717
FXUS01 KWBC 210756
PMDSPD
SHORT RANGE FORECAST DISCUSSION
NWS HYDROMETEOROLOGICAL PREDICTION CENTER CAMP SPRINGS MD
355 AM EDT SUN SEP 21 2008
VALID 12Z SUN SEP 21 2008 - 00Z TUE SEP 23 2008
THE LONGWAVE PATTERN FEATURES A DEEP VORTEX OVER THE HUDSON BAY
AREA...RIDGING OVER THE GREAT PLAINS/CANADIAN PRAIRIE...AND AN
ACTIVE MID TO UPPER LEVEL TROUGH OVER THE PACIFIC NORTHWEST AND
GREAT BASIN. THE MAIN FOCUS FOR PRECIPITATION FOR SUNDAY AND
MONDAY WILL BE ACROSS THE PACIFIC NORTHWEST...NORTHERN
ROCKIES...AND NORTHERN PLAINS. A SERIES OF SHORTWAVES WILL MOVE
ABOUT THE BASE OF THE LONGWAVE TROUGH...BRINGING UPPER LEVEL
DIFFLUENCE...COLD AIR ALOFT AND MOISTURE TO THE NORTHWESTERN
QUADRANT OF THE NATION. WHILE THE FRONTAL BOUNDARY MAY A LITTLE
BIT CHALLENGING TO DELINEATE AT THE SURFACE EARLY IN THE FORECAST
PERIOD...THE IMPACTS ON SENSIBLE WEATHER WILL BE STRAIGHT FORWARD.
EXPECT A BATCH OF UNSETTLED WEATHER CONDITIONS WITH THIS MODIFIED
MARITIME AIRMASS AS IT GRADUALLY MOVES EASTWARD. ONCE THIS SYSTEM
EMERGES INTO THE PLAINS...ORGANIZED CONVECTION WILL BREAKOUT IN
THE WARM SECTOR...OVER THE DAKOTAS AND NEBRASKA. OTHERWISE...THE
HEAVIEST PRECIPITATION IS EXPECTED ALONG THE CANADIAN ROCKIES AS
ENHANCED UPSLOPE FLOW DEVELOPS.
SCATTERED LIGHT SHOWERS AND THUNDERSTORMS ARE FORECAST FOR SUNDAY
AND MONDAY ACROSS THE LOWER AND MIDDLE MISSISSIPPI VALLEYS...AS A
WEAK IMPULSE PROPAGATES ACROSS THE REGION WEARING AWAY AT ANY
CAPPING MECHANISM AND THEREBY DESTABILIZING THE ATMOSPHERE.
HEADING EAST...CLOCKWISE FLOW AROUND THE SURFACE HIGH PRESSURE
SYSTEM WILL RESULT IN EASTERLY FLOW OFF THE ATLANTIC AND INTO THE
SOUTHEAST. THE BEST CHANCE FOR CONVECTION TO MATERIALIZE WILL BE
IN THE AFTERNOON/EVENING...WHEN THE ENVIRONMENT WILL BE MOST
BUOYANT. ELSEWHERE...ONSHORE FLOW INTO THE WESTERN GULF OF MEXICO
WILL YIELD PRECIPITATION ACROSS THE TEXAS COAST AND THE MEXICAN
STATE TAMAULIPAS.
SZATANEK
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