Prognostic Meteorological Discussion
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FXAK02 KWNH 241843
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ALASKA EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
242 PM EDT SUN SEP 24 2017

VALID 12Z THU SEP 28 2017 - 12Z MON OCT 02 2017

ENSEMBLE MEANS CONTINUE TO BE IN VERY GOOD AGREEMENT WITH THE
LONGWAVE PATTERN OVER ALASKA DURING THE LATER HALF OF THE WEEK,
WHICH SHOULD FEATURE AN ANOMALOUS UPPER LOW LIFTING NORTHEASTWARD
THROUGH THE GULF OF ALASKA WHILE UPSTREAM RIDGING PROGRESSES
THROUGH THE BERING SEA.  THE MEANS MAINTAIN AGREEMENT EVEN INTO
NEXT WEEKEND AND EARLY NEXT WEEK WITH ADVERTISING RIDGING IN
BERING SEA SHOULD SUPPRESS AND SHIFT EASTWARD AS A LARGE SCALE
UPPER TROUGH APPROACHES FROM THE WEST.

THE WPC FORECAST STARTED WITH A BLEND OF THE 00Z
GFS/ECMWF/ENSEMBLE MEANS, WHICH ACCOUNTED FOR THE REASONABLE
AGREEMENT AMONG DETERMINISTIC GUIDANCE EARLY IN THE PERIOD WHILE
ALSO SMOOTHING OUT SOME OF THE LINGERING DETAIL DIFFERENCES WITH A
STRONG SURFACE LOW AND COLD FRONT LIFTING THROUGH THE GULF AND
INTO THE SOUTHERN COAST ON THURSDAY.  BY DAY 6(SATURDAY), ENSEMBLE
SPREAD AND UNCERTAINTY QUICKLY INCREASES WITH THE EVOLUTION OF THE
UPPER LOW IN THE GULF AS WELL AS WITH THE TIMING OF THE LEADING
EDGE OF HEIGHT FALLS FROM THE LARGE SCALE UPPER TROUGH EDGING INTO
THE BERING SEA, SO THE WPC FORECAST TRANSITIONED TO PREDOMINANTLY
ENSEMBLE MEANS (00Z GEFS/NAEFS/EC MEAN) FOR THE DAY
6-8(SATURDAY-MONDAY) FORECAST.

IN TERMS OF SENSIBLE WEATHER OVER ALASKA DURING THE DAY 4-8
PERIOD, MODELS CONTINUE TO SHOW A STRONG SIGNAL THAT A PERSISTENT
STREAM OF MOISTURE AHEAD OF THE DEEP SURFACE LOW AND COLD FRONT
LIFTING THROUGH THE GULF WILL BRING HEAVY PRECIPITATION TO THE
PANHANDLE DURING THE MIDDLE OF THE WEEK, WITH THE WET CONDITIONS
EXPECTED TO SPREAD INTO THE SOUTHCENTRAL PORTION OF THE STATE.
ALSO, STRONG WINDS AND AN INCREASED CHANCE OF PRECIPITATION WILL
BE POSSIBLE WITH HEIGHT FALLS EDGING INTO THE BERING SEA AND
WESTERN/CENTRAL ALEUTIANS NEXT WEEKEND.


GERHARDT


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