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Alaska Extended Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 1912Z Dec 05, 2017)
 
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ALASKA EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
212 PM EST TUE DEC 05 2017

VALID 12Z SAT DEC 09 2017 - 12Z WED DEC 13 2017

THE ENSEMBLE MEANS AND MOST OPERATIONAL MODEL RUNS AGREE EXTREMELY
WELL ON THE LARGE SCALE FORECAST.  THE EXPECTED PATTERN WILL
FEATURE A VERY STRONG EASTERN PACIFIC/CONUS WEST COAST RIDGE
EXTENDING INTO MAINLAND ALASKA, WHILE SYSTEMS EMERGING OVER THE
NORTHWEST PACIFIC FEED INTO AND THEN EJECT FROM AN EAST-CENTRAL
PACIFIC TROUGH.  FLOW BETWEEN THE TROUGH AND RIDGE WILL FAVOR A
STORM TRACK AIMED TOWARD OR SLIGHTLY ON EITHER SIDE OF THE ALASKA
PENINSULA, YIELDING PERIODS OF HEAVY PRECIP AND STRONG WINDS ALONG
THE SOUTHERN COAST OF THE MAINLAND AND PERHAPS EXTENDING INTO THE
PANHANDLE. 

IN CONTRAST TO THE RELATIVELY HIGH CONFIDENCE IN THE MEAN PATTERN,
TIMING/STRENGTH DETAILS OF EMBEDDED FEATURES ARE SIGNIFICANTLY
MORE UNCERTAIN IN LIGHT OF SPREAD AND RUN TO RUN DIFFERENCES IN
THE MODELS ALONG WITH POOR ENSEMBLE CLUSTERING.  NOT SURPRISINGLY
THE MOST AGREEABLE SYSTEM EXISTS AT THE START OF THE PERIOD 12Z
SAT, SOUTH OF THE ALASKA PENINSULA.  AFTER THAT VALID TIME THERE
ARE STILL MIXED MESSAGES ON HOW FAR NORTH THIS SYSTEM WILL REACH
BEFORE WEAKENING, WITH AN AVERAGE OF LATEST OPERATIONAL RUNS (AS
OF 12Z GFS ARRIVAL) PROVIDING THE BEST TEMPLATE.  THEN THERE IS A
DECENT SIGNAL FOR A STRONGER SYSTEM THAT MAY TRACK FARTHER NORTH
THAN ITS PREDECESSOR.  HOWEVER MODELS ARE AT LEAST 24 HOURS APART
FOR TIMING.  CONFIDENCE IS LOW IN EVOLUTION SPECIFICS BUT THE 12Z
GFS HAS TRENDED FASTER TO A POSITION ABOUT HALFWAY BETWEEN 00Z/06Z
GFS RUNS AND THE FASTER 00Z ECMWF.  AN AVERAGE OF THE 12Z GFS/00Z
ECMWF TIMING IS THE CLOSEST OPERATIONAL MODEL IDEA TO WHAT THE
WEAKER ENSEMBLE MEANS ARE HINTING.  A BLEND OF THESE RUNS WITH THE
ENSEMBLE MEANS APPEARS BEST AT THIS TIME FOR INTRODUCING THE
POTENTIAL OF A SIGNIFICANT SYSTEM AROUND SUN-MON WHILE ACCOUNTING
FOR ONGOING UNCERTAINTY.  GFS/ECMWF RUNS INDICATE THIS AREA OF LOW
PRESSURE SHOULD ULTIMATELY WEAKEN AND SETTLE WITHIN AN AREA OF
OVERALL SURFACE TROUGHING NEAR THE BERING STRAIT LATE IN THE
PERIOD.  THE ENSEMBLE MEANS PROVIDE THE MOST AGREEABLE DEPICTION
OF TRAILING LOW PRESSURE THAT MAY APPROACH SOUTH OF THE PENINSULA
BY DAY 8 WED. 

BETWEEN SOUTHERN KAMCHATKA AND THE WESTERN ALEUTIANS, JUST OUTSIDE
OF THE FORECAST DOMAIN THERE IS GOOD OPERATIONAL MODEL AGREEMENT
ON A DEEP/CONCENTRATED DAY 4 SAT STORM WHICH WILL QUICKLY ELONGATE
SOUTH OF THE ALEUTIANS.  THEN ANOTHER SYSTEM COMES INTO THE
PICTURE NEAR THE WESTERN ALEUTIANS BY NEXT WED WITH THE 12Z
GFS/00Z ECMWF AND RECENT ENSEMBLE MEANS SIMILAR IN PRINCIPLE.  AN
AVERAGE BETWEEN THE GEFS/ECMWF MEAN TRACKS SHOULD PROVIDE THE BEST
STARTING POINT.

BASED ON THE RELATIVE MODEL AGREEMENT TO START THE FORECAST, DAY 4
SAT USED AN OPERATIONAL MODEL BLEND CONSISTING OF THE 00Z
ECMWF/12Z-06Z GFS/00Z CMC.  AFTER SAT OPERATIONAL INPUT WAS
PRIMARILY CONFINED TO THE 12Z GFS/00Z ECMWF GIVEN COMPARISONS FOR
THE POTENTIAL WESTERN MAINLAND STORM LATE SUN-MON AND SYSTEM NEAR
THE WESTERN ALEUTIANS NEXT WED.  TOTAL 06Z GEFS/00Z ECMWF MEAN
WEIGHT WAS 40 PCT FOR DAY 5 SUN, 60 PCT DAY 6 MON, AND 70 PCT
THEREAFTER.

RAUSCH