Prognostic Meteorological Discussion
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FXAK02 KWNH 221949
PMDAK

ALASKA EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
248 PM EST WED NOV 22 2017

VALID 12Z SUN NOV 26 2017 - 12Z THU NOV 30 2017

MODEL AGREEMENT HAS IMPROVED A BIT ACROSS ALASKA COMPARED TO THIS
TIME YESTERDAY. IMPROVED ENSEMBLE CLUSTERING WAS NOTED THROUGHOUT
THE MEDIUM RANGE, BUT PARTICULARLY LATER IN THE PERIOD WHERE
LITTLE TO NO CLUSTERING/CONSENSUS WAS NOTED PREVIOUSLY. THE
BREAKDOWN OF CENTRAL/NORTH PACIFIC REX BLOCK WILL ALLOW FOR A
HIGHLY PROGRESSIVE FLOW PATTERN TO SET UP. SOMEWHAT DECREASED
UNCERTAINTY SURROUNDING THE DETAILS OF THE LARGE SCALE PATTERN
EVOLUTION HAS RESULTED IN LESS CHAOS BEING INTRODUCED INTO THE
MODEL GUIDANCE AND SOMEWHAT LESS SPREAD SURROUNDING THE
SMALLER-SCALE DETAILS (ALTHOUGH PLENTY OF SPREAD STILL REMAINS).
GUIDANCE IS NOW IN GENERAL AGREEMENT THAT A DEEP CYCLONE WILL BE
IN PLACE ACROSS THE BERING SEA NORTH OF THE ALEUTIANS DAYS 4-5
(SUN-MON). THE 00Z ECMWF/GFS SEEMED REASONABLY WELL CLUSTERED
AROUND THE ENSEMBLE MEANS, SO A MAJORITY DETERMINISTIC BLEND OF
THESE TWO SOLUTIONS (MORE WEIGHTED TOWARD THE ECMWF) SERVED AS A
GOOD STARTING POINT FOR THE FORECAST DURING DAYS 4-5. SOME
UNCERTAINTY REMAINS SURROUNDING WHETHER A TRIPLE POINT LOW WILL
DEVELOP ACROSS THE GULF OF AK BY LATE DAY 5 INTO DAY 6 (TUE), BUT
GUIDANCE SEEMS TO BE TRENDING IN THAT DIRECTION, AND THE 00Z
ECMWF/GFS SHOWED VERY SIMILAR SOLUTIONS IN THAT RESPECT, WITH THE
NEW LOW MOVING INTO SOUTHEAST AK ON DAY 6. AS SHORTWAVES EJECT
FROM A PERSISTENT AREA OF NEGATIVE HEIGHT ANOMALIES ACROSS EASTERN
RUSSIA IN RAPID SUCCESSION, ANOTHER CYCLONE WILL QUICKLY MOVE
ACROSS/NEAR THE ALEUTIANS BY DAY 6. SPREAD IS A BIT HIGHER WITH
THIS SYSTEM, BUT MODEL/ENSEMBLE CONSENSUS HAS IMPROVED AND
ENSEMBLE SCATTER PLOTS SHOW INCREASED CLUSTERING AROUND THE 00Z
ECMWF/GFS. THIS SYSTEM SHOULD MOVE TOWARD SOUTHERN MAINLAND ALASKA
BY DAYS 7-8 (WED-THU), AND THE 00Z ECMWF/GFS REMAIN WELL CENTERED
WITHIN THE ENSEMBLE SPREAD. THUS, WHILE A GRADUAL TREND TOWARD
HEAVIER ENSEMBLE (00Z ECENS/NAEFS) MEAN WEIGHTING WAS SHOWN FROM
DAY 6 ONWARD, MINORITY COMPONENTS OF THE 00Z ECMWF/GFS WERE USABLE
THROUGH DAY 8.

IN TERMS OF SENSIBLE WEATHER, THIS PATTERN WILL RESULT IN PERIODS
OF INCREASED SNOW LIKELIHOOD FOR SOUTHERN/SOUTHEASTERN AK, WITH
DRIER CONDITIONS PREVALENT NORTH OF THE ALASKA RANGE. TEMPERATURES
WILL START OUT THE PERIOD BELOW AVERAGE ESPECIALLY ACROSS THE
INTERIOR, BUT WILL GRADUALLY MODERATE A BIT THROUGH NEXT WEEK TO
NEAR OR EVEN SLIGHTLY ABOVE NORMAL VALUES.

RYAN

$$





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