NWS STORM PREDICTION CENTER NORMAN OK 1107 AM CDT FRI SEP 06 2013 VALID 061630Z - 071200Z ...THERE IS A SLGT RISK OF SVR TSTMS OVER PARTS OF WRN AND CNTRL MT... ...SYNOPSIS... A MID TO UPPER-LEVEL ANTICYCLONE WILL REMAIN ANCHORED OVER THE CNTRL HIGH PLAINS WITH THE PRIMARY BELT OF POLAR WESTERLIES CONFINED TO CANADA. A SEPARATE RIBBON OF HIGHER MOMENTUM MID TO UPPER-TROPOSPHERIC FLOW WILL PREVAIL ACROSS THE NRN ROCKIES IN CONJUNCTION WITH AN UPPER LOW WHICH WILL SLOWLY PROGRESS NEWD ACROSS THE PACIFIC NW INTO SRN BRITISH COLUMBIA. SEVERAL LOW-AMPLITUDE VORTICITY MAXIMA WILL PROGRESS THROUGH THE BROADER SYNOPTIC PATTERN...ONE OF WHICH WILL MOVE FROM THE PACIFIC NW THROUGH THE NRN INTERMOUNTAIN REGION...AND AT LEAST ONE OTHER WHICH WILL CREST THE RIDGE AXIS OVER THE NRN PLAINS BEFORE SHIFTING SEWD INTO THE UPPER MIDWEST. IN THE LOW LEVELS...MID-MORNING MESOANALYSIS INDICATED A WEAK FRONT FROM CNTRL MANITOBA SWWD TO LOW PRESSURE OVER CNTRL SD AND THEN INTO CNTRL WY. ANOTHER BOUNDARY EXTENDED FROM THE INTERSECTION WITH THE FORMER OVER CNTRL MANITOBA SWWD INTO NWRN MT. LATEST MODEL GUIDANCE SUGGESTS THAT THE SRN BOUNDARY MAY WEAKEN WITH TIME WHILE SAGGING SLOWLY SEWD AHEAD OF THE NRN BAROCLINIC ZONE WHICH WILL INTENSIFY WHILE PROGRESSING INTO THE NRN PLAINS. ...WRN/CNTRL MT THIS AFTERNOON INTO TONIGHT... MIDLEVEL HEIGHT FALLS/DCVA AHEAD OF THE PACIFIC NW SYSTEM WILL AUGMENT LOW-LEVEL CONVERGENCE ALONG THE NRN-MOST FRONT AND TERRAIN-INDUCED CIRCULATIONS TO FOSTER SCATTERED TSTMS THIS AFTERNOON. CURRENT SATELLITE-DERIVED PW IMAGERY INDICATES THAT A DRIER AIR MASS RESIDES ACROSS THE SWRN PORTION OF THE STATE WITH AN AXIS OF HIGHER MOISTURE CONTENT /I.E. 1.0-1.3 INCH PW/ ACROSS NRN MT INTO THE DAKOTAS. THIS MOISTURE WILL COMBINE WITH STEEP LAPSE RATES TO PROMOTE A MODERATELY UNSTABLE AIR MASS BY AFTERNOON WITH MLCAPE APPROACHING 1000 J/KG. THE STRONGEST MID AND HIGH-LEVEL FLOW --AND RESULTANT VERTICAL SHEAR-- WILL RESIDE ACROSS WRN MT WHERE A FEW SUPERCELLS APPEAR LIKELY WITH LARGE HAIL AND DAMAGING WINDS BEING THE PRIMARY HAZARDS. WITH TIME...STORMS MAY GROW UPSCALE INTO ONE OR MORE FORWARD-PROPAGATING CLUSTERS WITH THE THREAT FOR DAMAGING WINDS AND HAIL PERSISTING INTO TONIGHT...MAINLY ACROSS N-CNTRL MT. ...NRN PLAINS INTO UPPER GREAT LAKES THIS AFTERNOON AND EVENING... WIDELY SCATTERED ELEVATED TSTMS ACROSS THE WRN DAKOTAS APPEAR TO BE ASSOCIATED WITH A WEAK MIDLEVEL PERTURBATION CRESTING THE MEAN RIDGE AXIS. A SUBSET OF THIS ACTIVITY MAY PERSIST INTO AFTERNOON WITH ADDITIONAL STORMS POSSIBLE ALONG EITHER OF THE TWO SURFACE BOUNDARIES MENTIONED IN THE SYNOPSIS. GIVEN THE PRESENCE OF MODEST AMOUNTS OF INSTABILITY AND VERTICAL SHEAR...A LOW-PROBABILITY RISK FOR DAMAGING WINDS AND HAIL WILL EXIST WITH THE MOST INTENSE TSTMS INTO THIS EVENING. ...CNTRL/SRN FL THIS AFTERNOON INTO EARLY EVENING... 12Z REGIONAL SOUNDINGS INDICATE THE PRESENCE OF SEASONABLY COOL MIDLEVEL TEMPERATURES ATOP A VERY MOIST BOUNDARY LAYER WHICH WILL PROMOTE A MODERATE TO STRONGLY UNSTABLE AIR MASS BY AFTERNOON WITH MLCAPE VALUES OF 2000-3000 J/KG. WHILE LARGE-SCALE FORCING FOR ASCENT WILL REMAIN NEGLIGIBLE THROUGH THE PEAK OF THE DIURNAL HEATING CYCLE...LATEST CONVECTION-ALLOWING MODEL GUIDANCE REMAINS CONSISTENT WITH PRIOR RUNS IN SUGGESTING THE DEVELOPMENT OF NUMEROUS STORMS BY AFTERNOON ALONG THE EVOLVING SEA BREEZE CIRCULATIONS. GIVEN THE MAGNITUDE OF INSTABILITY...LOCALLY DAMAGING WET MICROBURSTS AND PERHAPS SOME SMALL HAIL MAY ACCOMPANY THE STRONGEST
Posted on: Fri, 06 Sep 2013 16:50:48 +0000
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