Prognostic Meteorological Discussion
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FXAK02 KWNH 172015
PMDAK

ALASKA EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
414 PM EDT TUE OCT 17 2017

VALID 12Z SAT OCT 21 2017 - 12Z WED OCT 25 2017

...TROPICAL CYCLONE LAN OVER THE WESTERN PACIFIC COULD BRING AN
EXTRA LAYER OF COMPLEXITY AND UNCERTAINTY TO THE LARGE-SCALE
PATTERN OVER ALASKA BY THE MIDDLE OF NEXT WEEK...

TWO OCCLUDED CYCLONES WILL APPEAR TO IMPACT ALASKAN WEATHER DURING
THE MEDIUM-RANGE PERIOD.  THE FIRST CYCLONE SHOULD APPROACH THE
SOUTH COAST OF ALASKA AND FARTHER SOUTH ALONG THE PANHANDLE DURING
THE WEEKEND AS AN UPPER-LEVEL VORTEX FROM THE ARCTIC OCEAN DROPS
SOUTH ALONG THE WEST COAST OF MAINLAND ALASKA.  THE CYCLONE SHOULD
QUICKLY DISSIPATE ON MONDAY OVER THE HIGH TERRAIN OF THE SOUTHERN
PERIPHERY OF ALASKA AS THE NEXT BAROCLINIC TROUGH SWINGS IN FROM
THE SOUTH TOWARD THE ALASKAN PENINSULA ON TUESDAY BEFORE
DISSIPATING ON WEDNESDAY DAY 8.

THE NEXT CYCLONE OF INTEREST COMING FROM SIBERIA SHOULD MOVE
ACROSS KAMCHATKA LATE SUNDAY/EARLY MONDAY BEFORE MOVING ACROSS THE
ALEUTIAN ISLAND CHAIN THROUGH WEDNESDAY WITH THE CENTER OF THE
CYCLONE FORECAST TO SKIRT SOUTH OF THE ISLANDS.

THE WPC WEATHER GRIDS WERE BASED ON A COMBINATION OF THE 00Z ECMWF
AND THE 12Z GFS TOGETHER WITH THEIR CORRESPONDING ENSEMBLES.  MORE
ENSEMBLE MEANS WERE USED FOR DAYS 6 THROUGH 8.  THIS PARTICULAR
COMBINATION WAS BASED ON GOOD AGREEMENT REGARDING THE UPSTREAM
DEVELOPMENT AND SUBSEQUENT TRACK OF TYPHOON LAN...WHICH IS
CURRENTLY LOCATED EAST OF THE PHILIPPINES AND CAN POTENTIALLY
IMPOSE A SIGNIFICANT INFLUENCE ON THE LARGE-SCALE PATTERN NEAR
ALASKA BY THE MIDDLE OF NEXT WEEK.  THE 00Z CANADIAN AND 12Z UKMET
WERE BOTH MUCH FASTER THAN THE 00Z/06Z/12Z GFS RUNS AS WELL AS THE
00Z ECMWF REGARDING THE FORWARD MOTION OF LAN...AND THUS...WERE
TAKEN OUT OF CONSIDERATION.  THE 00Z ECMWF WAS THE SLOWEST AMONG
THE GUIDANCE.  ALTHOUGH THE FASTER 06Z GFS APPEARED TO BE A GOOD
MIDDLE GROUND...THE 12Z GFS SLOWED DOWN LAN AND RESULTED IN A
BETTER AGREEMENT WITH THE 00Z ECMWF THROUGH DAY 4.  BY DAY 5...THE
12Z GFS BEGAN TO ACCELERATE LAN TOWARD JAPAN FASTER THAN THE 00Z
ECMWF.  IT APPEARED THAT THE FASTER 12Z WAS MORE REASONABLE.  THE
NEW 12Z ECMWF SUPPORTED THIS FASTER SCENARIO ACROSS JAPAN IN DAY
5...BRINGING THE EXTRATROPICAL CIRCULATION OF LAN TOWARD THE
OCCLUDED CYCLONE NEAR THE ALEUTIANS BY DAY 8.

KONG

$$




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