MESOSCALE DISCUSSION 0515
   NWS STORM PREDICTION CENTER NORMAN OK
   0351 PM CDT WED APR 18 2007
   
   AREAS AFFECTED...NRN WY AND SRN MT
   
   CONCERNING...SEVERE POTENTIAL...WATCH POSSIBLE 
   
   VALID 182051Z - 182215Z
   
   A SEVERE THREAT WILL LIKELY DEVELOP OVER NRN WY AND SRN MT OVER THE
   NEXT FEW HOURS. LARGE HAIL WILL BE THE PRIMARY THREAT ALTHOUGH
   ISOLATED WIND DAMAGE WILL ALSO BE POSSIBLE. A WW MAY BECOME
   NECESSARY LATE THIS AFTERNOON ACROSS THE REGION.
   
   LATEST SFC ANALYSIS SHOWS A SFC LOW NEAR RIVERTON WY WITH A COLD
   FRONT EXTENDING SWD ACROSS SW WY INTO ERN UT. A LOW-LEVEL MOIST AXIS
   EXISTS ACROSS WRN SD...ERN MT AND NE WY WHERE SFC DEWPOINTS RANGE
   FROM THE 30S F TO THE LOWER 40S F. SFC HEATING ALONG THE MOIST AXIS
   IS RESULTING IN INSTABILITY WITH MLCAPE VALUES GENERALLY AROUND 500
   J/KG. IN ADDITION...RUC ANALYSIS AND WATER VAPOR IMAGERY SHOW A
   SERIES OF SUBTLE SHORTWAVE TROUGHS EMBEDDED IN A LARGER WRN U.S.
   UPPER-LEVEL TROUGH. AS THESE FEATURES MOVE NNEWD ACROSS THE REGION
   WILL INCREASE LARGE-SCALE ASCENT PROVIDING A FOCUS FOR SCATTERED
   THUNDERSTORM DEVELOPMENT OVER THE NEXT FEW HOURS. AS THUNDERSTORMS
   INCREASE IN COVERAGE AND MATURE...MODERATE VERTICAL SHEAR AND LAPSE
   RATES EXCEEDING 9.0 C/KM SHOWN ON FORECAST SOUNDINGS SHOULD BE
   SUFFICIENT FOR ISOLATED SUPERCELLS WITH LARGE HAIL. WIND DAMAGE
   SHOULD BE MORE ISOLATED BUT MAY ALSO ACCOMPANY ANY SUPERCELLS THAT
   DEVELOP.
   
   ..BROYLES.. 04/18/2007
   
   
   ATTN...WFO...UNR...CYS...BYZ...RIW...
   
   43330757 43790910 44171040 44661072 45381067 45980996
   46120879 45640682 45100554 44160444 42920478 42730615