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000 FXUS01 KWBC 220811 PMDSPD SHORT RANGE FORECAST DISCUSSION NWS HYDROMETEOROLOGICAL PREDICTION CENTER CAMP SPRINGS MD 311 AM EST SUN NOV 22 2009 VALID 12Z SUN NOV 22 2009 - 00Z TUE NOV 24 2009 THERE WILL FINALLY BE A PATTERN CHANGE ACROSS THE UNITED STATES WHERE A MEAN UPPER TROUGH HAS BEEN A MAINSTAY FEATURE OVER THE WESTERN U.S. FOR MUCH OF THE MONTH. BEFORE THIS OCCURS...A WEATHER SYSTEM WHICH IS CURRENTLY PUSHING ONSHORE THIS MORNING WILL CARRY A BRIEF BURST OF MODERATE TO HEAVY PRECIPITATION ACROSS THE WESTERN HALVES OF WASHINGTON AND OREGON. FUELING THIS AXIS OF RAIN AND SNOW IS A STRONG FETCH OF PACIFIC MOISTURE WHICH WILL SURGE INLAND VIA A 35 TO 40 MPH ONSHORE FLOW. AFTER THE FRONTAL BOUNDARY QUICKLY PUSHES INLAND...THE FLOW OFF THE PACIFIC WILL WEAKEN WHICH WILL THEN CUT-OFF A SUFFICIENT MOISTURE SOURCE FOR INLAND LOCATIONS. CONSEQUENTLY...ONLY SCATTERED SNOW SHOWERS ARE EXPECTED ACROSS THE UPPER/CENTRAL INTERMOUNTAIN WEST AND INTO THE NORTHERN ROCKIES. THE BEST CHANCE FOR ISOLATED HEAVIER SNOWS WILL BE ACROSS THE TETONS AND SOUTHWARD INTO THE WASATCH. BY EARLY MONDAY...MUCH OF THE ACTIVITY SHOULD WIND DOWN AS THE SYSTEM ENTERS THE PLAINS. MEANWHILE...ANOTHER UPPER TROUGH WILL IMPACT THE NORTHWESTERN U.S. BY MIDDAY MONDAY...BUT THIS TIME THE BEST FETCH OF MOISTURE WILL BE DIRECTED FARTHER NORTH INTO WESTERN CANADA. THE OTHER MAJOR WEATHER SYSTEM OF INTEREST IS A DISTURBANCE MOVING ACROSS THE SOUTHERN U.S. WHILE THIS SOUTHERN STREAM SYSTEM HAS A HISTORY OF DROPPING VERY HEAVY RAINFALL ACROSS THE WESTERN GULF COAST...SUCH AMOUNTS ARE NOT TO BE EXPECTED AS A GENERAL WEAKENING TREND IS FORECAST. THE SWATH OF SHOWERS AND THUNDERSTORMS WILL CONTINUE MOVING EASTWARD TOWARD THE SOUTHEASTERN U.S. AND EVENTUALLY MOVE UP THE EAST COAST. GIVEN THE TRAJECTORY OF THE UPPER-LEVEL WAVE RESPONSIBLE FOR THIS FRONTAL SYSTEM...A BULK OF THE HEAVIER PRECIPITATION SHOULD REMAIN CONFINED TO THE COAST...REACHING THE OUTER BANKS BY MONDAY MORNING. THE MORE PROGRESSIVE NATURE OF THIS SYSTEM WILL SHOW AS IT QUICKLY DRAWS RAINFALL UP INTO LOWER NEW ENGLAND BY MONDAY EVENING. ACROSS THE UPPER MIDWEST...A STALLED FRONTAL BOUNDARY WILL BRING WIDELY SCATTERED RAIN SHOWERS TO THE REGION AND BACK INTO THE MIDDLE MISSISSIPPI VALLEY. THE INTENSITY SHOULD REMAIN LIGHT FOR THE MOST PART...UNTIL THE PACIFIC SYSTEM ARRIVES ON MONDAY EVENING....THEREBY ENHANCING SOME OF THE ACTIVITY ACROSS THE CENTRAL AND NORTHERN PLAINS. AT THIS POINT IN TIME...THE MEAN UPPER TROUGH WILL BEGIN TO SET UP ACROSS THE CENTRAL U.S...WITH RIDGING FINALLY EXPECTED OUT WEST. RUBIN-OSTER GRAPHIC AVAILABLE AT WWW.HPC.NCEP.NOAA.GOV $$
