SWODY1
SPC AC 191242
DAY 1 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
0742 AM CDT SAT MAY 19 2007
VALID 191300Z - 201200Z
..THERE IS A SLGT RISK OF SVR TSTMS THIS AFTERNOON INTO EARLY
TONIGHT FOR CENTRAL AND SRN MT TO EXTREME NE WY AND WRN SD...
..MT AREA THIS AFTERNOON INTO EARLY TONIGHT...
A MID LEVEL LOW W OF VANCOUVER WILL MOVE SLOWLY EWD TOWARD SRN
BRITISH COLUMBIA BY EARLY SUNDAY...WITH DOWNSTREAM HEIGHT FALLS
EXPECTED OVER THE NRN ROCKIES AS AN EMBEDDED SHORTWAVE TROUGH
ROTATES AROUND THE SERN PERIPHERY OF THE LOW FROM WA/ORE TO MT. A
WEAK COLD FRONT WILL MOVE EWD INTO WRN MT IN CONJUNCTION WITH THE
MID LEVEL TROUGH...WHILE A WEAK SURFACE LOW DEVELOPS EWD ACROSS S
CENTRAL INTO SE MT. MEANWHILE...A SEPARATE FRONT ACROSS NRN AND ERN
MT WILL LIKELY STALL TODAY...AND LIFT A LITTLE TO THE NE BY EARLY
TONIGHT.
LOW-LEVEL MOISTURE IS LIMITED ACROSS MT WITH BOUNDARY LAYER
DEWPOINTS ONLY IN THE MID-UPPER 40S ACROSS SRN MT THIS
MORNING...WITH LESSER MOISTURE ACROSS N/NE MT. STILL...STEEP
LOW-MID LEVEL LAPSE RATES AND AFTERNOON TEMPERATURES NEAR 80 F WILL
CONTRIBUTE TO MODERATE INSTABILITY /MLCAPE VALUES OF 750-1500 J/KG/
OVER CENTRAL AND SRN MT. INITIAL THUNDERSTORM DEVELOPMENT IS
EXPECTED BY MID AFTERNOON OVER THE HIGH TERRAIN OF SW MT...AND
CONVECTION WILL THEN SPREAD EWD ACROSS CENTRAL/SRN MT THROUGH THE
EVENING. VERTICAL SHEAR WILL ALSO INCREASE FROM W TO E AS THE
HEIGHT FALLS AND STRONGER MID LEVEL HEIGHT GRADIENT SPREAD EWD...AND
THE ENVIRONMENT WILL BECOME FAVORABLE FOR A FEW HIGH-BASED
SUPERCELLS. GIVEN THE RELATIVELY DRY BOUNDARY LAYER AND THE
POTENTIAL FOR STRONG COLD POOL DEVELOPMENT...CONVECTION MAY GROW
UPSCALE INTO A LINEAR MCS THROUGH OUTFLOW MERGERS. LARGE HAIL AND
DAMAGING WINDS WILL BE THE PRIMARY SEVERE THREATS. BY LATER
TONIGHT...THE SEVERE THREAT SHOULD DIMINISH AS THE CONVECTION
ENCOUNTERS THE MORE STABLE LOW-LEVEL AIR MASS ACROSS NE MT AND ND.
..NRN NEB/SRN SD TO CENTRAL WI THIS AFTERNOON...
A SURFACE COLD FRONT WILL SAG SWD ACROSS THE UPPER MS VALLEY...WHILE
THE BOUNDARY SHOULD STALL THIS AFTERNOON ACROSS NEB/SD. S OF THE
FRONT...A NARROW CORRIDOR OF BOUNDARY LAYER DEWPOINTS IN THE MID 50S
F PERSISTS THIS MORNING. THE NAM/GFS FORECASTS OF SURFACE DEWPOINTS
IN THE 60S F THIS AFTERNOON FROM SD TO WI APPEAR OVERDONE...WITH
MEAN VALUES IN THE MID-UPPER 50S MORE PROBABLE. THIS MOISTURE AND
AFTERNOON TEMPERATURES AOA 80 F WILL CONTRIBUTE TO MLCAPE VALUES OF
750-1500 J/KG...AND THE SURFACE FRONT WILL PROVIDE A FOCUS FOR STORM
DEVELOPMENT THIS AFTERNOON. STORM ORGANIZATION/PERSISTENCE WILL BE
LIMITED BY RELATIVELY WEAK VERTICAL SHEAR...BUT A FEW STORMS COULD
BRIEFLY PRODUCE OUTFLOW WINDS AND HAIL NEAR SEVERE CRITERIA.
.THOMPSON/TAYLOR.. 05/19/2007
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