SWODY1
SPC AC 311941
DAY 1 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
0241 PM CDT WED AUG 31 2011
VALID 312000Z - 011200Z
...THERE IS A SLGT RISK OF SVR TSTMS LATE THIS AFTN AND TONIGHT
ACROSS PARTS OF THE NORTHERN PLAINS...
...20Z UPDATE...
SLIGHT RISK PROBABILITIES ACROSS THE NORTHERN PLAINS HAVE BEEN
MAINTAINED AS IS...BUT A NUMBER OF COMPLICATING FACTORS ARE
CONTRIBUTING TO UNCERTAIN SEVERE POTENTIAL. A PLUME OF VERY WARM
AND CAPPING ELEVATED MIXED LAYER AIR IS ADVECTING NORTHEASTWARD OFF
THE HIGHER TERRAIN...AHEAD OF THE APPROACHING UPPER TROUGH. AND
STRONGER MID-LEVEL HEIGHT FALLS ASSOCIATED WITH AN INCREASINGLY
SHEARED IMPULSE LIFTING OUT OF THE TROUGH ARE EXPECTED TO SPREAD
ACROSS THE MOUNTAINS OF WESTERN MONTANA INTO SOUTHERN
SASKATCHEWAN/MANITOBA. WITH THE EXCEPTION OF THE GLASGOW AREA OF
MONTANA INTO THE WILLISTON AND MINOT AREAS OF NORTH DAKOTA ...THIS
IS GENERALLY TO THE NORTHWEST OF THE DESTABILIZING BOUNDARY LAYER
ACROSS THE NORTHERN PLAINS.
VIGOROUS STORM DEVELOPMENT IS NOW UNDERWAY IN THE PRESENCE OF A
FAIRLY UNSTABLE ENVIRONMENT ACROSS THE HIGHER TERRAIN OF THE
NORTHERN ROCKIES...PARTICULARLY AROUND THE LIVINGSTON MT AREA.
HOWEVER...A SIGNIFICANT SURFACE COLD FRONT HAS ALREADY NOSED THROUGH
LEWISTOWN AND WILL CONTINUE SOUTHEASTWARD THROUGH MUCH OF THE
REMAINDER OF CENTRAL INTO EASTERN MONTANA BY EARLY EVENING.
SO...SEVERE POTENTIAL WITH STORMS DEVELOPING OFF THE HIGHER TERRAIN
ACROSS THIS AIR MASS...AND ACROSS THE WEAKLY UNSTABLE DEEPLY MIXED
PRE-FRONTAL BOUNDARY LAYER OVER SOUTHEASTERN MONTANA REMAINS A BIT
UNCLEAR.
THE PRIMARY FOCUS FOR STRONG/SEVERE STORM DEVELOPMENT MAY BE A
FRONTAL WAVE EXPECTED TO CONTINUE EVOLVING ACROSS NORTHEASTERN
MONTANA INTO NORTHWESTERN NORTH DAKOTA LATER THIS AFTERNOON AND
EVENING. IT STILL APPEARS THAT THE ENVIRONMENT ACROSS THIS REGION
WILL BE FAVORABLE FOR SUPERCELLS...WITH FORCING PERHAPS BECOMING
SUPPORTIVE OF AN EVOLVING STRONG/SEVERE MESOSCALE CONVECTIVE SYSTEM
NEAR AND NORTH OF THE NORTH DAKOTA/CANADIAN BORDER TONIGHT.
..KERR.. 08/31/2011
.PREV DISCUSSION... /ISSUED 1124 AM CDT WED AUG 31 2011/
...NRN HIGH PLAINS EWD/SEWD THROUGH THE DAKOTAS AND NEB...
WELL-DEFINED VORTICITY MAXIMUM AND ASSOCIATED 50-60 KT MIDLEVEL JET
STREAK OVER THE COLUMBIA PLATEAU INTO NRN INTER-MOUNTAIN REGION WILL
EJECT NEWD THROUGH THE NRN HIGH PLAINS INTO SK/MB BY TONIGHT. THIS
SYSTEM WILL BE PRECEDED BY A WEAKENING MIDLEVEL IMPULSE PROGRESSING
ENEWD FROM WY INTO THE DAKOTAS. IN THE LOW LEVELS...SURFACE LOW
OVER ERN MT WILL DEVELOP NEWD ALONG ATTENDANT QUASI-STATIONARY
BOUNDARY INTO SRN MB WHILE TRAILING EXTENSION OF BAROCLINIC ZONE
ADVANCES THROUGH THE NRN HIGH PLAINS AS A COLD FRONT. ELSEWHERE...A
LEE TROUGH WILL STRENGTHEN TODAY...EXTENDING FROM THE ERN MT SURFACE
LOW SWD THROUGH WRN NEB.
AN EML AND ASSOCIATED CAP WILL BE ADVECTED NEWD ACROSS THE NRN/CNTRL
PLAINS INTO UPPER MS VALLEY AS MIDLEVEL HEIGHT FIELD TIGHTENS IN
ADVANCE OF NRN ROCKIES IMPULSE. AT THE SAME TIME...A MOIST BOUNDARY
LAYER /I.E. DEWPOINTS IN THE MID TO UPPER 60S/ CURRENTLY OBSERVED
FROM ERN SD INTO THE MID-MO VALLEY WILL BE ADVECTED NWWD. THESE
DIFFERENTIAL ADVECTIONS WILL YIELD A MODERATELY UNSTABLE /BUT
CAPPED/ AIR MASS BY AFTERNOON FROM THE VICINITY OF THE SURFACE LOW
OVER ERN MT SWD ALONG LEE TROUGH WHERE MLCAPE WILL APPROACH
1500-2500 J/KG. FARTHER W ALONG FRONT OVER CNTRL MT...A DRIER
BOUNDARY LAYER WILL LIMIT AFTERNOON MLCAPE TO 500-1000 J/KG.
THE HIGHEST PROBABILITY OF SURFACE-BASED TSTM DEVELOPMENT APPEARS TO
BE ALONG OR PERHAPS ON THE IMMEDIATE COOL SIDE OF SURFACE FRONT OVER
MT LATER TODAY AHEAD OF EJECTING MIDLEVEL SYSTEM. HERE...
STRENGTHENING MIDLEVEL WIND FIELD WILL ALIGN WITH A STEEP-LAPSE RATE
ENVIRONMENT...SUPPORTING A THREAT FOR HIGH-BASED SUPERCELLS AND/OR
BOWING STRUCTURES CAPABLE OF DAMAGING WINDS AND HAIL. UPSCALE
GROWTH INTO AN MCS IS ANTICIPATED TONIGHT AS MOIST INFLOW IS
ENHANCED BY A 50+ KT NOCTURNAL LLJ. THIS MAY RESULT IN A CONTINUED
THREAT FOR DAMAGING WINDS AND HAIL...PRIMARILY ACROSS THE NRN HALF
OF ND.
FARTHER SE...STRONG DAYTIME HEATING AND POTENTIAL INFLUENCE OF
WEAKENING WY SHORT-WAVE TROUGH MAY CONTRIBUTE TO ISOLATED STORM
DEVELOPMENT ALONG THE LEE TROUGH THIS AFTERNOON/EVENING FROM CNTRL
SD INTO WRN NEB. HERE...THE COMBINATION OF MODERATE INSTABILITY AND
30-35 KT OF DEEP WLY SHEAR WILL SUPPORT SUPERCELLS CAPABLE OF HAIL
AND DAMAGING WINDS. HOWEVER...GIVEN THE STRONG CAP AND UNCERTAINTY
IN CONVECTIVE STORM COVERAGE...ONLY LOW SEVERE PROBABILITIES WILL BE
INCLUDED.
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