Prognostic Meteorological Discussion
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FXAK02 KWNH 201911
PMDAK

ALASKA EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
310 PM EDT FRI OCT 20 2017

VALID 12Z TUE OCT 24 2017 - 12Z SAT OCT 28 2017

DYNAMIC FORECAST FOR THE HIGH LATITUDES AS THE EXTRATROPICAL
TRANSITION OF TYPHOON LAN WILL DRIVE THE PATTERN EVOLUTION. THE
MODELS SEEM TO HAVE A LIMIT OF PREDICTABILITY OF ABOUT 4-5 DAYS IN
THIS PATTERN AND HAVE MOSTLY CONVERGED ON HOW LAN TRANSITIONS INTO
THE MID-LATITUDE FLOW AND BECOMES ABSORBED BY A NORTHERN STREAM
SYSTEM ROLLING OFF KAMCHATKA NEXT MONDAY. IT SHOULD THEN CROSS
THROUGH THE SOUTHERN BERING SEA JUST NORTH OF THE ALEUTIANS AND
HEAD TOWARD SW OR W ALASKA BY LATE WED INTO THU. THIS IS WHEN THE
MODELS/ENSEMBLES DIVERGE MOST NOTABLY. THROUGH THAT POINT, A BLEND
OF THE 12Z GFS WITH THE 00Z ECMWF AND THEIR ENSEMBLE MEANS OFFERED
A GOOD STARTING POINT. AFTER THAT, RELIED ON THE 06Z GEFS MEAN AND
THE 00Z ECMWF ENSEMBLE MEAN TO ROUND OUT THE FORECAST. MIN PMSL
WITH THE SFC LOW SEEMS TO BE IN THE 940S MB PER THE DETERMINISTIC
GUIDANCE WITH WINDS LIKELY IN EXCESS OF 50KTS IN ADVANCE OF THE
OCCLUSION AND AROUND THE SW SIDE OF THE SFC LOW (ECMWF SHOWS A
LARGE AREA OF GUSTS OVER 60KTS). PRECIPITATION WILL BECOME
WIDESPREAD ACROSS THE ALEUTIANS AND THEN INTO SW ALASKA, BUT HOW
FAR NORTH ALONG THE WEST COAST REMAINS A QUESTION AS THE PARENT
LOW WEAKENS AND THE TRIPLE POINT TAKES OVER AS IT HEADS INTO THE
GULF AROUND NEXT FRIDAY. FAVORED COASTAL AREAS FROM THE SE KENAI
TO YAKUTAT WILL LIKELY SEE SEVERAL INCHES OF RAIN (LOCAL MAXIMA
OVER TEN INCHES). NEXT SYSTEM, MUCH WEAKER, WILL PUSH THROUGH THE
BERING LATER NEXT WEEK INTO NEXT WEEKEND.


FRACASSO


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