Prognostic Meteorological Discussion
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FXAK02 KWNH 221906
PMDAK

ALASKA EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
306 PM EDT SUN OCT 22 2017

VALID 12Z THU OCT 26 2017 - 12Z MON OCT 30 2017

TYPHOON LAN WILL CONTINUE TO MOVE NORTHEASTWARD AND MERGE INTO A
NORTHERN STREAM UPPER LOW OVER KAMCHATKA IN THE SHORT RANGE. THAT
WILL BE MANIFEST AS THE LOW IN THE SOUTHERN BERING SEA ON THURSDAY
WITH THE TRIPLE POINT BECOMING THE NEW PARENT LOW IN THE GULF ON
FRIDAY. THE GFS/ECMWF CONTINUE TO WOBBLE AROUND THEIR ENSEMBLE
MEANS WITH THE DETAILS AND A BLEND OF THEM OFFER A GOOD STARTING
POINT FOR THE FIRST COUPLE OF DAYS OF THE FORECAST.

ONCE THE EX-LAN SYSTEM MOVES INTO CANADA, THE PATTERN BECOMES LESS
PREDICTABLE REGARDING SYSTEM DEVELOPMENT NEAR THE WESTERN
ALEUTIANS. THE ECMWF ENSEMBLES WERE MORE AGGRESSIVE THAN THE GEFS
MEMBERS LATE THIS WEEK INTO THE WEEKEND WHICH RESULTS IN ABOUT A
24-36 HR LAG IN THE TROUGH AXIS PAST 160W NEXT SUNDAY. TYPICALLY
THE ECMWF ENSEMBLES HANDLE AN AMPLIFIED FLOW PATTERN BETTER THAN
THE GEFS MEMBERS, AND GIVEN THAT A BLEND WOULD YIELD A FLATTER
FLOW THAN DESIRED, OPTED TO USE THE 00Z ECMWF ENSEMBLE MEAN AS THE
BASE FOR THE LAST FEW DAYS OF THE FORECAST, WHICH RESULTED IN A
SLOWER PROGRESSION OF THE SYSTEM (WHICH HAS BEEN THE TREND IN
RECENT DAYS FOR MOST OF THE SYSTEMS). THE 12Z GFS LIED IN BETWEEN
THE QUICKER 06Z GEFS MEAN AND THE 00Z ECMWF ENSEMBLE MEAN WHILE
THE 00Z ECMWF WAS SLOWER THAN ITS MEAN, AND NOT DESIRED AT THIS
POINT. THEREFORE, THE 00Z ECMWF ENSEMBLE MEAN WAS NICELY PLACED IN
BETWEEN THE 12Z GFS AND 00Z ECMWF.


FRACASSO


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