Prognostic Meteorological Discussion
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FXAK02 KWNH 191945
PMDAK

ALASKA EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
345 PM EDT SAT AUG 19 2017

VALID 12Z WED AUG 23 2017 - 12Z SUN AUG 27 2017

...GUIDANCE ASSESSMENT AND PREDICTABILITY...

WPC ALASKAN MEDIUM RANGE SURFACE FRONTS/PRESSURES...500 MB
PROGS...AND SENSIBLE WEATHER GRIDS WERE MAINLY DERIVED FROM A
COMPOSITE BLEND OF THE 12 UTC GFS/00 UTC ECMWF AND 06 UTC GEFS/00
UTC ECMWF ENSEMBLE MEANS. DAY4/WED USED HIGH DETERMINISTIC MODEL
WEIGHTING. THIS WAS DESIGNED TO PROVIDE A MORE DETAILED TRANSITION
FROM SHORT TO MEDIUM RANGE TIME SCALES EVEN AS THOSE GFS/ECMWF
RUNS OFFER DIFFERENCES...ESPECIALLY OVER THE ARCTIC. RAPIDLY
INCREASED USAGE OF MORE COMPATABLE ENSEMBLE MEANS THROUGH THE
REMAINDER OF THE WEEK AMID GROWING GUIDANCE SPREAD. THE NEW 12 UTC
ECMWF SEEMS TO GOOD MATCH WITH THIS WPC BLEND. THIS BOLSTERS
FORECAST CONFIDENCE TO SOME DEGREE GIVEN FAVORBALE TRENDING.

...FORECAST PATTERN AND WEATHER HIGHLIGHTS...

A MIDWEEK SRN ARCTIC CLOSED LOW AND ASSOCIATED SURFACE FRONTAL
SYSTEM TRAILING INTO THE NORTH SLOPE AND NRN INTERIOR IS EXPECTED
TO LIFT TOWARD THE POLE THEREAFTER...SO EXPECT A PERIOD OF
UNSETTLED WEATHER WILL THEN DIMINISH UNDER SLOWLY BUILDING HEIGHTS
ALOFT.

WELL UNDERNEATH...A CLOSED LOW ALOFT AND SURFACE SYSTEM OVER THE
SERN GULF OF AK OFFER HIGHLY ANOMOLOUS LEAD DEEP MOISTURE AND
LINGERING HEAVY PRECIPITATION POTENTIAL FOR FAR SERN AK INTO
MIDWEEK...BUT WORKS INCREASING AWAY FROM THE STATE AND INLAND TO
BC THU.

UPSTREAM...AN AMPLIFIED SRN BERING SEA MID-UPPER LEVEL TROUGH AND
DEEPENED SURFACE LOW/FRONTAL SYSTEM WILL PRODUCE HIGHLY UNSETTLED
SEAS AND WEATHER MIDWEEK...WITH ACTIVITY WORKING INLAND ACROSS THE
ALEUTIANS AND SW AK. ENERGY TRANSFER SHOULD FORCE WELL ORGANIZED
LOW DEVELOPMENT SEWD INTO THE GULF OF AK BY FRI-NEXT
SUN...PROMPTING HEAVY RAINFALL POTENTIAL INTO SRN THEN SERN AK
LATER WEEK ALONG WITH UNSETTLED WEATHER/SEAS.

SCHICHTEL


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