Prognostic Meteorological Discussion
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FXAK02 KWNH 192058
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ALASKA EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
457 PM EDT THU OCT 19 2017

VALID 12Z MON OCT 23 2017 - 12Z FRI OCT 27 2017

FORECAST CONTINUES TO EVOLVE OVER ALASKA IN THE MEDIUM RANGE, BUT
WITH LITTLE AGREEMENT ON SPECIFICS. THE DETAILS ARE DEPENDENT ON
AT LEAST TWO PLAYERS: 1) EXTRATROPICAL TRANSITION OF TYPHOON LAN
IN THE WESTERN PACIFIC AND 2) EVOLUTION OF A TROUGH ALONG 160W
SOUTH OF THE ALEUTIANS EARLY IN THE PERIOD. NO MODEL HAS BEEN
CONSISTENT IN THE PAST 24-48 HRS BUT AT LEAST THE ENSEMBLE MEANS
(06Z GEFS MEAN AND 00Z ECMWF EPS) WERE CLOSE ENOUGH TO EACH OTHER
TO BE CONSIDERED USABLE.

REGARDING TYPHOON LAN, THE MODELS HAVE AT LEAST CONVERGED ON A
CLOSER TRACK NEAR THE SE COAST OF JAPAN AND THEN A NORTHEAST TURN
RATHER QUICKLY, BUT ALL WERE FASTER THAN THE JTWC TRACK.
REGARDLESS, THIS ALLOWS THE SYSTEM TO BECOME EMBEDDED/WRAPPED UP
INTO A NORTHERN STREAM UPPER LOW OVER KAMCHATKA LATE MONDAY AND
POTENTIALLY DEEPEN INTO A STRONG EXTRATROPICAL CYCLONE OVER THE
BERING. EVENTUAL TRACK HAS SHIFTED NORTH FROM YESTERDAY WHEN THE
TIMING WAS FOR A SLOWER EX-LAN. GFS/ECMWF RUNS WERE MUCH FARTHER
NORTH BY LATER IN THE WEEK INTO SW OR W ALASKA WHILE THE ENSEMBLES
WERE CONCENTRATED ALONG THE ALEUTIANS (STILL FARTHER NORTH THAN 24
HRS AGO). RELIED ON THE ENSEMBLE MEANS TO MINIMIZE DAY-TO-DAY
CHANGES. SYSTEM HAS THE POTENTIAL TO BE QUITE DEEP -- MIN PMSL
AMONG THE GEFS/ECMWF/CANADIAN MEMBERS WAS IN THE HIGH 920S OR LOW
930S MB.

FARTHER EAST, ECMWF HAS BEEN MUCH LESS CONSISTENT THAN USUAL WITH
THE DIGGING TROUGH, NOW FORECAST TO LEAVE BEHIND AN UPPER LOW NEAR
HAWAI`I. GFS RUNS HAVE VARIED BUT HAVE MOSTLY FORECAST A CLOSED
LOW THAT REMAINS MOSTLY PROGRESSIVE, SUPPORTING A SFC LOW ALONG A
FRONT THAT LIFTS INTO THE NORTHEAST GULF LATE MONDAY. ECMWF,
HOWEVER, ALLOWED PART OF THE TROUGH TO LIFT NORTHEASTWARD LIKE THE
GFS AND CARRIED A WEAKER/QUICKER SYSTEM INTO THE PANHANDLE. SPLIT
THE DIFFERENCE BETWEEN THE 12Z GFS AND 00Z ECMWF WITH THEIR
ENSEMBLE MEANS TO MAINTAIN DECENT CONTINUITY (ALBEIT A BIT
QUICKER).


FRACASSO


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