008
FXUS01 KWBC 200731
PMDSPD
SHORT RANGE FORECAST DISCUSSION
NWS HYDROMETEOROLOGICAL PREDICTION CENTER CAMP SPRINGS MD
330 AM EDT SUN SEP 20 2009
VALID 12Z SUN SEP 20 2009 - 00Z TUE SEP 22 2009
...COLDER AIR AND UNSETTLED WEATHER TO AIM FOR THE ROCKIES AND
HIGH PLAINS...
SIGNIFICANT CHANGES ARE EXPECTED TO UNFOLD OVER THE NEXT COUPLE OF
DAYS AND IN TIME FOR THE START OF THE NEW WEEK AS A VERY STRONG
UPPER TROF IS EXPECTED TO DEVELOP OVER THE NORTHERN AND CENTRAL
ROCKIES AND THEN MOVE INTO THE HIGH PLAINS. THIS DEVELOPING UPPER
TROF IS EXPECTED TO BECOME A PART OF AN OTHERWISE INCREASINGLY
AMPLIFIED JET STREAM PATTERN ALOFT...WITH THE CENTRAL U.S. UPPER
TROF EXPECTED TO BECOME BORDERED BY TWO LARGER SCALE UPPER LEVEL
RIDGES WITH ONE DEVELOPING ALONG THE WEST COAST AND THE OTHER ONE
ALONG AND JUST OFFSHORE THE EAST COAST.
IN THE SHORT TERM TO ROUND OUT THE WEEKEND...THE REMAINS OF AN
UPPER LOW WHICH HAD RESIDED ACROSS THE SOUTHERN U.S. FOR WELL OVER
A WEEK WILL FINALLY WEAKEN AND LIFT NORTHWARD INTO THE OHIO VALLEY
AND GREAT LAKES REGION. THIS ENERGY THOUGH WILL BE INTERACTING
WITH ADDITIONAL ENERGY UPSTREAM THAT WILL BE PUSHING OUT OF THE
LOWER AND MIDDLE MISSISSIPPI VALLEY AND HENCE SHOULD ALLOW FOR AN
ADDITIONAL THREAT OF HEAVIER SHOWERS AND THUNDERSTORMS ACROSS THE
TENNESSEE VALLEY INTO AT LEAST EARLY MONDAY...BUT WITH SOME OF THE
MORE CONCENTRATED RAINS EXPECTED TO BE LIFTING NORTHWARD ACROSS
THE OHIO VALLEY THROUGH THE PERIOD. THERE WILL BE PLENTY OF
SUBTROPICAL MOISTURE IN PLACE AND SO THERE WILL AGAIN BE A THREAT
OF LOCALLY EXCESSIVE RAINS AS THIS MOISTURE AND ENERGY LIFTS
NORTH. MEANWHILE...CANADIAN HIGH PRESSURE THAT RECENTLY SETTLED
INTO THE NORTHEASTERN U.S. THIS WEEKEND AND IS CURRENTLY YIELDING
SOME OF THE COLDEST AIR OF THE SEASON TO THE INTERIOR OF NEW
ENGLAND IS EXPECTED TO NOSE SUFFICIENTLY FAR SOUTH ALONG THE EAST
COAST TO KEEP AT LEAST THE NORTHEAST AND MOST OF THE MID-ATLANTIC
REGION DRY THROUGH THE PERIOD. ALTHOUGH THAT WILL NOT BE THE CASE
FOR THE SOUTHEAST AND SOUTHERN APPALACHIANS WHERE SUFFICIENT
ONSHORE FLOW AND CLOSER PROXIMITY TO A FRONT WILL SUPPORT
SCATTERED SHOWERS AND THUNDERSTORMS. THIS HIGH PRESSURE RIDGE
THOUGH SHOULD BEGIN TO ADVANCE OFFSHORE EARLY THIS WEEK AND THAT
WILL ALLOW TEMPERATURES TO AGAIN BEGIN TO MODERATE.
THE MAIN FOCUS OF ATTENTION THOUGH AGAIN WILL BE ON THE ROCKIES
AND HIGH PLAINS AS AN UPPER TROF AND AN ASSOCIATED COLD FRONT
ALREADY PUSHING THROUGH THE PACIFIC NORTHWEST AND NORTHERN GREAT
BASIN MOVES EASTWARD AND STRENGTHENS. MUCH COLDER AIR SHOULD
ARRIVE OVER THE ROCKIES BY SUNDAY NIGHT AND MONDAY WHICH WILL
SUPPORT SOME OF THE HIGHEST TERRAIN POSSIBLY SEEING ITS FIRST
SNOWFALL OF THE SEASON. THE ASSOCIATED FRONT AS IT MOVES INTO THE
PLAINS SHOULD BECOME CONVECTIVELY ACTIVE AS IT ENCOUNTERS
SUFFICIENT MOISTURE AND INSTABILITY FOR AN EXPANDING AXIS OF HEAVY
SHOWERS AND THUNDERSTORMS...AND POSSIBLY SOME SEVERE THUNDERSTORMS
LOCALLY. HIGH PRESSURE MEANWHILE SHOULD BEGIN TO DEVELOP ALONG THE
WEST COAST AND SO THIS SHOULD YIELD DRY WEATHER HERE ALONG WITH
SOME MUCH WARMER TEMPERATURES. IT SHOULD BE NOTED TOO THAT WITH
FAIRLY STRONG HIGH PRESSURE BUILDING DOWN OVER THE GREAT BASIN
THAT THERE WILL BE A NOTABLE INCREASE IN THE VERY WARM AND DRY
SANTA ANA WINDS FOR SOUTHERN CALIFORNIA...SO THE FIRE DANGER
ACROSS THIS REGION IS LIKELY TO BE ENHANCED.
ORRISON
GRAPHICS AVAILABLE AT WWW.HPC.NCEP.NOAA.GOV
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