Prognostic Meteorological Discussion
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FXUS02 KWBC 241558
PMDEPD

EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1058 AM EST WED DEC 24 2014

VALID 12Z SAT DEC 27 2014 - 12Z WED DEC 31 2014

...GENERAL FLOW PATTERN...

A MID-UPPER LEVEL RIDGE WILL BUILD INTO THE NORTHEAST
PACIFIC/WESTERN CANADA/ALASKA THIS WEEKEND AND EARLY NEXT WEEK
WHILE DOWNSTREAM...BROAD TROUGHING IS EXPECTED TO CARVE OUT OVER
THE LOWER 48.  AT THE LEADING EDGE OF HEIGHT FALLS...A COLD FRONT
WILL STEADILY PUSH FROM THE MISSISSIPPI VALLEY TO THE APPALACHIANS
ON SATURDAY.  THE NORTHERN PORTION OF THE BOUNDARY SHOULD PUSH OFF
THE NEW ENGLAND COAST BY MONDAY...BUT A POSSIBLE SURFACE WAVE
DEVELOPING FARTHER SOUTH ALONG THE BOUNDARY COULD STALL FRONTAL
PROGRESSION THROUGH THE LOWER MISSISSIPPI VALLEY...TENNESSEE
VALLEY...AND SOUTHEAST.  OUT WEST...STRONG SURFACE RIDGING NOSING
DOWN FROM WESTERN CANADA LATE THIS WEEKEND WILL SPREAD OUT OVER
THE NORTHWEST/ROCKIES/NORTH CENTRAL U.S. EARLY NEXT WEEK.


...MODEL EVALUATION/PREFERENCES...

ALTHOUGH MODELS ARE IN DECENT AGREEMENT WITH THE MEAN FLOW PATTERN
OVER THE CONUS...A LARGE AMOUNT OF SPREAD DEVELOPS EARLY IN THE
FORECAST PERIOD WITH SOME OF THE SMALLER SCALE FEATURES AND
DETAILS.  THE GFS STRAYS FROM MODEL CONSENSUS BY DAY 3 AND
AGGRESSIVELY AMPLIFIES SHORTWAVE ENERGY LIFTING INTO THE UPPER
MIDWEST/GREAT LAKES ON SATURDAY.  MODELS REALLY BEGIN TO DIVERGE
WITH FLOW OVER THE SOUTHERN TIER LATE THIS WEEKEND...RESULTING IN
WIDELY VARYING SOLUTIONS WITH A POSSIBLE SURFACE WAVE DEVELOPING
IN THE WESTERN GULF AND TRACKING TOWARDS THE SOUTHERN MID-ATLANTIC
STATES EARLY NEXT WEEK.  TOWARDS THE LATER HALF OF THE FORECAST
PERIOD...EVEN ENSEMBLE MEAN GUIDANCE SHOWS SLIGHT DIFFERENCES WITH
THE EXACT AMPLITUDE/AXIS OF THE UPPER RIDGE OVER THE NORTHEAST
PACIFIC AND DOWNSTREAM TROUGH OVER THE WESTERN U.S..

GIVEN THE LARGE AMOUNT OF MODEL SPREAD AND WITH EACH PIECE OF
GUIDANCE HAVING ITS OWN RUN-TO-RUN CONTINUITY ISSUES...THE WPC DAY
3-7 FORECAST RELIED HEAVILY ON THE ENSEMBLE MEANS.  EARLY IN THE
FORECAST PERIOD...THE ECMWF ENSEMBLE MEAN APPEARED TO OFFER A
MIDDLE-OF-THE-ROAD SOLUTION WITH THE COLD FRONT AND POSSIBLE
SURFACE LOW MOVING THROUGH THE EASTERN HALF OF THE COUNTRY...BUT
BY DAY 5...A BLEND OF THE ECMWF ENSEMBLE/GEFS MEAN WAS USED TO
ACCOUNT FOR UNCERTAINTY IN THE WEST.


...SENSIBLE WEATHER HIGHLIGHTS...

AN ELONGATED AXIS OF PRECIPITATION IS EXPECTED WITH THE COLD FRONT
PRESSING FROM THE MISSISSIPPI VALLEY TO THE EASTERN SEABOARD.
TEMPERATURES WILL BE LOW ENOUGH TO SUPPORT SNOW WITHIN THE
NORTHWESTERN FRINGE OF THE PRECIPITATION SHIELD AND MODERATE TO
HEAVY RAINS WILL BE POSSIBLE FARTHER SOUTH; HOWEVER...THE LARGE
AMOUNT OF UNCERTAINTY IN THE FORECAST...ESPECIALLY WITH THE
POSSIBLE SURFACE WAVE DEVELOPMENT...MAKES IT DIFFICULT TO PINPOINT
THE EXACT LOCATION/AMOUNT OF SIGNIFICANT PRECIPITATION.  BEHIND
THE FRONT...ARCTIC AIR SPREADING DOWN OVER MUCH OF THE NATION WILL
KEEP TEMPERATURES WELL BELOW NORMAL ACROSS THE
NORTHWEST...ROCKIES...AND NORTH CENTRAL U.S..


GERHARDT


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