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Alaska Extended Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 1922Z Dec 06, 2017)
 
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ALASKA EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
222 PM EST WED DEC 06 2017

VALID 12Z SUN DEC 10 2017 - 12Z THU DEC 14 2017

GUIDANCE CONTINUES TO AGREE ON THE LARGE SCALE PATTERN WITH A
STRONG EASTERN PACIFIC/CONUS WEST COAST RIDGE EXTENDING INTO THE
MAINLAND WHILE MEAN TROUGHING PERSISTS OVER THE EAST-CENTRAL
PACIFIC AND BERING SEA/EASTERN TIP OF SIBERIA.  LATE IN THE PERIOD
THE MODELS/ENSEMBLES SUGGEST THE TROUGH SHOULD BEGIN TO FILL
SOMEWHAT WITH THE APPROACH OF FLOW AROUND A BROAD UPPER LOW
EMERGING INTO THE EXTREME NORTHWEST PACIFIC.

THE BEST DEFINED SYSTEM OF INTEREST DURING THE PERIOD IS MOST
LIKELY TO TRACK NORTHWARD INTO THE SOUTHWEST CORNER OF THE
MAINLAND BY AROUND EARLY DAY 5 MON.  AS OF THAT TIME A STEADY
EASTWARD TREND OF 6-HOURLY GFS RUNS BETWEEN 00Z AND 12Z CYCLES HAS
YIELDED GOOD TRACK CLUSTERING WITH THE 00Z ECMWF/CMC.  HOWEVER
THERE IS STILL A WIDE SPREAD FOR CENTRAL PRESSURE WITH THE 12Z GFS
DEEPER THAN PREVIOUS RUNS AND THE 00Z ECMWF WEAKER THAN ITS PRIOR
TWO RUNS.  BOTH ARE WITHIN THE BROAD SPREAD AMONG INDIVIDUAL
ENSEMBLE MEMBERS, AND ENSEMBLES STILL VARY IN TRACK AND TIMING. 
SOME DEGREE OF CONTINUITY FROM YESTERDAY'S FORECAST AND 12Z
GFS/00Z ECMWF-CMC CLUSTERING LEAD TO GRADUALLY INCREASING
CONFIDENCE IN A SIGNIFICANT STORM WITH A PERIOD OF STRONG WINDS
AND HEAVY COASTAL/PANHANDLE PRECIP, BUT WITH CONTINUING
UNCERTAINTY FOR IMPORTANT DETAILS.  FOR THE 12Z MON VALID TIME A
COMPROMISE DEPTH IN THE 970'S MB APPEARS MOST REASONABLE BASED ON
THE FULL RANGE OF GUIDANCE.  ONCE THE SYSTEM MOVES INLAND THE 00Z
ECMWF DIVERGES FROM ITS PAST TWO RUNS AND MOST OTHER GLOBAL
MODELS, TAKING NEARLY ALL OF THE WEAKENING LOW PRESSURE ACROSS THE
CENTRAL/EASTERN MAINLAND INSTEAD OF BRINGING AT LEAST SOME LOWER
PRESSURES TOWARD THE BERING STRAIT.  GFS RUNS ARE ON THE STRONGER
SIDE OF THE SPREAD BY 12Z TUE BUT PROXIMITY OF AN UPPER
LOW/TROUGHING OVER THE BERING SEA/EXTREME EASTERN SIBERIA SEEM TO
ARGUE FOR AT LEAST A BETTER DEFINED SURFACE TROUGH/EMBEDDED WAVE
AT THAT TIME.  LOOKING AT REMAINING 12Z GUIDANCE, THE ECMWF IS
STILL THE WEAKEST SOLUTION WHILE THE UKMET IS SLOWEST.

THE FORECAST FOR ONE OR MORE TRAILING FEATURES THAT MAY TRACK INTO
THE EASTERN ALEUTIANS/ALASKA PENINSULA TUE ONWARD IS STILL FAIRLY
AMBIGUOUS.  THE 06Z GFS/00Z ECMWF DIFFER IN SPECIFICS BUT ARE
CLOSER IN PRINCIPLE TO THE ENSEMBLE MEANS VERSUS THE 12Z GFS/00Z
CMC WHICH BRING LOW PRESSURE MUCH FARTHER NORTHWARD, AGAIN TO THE
BERING STRAIT, BY WED.  MOST GUIDANCE IS SIGNALING THAT THE UPPER
RIDGE MAY STRENGTHEN SLIGHTLY IN THE WAKE OF THE FIRST SYSTEM SO
THE 06Z GFS/00Z ECMWF AND ENSEMBLE MEANS SEEM MORE PROBABLE.  IN
SPITE OF THIS THERE IS ACTUALLY A SMALL MINORITY OF ENSEMBLES
SHOWING A FARTHER EASTWARD TRACK SUCH AS DEPICTED BY THE NEW 12Z
CMC.

AS WAS THE CASE YESTERDAY, RELATIVE TO THE MEANS AN OPERATIONAL
MODEL BLEND BETTER DEPICTS A DEEP SYSTEM BETWEEN SOUTHERN
KAMCHATKA AND THE WESTERN ALEUTIANS AT THE START OF THE FORECAST
ON SUN.  ON THE OTHER HAND THE ENSEMBLE MEANS PROVIDE THE BEST
CONTINUITY AND RELATIVE SIMILARITY FOR THE NEXT NORTHWEST PACIFIC
SYSTEM AND ASSOCIATED FRONT COMING INTO THE PICTURE BY DAYS 7-8
WED-THU.  OPERATIONAL MODELS/INDIVIDUAL ENSEMBLE MEMBERS SHOW A
GENERAL AREA OF LOW PRESSURE BUT WITH LITTLE CLUSTERING FOR
EVOLUTION OF INDIVIDUAL CENTERS.

BASED ON DATA AVAILABLE THROUGH ARRIVAL OF THE 12Z GFS, A
COMPROMISE BETWEEN THE 00Z ECMWF AND 12Z GFS/00Z CMC PROVIDES THE
BEST BALANCE OF SYSTEM DETAIL AND ACCOUNT FOR ONGOING UNCERTAINTY
FOR DAYS 4-5 SUN-MON.  THEN AS SPECIFICS BECOME MORE UNCERTAIN THE
06Z GEFS/ECMWF MEANS PROVIDE ABOUT HALF OF THE INPUT FOR THE
FORECAST BLEND DAYS 6-7 TUE-WED, FOLLOWED BY 100 PERCENT FOR DAY 8
THU.

RAUSCH