Prognostic Meteorological Discussion
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FXAK02 KWNH 181904
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ALASKA EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
203 PM EST SAT NOV 18 2017

VALID 12Z WED NOV 22 2017 - 12Z SUN NOV 26 2017

TODAY`S GUIDANCE GENERALLY AGREES ON PROGRESSIVE FLOW DURING THE
PERIOD THOUGH WITH SOME CHANGE IN CHARACTER TOWARD NEXT WEEKEND.
SHORT RANGE PATTERN OF ENERGY APPROACHING MAINLAND ALASKA FROM THE
WEST AND THEN DROPPING INTO A NORTHEASTERN PACIFIC MEAN TROUGH
SHOULD CONTINUE INTO LATE WEEK.  THE HIGHLIGHTED SYSTEM WILL BE A
DEEP WESTERN-NORTHERN BERING SEA LOW THAT SHOULD TRACK SOUTHEAST
OVER/NEAR THE SOUTHWEST CORNER OF THE MAINLAND AND SETTLE INTO THE
NORTHEAST PACIFIC.  BEHIND THIS SYSTEM, MOST SOLUTIONS SIGNAL AN
EASTWARD BROADENING OF THE CYCLONIC FLOW ALOFT AHEAD OF EASTERN
ASIA MEAN TROUGHING BUT WITH A FAIR AMOUNT OF AMBIGUITY REGARDING
DETAILS OF PARTICULAR SYSTEMS.

REGARDING THE STRONG BERING STORM AS OF EARLY IN THE PERIOD, THE
00Z ECMWF IS THE ODD MODEL OUT RELATIVE TO OTHER SOLUTIONS AS IT
TAKES LONGER TO CLOSE OFF AN UPPER LOW, PERHAPS CONTRIBUTING TO
ITS SOMEWHAT FASTER TIMING FOR A PORTION OF THE FORECAST.  OTHER
GUIDANCE SHOWS SOME CONFLICTING TRENDS FOR TIMING.  GEFS/ECMWF
MEANS OVER THE PAST 1-2 DAYS HAVE BEEN DRIFTING TOWARD A
SLOWER/NORTHWARD SYSTEM BY DAY 5 THU.  HOWEVER 06Z/12Z GFS RUNS
HAVE ADJUSTED STEADILY FASTER AND THE POSSIBLY WEAK 00Z CMC IS
ALSO AHEAD OF THE SLOW SIDE OF THE ENVELOPE.  THESE CONSIDERATIONS
FAVOR AN OPERATIONAL COMPROMISE EMPHASIZING THE 06Z-12Z GFS/00Z
UKMET AND 12Z/17 ECMWF, WITH ONLY MINIMAL INCLUSION OF THE 00Z
ECMWF.  ECMWF RUNS CONVERGE ONCE THE SYSTEM REACHES THE GULF OF
ALASKA/NORTHEAST PACIFIC BY EARLY DAY 6 FRI, LEVELING OUT
PREFERENCES FOR THE ECMWF COMPONENT OF THE BLEND AT THAT TIME.

UPSTREAM PREFER THE PAST COUPLE ECMWF RUNS AND GEFS/ECMWF MEANS
FOR THE COLD FRONT CROSSING THE ALEUTIANS/BERING SEA BY DAY 7 SAT.
 SUCH A SOLUTION PROVIDES GOOD CONTINUITY FROM YESTERDAY WHILE
WAITING FOR BETTER AGREEMENT REGARDING POTENTIAL SURFACE
DEVELOPMENT ALONG THE FRONT.  THE 00Z CMC AND SOME GFS RUNS ALONG
WITH A SCATTERING OF ENSEMBLE MEMBERS SHOW THIS POTENTIAL WAVINESS
BUT ENSEMBLE MEMBERS ARE SUFFICIENTLY DIVERSE TO YIELD NO WELL
DEFINED WAVE THUS FAR.  IMMEDIATELY BEHIND THIS FRONT THERE ARE
ALSO SIGNALS THAT UPSTREAM ENERGY MAY SUPPORT SURFACE DEVELOPMENT.
 ENSEMBLE MEANS HAVE BEEN WAFFLING ON THE STRENGTH OF SUCH A
SYSTEM WHILE OPERATIONAL RUNS HAVE BEEN ERRATIC IN DEPICTING
EXISTENCE/TRACK.

ELSEWHERE, THE ECMWF/NAVGEM ARE THE ONLY SOLUTIONS THAT ARE AS
STRONG/CONCENTRATED WITH LOW PRESSURE OVER/NEAR THE CHUKCHI SEA
EARLY IN THE PERIOD.  MEANWHILE THE ENSEMBLE MEANS PROVIDE THE
BEST INTERMEDIATE SOLUTION FOR A NORTHEASTERN PACIFIC SYSTEM THAT
MAY TRACK SOMEWHERE BETWEEN THE SOUTHERN TIP OF THE PANHANDLE AND
HAIDA GWAII.

BASED ON THE ABOVE CONSIDERATIONS TODAY`S FORECAST STARTED WITH A
BLEND OF THE 06Z-12Z GFS, MORE 12Z/17 ECMWF THAN 00Z ECMWF, AND
00Z UKMET DAYS 4-5 WED-THU FOLLOWED BY INCREASING WEIGHT OF THE
06Z GEFS/00Z ECMWF MEANS SUCH THAT DAY 8 SUN INCORPORATED THE
MEANS EXCLUSIVELY.  THIS COMBINATION OF GUIDANCE PROVIDED BETTER
DEFINITION THAN THE MEANS FOR THE DEEP BERING SYSTEM, DOWNPLAYED
ECMWF STRENGTH OF THE CHUKCHI SEA LOW, AND KEPT AN EVOLUTION CLOSE
TO THE ENSEMBLE MEANS NEAR THE PANHANDLE.  UPSTREAM LATE IN THE
PERIOD THERE IS LOW CONFIDENCE IN SPECIFICS BUT THE ENSEMBLE MEAN
EMPHASIS PROVIDED BETTER CONTINUITY THAN OTHER PIECES OF GUIDANCE.

RAUSCH

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