SWODY1
SPC AC 181244
DAY 1 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
0744 AM CDT FRI MAY 18 2007
VALID 181300Z - 191200Z
..THERE IS A SLGT RISK OF SVR TSTMS LATE THIS AFTERNOON INTO
TONIGHT ACROSS ND AND NW MN...
..SYNOPSIS...
ONLY SLOW EWD PROGRESSION OF THE LARGE SCALE PATTERN IS EXPECTED
TODAY INTO EARLY SAT AS AN INITIAL SHORTWAVE TROUGH MOVES OVER THE
PAC NW...WHILE A DOWNSTREAM RIDGE OVER THE NRN HIGH PLAINS AND A LOW
OVER THE CENTRAL APPALACHIANS DRIFT EWD. A SMALLER SCALE EMBEDDED
WAVE OVER SRN ALBERTA/SASKATCHEWAN WILL CREST THE NRN HIGH PLAINS
RIDGE AND MOVE ESEWD OVER NRN MN BY LATE TONIGHT...WHILE A BROADER
SCALE NRN STREAM WAVE MOVES ACROSS HUDSON BAY. IN THE WAKE OF THE
HUDSON BAY WAVE...A SURFACE RIDGE WILL BUILD OVER MANITOBA AND DRIVE
A SURFACE COLD FRONT SWD ACROSS NE MT/ND/NW MN BY EARLY SAT. THIS
COLD FRONT WILL SERVE AS THE PRIMARY FOCUS FOR STRONG/SEVERE
THUNDERSTORM DEVELOPMENT LATE TODAY INTO TONIGHT ACROSS ND/MN.
..ND/NW MN AREA LATE THIS AFTERNOON INTO TONIGHT...
SURFACE RIDGING WILL PERSIST FROM THE SRN PLAINS TO THE OH
VALLEY...WHICH WILL INHIBIT THE NWD RETURN OF THE MOIST MARITIME AIR
MASS FROM THE GULF BASIN TO THE PLAINS. THEREFORE...INSTABILITY
WITHIN THE WARM SECTOR WILL DEPEND LARGELY ON STRONG SURFACE HEATING
AND RESIDUAL BOUNDARY LAYER DEWPOINTS IN THE LOW-MID 50S. THE
SOMEWHAT LIMITED MOISTURE AND A RELATIVELY WARM ELEVATED MIXED LAYER
WILL CONFINE DEEP CONVECTION TO THE AREAS OF DEEPEST MIXING AND
STRONGEST LOW-LEVEL FORCING BY LATE AFTERNOON.
INITIAL THUNDERSTORM DEVELOPMENT IS EXPECTED LATE THIS AFTERNOON
ACROSS CENTRAL ND WHERE SURFACE TEMPERATURES OF 88-90 F WILL RESULT
IN SUFFICIENT MIXING TO WEAKEN THE CAP /PER 12Z BIS AND ABR
SOUNDINGS/...AND WHERE LOW-LEVEL ASCENT WILL BE FOCUSED NEAR THE
REMNANT LEE CYCLONE/TRIPLE POINT. CONVECTION WILL THEN DEVELOP
ENEWD ALONG THE FRONT AND SPREAD SEWD INTO THE OVERNIGHT HOURS
ACROSS ERN ND AND NW MN. WARM SECTOR MLCAPE VALUES WILL LIKELY
RANGE FROM 1000-2000 J/KG AFTER ACCOUNTING FOR AFTERNOON
MIXING...WHILE VERTICAL SHEAR IS FORECAST TO BE MARGINALLY
SUFFICIENT FOR SUPERCELLS ALONG THE SURFACE FRONT. THUS...EXPECT A
MIX OF MULTICELL CLUSTERS AND MARGINAL SUPERCELL TYPE STORMS. THE
RELATIVELY DRY BOUNDARY LAYER...STEEP LOW-MID LEVEL LAPSE
RATES...AND WEAK TO MODEST VERTICAL SHEAR SUGGEST THAT LARGE HAIL
AND DAMAGING OUTFLOW WINDS WILL BE THE PRIMARY SEVERE STORM THREATS.
..SRN MT THIS AFTERNOON/EVENING...
DESPITE BOUNDARY LAYER DEWPOINTS ONLY IN THE LOWER 40S...STRONG
SURFACE HEATING OVER THE HIGH TERRAIN OF SW AND S CENTRAL MT SHOULD
RESULT IN WEAK INSTABILITY AND THE DEVELOPMENT OF A FEW
THUNDERSTORMS THIS AFTERNOON. STEEP LAPSE RATES AND PROXIMITY TO
THE SRN FRINGE OF THE 25-30 KT MID LEVEL WLYS COULD ALLOW A FEW OF
THE STORMS TO PRODUCE GUSTY WINDS AND SMALL HAIL THIS
AFTERNOON...BEFORE THE STORMS WEAKEN BY LATE EVENING.
..SRN NM THIS AFTERNOON...
A DISTINCT SRN STREAM PERSISTS OVER NRN MEXICO AND DEEP S TX...BUT
SURFACE RIDGING ACROSS TX AND THE NW GULF OF MEXICO HAS SHUNTED THE
RICHER LOW-LEVEL MOISTURE SWWD TO THE MOUNTAINS OF NE MEXICO.
HOWEVER...BOUNDARY LAYER DEWPOINTS IN THE LOW-MID 50S HAVE SPREAD
INTO S/SW NM ON ELY POST FRONTAL FLOW. DESPITE RELATIVELY WEAK
VERTICAL SHEAR TO THE N OF THE MID-UPPER JET...THE RESIDUAL MOISTURE
AND STEEP LAPSE RATES COULD SUPPORT A FEW STORMS WITH GUSTY OUTFLOW
WINDS AND HAIL...ALONG AND E OF THE MID LEVEL CYCLONIC SHEAR AXIS.
.THOMPSON/CROSBIE.. 05/18/2007
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