DAY1SVR: Day 1 Convective Outlook

ACUS01 KWNS 200100
SWODY1
SPC AC 200057

DAY 1 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
0757 PM CDT WED SEP 19 2007

VALID 200100Z - 201200Z

..THERE IS A SLGT RISK OF SVR TSTMS LATE TONIGHT ACROSS PARTS OF
THE CNTRL/ERN DAKOTAS....

..PLAINS...
A WEAK IMPULSE EMANATING FROM THE TROUGH IN A SOUTHERN BRANCH OF
POLAR WESTERLIES...ACROSS THE WESTERN U.S...IS EVIDENT IN WATER
VAPOR IMAGERY LIFTING ACROSS THE NORTHERN ROCKIES. THIS FEATURE IS
EXPECTED TO ACCELERATE EAST NORTHEASTWARD TONIGHT...GRADUALLY
MERGING INTO THE BROAD PRIMARY POLAR TROUGH ACROSS THE CANADIAN
PROVINCES...AS AN UPSTREAM SHORT WAVE TROUGH DIGS NEAR/TO THE LEE OF
THE CANADIAN ROCKIES. AS THIS OCCURS...UPPER FLOW IS PROGGED TO
BECOME INCREASINGLY DIVERGENT OVERNIGHT ACROSS THE DAKOTAS
..CONTRIBUTING TO DEVELOPING LARGE-SCALE UPWARD VERTICAL MOTION
FIELD...IN CONCERT WITH A STRENGTHENING SOUTHERLY LOW-LEVEL JET.

MODELS SUGGEST THAT AN 850 MB SPEED MAXIMUM WILL FORM BY 06Z ACROSS
THE NORTH CENTRAL PLAINS...ALLOWING A RAPID ELEVATED MOISTURE RETURN
FROM THE CENTRAL PLAINS...ABOVE A WARM FRONTAL/RADIATIONAL INVERSION
LAYER. AS THE LOW-LEVEL JET VEERS TO A MORE SOUTHWESTERLY
COMPONENT...THIS MOISTURE IS EXPECTED TO REACH THE CENTRAL/EASTERN
DAKOTAS...WHERE WEAKER INHIBITION IS EXPECTED TO ALLOW FOR
INCREASING CONVECTIVE DEVELOPMENT.

SCATTERED THUNDERSTORM ACTIVITY IS ALREADY FORMING ACROSS PARTS OF
NORTH CENTRAL/NORTHEASTERN SOUTH DAKOTA. BUT...THE INITIATION OF
MORE VIGOROUS STORMS SEEMS MORE LIKELY DURING THE 06-09Z TIME
FRAME...AS MOST UNSTABLE CAPE INCREASES IN EXCESS OF 1000 J/KG.
GIVEN INCREASING INSTABILITY...DEEP LAYER SHEAR IN THE CONVECTIVE
LAYER IS EXPECTED TO BE SUFFICIENT FOR UPDRAFT ROTATION. AND...
ISOLATED SUPERCELLS ARE POSSIBLE...PRIMARILY WITH A LARGE HAIL
THREAT...WHICH WILL CONTINUE ACROSS THE EASTERN DAKOTAS THROUGH
DAYBREAK.

FARTHER SOUTH...THE BULK OF ONGOING STORMS ACROSS THE CENTRAL
PLAINS/SOUTH CENTRAL HIGH PLAINS ARE EXPECTED TO DIMINISH WITH THE
LOSS OF DAYTIME HEATING.

..NORTHERN CALIFORNIA...
THE RISK FOR AN ISOLATED STRONG OR SEVERE STORM MAY CONTINUE THROUGH
02-03Z ACROSS PARTS OF THE SACRAMENTO VALLEY. BUT...LIMITED THREAT
IS EXPECTED TO DIMINISH WITH THE LOSS OF DAYTIME HEATING.
AND...THUNDERSTORM ACTIVITY SHOULD DIMINISH OVERNIGHT AS THE
SOUTHERN BRANCH MID-LEVEL CIRCULATION DEVELOPS
SOUTH/SOUTHWESTWARD...TO THE WEST OF CENTRAL/SOUTHERN CALIFORNIA
COASTAL AREAS.

..FLORIDA...
LOW-LEVEL FLOW/SHEAR APPEARS TO BE SLOWLY WEAKENING ACROSS NORTHERN
FLORIDA ATLANTIC COASTAL AREAS. THIS TREND IS EXPECTED TO
CONTINUE...AS THE SURFACE LOW REFORMS TO THE WEST OF THE PENINSULA
OVERNIGHT. HOWEVER...AT LEAST A LIMITED RISK FOR AN ISOLATED
TORNADO STILL EXISTS THIS EVENING...WITH INLAND PROGRESSING
CONVECTIVE BANDS/CELLS.

.KERR.. 09/20/2007

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