Prognostic Meteorological Discussion
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FXAK02 KWNH 241859
PMDAK

ALASKA EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
258 PM EDT MON JUL 24 2017

VALID 12Z FRI JUL 28 2017 - 12Z TUE AUG 01 2017

A BROAD UPPER LOW WILL REMAIN DOMINANT OVER THE GULF OF ALASKA
THROUGH THE MEDIUM RANGE, WITH A PERSISTENT UPPER RIDGE ANCHORED
OVER THE NORTH PAC. MUCH OF AK WILL REMAIN IN RELATIVELY
PROGRESSIVE WESTERLIES WITH A SERIES OF SHORTWAVES OF VARIOUS
AMPLITUDES MOVING EAST FROM NERN RUSSIA ACROSS THE BERING STRAIT
INTO AK BEFORE DIVING SOUTHEAST AND REINFORCING THE UPPER LOW
ACROSS THE GULF. MODELS/ENSEMBLES GENERALLY SHOW GOOD AGREEMENT ON
THIS LARGE SCALE PATTERN PERSISTING THROUGH THE MEDIUM RANGE. THE
FIRST SIGNIFICANT TROUGH DURING THE FCST PERIOD SHOULD REACH
WESTERN AK ON FRI. GFS/ECMWF/GEM SHOW GENERALLY GOOD AGREEMENT ON
THE TIMING OF THIS WAVE. THE GFS/GEM SHOW THIS WAVE LINGERING
ACROSS S-CNTRL AK THROUGH SUN AS IT GETS CAUGHT IN A ZONE OF FLOW
DEFORMATION, WHILE THE ECMWF NOW TAKES IT SOUTHWARD AND MERGES IT
INTO THE LARGE SCALE CIRCULATION OVER THE GULF. THE NEXT SHORTWAVE
IS SHOWN BY THE GFS/ECMWF CROSSING THE BERING STRAIT ON DAY 7
(MON). ADDITIONALLY, THE GFS/ECMWF/GEM ALL SHOW A BLOCKING
MID/UPPER LEVEL RIDGE EXPANDING ACROSS EASTERN/CENTRAL AK FROM
NWRN CANADA BY DAYS 6-8 (SAT-TUE). THE GEM IS SUBSTANTIALLY
STRONGER THAN THE GFS/ECMWF WITH THIS RIDGE, WHILE THE ECMWF IS
THE WEAKEST. THE WPC FORECAST WAS BASED HEAVILY ON THE
DETERMINISTIC GFS/ECMWF EARLY IN THE MEDIUM RANGE, WITH A GRADUAL
TRANSITION TOWARD HEAVIER ENSEMBLE WEIGHTING (ECMWF ENS, GEFS,
NAEFS) IN THE LATTER HALF OF THE FORECAST PERIOD.

GIVEN THE ACTIVE FLOW PATTERN THROUGH MUCH OF THE PERIOD, MOST OF
THE MEDIUM RANGE SHOULD REMAIN RATHER UNSETTLED ACROSS MUCH OF THE
STATE, WITH NUMEROUS UPPER DISTURBANCES RESULTING IN FREQUENT
PRECIP. MEANWHILE, THE UPPER LOW IN THE GULF WILL KEEP MOIST
SOUTHEASTERLY FLOW IN PLACE ACROSS SE AK, KEEPING OCCASIONAL
SHOWERS AND PREVALENT. BY EARLY NEXT WEEK, DEPENDING ON THE
STRENGTH OF THE DEVELOPING RIDGE FROM EASTERN AK INTO YUKON
TERRITORY, DRIER CONDITIONS COULD DEVELOP ACROSS MUCH OF EAST
CENTRAL AK.


RYAN

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