Prognostic Meteorological Discussion
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000 FXUS02 KWBC 251557 PMDEPD EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 1157 AM EDT SAT OCT 25 2014 VALID 12Z TUE OCT 28 2014 - 12Z SAT NOV 01 2014 ...REMNANTS OF `ANA` TO IMPACT BRITISH COLUMBIA AND THE PACIFIC NORTHWEST... ...OVERVIEW... AN `ATMOSPHERIC RIVER` LOOKS TO SET UP ALONG THE PACIFIC NORTHWEST COAST DURING THE FIRST HALF OF THIS MEDIUM RANGE PERIOD. WITH `ANA` IN AN EXTRATROPICAL TRANSITION PHASE OVER THE CENTRAL PACIFIC PRIOR TO THE START OF THE MEDIUM RANGE---SEE THE CPHC FORECAST FOR MORE DETAILS---THE EXPECTED JET-LEVEL WINDS WILL PLAY AN IMPORTANT ROLE IN THE EVOLUTION OF THE WAVE AMPLITUDE DOWNSTREAM ACROSS THE LOWER 48 THROUGH NEXT WEEK. ENSEMBLES POINT TO A REAMPLIFICATION OF THE PATTERN BY NEXT SAT/D7 WITH TROUGHING NEAR BOTH COASTS AND RIDGING ALONG 100W. ...UNCERTAINTY ASSESSMENT... THOUGH THE ENSEMBLES HAVE SETTLED DOWN FROM A FEW DAYS AGO... QUESTIONS REMAIN OFF THE WEST/EAST COASTS AS THE PATTERN SHARPENS AS TO HOW MUCH SEPARATION IN THE FLOW WILL OCCUR. CONTINUITY AMONG THE DETERMINISTIC MODELS IS LACKING AND THE SENSIBLE WEATHER DIFFERENCES ARE SIGNIFICANT. THE MULTI-CENTER ENSEMBLES MEANS SHOW BETTER CLUSTERING OFF THE WEST COAST THAN THE EAST COAST /WHERE THE ECENS MEAN LAGS BEHIND THE OTHERS IN THE SOUTHEAST/ BUT SMALLER SCALE INFLUENCES UPSTREAM OVER ALASKA /WHERE CONTINUITY CONTINUES TO BE POOR/ WILL AFFECT THE ORIENTATION OF SFC WAVES NEAR OR APPROACHING THE PAC NW. ...MODEL PREFERENCES... A BLEND OF THE 00Z GFS/GEFS AND ECMWF/ECENS MEANS OFFERED A REASONABLE STARTING POINT FOR TUE/D3-THU/D5. THE 06Z GFS/GEFS SEEMED TOO QUICK WITH THE REMNANTS OF `ANA` PER EXTRAPOLATION OF THE LAST POINT OF THE CPHC FORECAST. BY FRI/D6... THE 00Z GFS MAY BE TOO QUICK WITH HEIGHT FALLS INTO THE WEST WHILE IN THE EAST THE ECMWF IS ON THE DEEPER SIDE OF THE ENSEMBLE SPREAD AS IT CLOSES OFF ENERGY IN THE SOUTHEAST. THOUGH THE AMPLIFYING PATTERN SUPPORTS SUCH A SCENARIO... WOULD PREFER TO SEE MORE ENSEMBLE SUPPORT AND DETERMINISTIC CONTINUITY BEFORE JUMPING TOWARD THIS SOLUTION. FOR NOW... CONTINUED ALONG THE LINES OF WPC CONTINUITY WITH A GEFS/ECENS COMPROMISE. SENSIBLE WEATHER HIGHLIGHTS... SURFACE CYCLONE MIGRATING FROM THE UPPER GREAT LAKES ON DAY 3 TO NORTH CENTRAL QUEBEC ON DAY 5 IS FORECAST TO PRODUCE A BROAD AREA OF WARM ADVECTION AND ABOVE AVERAGE TEMPERATURES FOR A LARGE PORTION OF THE EAST... A NARROW BAND OF MODERATE RAINFALL INTENSITY ALONG ITS COLD FRONT... AND A SOUTHEASTWARD SURGE OF COOLER CANADIAN AIR BEHIND ITS COLD FRONT. THE COLDER AIRMASS SHOULD SPREAD ACROSS THE GREAT LAKES AND THE NORTHEAST AND ULTIMATELY THE EASTERN HALF OF THE CONUS BY THE END OF THE WEEK. THIS FROPA COULD PROVIDE A NOTABLE DROP IN DEW POINTS TO ALL OF THE SOUTHEAST AND MOST OF FLORIDA... PERHAPS DIPPING BELOW 40F ALL THE WAY TO THE GULF COAST. IN THE WEST... CONSIDERABLE PERIODS OF STRONG SOUTH AND SOUTHEAST WIND BURSTS AT THE COAST AND WINDWARD COASTAL RANGES---WITH PERIODS OF WARM ADVECTION STRATIFORM PRECIPITATION SPREADING LEE OF THE CASCADES. THE JET-DRIVEN HEAVY RAINS GENERALLY CONCENTRATING FROM THE US/CANADA BORDER ON DAY 3... FOLLOWED BY HEIGHT FALLS DOWN THE COAST INTO SOUTHWEST OREGON BY DAY 5... SIGNAL THE ARRIVAL OF THE POLAR JET AND THE MODIFIED MARITIME POLAR AIRMASS ASSOCIATED WITH THE GULF OF ALASKA. NORTHERN ROCKIES AND ADJACENT HIGH PLAINS SHOULD SEE WIDESPREAD DOWNSLOPE CONDITIONS... INCLUDING NOTABLY HIGHER WIND SPEEDS AND RAPIDLY FLUCTUATING AMBIENT AIR TEMPERATURES BUT LITTLE IN THE WAY OF RAINFALL. ASIDE FROM THE WESTERN SLOPES OF THE NORTHERN DIVIDE WHERE THE MID-LEVEL WARM ADVECTION RAINS ARE LIKELY... THE ONLY ADDITIONAL PRECIPITATION SOURCE WILL BE GENERATED BY THE ALBERTA CLIPPER RACING THROUGH THE MISSOURI AND OHIO RIVER VALLEYS ON THURSDAY. ITS LINGERING UPPER SUPPORT COULD TOUCH OF SOME PRECIPITATION IN THE EASTERN LAKES AND ALONG/WEST OF THE NORTHERN APPALACHIANS IN A MARGINALLY COLD AIRMASS... SUPPORTING THE CHANCE FOR SOME SNOWFLAKES WELL NORTHWEST OF THE COASTAL PLAIN AS WE ENTER NOVEMBER ON SATURDAY. FRACASSO/VOJTESAK $$

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