SWODY2
SPC AC 190438
DAY 2 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
1138 PM CDT SUN SEP 18 2011
VALID 201200Z - 211200Z
...NO SVR TSTM AREAS FORECAST...
...SYNOPSIS...
A COMPACT BUT INTENSE UPPER LOW IS FORECAST TO DEVELOP OVER ND LATE
IN THE DAY 1 PERIOD AND MOVE EWD ACROSS NRN MN ON TUESDAY AS A
STRONG MID/UPPER LEVEL JET ROTATES CYCLONICALLY THROUGH THE BASE OF
THE SYSTEM. THE CYCLONE IS EXPECTED TO OCCLUDE AS IT MOVES
EWD...WITH THE ASSOCIATED SURFACE LOW BECOMING ALIGNED VERTICALLY
WITH THE UPPER SYSTEM. A FRONT WILL ARC SWD FROM THE LOW...MOVING
FROM THE UPPER MS VALLEY/CENTRAL PLAINS EWD TO AN ERN UPPER
MI/CENTRAL IL/NWRN AR/TX SOUTH PLAINS LINE BY 21/12Z.
...WI/UPPER MI AREA...
FAVORABLE DYNAMIC FORCING FOR LARGE SCALE ASCENT WILL ACCOMPANY THE
CYCLONE AS IT MOVES EWD ACROSS MN TOWARD THE WRN GREAT LAKES.
STRONG WINDS ALOFT WITH A 40-50 KT LOW LEVEL JET AND A 60-90 KT
MID/UPPER LEVEL JET WILL PROVIDE SUBSTANTIAL LOW LEVEL AND DEEP
LAYER VERTICAL SHEAR ALONG AND IMMEDIATELY AHEAD OF THE ADVANCING
FRONT. HOWEVER...MARGINAL LOW LEVEL MOISTURE WITH SURFACE DEW POINTS
IN THE 50S ALONG/AHEAD OF THE FRONT COUPLED WITH MODEST MID LEVEL
LAPSE RATES WILL CONTRIBUTE TO WEAK INSTABILITY /SBCAPE AOB 500 J
PER KG/...AND THIS IS EXPECTED TO BE THE PRIMARY LIMITING FACTOR FOR
STRONG/SEVERE THUNDERSTORM POTENTIAL.
FOCUSED CONVERGENCE ALONG THE FRONT MAY BE SUFFICIENT FOR ISOLATED
TO WIDELY SCATTERED LOW-TOPPED THUNDERSTORMS TO DEVELOP...MAINLY
DURING THE AFTERNOON AND EVENING HOURS WHEN THE EFFECTS OF DIABATIC
HEATING WILL BE MOST PRONOUNCED. A FEW CONVECTIVE CELLS MAY BE
CAPABLE OF PRODUCING STRONG WIND GUSTS GIVEN THE STRENGTH OF THE
WINDS ALOFT AND LOCALLY STEEPER LOW LEVEL LAPSE RATES...WHICH MAY
AID IN DOWNWARD TRANSFER OF HORIZONTAL MOMENTUM. HOWEVER...THE
OVERALL SEVERE THREAT IS EXPECTED TO BE LIMITED BY WEAK INSTABILITY.
..WEISS.. 09/19/2011
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