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Mesoscale Discussion 1697
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   MESOSCALE DISCUSSION 1697
   NWS STORM PREDICTION CENTER NORMAN OK
   0210 PM CDT MON AUG 17 2015

   AREAS AFFECTED...PORTIONS OF SRN MT...NRN WY

   CONCERNING...SEVERE POTENTIAL...WATCH POSSIBLE 

   VALID 171910Z - 172145Z

   PROBABILITY OF WATCH ISSUANCE...60 PERCENT

   SUMMARY...THE AREA IS BEING MONITORED FOR AN INCREASING RISK OF
   SEVERE THUNDERSTORMS WITH HAIL AND WIND HAZARDS. THERE IS
   UNCERTAINTY REGARDING THE OVERALL MAGNITUDE/EXTENT OF THE SVR RISK
   AND ITS TIME OF MORE SUBSTANTIAL INCREASE. HOWEVER...TRENDS WILL BE
   MONITORED FOR POSSIBLE SEVERE THUNDERSTORM WATCH ISSUANCE THROUGH
   THE AFTERNOON.

   DISCUSSION...WATER VAPOR LOOPS IMPLY A COMPACT MID-LEVEL IMPULSE
   ADVANCING ESEWD ALONG THE WRN-MT/ERN-ID BORDER. CG LIGHTNING HAS
   BEEN INCREASING WITHIN THE IMPULSE-PRECEDING PLUME OF DCVA -- IN
   ASSOCIATION WITH ELEVATED CONVECTION ACROSS CNTRL MT. THIS IS
   OCCURRING AS THE FORCING FOR ASCENT IS OVERSPREADING A FEED OF
   MOISTURE RETURN CHARACTERIZED BY MIDDLE 40S-LOWER 50S SFC DEWPOINTS
   ACROSS THE NRN HIGH PLAINS/VICINITY. AS THE SPATIAL GAP BETWEEN
   RELATED MODEST MOISTURE/BUOYANCY AND ASCENT CONTINUES TO
   NARROW...AND IMPULSE-PRECEDING FRONTOGENETIC CIRCULATIONS
   STRENGTHEN...CONVECTION WILL CONTINUE TO INCREASE/DEVELOP INTO SRN
   MT AND NRN WY. DEEPER CONVECTION MAY ALSO EMANATE FROM ONGOING
   CUMULUS DEVELOPMENT ALREADY OCCURRING OVER THE BIG HORNS AS DETECTED
   BY VIS IMAGERY. THIS IS WHERE CLOUD BREAKS/CLOUD TRANSLUCENCE AND
   RELATED SFC DIABATIC HEATING ARE PERMITTING INSOLATION TO BASE
   EFFECTIVE-INFLOW LAYERS AT THE SFC.

   BUOYANCY IS FORECAST TO REMAIN ONLY MARGINAL...AS LOW-LEVEL MOISTURE
   REMAINS RELATIVELY NON-ROBUST AND THE UPSTREAM GREAT FALLS 12Z RAOB
   INDICATES ONLY MODEST H7-H5 LAPSE RATES AROUND 6.7 C/KM.
   REGARDLESS...THE FORCING FOR ASCENT...COUPLED WITH NON-ZERO BUOYANCY
   AND 45-50 KT OF DEEP SHEAR SAMPLED BY THE BILLINGS VWP...SUGGEST
   THAT STORM-SCALE HYDRODYNAMIC PERTURBATION PRESSURE GRADIENTS MAY
   ENHANCE CONVECTIVE ACCELERATIONS IN ROTATING UPDRAFTS...AS
   SUPERCELLS POTENTIALLY EVOLVE ACROSS THE REGION. SVR HAIL/WIND WILL
   BE POSSIBLE. THE GREATEST POTENTIAL FOR SUCH ACTIVITY WILL BE INVOF
   RICHER MOISTURE E/SE OF THE BIG HORNS...THOUGH SOME SVR RISK WILL
   EXIST AREA-WIDE. ALSO...GIVEN AMPLE DIRECTIONAL SHEAR INDICATED BY
   THE BILLINGS VWP...SUSTAINED SUPERCELLS MAY ALSO OFFER THE POTENTIAL
   FOR A TORNADO -- PARTICULARLY INVOF TERRAIN-FACILITATED
   CHANNELED-FLOW REGIMES THAT AUGMENT STREAMWISE VORTICITY ON
   MESO-BETA/GAMMA SCALES.

   IT IS CERTAINLY POSSIBLE THAT A WW MAY BE NEEDED ACROSS THE REGION
   AT SOME POINT THIS AFTERNOON. HOWEVER...THE TIMING OF MORE
   SUBSTANTIAL SVR-RISK DEVELOPMENT...AND ITS OVERALL
   MAGNITUDE/EXTENT...ARE SOMEWHAT UNCERTAIN -- AS POCKETS OF STRATUS
   CLOUDS ARE PRESENTLY STUNTING SFC HEATING IN THE REGIME OF RETURNING
   MOISTURE. LIMITED LOW-LEVEL MOISTURE MAY ALSO PLAY A ROLE IN
   MITIGATING THE OVERALL SVR RISK. DESPITE THESE UNCERTAINTIES...A
   RISK FOR SVR-HAIL/WIND-PRODUCING SUPERCELLS WILL EXIST...CAPABLE OF
   A TORNADO OR TWO...AND THE AREA WILL BE MONITORED FOR POSSIBLE WW
   ISSUANCE THROUGH THE AFTERNOON.

   ..COHEN/CORFIDI.. 08/17/2015


   ATTN...WFO...UNR...CYS...BYZ...RIW...

   LAT...LON   45231033 45670884 45780498 44930402 43240423 42940544
               43290700 44090994 45231033 

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