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Extended Forecast Discussion
 
(Latest Discussion - Issued 1557Z Oct 25, 2014)
 
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Abbreviations and acronyms used in this product
 
Geographic Boundaries -  Map 1: Color  Black/White       Map 2: Color  Black/White


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