SWODY1
SPC AC 181248
DAY 1 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
0748 AM CDT WED APR 18 2007
VALID 181300Z - 191200Z
..NO SVR TSTM AREAS FORECAST...
..SYNOPSIS...
A LARGE SCALE OMEGA-TYPE BLOCKING PATTERN IS BECOMING ESTABLISHED
OVER N AMERICA WITH THE PRIMARY RIDGE AXIS FROM THE ERN PLAINS NWD
TO A DEVELOPING CLOSED HIGH OVER WRN ONTARIO. FLANKING RIDGE ARE A
DOWNSTREAM TROUGH INVOF THE ATLANTIC COAST...AND AN UPSTREAM MEAN
TROUGH OVER THE GREAT BASIN. WITHIN THE WRN MEAN TROUGH...ONE
STRONG SHORTWAVE TROUGH OVER NV THIS MORNING WILL MOVE EWD TO THE
CENTRAL ROCKIES THIS EVENING AND NEWD TO THE CENTRAL/NRN HIGH PLAINS
BY EARLY THURSDAY...AS ANOTHER UPSTREAM TROUGH APPROACHES NRN
CA/ORE. CYCLOGENESIS WILL OCCUR IN THE IMMEDIATE LEE OF THE NRN
ROCKIES IN ADVANCE OF THIS MID LEVEL WAVE...WHILE A COLD FRONT
SURGES EWD ACROSS CO/WY TO THE S OF THE SURFACE CYCLONE IN CENTRAL
MT.
..CO/WY/MT THIS AFTERNOON INTO TONIGHT...
THUNDERSTORMS WILL BE POSSIBLE TODAY FROM NRN UT/NWRN CO NWD INTO
SRN MT WHERE STRONG ASCENT IS EXPECTED ALONG THE DEEP LAYER
BAROCLINIC ZONE NOW PROGRESSING EWD INTO UT/ID. HOWEVER... LIMITED
LOW-LEVEL MOISTURE...AS WELL AS INCREASING CLOUDS AND RELATIVELY
EARLY ARRIVAL OF THE FRONT WILL TEND TO LIMIT INSTABILITY IN THIS
AREA. STRONG VERTICAL SHEAR AND STEEP LAPSE RATES COULD STILL
SUPPORT STORMS WITH STRONG WINDS TODAY. FARTHER E OVER THE HIGH
PLAINS...STRONGER SURFACE HEATING /AFTERNOON TEMPERATURES REACHING
65-75 F/ AND STEEP MID LEVEL LAPSE RATES WILL SUPPORT LATE AFTERNOON
MLCAPE VALUES AOA 500 J/KG FROM THE HIGH PLAINS OF N CENTRAL CO NWD
ACROSS ERN WY INTO SE MT...DESPITE BOUNDARY LAYER DEWPOINTS ONLY IN
THE MID-UPPER 30S. THUNDERSTORM COVERAGE SHOULD INCREASE OVER AND E
OF THE MOUNTAINS BY LATE EVENING AS THE COLD FRONT OVERTAKES THE LEE
TROUGH AND ASCENT INCREASES. THE INITIALLY SURFACE-BASED STORMS
SHOULD PERSIST AND SPREAD EWD OVER THE HIGH PLAINS AS ELEVATED
CONVECTION.
THE STEEP MID LEVEL LAPSE RATES AND RELATIVELY COOL MID LEVEL
TEMPERATURES...ALONG WITH INCREASING VERTICAL SHEAR FROM N TO S IN
CLOSER PROXIMITY TO THE MID LEVEL JET...SHOULD SUPPORT A FEW STORMS
WITH MARGINALLY SEVERE HAIL. ALSO...INVERTED-V PROFILES IN THE LOW
LEVELS SHOULD BE CONDUCIVE TO STRONG OUTFLOW WINDS AS THUNDERSTORM
COVERAGE INCREASES DURING THE EVENING.
..LOWER OH VALLEY THIS AFTERNOON...
A COMPLEX INTERACTION WILL OCCUR BETWEEN MULTIPLE MID LEVEL WAVES
OVER THE MS/OH VALLEYS AND THE SE STATES TODAY. THE SRN STREAM
TROUGH OVER THE LOWER MS VALLEY THIS MORNING WILL LOSE AMPLITUDE AND
ACCELERATE INTO THE CONFLUENT FLOW REGIME OFF THE SE ATLANTIC COAST
BY THIS EVENING. MEANWHILE...A COMPACT NRN STREAM CLOSED LOW OVER
SRN LAKE MI WILL ROTATE SWD TO THE LOWER OH VALLEY AS PART OF THE
LARGER SCALE TROUGH ON THE ERN FLANK OF THE OMEGA BLOCK. THE THICK
CLOUDS WITH THE SRN STREAM WAVE SHOULD CLEAR THE LOWER OH VALLEY BY
MIDDAY...ALLOWING SOME SURFACE HEATING OF THE BOUNDARY LAYER WITH
RESIDUAL DEWPOINTS OF 45-50 F NEAR A WEAK SURFACE LOW AND ALONG A
WEAK COLD FRONT MOVING SWD TOWARD THE OH RIVER. THOUGH TOTAL CAPE
SHOULD REMAIN AOB 500 J/KG AND VERTICAL SHEAR WILL BE WEAK...THE
RELATIVELY STEEP LAPSE RATES AND COLD MID LEVEL TEMPERATURES COULD
SUPPORT MARGINALLY SEVERE HAIL FOR A FEW HOURS THIS AFTERNOON.
.THOMPSON/GRAMS.. 04/18/2007
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