Prognostic Meteorological Discussion
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FXAK02 KWNH 121902
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ALASKA EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
201 PM EST TUE DEC 12 2017

VALID 12Z SAT DEC 16 2017 - 12Z WED DEC 20 2017

...STRONG SYSTEM IN THE BERING SEA LIKELY NEXT WEEK...


LATEST GFS/ECMWF AND THEIR ENSEMBLE MEANS SHOWED GOOD CLUSTERING
THROUGH NEXT WEEK WITH EXPECTED DETAIL DIFFERENCES. THE PACIFIC
UPPER PATTERN IS STILL FORECAST TO TRANSITION TO A STRONG W PAC
JET (BETWEEN AN UPPER LOW OVER THE SEA OF OKHOTSK AND AN UPPER
HIGH NORTHEAST OF GUAM) WITH SPLIT FLOW EAST OF THE DATELINE. THE
NORTHERN EXTENT OF THIS WILL LEAD TO A ROBUST BERING SEA SYSTEM
LIFTING NORTHEASTWARD OR NORTH-NORTHEASTWARD AS UPPER RIDGING
BUILDS BACK INTO THE INTERIOR OF ALASKA (AROUND 150W).

A LEAD SYSTEM WILL PUSH THROUGH THE GULF/PANHANDLE SATURDAY THOUGH
THE MODELS HAVE YET TO PIN DOWN HOW IT TRANSLATES EASTWARD. THE
12Z GFS WAS MUCH STRONGER THAN ITS 06Z RUN OR THE 00Z ECMWF WITH A
TRAILING SFC WAVE BEHIND THE LEAD LOW BUT GIVEN THE RATHER STRONG
SIGNAL IN THE 00Z UKMET/CANADIAN OPTED TO INCLUDE IT AS PART OF
THE BLEND (20%). THEREAFTER, THE GFS/ECMWF CONTINUE TO SHOW A
STRONG SYSTEM IN THE WESTERN BERING SEA NEXT WEEK, LIKELY TRACKING
TOWARD THE BERING STRAIT (PERHAPS JUST TO THE WEST) WHICH WOULD
BRING STRONG WINDS (ECMWF SHOWED 50KTS SUSTAINED AND GUSTS OVER
70KTS) TO THE UNFROZEN OPEN BERING AND MODEST TO LOCALLY HEAVY
RAIN/SNOW TO THE ALEUTIANS AND WESTERN ALASKA. ENSEMBLE MEANS
LAGGED BEHIND THE GFS/ECMWF WITH THE SPEED OF THE PARENT LOW BUT
FELT IT SHOULD BE PROGRESSIVE ENOUGH TO KEEP THE PARENT LOW MOVING
TOWARD 70N WHILE SUBSEQUENT SYSTEMS MAY THEN ROTATE AROUND ITS
BROAD CIRCULATION. FRONTAL BOUNDARY MAY GET STUCK OVER WESTERN
ALASKA AS UPPER HEIGHTS BUILD RATHER QUICKLY INTO THE GULF AND
INTERIOR.


FRACASSO


$$





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