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Alaska Extended Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 1910Z Dec 09, 2017)
 
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ALASKA EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
210 PM EST SAT DEC 09 2017

VALID 12Z WED DEC 13 2017 - 12Z SUN DEC 17 2017

MODELS/ENSEMBLES SHOW A GENERAL CONSENSUS THAT PERSISTENT UPPER
RIDGE ACROSS WESTERN CANADA AND INTO ALASKA SHOULD BEGIN TO WEAKEN
SOME BY LATE NEXT WEEK, ALTHOUGH DIFFERENCES REMAIN AS TO HOW
QUICKLY AND TO WHAT DEGREE THIS OCCURS. AT LEAST DURING THE EARLY
PORTION OF THE MEDIUM RANGE, MODELS SHOW AVERAGE AGREEMENT WITH
RESPECT TO A SURFACE LOW INITIALLY CROSSING THE GULF OF AK ON DAY
4 (WED) THAT THEN MOVES QUICKLY NORTH INTO SOUTHERN MAINLAND AK ON
DAY 5 (THU) BEFORE WEAKENING. WHILE SOME SPREAD REMAINS, THE ECMWF
AND GFS SEEMED REASONABLY WELL CLUSTERED WITHIN CONSENSUS, AND A
HEAVILY DETERMINISTIC BLEND INCLUDING THESE TWO SOLUTIONS SERVED
AS A FORECAST STARTING POINT DURING DAYS 4-5.

FROM DAY 6 (FRI) ONWARD, SPREAD BEGINS TO INCREASE WITH RESPECT TO
THE NEXT SYSTEM CROSSING THE ALEUTIANS. MODELS SHOW QUITE A BIT OF
RUN-TO-RUN VARIABILITY ESPECIALLY BY DAYS 7-8 (SAT-SUN)
SURROUNDING WHETHER THE TRIPLE POINT LOW TAKES OVER AS THE PRIMARY
LOW, TAKING A MORE EASTERLY TRACK AS SHOWN BY THE ECMWF, OR WRAPS
NORTH INTO THE BERING SEA AS THE ECENS/GEFS MEANS SUGGEST. GIVEN
THE UNCERTAINTY, THE FORECAST WAS TRENDED TOWARD SIGNIFICANTLY
HEAVIER ECENS/GEFS WEIGHTING BY LATE NEXT WEEK. ADDITIONALLY,
ENSEMBLE MEANS ARE ALL A BIT SLOWER TO WEAKEN THE UPPER RIDGE
RELATIVE TO THE DETERMINISTIC GUIDANCE, SO MORE ENSEMBLE WEIGHTING
IN THE FORECAST WOULD SUPPORT A SLIGHTLY STRONGER/MORE PERSISTENT
RIDGE THAN INDICATED BY MOST DETERMINISTIC SOLUTIONS.

IN TERMS OF SENSIBLE WEATHER, THE PERSISTENT ONSHORE FLOW AND
RAPID SUCCESSION OF SHORTWAVES AND SURFACE CYCLONES WILL RESULT IN
A CONTINUED STORMY PERIOD FOR SOUTHERN AND SOUTHEASTERN AK, WITH
HEAVY PRECIPITATION POSSIBLE ESPECIALLY IN FAVORED WINDWARD
TERRAIN AREAS. FARTHER NORTH, SOMEWHAT DRIER CONDITIONS WILL
PREVAIL NORTH OF THE ALASKA RANGE, BUT STILL EXPECT SCATTERED SNOW
SHOWERS THROUGH MUCH OF THE PERIOD AS ROUNDS OF SHORTWAVE ENERGY
PASS OVERHEAD. GRADUAL WEAKENING OF UPPER RIDGING BY LATE NEXT
WEEK SHOULD RESULT IN DECREASING TEMPERATURES FROM ABOVE NORMAL
ACROSS MUCH OF THE STATE SOUTH OF THE BROOKS RANGE, TO CLOSER TO
CLIMATOLOGICAL NORMS AS ARCTIC AIR IS ABLE TO SPREAD FARTHER SOUTH
ACROSS MUCH OF THE STATE.

RYAN