMAINS IN THE SHORT TERM.

   AS INDICATED IN THE SUMMARY...DEEP-LAYER SHEAR AND INSTABILITY
   WEAKEN WITH EASTWARD EXTENT INTO NORTHERN INDIANA...SUCH THAT
   PERHAPS ONLY THE WFO-LOT COUNTIES IN NORTHWEST INDIANA MAY NEED TO
   BE INCLUDED IN WW 494.  THIS SCENARIO IS SUPPORTED BY THE 23Z HRRR
   AND ESRL HRRR EACH SUGGESTING THE ONGOING QLCS WILL BEGIN TO WEAKEN
   AS IT EXITS IL.

   ..PETERS.. 08/19/2015


   ATTN...WFO...IWX...GRB...LOT...ILX...MKX...

   LAT...LON   40408848 41548869 42188922 42698919 43418960 43838958
               44028884 43568830 42798824 42488785 41678735 40888705
               40518718 40408848 

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Page last modified: August 19, 2015
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