Prognostic Meteorological Discussion
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FXAK02 KWNH 251844
PMDAK

ALASKA EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
244 PM EDT SUN SEP 25 2016

VALID 12Z THU SEP 29 2016 - 12Z MON OCT 03 2016

LATEST GUIDANCE CONTINUES TO SHOW A SHARP AND AMPLIFIED RIDGE
BUILDING OVER THE MAINLAND LATE THIS WEEK WHILE AN ELONGATED MEAN
TROUGH SHOULD EXTEND SEWD ACROSS THE BERING SEA FROM AN UPR LOW
OVER/NEAR RUSSIA.  THE PRIMARY FCST UNCERTAINTIES INVOLVE THE
EVOLUTION OF INITIAL WRN PACIFIC MID LVL ENERGY/ASSOC SFC
REFLECTION CROSSING THE NRN PAC/ALEUTIANS/BERING SEA... AND
WHETHER THE MAINLAND RIDGE CLOSES OFF AN UPR HIGH AFTER EARLY DAY
6 SAT.

LATEST OPERATIONAL RUNS ARE SHOWING STRONGER SIGNALS TOWARD A WELL
DEFINED SFC SYSTEM TRACKING FROM THE NRN PAC ACROSS THE ALEUTIANS
AND INTO THE SERN BERING SEA FROM THU ONWARD.  SOME ENSEMBLE MEAN
RUNS HAVE BEEN SHOWING SUBTLE HINTS OF SUCH A WAVE BUT INDIVIDUAL
MEMBERS ARE STILL SUFFICIENTLY SPREAD OUT THAT THE RESULTING MEANS
SUGGEST A MERE WAVY FRONT AT MOST FCST HRS.  00Z-06Z MODELS ARE
EVENLY SPLIT BTWN THE GFS RUNS/CMC AND FASTER ECMWF/UKMET.
INCREASED CONFIDENCE IN EXISTENCE OF THIS WAVE FAVORS ADJUSTING
CONTINUITY MORE TOWARD A COMPROMISE OF LATEST OPERATIONAL SOLNS TO
PROVIDE REASONABLE DETAIL BUT ACCOUNTING FOR THE TIMING
UNCERTAINTY.  12Z GUIDANCE THUS FAR GENERALLY KEEPS CONFIDENCE
AROUND THE SAME LEVEL.  THE NEW GFS IS SLOWER THAN OTHER SOLNS
BEFORE 12Z SAT WHILE THE UKMET REMAINS FASTER.  THE GEFS MEAN
SHOWS A BETTER DEFINED SFC LOW THAN PREVIOUS RUNS.

BY THE LATTER HALF OF THE PERIOD GFS/ECMWF RUNS ALONG WITH THE CMC
SHOW THE MAINLAND PART OF THE RIDGE CLOSING OFF AN UPR HIGH WHICH
TRACKS NEWD BY DAYS 7-8 SUN-MON... OR TO THE E THEN N OF THE
MAINLAND AS IN THE CASE OF THE 12Z GFS.  AT THE VERY LEAST BY THE
VALID TIME 12Z SAT THE NEW UKMET/CMC/GEFS ALL LEAN AWAY FROM THE
EWD EXTENSION OF THE RIDGE FCST BY THE 12Z GFS.  TYPICAL GUIDANCE
ERROR AT THE TIME FRAME INVOLVED PROVIDES SOME RESERVATIONS ON
FOLLOWING SOME VARIATION OF THE OPERATIONAL MODEL EVOLUTION...
THOUGH IT IS PLAUSIBLE GIVEN THE SHARPNESS AND AMPLITUDE OF THE
RIDGE LEADING INTO NEXT SAT.  WOULD LIKE TO LEAN MORE TOWARD THE
ENSEMBLE MEANS THAT KEEP THE RIDGE MORE OPEN FOR THE TIME BEING
WHILE AWAITING OPERATIONAL MODEL CONTINUITY TO INCREASE CONFIDENCE
IN THE CLOSED HIGH SCENARIO.

ALSO BY LATE IN THE PERIOD MOST MODELS AND ENSEMBLE MEANS SUGGEST
A FEATURE SHOULD TRACK INTO THE WRN-CNTRL ALEUTIANS/BERING SEA...
BUT WITH A LOT OF SPREAD FOR THE DETAILS.  THUS A FAIRLY DIFFUSE
DEPICTION ANCHORED BY WRN BERING SEA LOW PRESSURE... AS DEPICTED
BY THE MEANS... SEEMS MOST REASONABLE AT THIS TIME.

TO REFLECT THE DESIRED LEVEL OF DETAIL EARLY IN THE FCST... DAY 4
THU STARTS WITH A 00Z-06Z GFS/00Z ECMWF COMPROMISE.  GRADUALLY
MORE 06Z GEFS/00Z ECMWF ENSEMBLE MEAN GUIDANCE IS ADDED WITH TIME
SO THAT BY DAY 8 MON THE FCST CONSISTS ENTIRELY OF THE MEANS WITH
A LITTLE 00Z NAEFS INCLUDED AS WELL.

RAUSCH

$$




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