SWODY2
SPC AC 161721
DAY 2 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
1221 PM CDT FRI OCT 16 2009
VALID 171200Z - 181200Z
...NO SVR TSTM AREAS FORECAST...
...SYNOPSIS...
MODELS INDICATE THAT STRONG ZONAL FLOW OVER THE NORTHERN PACIFIC
WILL BEGIN TO DEVELOP EASTWARD TOWARD THE PACIFIC COAST DURING THIS
FORECAST PERIOD. AT THE SAME TIME...THE DOWNSTREAM PATTERN APPEARS
LIKELY TO REMAIN AMPLIFIED...WITH INTENSIFYING FLOW IN AN
ANTICYCLONIC BELT FROM MID/NORTHERN ATLANTIC COASTAL AREAS EAST
NORTHEASTWARD AROUND THE NORTHERN PERIPHERY OF A STRONG SUBTROPICAL
RIDGE EXTENDING FROM THE CARIBBEAN INTO THE NORTH CENTRAL ATLANTIC.
SOME SHARPENING OF THE POSITIVELY TILTED UPSTREAM TROUGH OVER THE
EASTERN U.S. IS PROGGED...WITH ONE IN A SERIES OF EMBEDDED SHORT
WAVE IMPULSES DIGGING INTO ITS BASE EARLY SATURDAY...BEFORE LIFTING
THROUGH SOUTHERN AND MID ATLANTIC COASTAL AREAS IN INCREASINGLY
SHEARED FASHION LATE SATURDAY THROUGH EARLY SUNDAY. THIS LATTER
FEATURE MAY BE ACCOMPANIED BY WEAK CYCLOGENESIS ALONG A BAROCLINIC
ZONE OFF THE MID ATLANTIC COAST. HOWEVER...LOW-LEVEL COOLING AND
DRYING IN THE WAKE OF THE ASSOCIATED COLD FRONT WILL CONTINUE TO
CONTRIBUTE TO DIMINISHING CONVECTIVE POTENTIAL ACROSS THE SOUTHEAST
STATES.
THE EVOLVING PATTERN IS ALSO EXPECTED TO MAINTAIN NEARLY NEGLIGIBLE
CONVECTIVE POTENTIAL ELSEWHERE EAST OF THE ROCKIES...EVEN AS A LEE
SURFACE TROUGH DEVELOPS/DEEPENS LATE IN THE PERIOD IN RESPONSE TO A
SIGNIFICANT SHORT WAVE IMPULSE LIFTING ACROSS THE CANADIAN/NORTHERN
U.S. ROCKIES. BUT...THIS UPPER FEATURE...AND AN ASSOCIATED SURFACE
LOW...COULD BE ACCOMPANIED BY SOME THUNDERSTORM ACTIVITY ACROSS
PARTS OF THE PACIFIC NORTHWEST EARLIER IN THE PERIOD.
...PACIFIC NORTHWEST...
A 70-90 KT SOUTHWESTERLY 500 MB JET STREAK IS PROGGED TO ACCOMPANY
THE INLAND PROGRESSING SHORT WAVE IMPULSE...CONTRIBUTING TO
STRENGTHENING DEEP LAYER FLOW FIELDS AND SHEAR ACROSS THE WESTERN
SLOPES OF THE CASCADES DURING THE DAY SATURDAY. THIS LIKELY WILL
INCLUDE THE WILLAMETTE VALLEY AREA...WHERE GUIDANCE INDICATES A
MOISTENING BOUNDARY LAYER WILL DESTABILIZE AND BECOME SUPPORTIVE OF
THUNDERSTORM ACTIVITY. WHILE INSTABILITY MAY BE LIMITED DUE TO LACK
OF STRONGER COOLING AND STEEPENING LAPSE RATES ALOFT...PROFILES SEEM
TO SUGGEST THAT IT IS NOT OUT OF THE QUESTION THAT A STRONG/SEVERE
STORM OR TWO COULD FORM...WHICH COULD BE CAPABLE OF PRODUCING
POTENTIALLY DAMAGING WIND GUSTS...PERHAPS A BRIEF TORNADO.
...SOUTHERN FLORIDA PENINSULA...
WEAK LARGE-SCALE FORCING FOR UPWARD VERTICAL MOTION AND MODEST SHEAR
ARE EXPECTED TO LIMIT CONVECTIVE POTENTIAL. BUT...ISOLATED OR
WIDELY SCATTERED STORMS ARE POSSIBLE SATURDAY WITHIN THE WARM AND
MOIST PRE-FRONTAL ENVIRONMENT.
...MID ATLANTIC STATES...
WEAK CAPE EVIDENT IN FORECAST SOUNDINGS...BASED WITHIN A MOIST
MID-LEVEL ENVIRONMENT...AND POSSIBLY EXTENDING THROUGH THE FAVORABLE
MIXED PHASE LAYER FOR CHARGE SEPARATION...APPEARS SUFFICIENT FOR
OCCASIONAL WEAK THUNDERSTORM ACTIVITY IN THE PRESENCE OF FAVORABLE
LARGE-SCALE ASCENT THROUGH THE PERIOD. THE BEST CHANCE MAY BE AS
LIFT INCREASES DOWNSTREAM OF THE IMPULSE LIFTING OUT OF THE BASE OF
THE UPPER TROUGH LATE SATURDAY AFTERNOON/EVENING.
..KERR.. 10/16/2009
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