SWODY1
SPC AC 170549
DAY 1 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
1249 AM CDT WED SEP 17 2008
VALID 171200Z - 181200Z
...NO SVR TSTM AREAS FORECAST...
...SYNOPSIS...
WEAK UPPER FORCING IS EXPECTED TO CONTINUE DOMINATING MUCH OF THE
LOWER 48 STATES THROUGH THIS FORECAST PERIOD...AS STRONGER UPPER
LEVEL JET STREAMS RESIDE ALONG 1) THE FAR SRN TIER OF THE U.S. AND
2) N OF THE U.S./CANADIAN BORDER. MID/UPPER LEVEL TROUGH WILL TRACK
EWD FROM ONTARIO INTO QUEBEC...WHILE DIFFUSE/POSITIVELY-TILTED SRN
PORTION OF THIS TROUGH WILL MOVE EWD INTO THE SERN STATES BY 12Z
THURSDAY. MEANWHILE...A PACIFIC SHORT WAVE TROUGH LOCATED OVER NRN
CA AT 12Z TODAY IS FORECAST TO WEAKEN AS IT TRACKS NEWD ACROSS THE
PACIFIC NW. A BROAD AND DIFFUSE NEGATIVELY TILTED MID LEVEL TROUGH
WILL SHIFT SLOWLY EWD ACROSS THE GREAT BASIN AND SWRN STATES
REACHING THE NRN INTERMOUNTAIN WEST TO SRN HIGH PLAINS BY END OF
PERIOD.
...SERN GA/NRN AND CENTRAL FL...
SURFACE FRONT RESIDING ALONG THE GA/FL BORDER AT 12Z TODAY SHOULD
WEAKEN AS IT MOVES S INTO NRN FL. THE LACK OF STRONGER FORCING
ALOFT SUGGESTS THAT THE FOCI FOR CONVECTIVE DEVELOPMENT TODAY SHOULD
INITIALLY BE ALONG THE FRONT AND SEA BREEZE BOUNDARIES AND THEN WITH
BOUNDARY INTERACTIONS. SURFACE HEATING WITHIN A MOIST...WEAKLY
CAPPED AIR MASS AND BENEATH COOLER MID LEVEL TEMPERATURES /-8 TO -10
C AT 500 MB/ WILL RESULT IN MODERATE INSTABILITY FROM SRN GA SWD
ACROSS MUCH OF FL. ALTHOUGH LOWER TROPOSPHERIC WINDS SHOULD REMAIN
WEAK...STRONGER FLOW ABOVE 500 MB WILL CONTRIBUTE TO EFFECTIVE BULK
SHEAR AROUND 35 KT. ALTHOUGH THIS WILL BE SUFFICIENT FOR ORGANIZED
STORMS...THE LACK OF STRONGER FORCING ALOFT MAY LIMIT STORM COVERAGE
AND THE NEED FOR HIGHER SEVERE PROBABILITIES AT THIS TIME. THE
STRONGEST STORMS WILL BE CAPABLE OF PRODUCING A MARGINAL SEVERE
WEATHER THREAT WITH STRONG GUSTY WINDS AND SOME HAIL.
...WRN CONUS...
NWD MOISTURE RETURN WILL CONTINUE TODAY ACROSS SRN CA/LOWER CO RIVER
VALLEY. THIS COMBINED WITH SURFACE HEATING SHOULD SUPPORT TSTM
DEVELOPMENT ACROSS THESE AREAS DURING THE AFTERNOON INTO THE
EVENING. MEANWHILE...COOLER MID LEVEL TEMPERATURES /-12 TO -14 C AT
500 MB/ WITHIN THE NEGATIVELY TILTED TROUGH COMBINED WITH DIABATIC
HEATING WILL RESULT IN STEEP LOW-MID LEVEL LAPSE RATES ACROSS MUCH
OF THE FOUR CORNERS REGION INTO INTERMOUNTAIN WEST. MARGINAL LOW
LEVEL MOISTURE WILL TEND TO INHIBIT THE DEVELOPMENT OF STRONGER
INSTABILITY...BUT TSTMS WILL BE POSSIBLE MAINLY OVER THE HIGHER
TERRAIN THIS AFTERNOON/EARLY EVENING.
FARTHER NW...FORECAST SOUNDINGS CONTINUE TO INDICATE PRIMARILY
MID-LEVEL MOISTENING ASSOCIATED WITH ASCENT/COOLING AHEAD OF THE
WEAKENING TROUGH MOVING NEWD ACROSS THE PACIFIC NW. PWAT VALUES AOB
0.50 INCH WILL RESULT IN HIGH-BASED TCU/CBS OVER THE MOUNTAINS WITH
GENERALLY DRY LIGHTNING/GUSTY WINDS EXPECTED FROM THE NRN GREAT
BASIN INTO PARTS OF THE PACIFIC NW.
...SRN GREAT LAKES REGION...
A COLD FRONT ATTENDANT TO THE ONTARIO/QUEBEC TROUGH IS EXPECTED TO
MOVE SEWD THROUGH THE SAINT LAWRENCE RIVER VALLEY AND SRN GREAT
LAKES REGION DURING PEAK HEATING. MID LEVEL LAPSE RATES RANGING
FROM 6-6.5 C/KM SHOULD PRECLUDE THE DEVELOPMENT OF STRONGER
INSTABILITY ACROSS THIS REGION. HOWEVER...FORECAST SOUNDINGS
SUGGEST THE POTENTIAL FOR SOME CONVECTION...DEEP ENOUGH FOR
LIGHTNING ALONG THE FRONT INTO THE AFTERNOON.
..PETERS/CROSBIE.. 09/17/2008
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