MESOSCALE DISCUSSION 1074

        MESOSCALE DISCUSSION 1074     NWS STORM PREDICTION CENTER NORMAN OK     0256 PM CDT TUE JUN 05 2012          AREAS AFFECTED...PARTS OF CNTRL/N-CNTRL MONTANA          CONCERNING...SEVERE POTENTIAL...TORNADO WATCH LIKELY           VALID 051956Z - 052200Z          PROBABILITY OF WATCH ISSUANCE...95 PERCENT          SUMMARY...A TORNADO WATCH WILL BE ISSUED BY 2100Z ACROSS PARTS OF     CNTRL AND N-CNTRL MONTANA...AS THE THREAT FOR TORNADIC SUPERCELLS     INCREASES THIS AFTERNOON AND CONTINUES INTO THE EVENING.          DISCUSSION...A 1000-MB SUB-SYNOPTIC SCALE LOW IS BECOMING     INCREASINGLY BETTER-DEFINED PER SFC OBSERVATIONS ACROSS N-CNTRL     MT...WITH A CLOSED CIRCULATION CENTERED OVER CHOUTEAU COUNTY.     CYCLOGENESIS IS BEING SUPPORTED IN THIS AREA BY INCREASING     LARGE-SCALE ASCENT -- HEIGHT FALLS APPROACHING 60 GEOPOTENTIAL     METERS PER 12 HOURS -- IN ADVANCE OF A POTENT/NEGATIVELY TILTED     SHORTWAVE TROUGH MOVING ACROSS THE NRN/CNTRL GREAT BASIN. LOW-LEVEL     MOISTURE TRAJECTORIES EMANATING FROM THE LOWER PLAINS OF MT/DAKOTAS     ARE MAINTAINING A ZONE OF DEWPOINTS IN THE MIDDLE/UPPER 50S TO THE     NORTH/NORTHEAST OF A FRONTOGENETIC BOUNDARY/WARM FRONT ARCING     EWD/SEWD FROM THE LOW INTO SERN MT. THIS ZONE OF ABNORMALLY RICH     LOW-LEVEL MOISTURE FOR SUCH HIGH ELEVATIONS IS CIRCULATING AROUND     THE WRN AND SRN SEMICIRCLE OF THE LOW JUST EAST OF THE NRN ROCKY     MOUNTAIN FRONT AND INTO AREAS JUST NORTH OF THE LITTLE AND BIG BELT     MOUNTAINS NEAR GREAT FALLS...WITH ONLY MODESTLY LOWER DEWPOINTS     FARTHER SOUTH INTO THE HELENA VALLEY.          AREAS OF STRONG INSOLATION/DIABATIC HEATING EAST OF THE CONTINENTAL     DIVIDE AMIDST THE RICH LOW-LEVEL MOISTURE WILL ALLOW MLCAPE VALUES     TO CLIMB TO 1000-1750 J/KG. MEANWHILE...THE SHALLOW/STABLE NATURE OF     THE RELATIVELY COOL AIR MASS WEST OF THE CONTINENTAL DIVIDE --     DELINEATED BY THE OVERCAST LOW-CLOUD DECK -- IS PRESENTLY BEING     BLOCKED BY THE NRN ROCKIES AND IS NOT EXPECT TO EMERGE ONTO THE     PLAINS UNTIL DEEPER COLD AIR ASSOCIATED WITH THE TROUGH MAKES MORE     SUBSTANTIAL EWD PROGRESS TONIGHT. FURTHERMORE...THE BARRIER-JET-LIKE     NATURE OF THE SFC FLOW ON THE WRN/SWRN FLANK OF THE LOW WILL LIKELY     MITIGATE DOWNSLOPE EFFECTS OVER AREAS JUST EAST OF THE ROCKY     MOUNTAIN FRONT. THESE FACTORS WILL ALLOW MODERATE INSTABILITY TO BE     MAINTAINED AND JUXTAPOSED WITH FAVORABLE VERTICAL WIND PROFILES FOR     SUSTAINED SUPERCELLS THIS AFTERNOON INTO THIS EVENING...WITH NEARLY     9-C/KM 700-500 MB LAPSE RATES EVIDENT IN THE 18Z GLASGOW AND 19Z     GREAT FALLS SPECIAL RAOBS AND 50-65 KT OF EFFECTIVE BULK SHEAR.     THESE SPECIAL RAOBS ALREADY INDICATE APPRECIABLE LOW-LEVEL SHEAR.          STORMS WILL CONTINUE TO DEVELOP OVER THE HIGHER TERRAIN OF     SWRN/CNTRL MONTANA DURING THE NEXT COUPLE OF HOURS BEFORE ADVANCING     NWD/NEWD INTO N-CNTRL MT THROUGH THE EVENING. DAMAGING WIND GUSTS     AND SEVERE HAIL...SOME POTENTIALLY SIGNIFICANT...WILL BE LIKELY WITH     THE SUPERCELLS. STRONG VERTICAL DIRECTIONAL/SPEED SHEAR WILL EXIST     OWING TO AN INCREASING SELY/ELY CURRENT WITHIN THE 850-700-MB LAYER     FEATURING SPEEDS EXCEEDING 40 KT BY 00Z ATOP SFC WINDS RELATIVELY     BACKED COMPARED TO THOSE ALOFT. THIS WOULD SUPPORT EFFECTIVE     STORM-RELATIVE HELICITY VALUES IN EXCESS OF 400 M2/S2. THE MOST     SUBSTANTIAL TORNADO THREAT WILL EXIST WITHIN AN AREA BOUNDED BY     GREAT FALLS...CUT BANK...AND HAVRE INVOF MAXIMIZED SFC VORTICITY     NEAR THE LOW. STORMS COULD EVENTUALLY EVOLVE INTO BOWING LINE     SEGMENTS WITH LINE ECHO WAVE PATTERN STRUCTURES THROUGH THE EVENING.     THE STRONGEST STORMS WILL LIKELY HAVE CROSSED NORTH OF THE     INTERNATIONAL BORDER INTO ALBERTA AND SASKATCHEWAN BY 05Z...THOUGH A     FEW STRONG/ELEVATED STORMS MAY PERSIST IN THE WAKE OF THE INITIAL     ACTIVITY THROUGH THE NIGHT.          ..COHEN/MEAD.. 06/05/2012               ATTN...WFO...GGW...TFX...