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Mesoscale Discussion 675
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   MESOSCALE DISCUSSION 0675
   NWS STORM PREDICTION CENTER NORMAN OK
   0209 PM CDT SAT MAY 18 2013
   
   AREAS AFFECTED...NWRN KS...NERN CO AND WRN NEB
   
   CONCERNING...SEVERE POTENTIAL...WATCH LIKELY 
   
   VALID 181909Z - 182045Z
   
   PROBABILITY OF WATCH ISSUANCE...80 PERCENT
   
   SUMMARY...STORMS ARE EXPECTED TO INITIATE BY 20-21Z FROM NERN CO
   INTO NWRN KS AND EXTREME SERN WY...THEN SUBSEQUENTLY DEVELOP NEWD
   INTO WRN NEB DURING THE EVENING. PRIMARY THREATS WILL BE VERY LARGE
   HAIL AND ISOLATED TORNADOES. STORMS MAY EVENTUALLY GROW UPSCALE INTO
   ONE OR MORE CLUSTERS LATER IN THE EVENING WITH AN INCREASING THREAT
   FOR DAMAGING WIND. A TORNADO WATCH WILL LIKELY BE NEEDED PRIOR TO
   21Z.
   
   DISCUSSION...THIS AFTERNOON A DRYLINE EXTENDS FROM SERN WY TO A
   MESOLOW NEAR THE KS/CO BORDER THEN FARTHER SWD THROUGH SWRN KS. THE
   LATEST OBJECTIVE ANALYSIS INDICATES THE WARM SECTOR HAS BECOME VERY
   UNSTABLE IN CORRIDOR FROM NWRN KS THROUGH EXTREME NERN CO AND WRN
   NEB WHERE VERY STEEP LAPSE RATES /APPROACHING 9 C/KM/ HAVE
   OVERSPREAD THE MOIST AXIS. SPECIAL 18Z RAOB FROM NORTH PLATTE NEB
   INDICATES A STRONG CAPPING INVERSION IS STILL IN PLACE JUST BELOW
   700 MB IN ASSOCIATION WITH WARM AIR AT BASE OF THE EML. THIS
   SUGGESTS THUNDERSTORM INITIATION WILL LIKELY BE FARTHER WEST ACROSS
   NERN CO AND NWRN KS. SATELLITE IMAGERY SHOWS MID-LEVEL CONVECTION
   SPREADING RAPIDLY NWD ACROSS NERN CO AND NWRN KS AND APPEARS TO BE
   IN ASSOCIATION WITH MID-LEVEL ASCENT ACCOMPANYING A NEWD EJECTING
   SHORTWAVE TROUGH. FORCING FROM THIS FEATURE AND DEEPER MIXING OVER
   THE HIGH PLAINS OF NERN CO THROUGH WRN KS WILL LIKELY SUPPORT
   THUNDERSTORM INITIATION ALONG THE DRYLINE SOMETIME DURING THE NEXT
   COUPLE HOURS. 
   
   VERTICAL SHEAR THROUGH A DEEP LAYER WILL REMAIN SUFFICIENT FOR
   SUPERCELLS CAPABLE OF VERY LARGE HAIL GIVEN THE FAVORABLE LAPSE
   RATES AND STRONG INSTABILITY. ALSO...THE THREAT FOR ISOLATED
   TORNADOES IS EXPECTED TO INCREASE IN WINDOW FROM LATE AFTERNOON INTO
   EARLY EVENING AS THE LLJ STRENGTHENS AND BEFORE THE BOUNDARY LAYER
   DECOUPLES AND WHILE SOME STORMS MAY STILL BE DISCRETE. STORMS MAY
   EVENTUALLY CONGEAL INTO ONE OR MORE CLUSTERS DURING THE
   EVENING...WITH AN ATTENDANT INCREASE IN DAMAGING WIND THREAT.
   
   ..DIAL/KERR.. 05/18/2013
   
   
   ATTN...WFO...GID...LBF...GLD...BOU...CYS...
   
   LAT...LON   39030147 39950243 40740317 42060386 42720319 42270080
               39630003 38840064 39030147 
   
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