SPC 1300Z Day 1 Outlook
DAY 1 CONVECTIVE OUTLOOK NWS STORM PREDICTION CENTER NORMAN OK 0757 AM CDT WED AUG 21 2013 VALID 211300Z - 221200Z ...THERE IS A SLGT RISK OF SVR TSTMS ACROSS PARTS OF THE UPPER MS VALLEY AND ADJACENT WRN UPPER GREAT LAKES REGION... ...THERE IS A SLGT RISK OF SVR TSTMS OVER PORTIONS OF THE NEB VICINITY... ...SYNOPSIS... STRONG/CYCLONIC FLOW ALOFT IS FORECAST ACROSS SRN CANADA...WITH THE SRN FRINGE OF THIS ENHANCED FLOW FIELD FORECAST TO SPREAD ESEWD ACROSS THE N CENTRAL U.S. AND EVENTUALLY INTO THE UPPER GREAT LAKES REGION. ELSEWHERE...MUCH WEAKER WINDS ALOFT ARE EXPECTED...WITH A BROAD UPPER RIDGE TO PREVAIL ACROSS A LARGE PORTION OF THE CONUS. EXCEPTIONS TO THE RIDGING WILL EXIST ACROSS THE OH VALLEY -- WHERE AN UPPER TROUGH WILL CONTINUE SHIFTING SLOWLY EWD WITH TIME...AND OVER CA -- WHERE AN ERN PACIFIC UPPER LOW WILL MOVE SLOWLY NEWD/ONSHORE. AT THE SURFACE...A COLD FRONT WILL MOVE SLOWLY ESEWD ACROSS THE UPPER GREAT LAKES/UPPER MS VALLEY REGION...WHILE SAGGING MORE SLOWLY SWD INTO THE NEB/NRN KS VICINITY. THIS BOUNDARY WILL PROVE TO BE THE PRIMARY SURFACE FEATURE WITH RESPECT TO THE DEVELOPMENT OF SEVERE WEATHER POTENTIAL THIS FORECAST. ...UPPER MS VALLEY/WRN UPPER GREAT LAKES REGION... DIURNAL HEATING AHEAD OF THE ADVANCING COLD FRONT SHOULD RESULT IN MODERATE DESTABILIZATION...WHICH WILL SUPPORT STORM DEVELOPMENT ALONG THE FRONT THIS AFTERNOON ACROSS WI AND ADJACENT UPPER MI. WITH FLOW ALOFT ATOP THE WARM SECTOR FORECAST TO REMAIN MODEST ACROSS MOST OF THIS REGION...OVERALL SEVERE THREAT SHOULD REMAIN TEMPERED. STILL...THE DEGREE OF CAPE ANTICIPATED SUGGESTS THAT MULTICELL STORMS WITH SOME SUPERCELL CHARACTERISTICS LOCALLY CAN BE EXPECTED DURING THE LATE AFTERNOON AND EARLY EVENING -- ALONG WITH ATTENDANT RISKS FOR HAIL AND LOCALLY DAMAGING WINDS. THREAT SHOULD GRADUALLY DIMINISH THROUGH THE EVENING AS THE AIRMASS DIURNALLY STABILIZES. ...NEB VICINITY... BACKED/ELY LOW-LEVEL FLOW N OF A SAGGING COLD FRONT BENEATH A BELT OF ENHANCED /25 KT/ MID-LEVEL WLYS WILL RESULT IN A VEERING/SHEARED FLOW FIELD SUPPORTIVE OF ORGANIZED CONVECTION. WITH DIURNAL HEATING LIKELY TO SUPPORT DEVELOPMENT OF 500 TO 1500 J/KG MIXED-LAYER CAPE INVOF THE FRONTAL ZONE...CONVECTION INITIATING NEAR THE BOUNDARY DURING THE AFTERNOON IS FORECAST TO GROW UPSCALE INTO CLUSTERS OF LOCALLY ORGANIZED STORMS. THIS UPSCALE GROWTH SHOULD CONTINUE INTO THE EVENING -- AS A SLY LOW-LEVEL JET DEVELOPS OVER THE PLAINS SUSTAINING ASCENT NEAR/N OF THE FRONT. GIVEN THE ADEQUATE CAPE/SHEAR ENVIRONMENT ANTICIPATED...THREAT FOR HAIL AND LOCALLY DAMAGING WINDS ACROSS PORTIONS OF NEB APPEARS SUFFICIENT ATTM TO WARRANT SLIGHT RISK UPGRADE ACROSS THIS AREA. ...PARTS OF NRN CA/SRN OREGON/NWRN NV... AS AN UPPER LOW SHIFTS SLOWLY NEWD TOWARD -- AND EVENTUALLY INTO -- NRN CA...A GRADUAL INCREASE IN BOTH LARGE-SCALE ASCENT AND FLOW ALOFT WILL OCCUR. DAYTIME HEATING OVER THE HIGHER TERRAIN BUT MODEST LOW-LEVEL MOISTURE WILL YIELD MARGINAL CAPE -- SUFFICIENT FOR DEVELOPMENT OF SHOWERS AND THUNDERSTORMS BUT GENERALLY UNSUPPORTIVE OF ROBUST/LONG-LIVED UPDRAFTS. STILL...WITH INCREASING SHEAR ACROSS THE AREA...A STRONGER STORM OR TWO MAY EVOLVE -- CAPABLE OF PRODUCING SMALL HAIL...ALONG WITH GUSTY WINDS THAT MAY LOCALLY REACH OR EXCEED SEVERE LEVELS. ..GOSS/MOSIER.. 08/21/2013
via SPC Forecast Products http://www.spc.noaa.gov/products/outlook/day1otlk_1300.html
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