MESOSCALE DISCUSSION 0841
   NWS STORM PREDICTION CENTER NORMAN OK
   0245 PM CDT SUN MAY 20 2007
   
   AREAS AFFECTED...PORTIONS SRN MT...NRN WY...WRN SD.
   
   CONCERNING...SEVERE POTENTIAL...WATCH LIKELY 
   
   VALID 201945Z - 202045Z
   
   CONDITIONS STEADILY WILL BECOME MORE FAVORABLE FOR SVR THROUGH LATE
   AFTERNOON/EARLY EVENING...INCLUDING SUPERCELLS...AND WW PROBABLY
   WILL BE REQUIRED.
   
   SFC MESOANALYSIS SHOWS LOW NEAR GEY...WITH WAVY FRONTAL ZONE EWD
   ACROSS AREA BETWEEN GCC-DGW...SRN BLACK HILLS...TO VICINITY MN/IA
   BORDER.  WY PORTION OF THIS BOUNDARY SHOULD RETREAT NWD AS WARM
   FRONT DURING NEXT FEW HOURS...S OF WHICH NEAR-SFC AIR MASS SHOULD
   BECOME MORE STRONGLY MIXED AND DRIER THAN THAT TO ITS N.  MOIST AXIS
   WILL BECOME EVEN MORE SHARPLY DEFINED N OF FRONT FROM NWRN SD  WWD
   OVER S-CENTRAL MT...SUPPORTING MLCAPES IN 800-1500 J/KG RANGE WITH
   ADDITIONAL INSOLATION.  MAIN FOCI FOR INITIATION WILL BE MOUNTAINS
   INVOF WY/MT BORDER...THOUGH A FEW STORMS MAY DEVELOP IN MOIST
   SECTOR.  THICKENING HIGH CLOUDS WITH NWWD EXTENT...ACROSS SWRN
   MT...MAY LIMIT HEATING AND CONVECTIVE DEVELOPMENT...AND THEREFORE
   MAKE SVR POTENTIAL ACROSS CENTRAL/W-CENTRAL MT MORE CONDITIONAL THAN
   THAT CLOSER TO WY BORDER.
   
   DEEP-LAYER SHEAR SHOULD REMAIN RELATIVELY WEAK OVER SD...WITH BLACK
   HILLS CONVECTION ALREADY PRODUCING OUTFLOW BOUNDARIES. 
   HOWEVER...SHEAR INCREASES WITH WWD EXTENT ACROSS THIS REGION...IN
   PROGRESSIVELY CLOSE PROXIMITY TO APCHG ID PERTURBATION EVIDENT IN
   MOISTURE CHANNEL IMAGERY.  AS TROUGH MOVES NEWD OVER NEXT SEVERAL
   HOURS...CONTINUED LOW LEVEL ISALLOBARIC/ISALLOHYPSIC RESPONSE WILL
   YIELD STRONGER LOW LEVEL FLOW...AND BY EXTENSION...MORE FAVORABLE
   DEEP-LAYER SHEAR AND STORM-RELATIVE INFLOW.  LOW LEVEL HODOGRAPHS
   APPEARED RATHER SMALL AT 19Z...WITH ONLY 10 KT FLOW OBSERVED THROUGH
   LOWEST 2 KM AGL IN VWP AROUND BIL...HOWEVER RUC FCST SOUNDINGS
   REASONABLY INDICATE DEVELOPMENT OF 0-3 KM SRH 200-400 J/KG BY 23-00Z
   TIME FRAME.  EFFECTIVE BULK SHEAR MAGNITUDE...ALREADY 50-60 KT OVER
   S-CENTRAL MT...MAY INCREASE ANOTHER 10-15 KT AS LOW LEVEL WINDS
   SHIFT TOWARD LARGER ELY COMPONENT AND STRENGTHEN.
   
   ..EDWARDS.. 05/20/2007
   
   
   ATTN...WFO...BIS...UNR...CYS...BYZ...GGW...RIW...TFX...
   
   45621039 45971123 46621079 47020946 47000609 46130420
   45990387 44840256 43880289 43480441 43520646 44070831