SWODY2
SPC AC 071724
DAY 2 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
1224 PM CDT MON SEP 07 2009
VALID 081200Z - 091200Z
...THERE IS A SLGT RISK OF SVR TSTMS TUE AFTN AND EVE ACROSS PARTS
OF THE NRN PLAINS....
...SYNOPSIS...
THE STRONGER WESTERLIES APPEAR LIKELY TO PERSIST IN AN AMPLIFIED
BELT ACROSS THE NORTHEAST PACIFIC THROUGH MUCH OF CANADA DURING THIS
FORECAST PERIOD. MODELS SUGGEST THAT A SIGNIFICANT SHORT WAVE
IMPULSE...ON THE SOUTHERN PERIPHERY OF ONE TROUGH EMBEDDED WITHIN
THIS REGIME...WILL LIFT THROUGH PARTS OF THE NORTHERN PLAINS AND
ADJACENT INTERNATIONAL BORDER AREA. OTHERWISE...A GENERALLY WEAK
WESTERLY MID/UPPER FLOW IS EXPECTED TO PREVAIL ACROSS MUCH OF THE
REMAINDER OF THE U.S...WITH AN ELONGATED CYCLONIC CIRCULATION
LINGERING ACROSS THE OHIO VALLEY AND MID ATLANTIC COAST
REGION...PERHAPS ANOTHER DEVELOPING NEAR THE CALIFORNIA COAST.
IN THE WAKE OF THE POLAR TROUGH CONTINUING TO LIFT INTO THE CENTRAL
CANADIAN PROVINCES...THUNDERSTORM POTENTIAL WILL REMAIN LOW OR
CONTINUE TO DIMINISH TUESDAY ACROSS MUCH OF THE GREAT BASIN AND
NORTHERN INTERMOUNTAIN REGION INTO THE NORTHERN ROCKIES.
BUT...THUNDERSTORM PROBABILITIES WILL PERSIST/INCREASE AHEAD OF AN
ASSOCIATED COLD FRONT ACROSS THE NORTHERN PLAINS/UPPER MISSISSIPPI
VALLEY REGION...SOUTHWESTWARD INTO THE SOUTHERN PLATEAU/ROCKIES.
SCATTERED THUNDERSTORMS ALSO APPEAR POSSIBLE ACROSS PARTS OF THE
GULF COAST STATES...AND NEAR THE UPPER LOW ACROSS PARTS OF THE
MIDDLE MISSISSIPPI/OHIO AND TENNESSEE VALLEYS...THE LOWER GREAT
LAKES AND MID ATLANTIC COAST.
...NORTHERN PLAINS...
STRONGER MID-LEVEL COOLING AND WIND FIELDS ASSOCIATED WITH THE SHORT
WAVE IMPULSE ARE PROGGED TO LAG BEHIND THE SURFACE COLD FRONT AS IT
ADVANCES EASTWARD THROUGH THE NORTHERN PLAINS TUESDAY.
HOWEVER...PRE-FRONTAL LOW-LEVEL MOISTENING AND HEATING ARE EXPECTED
TO CONTRIBUTE TO SUBSTANTIAL INSTABILITY IN AT LEAST A NARROW
CORRIDOR FROM PARTS OF WESTERN/NORTH CENTRAL NEBRASKA THROUGH THE
EASTERN DAKOTAS AND NORTHWEST MINNESOTA. BY THE MID TO LATE
AFTERNOON HOURS...SREF GUIDANCE INDICATES A HIGH LIKELIHOOD OF MIXED
LAYER CAPE EXCEEDING 2000 J/KG.
DESTABILIZATION...COUPLED WITH FAVORABLE LARGE-SCALE FORCING FOR
UPWARD VERTICAL MOTION...IS EXPECTED TO BECOME SUPPORTIVE OF
STRONG/SEVERE STORM DEVELOPMENT BY 08/21Z...WITH INCREASING
CONVECTION THEN LIKELY THROUGH THE REMAINDER OF THE
AFTERNOON...PRIMARILY ALONG OR JUST AHEAD OF THE FRONT. WHILE
MID-LEVEL FLOW WILL PROBABLY REMAIN RATHER WEAK...SHEAR/MOMENTUM
ASSOCIATED WITH A MODEST--ON THE ORDER OF 30+ KT AT 850
MB--SOUTHERLY LOW-LEVEL JET MAY ENHANCE THE RISK FOR DAMAGING WIND
GUSTS IN THE PRESENCE OF STEEP LAPSE RATES AND SIZABLE LOW-LEVEL
TEMPERATURE/DEW POINTS SPREADS. LARGE HAIL WILL ALSO BE POSSIBLE IN
STRONGEST ACTIVITY ...WHICH MAY BE SLOWEST TO DIMINISH IN THE
PRESENCE OF BETTER INSTABILITY...POSSIBLY A NOCTURNALLY
STRENGTHENING LOW-LEVEL SPEED MAXIMUM...ACROSS PARTS OF NORTH
CENTRAL/NORTHEAST NEBRASKA AND SOUTHEASTERN SOUTH DAKOTA TUESDAY
EVENING.
..KERR.. 09/07/2009
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