SPC AC 200534
DAY 1 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
1234 AM CDT SUN AUG 20 2006
VALID 201200Z - 211200Z
...NO SVR TSTM AREAS FORECAST...
MODELS INDICATE THAT THE GRADUAL RETROGRESSION AND WEAKENING OF THE
OZARK PLATEAU SUBTROPICAL HIGH CENTER WILL CONTINUE DURING THIS
FORECAST PERIOD. AND...A SLIGHT SOUTHWARD/SOUTHEASTWARD SHIFT OF
STRONGER WESTERLIES IS PROGGED FROM THE CENTRAL AND EASTERN
CANADIAN/U.S. BORDER AREA THROUGH PARTS OF THE UPPER MISSISSIPPI
VALLEY/GREAT LAKES AND NORTHEAST. EMBEDDED WITHIN A SOUTHERN BRANCH
OF THE POLAR JET...A SIGNIFICANT SHORT WAVE TROUGH IS FORECAST TO
SHIFT EAST OF THE LOWER GREAT LAKES AND OFF THE ATLANTIC COAST BY
12Z MONDAY. THIS WILL BE ACCOMPANIED BY AN EASTWARD MIGRATING
SURFACE LOW...ACROSS NORTHERN NEW ENGLAND DURING THE DAY
TODAY...THEN UP THE NORTH ATLANTIC COAST TONIGHT. INTRUSION OF
COOLER AND DRIER AIR WILL ADVANCE DEEPER INTO THE EASTERN U.S...WITH
ASSOCIATED FRONTAL ZONE REMAINING A FOCUS FOR CONVECTIVE
DEVELOPMENT...FROM PARTS OF NORTHERN/MID ATLANTIC COASTAL AREAS
THROUGH THE TENNESSEE VALLEY AND SOUTH CENTRAL PLAINS.
MOIST PLUME ADVECTING FROM THE LOWER LATITUDE EASTERN PACIFIC...
AROUND WESTERN PERIPHERY OF MID-LEVEL HIGH CENTER...WILL MAINTAIN
FAVORABLE ENVIRONMENT FOR THUNDERSTORM ACTIVITY BENEATH BROADER
UPPER RIDGE ACROSS THE CENTRAL/SOUTHERN ROCKIES AND INTERMOUNTAIN
REGION. SOME OF THIS MOISTURE WILL ADVECT INTO THE NORTHERN
PLAINS...AHEAD OF NEXT FRONTAL SURGE ASSOCIATED WITH ANOTHER
VIGOROUS SHORT WAVE TROUGH PROGRESSING WITHIN MAIN BELT OF
WESTERLIES ACROSS THE CENTRAL CANADIAN PROVINCES.
...SOUTHERN NEW ENGLAND INTO MID ATLANTIC...
UNCERTAINTIES STILL EXIST CONCERNING SEVERE POTENTIAL TODAY. 40 TO
50 KT MID-LEVEL JET WILL CONTRIBUTE TO FAVORABLE SHEAR PROFILES FOR
SUPERCELLS IN WARM SECTOR OF SURFACE CYCLONE...FROM SOUTHERN NEW
ENGLAND AS FAR SOUTH AS NORTHERN MID ATLANTIC COASTAL AREAS.
HOWEVER...INITIAL MID-LEVEL IMPULSE/WIND SHIFT ASSOCIATED WITH
BROADER SCALE SOUTHERN BRANCH TROUGH IS EXPECTED TO RESULT IN
DRYING/SUBSIDENT WESTERLY FLOW ACROSS THIS REGION BY MID DAY. THIS
MAY ALLOW FOR A PERIOD OF SURFACE HEATING OF RESIDUAL BOUNDARY LAYER
MOISTURE AHEAD OF MAIN COLD FRONT...BEFORE SECONDARY MID-LEVEL
IMPULSE PROGRESSES INTO NORTHERN ATLANTIC COASTAL AREAS BETWEEN
20/21Z-21/00Z.
ISOLATED CONVECTIVE DEVELOPMENT MAY OCCUR LATE THIS AFTERNOON ACROSS
SOUTHERN NEW ENGLAND/LONG ISLAND. HOWEVER...RELATIVELY WEAK
DESTABILIZATION MAY LIMIT OVERALL SEVERE THREAT. MODELS ARE
SUGGESTIVE OF STRONGER MID/UPPER FORCING DEVELOPING IN PRE-FRONTAL
MOIST TONGUE BY EARLY EVENING...WELL EAST OF COASTAL AREAS...WHERE A
SUBSTANTIAL SQUALL LINE COULD DEVELOP.
...NORTHERN PLAINS...
LOW-LEVEL MOISTENING IN THE WAKE OF PRIOR FRONTAL PASSAGE WILL BE
LIMITED. HOWEVER...LAPSE RATES ALONG PRE-FRONTAL SURFACE TROUGH
WILL BECOME QUITE STEEP IN CONJUNCTION WITH STRONG SURFACE HEATING
...AND MID-LEVEL COOLING. STRONGER COOLING ALOFT WILL PROBABLY
REMAIN NORTH OF THE INTERNATIONAL BORDER...BUT MODELS INDICATE THAT
IMPULSE NOW PROGRESSING INTO THE NORTHERN INTERMOUNTAIN
REGION...AROUND PERIPHERY OF SUBTROPICAL RIDGE...WILL CONTINUE EAST
OF THE NORTH CENTRAL ROCKIES THIS AFTERNOON.
MID-LEVEL MOISTENING WILL PROBABLY CONTRIBUTE TO AT LEAST WIDELY
SCATTERED CONVECTIVE DEVELOPMENT DURING THE PEAK AFTERNOON HEATING
TODAY. AND...MODERATE VEERING WIND FIELDS WITH HEIGHT DEVELOPING
ACROSS THE DAKOTAS SHOULD BE SUFFICIENT FOR UPDRAFT ROTATION...
WHICH WILL ENHANCE SEVERE POTENTIAL IN ENVIRONMENT WITH MIXED LAYER
CAPE ON THE ORDER OF 1500 J/KG. GIVEN THE UNCERTAINTY OF THE
STRENGTH OF THE LARGE-SCALE FORCING FOR UPWARD VERTICAL MOTION TO
THE SOUTH OF THE CANADIAN BORDER...POTENTIAL FOR ANYTHING MORE
SUBSTANTIAL THAN A SMALL RELATIVELY SHORT-LIVED CONVECTIVE CLUSTER
SEEMS LOW. THIS PRECLUDES A SLIGHT RISK OF SEVERE STORMS AT THE
PRESENT TIME.
..KERR.. 08/20/2006
CLICK TO GET WUUS01 PTSDY1 PRODUCT
NOTE: THE NEXT DAY 1 OUTLOOK IS SCHEDULED BY 1300Z
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