SPC AC 111248
DAY 1 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
0748 AM CDT WED JUL 11 2007
VALID 111300Z - 121200Z
...THERE IS A SLGT RISK OF SVR TSTMS OVER THE NORTHEAST INTO THE MID
ATLANTIC...
...THERE IS A SLGT RISK OF SVR TSTMS ACROSS THE CENTRAL HIGH
PLAINS...
...THERE IS A SLGT RISK OF SVR TSTMS OVER A SMALL PART OF NRN CA AND
SRN ORE...
...NORTHEAST ACROSS THE MID ATLANTIC INTO THE CAROLINAS...
ACTIVE DAY OF THUNDERSTORMS EXPECTED ACROSS THIS REGION TODAY AHEAD
OF APPROACHING SHORTWAVE TROUGH AND ASSOCIATED SURFACE COLD FRONT.
LARGE AREA OF RICH LOW LEVEL MOISTURE /I.E. SFC DEW POINTS IN THE
LOWER 70S/ IS ALREADY IN PLACE WHICH WILL BOOST INSTABILITY THROUGH
THE DAY. ALTHOUGH LAPSE RATES ARE FAIRLY WEAK AND EXTENSIVE AREA OF
CLOUDS IS EVIDENT EARLY THIS MORNING...VERY MOIST LOW LEVELS AND
MODEST HEATING WILL SUPPORT 1000 J/KG MLCAPE AS CAPPING /EVIDENT AT
ALB THIS MORNING/ ERODES BY THE EARLY AFTERNOON. ACTIVITY SHOULD
INITIATE IN THIS AREA BY 18Z WITH DEVELOPMENT INCREASING SWD ACROSS
THE CENTRAL APPALACHIANS DURING THE EARLY TO MID AFTERNOON. SEVERE
THREAT WILL LIKELY INCREASE AS HEATING ENHANCES INSTABILITY AND
STEEPENS LOW LEVEL LAPSE RATES THIS AFTERNOON. SEVERAL
LINES/CLUSTERS ARE EXPECTED TO EVOLVE ALONG THE FRONT WITH 25-35 KT
EFFECTIVE SHEAR...WHILE THE POTENTIAL FOR SUPERCELLS WILL INCREASE
WITH NWD EXTENT GIVEN PROXIMITY TO STRONGER FLOW ALOFT. DESPITE
MODEST SHEAR AND WEAK LAPSE RATES...STRENGTH OF APPROACHING UPPER
SYSTEM AND SURFACE COLD FRONT...COMBINED WITH VERY MOIST/MODERATELY
UNSTABLE AIR MASS...SUGGEST THUNDERSTORMS WILL BECOME NUMEROUS
THROUGH THE DAY. MUCH OF THE ACTIVITY MAY BE LOWER-END
SEVERE...THOUGH STORM COVERAGE SHOULD BE NUMEROUS AND HAVE INCREASED
PROBABILITIES OF DAMAGING WINDS/TREES DOWN ACCORDINGLY. STORMS WILL
MOVE EWD TOWARDS THE COAST THROUGH THE DAY...THOUGH ONSHORE FLOW
ALONG COASTAL NEW ENGLAND MAY REINFORCE LOW CLOUDS AND MAINTAIN LESS
INSTABILITY/SEVERE THREAT OVER THIS AREA.
ALTHOUGH STORMS ARE FORECAST TO DEVELOP FROM THE SRN APPALACHIANS
INTO THE NORTHEAST...SEVERE THREAT IS EXPECTED TO BECOME MORE
BRIEF-LIVED/SPORADIC WITH SWD EXTENT INTO THE CAROLINAS. THEREFORE
WILL MAINTAIN LOWER SEVERE PROBABILITIES ACROSS THIS REGION.
...CENTRAL HIGH PLAINS...
SEVERE THUNDERSTORMS SHOULD BE QUITE ACTIVE LATER TODAY AND THIS
EVENING ACROSS THIS AREA...AS SUBTLE MID LEVEL IMPULSE SHEARING SEWD
ALONG BACK SIDE OF LARGE RIDGE OVER THE INTERMOUNTAIN WEST WILL MOVE
ACROSS WRN/SRN WY THROUGH THE DAY. AHEAD OF THIS SYSTEM...SURFACE
UPSLOPE FLOW WILL INCREASE ACROSS ERN CO AND SERN WY MAINTAINING
INFLUX OF LOWER TO MID 50F SURFACE DEW POINTS. EXPECT STORMS WILL
INCREASE ALONG THE FRONT RANGE FROM SERN WY INTO CENTRAL CO...AND
POSSIBLY FARTHER EAST NEAR DENVER-CYCLONE...DURING THE MID TO LATE
AFTERNOON AS CAP WEAKENS AND LARGE SCALE ASCENT INCREASES. VERY
STEEP LAPSE RATES WILL BOOST MLCAPE INTO 1500-2500 J/KG RANGE AS
TEMPERATURES CLIMB INTO THE 80S ALONG AND EAST OF THE FOOTHILLS.
MODEST NWLY MID/UPPER LEVEL FLOW WILL SHIFT STORMS ESEWD THROUGH THE
LATE AFTERNOON/EVENING...WHERE SELY SURFACE WINDS WILL ENHANCE DEEP
LAYER SHEAR. FORECAST SOUNDINGS INDICATE EFFECTIVE SHEAR FROM 40-50
KT WILL BE COMMON OVER THE CENTRAL HIGH PLAINS WHICH WILL SUPPORT
SUPERCELLS...INCLUDING SOME LONGER-LIVED ONES. A FEW TORNADOES CAN
BE EXPECTED...ALONG WITH ISOLATED VERY LARGE HAIL... BEFORE STORMS
CONSOLIDATE INTO A MCS AND SHIFT ESEWD ACROSS WRN KS/OK PANHANDLE
AFTER DARK. A CORRIDOR OF INCREASED WIND DAMAGE MAY OCCUR JUST
PRIOR TO AND SHORTLY AFTER MCS FORMATION...WITH SEVERE THREAT
SPREADING EWD TOWARDS CENTRAL KS/NRN OK THROUGH THE LATE EVENING.
...NRN CA/SWRN ORE...
AS HAS BEEN WELL-ADVERTISED OVER THE PAST FEW DAYS...IMPULSE AND
ASSOCIATED PLUME OF MID LEVEL MOISTURE WILL CONTINUE TO SPREAD NWWD
ACROSS THIS REGION TODAY. CONVECTION OVER THE PAST COUPLE DAYS HAS
MOISTENED THE LOW LEVELS. SURFACE DEW POINTS AOA 50F ARE NOW COMMON
OVER THIS AREA...WHILE GPS PW VALUES ARE IN EXCESS OF AN INCH.
THUS...STORMS INCREASING OVER HIGHER TERRAIN THIS AFTERNOON WILL BE
STRONGER AND PRODUCE A GREATER POTENTIAL FOR DAMAGING WINDS AND
LARGE HAIL THAN THE PAST COUPLE DAYS AS THEY SPREAD NNWWD LATE TODAY
AND THIS EVENING. IN ADDITION...DEEP LAYER SHEAR IN EXCESS OF 40 KT
SUGGESTS A FEW SUPERCELLS ARE POSSIBLE.
...NRN MN INTO THE NRN GREAT LAKES...
WELL DEFINED MID LEVEL IMPULSE ON WV IMAGERY IS NOW DIGGING ACROSS
MANITOBA AND WILL SPREAD RAPIDLY SSEWD ACROSS THIS AREA TODAY. LOW
LEVEL CONVERGENCE WILL REMAIN WEAK WITHIN NW-FLOW REGIME.
HOWEVER...MODEST AFTERNOON HEATING AND COLD MID LEVEL TEMPERATURES
WILL STEEPEN LAPSE RATES AND SUPPORT MARGINAL INSTABILITY.
CONDITIONS SHOULD FAVOR SCATTERED...WEAKLY FOCUSED STORMS WITH
POTENTIAL FOR HAIL...INCLUDING A FEW MARGINALLY SEVERE HAIL STONES.
ACTIVITY SHOULD DIMINISH AFTER SUNSET.
..EVANS/TAYLOR.. 07/11/2007
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