MESOSCALE DISCUSSION 0804
   NWS STORM PREDICTION CENTER NORMAN OK
   0244 PM CDT WED MAY 18 2011
   
   AREAS AFFECTED...WRN/CNTRL OK...PORTIONS OF N TX
   
   CONCERNING...SEVERE POTENTIAL...WATCH POSSIBLE 
   
   VALID 181944Z - 182045Z
   
   THE POTENTIAL FOR CONVECTIVE INITIATION APPEARS TO BE INCREASING
   THIS AFTERNOON ACROSS WRN OK AND PORTIONS OF N TX...ALTHOUGH SOME
   UNCERTAINTY STILL REMAINS. THE ENVIRONMENT WILL BE FAVORABLE FOR
   ISOLATED SUPERCELLS CAPABLE OF PRODUCING VERY LARGE HAIL AND
   POSSIBLY TORNADOES. THE AREA IS BEING MONITORED FOR A POSSIBLE WW. 
   
   STRONG DIABATIC HEATING ACROSS WRN OK HAS RESULTED IN AN EWD
   ADVANCEMENT OF A SFC DRYLINE...POSITIONED FROM 40 SSW SPS TO FSI TO
   40 NE CSM /PER 19Z SUBJECTIVE ANALYSIS/. THIS FEATURE IS ATTENDANT
   TO A DEEPENING SFC LOW...AND A DIFFUSE BAROCLINIC ZONE/WARM FRONT
   PROTRUDING ESE ACROSS CNTRL AND SERN OK. THERE IS SOME UNCERTAINTY
   REGARDING CONVECTIVE INITIATION AS RECENT SFC OBSERVATIONS HAVE
   INDICATED WINDS IN THE POST-DRYLINE ENVIRONMENT HAVE BACKED TOWARDS
   THE S /LIMITING LOW-LEVEL CONVERGENCE/. THIS MAY BE OFFSET BY AN
   AREA OF IMPLIED ASCENT THAT APPEARS TO BE SPREADING EWD OUT OF THE
   SRN HIGH PLAINS...WHERE A BROAD SWATH OF CIRRUS HAS DEVELOPED IN
   RECENT VISIBLE SATELLITE/WATER VAPOR IMAGES. AS THIS APPROACHES WRN
   OK AND NW TX...WEAKENING CINH AND INCREASING MID-LEVEL ASCENT SHOULD
   LEAD TO A MORE FAVORABLE ENVIRONMENT FOR CONVECTIVE INITIATION THIS
   AFTERNOON.
   
   RICH BOUNDARY LAYER MOISTURE AHEAD OF THE DRYLINE /LOWER-MID 60S SFC
   DEW POINTS/ ALONG WITH STEEP MIDLEVEL LAPSE RATES IN EXCESS OF 7 DEG
   C PER KM HAS YIELDED MLCAPE VALUES OF 1500-2000 J/KG...WHICH WILL
   SUPPORT A THREAT FOR VERY LARGE HAIL. ADDITIONALLY...ENHANCED
   LOW-LEVEL WIND SHEAR RESULTING FROM BACKED FLOW /PARTICULARLY WITH
   NWD EXTENT/...ALONG WITH A FAVORED DISCRETE CONVECTIVE MODE DUE TO A
   SIGNIFICANT ORTHOGONAL COMPONENT OF THE EFFECTIVE SHEAR VECTOR
   RELATIVE TO THE DRYLINE...SHOULD SUPPORT A THREAT FOR TORNADOES IF
   LONG-LIVED SUPERCELLS CAN DEVELOP. HOWEVER...CLOUD COVER ACROSS
   CNTRL OK HAS LIMITED HEATING/INSTABILITY PRIMARILY E OF I-35...WITH
   SOME QUESTION REGARDING MAINTENANCE OF THUNDERSTORMS MOVING E OFF
   THE DRYLINE.
   
   ..ROGERS.. 05/18/2011
   
   
   ATTN...WFO...FWD...OUN...
   
   LAT...LON   36619876 36569810 36199732 34859686 33919697 33599725
               33679820 33849867 34339871 34969856 35659892 36239896
               36489880 36619876