SWODY2
SPC AC 181730
DAY 2 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
1230 PM CDT FRI MAY 18 2007
VALID 191200Z - 201200Z
..THERE IS A SLGT RISK OF SVR TSTMS ACROSS PORTIONS OF MT...NRN WY
AND EXTREME SWRN SD...
..SYNOPSIS...
STRONG UPPER TROUGH WILL MOVE ACROSS THE PACIFIC NW ON SATURDAY...
WITH HEIGHT FALLS SPREADING EWD ACROSS THE NRN ROCKIES AND NRN HIGH
PLAINS. DOWNSTREAM OF THIS SYSTEM...A WEAK SHORT WAVE TROUGH IS
EXPECTED TO MOVE SEWD ACROSS THE GREAT LAKES REGION WITH ATTENDANT
COLD FRONT SPREADING SSEWD THROUGH THE UPPER MS VALLEY AND
UPPER/CENTRAL GREAT LAKES REGION. BY SATURDAY AFTERNOON... THIS
FRONT SHOULD EXTEND FROM LOWER MI WWD THROUGH SRN MN TO ALONG THE
SD/NEB BORDER...AND THEN WNWWD INTO CENTRAL MT.
..MT...
APPROACHING UPPER TROUGH...AND AT LEAST ONE LEAD MID LEVEL IMPULSE
NOW TRANSLATING THROUGH BASE OF UPPER TROUGH...SHOULD RESULT IN
LARGE SCALE LIFTING ACROSS THE AREA...WITH COLD FRONT PROVIDING
FOCUS FOR TSTMS ACROSS WRN/CENTRAL MT BY LATE AFTERNOON. SURFACE
DEWPOINTS IN THE LOWER 50S COMBINED WITH STEEP LAPSE RATES ARE
EXPECTED TO RESULT IN MLCAPES FROM 1500-2000 J/KG ALONG THE SURFACE
BOUNDARY. WEAK ELY BOUNDARY LAYER WINDS VEERING AND INCREASING TO
WSWLY WITH HEIGHT ARE EXPECTED TO PROVIDE STRONG DEEP LAYER SHEAR
FOR SUPERCELLS. THE RELATIVELY DRY BOUNDARY LAYER AND STEEP LAPSE
RATES INDICATE THAT HAIL AND WIND DAMAGE SHOULD BE THE MAIN THREATS.
FORCING FOR ASCENT WITH LEAD IMPULSE SPREADING ENEWD ACROSS ERN
MT/ND SATURDAY NIGHT AND SSELY LLJ STRENGTHENING ACROSS THE NRN HIGH
PLAINS SHOULD MAINTAIN EWD PROGRESSION OF TSTMS INTO ERN MT/ WRN ND.
HOWEVER...THESE STORMS SHOULD BECOME PROGRESSIVELY MORE ELEVATED
AND WEAKEN AS THEY SHIFT EWD INTO THE DEEPER AND COOLER BOUNDARY
LAYER LOCATED NORTH OF THE COLD FRONT...ESPECIALLY AS ACTIVITY
REACHES WRN ND.
..NERN WY AND EXTREME SWRN SD...
ALTHOUGH FORCING AND SHEAR ARE EXPECTED TO BE WEAKER ACROSS THIS
REGION THAN IN MT...STRONG SURFACE HEATING AND STEEP LAPSE RATES
ALONG THE FRONT SHOULD PROVE FAVORABLE FOR TSTMS TO DEVELOP LATE IN
THE AFTERNOON. ONCE STORMS INITIATE...MODERATE INSTABILITY AND
STEEP LAPSE RATES WILL SUPPORT A FEW STORMS WITH A THREAT FOR SEVERE
HAIL AND WIND DAMAGE. THESE STORMS SHOULD DIMINISH BY MID/LATE
EVENING WITH THE LOSS OF DAYTIME HEATING/INSTABILITY.
..PARTS OF CENTRAL PLAINS TO UPPER MS VALLEY...
TSTMS ARE EXPECTED BE ONGOING AT THE START OF THE FORECAST PERIOD
ACROSS NRN MN. THIS ACTIVITY WILL BE LOCATED ALONG THE COLD FRONT
AND AIDED BY ASCENT WITH MID LEVEL IMPULSE TRACKING SEWD ACROSS THIS
REGION. THIS IMPULSE SHOULD MOVE MORE QUICKLY SEWD THAN THE COLD
FRONT. THIS WILL RESULT IN SHORT WAVE RIDGING ACROSS MUCH OF
CENTRAL/SRN MN AND WRN/CENTRAL WI DURING FRONTAL PASSAGE.
HOWEVER...SOME SURFACE HEATING BENEATH ERN EXTENT OF STEEP LAPSE
RATES SHOULD PROVED FAVORABLE FOR A FEW STRONG TO SEVERE STORMS...
BUT GENERALLY WEAK DEEP LAYER SHEAR WILL LIMIT STORM ORGANIZATION.
FARTHER W...DESPITE LACK OF LARGE SCALE FORCING AND RELATIVELY WEAK
CONVERGENCE ALONG COLD FRONT IN A CAPPED ENVIRONMENT... DIURNAL
HEATING MAY BE SUFFICIENT FOR A FEW STORMS TO DEVELOP ALONG THE
FRONT LATE SATURDAY AFTERNOON. THE STEEP LAPSE RATES...ESPECIALLY
ACROSS SD/NEB...WILL RESULT IN MODERATE INSTABILITY...SO ANY STORMS
THAT DEVELOP WILL HAVE THE POTENTIAL TO PRODUCE SEVERE WEATHER.
SINCE UNCERTAINTY EXISTS REGARDING HOW MANY STORMS WILL
DEVELOP...ONLY LOW PROBABILITIES ARE FORECAST ATTM.
..SRN FL PENINSULA...
SURFACE BOUNDARY IS EXPECTED TO MOVE SLOWLY SWD THROUGH THIS REGION
WHERE THE AIR MASS WILL BE MOIST AND MODERATELY UNSTABLE SUPPORTING
TSTMS. MODELS SUGGEST A WEAK MID LEVEL IMPULSE SHOULD MOVE EWD
ACROSS SRN FL ON SATURDAY AIDING IN TSTM DEVELOPMENT. DEEP LAYER
SHEAR MAY BECOME STRONG ENOUGH FOR ISOLATED SEVERE STORMS...GIVEN
20-25 KT OF WLY MID LEVEL ATOP ELY LOW LEVEL WINDS.
.PETERS.. 05/18/2007
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