Prognostic Meteorological Discussion
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FXAK02 KWNH 231900
PMDAK

ALASKA EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
259 PM EDT SUN APR 23 2017

VALID 12Z THU APR 27 2017 - 12Z MON MAY 01 2017

THE MOST CONSISTENT AND AGREEABLE PART OF THE FORECAST CONTINUES
TO BE WITH THE MEAN RIDGE ALOFT EXTENDING THROUGH THE MAINLAND
FROM WRN CANADA FOR MOST IF NOT ALL OF THE PERIOD, ALONG WITH THE
WEAKENING ALEUTIANS SYSTEM DRIFTING TOWARD THE SWRN MAINLAND
DURING THE LATTER HALF OF THE WEEK.  OVER THE PAST COUPLE DAYS
GUIDANCE HAS ALSO ACHIEVED FAIRLY SIMILAR FCSTS FOR A SEPARATE
ARCTIC RIDGE THAT PROGRESSES FROM N OF ERN SIBERIA TO N OF THE
MAINLAND.  THIS RIDGE MAY MERGE WITH THE MAINLAND RIDGE TO SOME
DEGREE.  THE TENDENCY FOR MEAN RIDGING OVER THE MAINLAND LEADS TO
VERY LOW CONFIDENCE IN THE COMPACT UPR LOW WHICH 00Z/06Z GFS RUNS
BROUGHT TO THE FAR NWRN CORNER OF THE MAINLAND BY DAY 8 MON.  IF
SUCH A LOW WERE TO EXIST THE 12Z GFS ADJUSTMENT TO KEEP IT OVER
ERN SIBERIA SEEMS MORE REASONABLE.  AT THE SFC EXPECT HIGH
PRESSURE TO EXTEND FROM THE ARCTIC INTO NRN CANADA.

CONFIDENCE REMAINS IN THE MID-LOWER PART OF THE SPECTRUM FOR THE
SYSTEM FCST TO AFFECT THE ALEUTIANS/SRN BERING FROM LATE WEEK
ONWARD.  INITIAL DIFFS ORIGINATE FROM HOW A LOWER LATITUDE WRN PAC
WAVE IN THE LATE SHORT RANGE TIME FRAME IS INCORPORATED INTO THE
LARGER SCALE SYSTEM.  THEN IT APPEARS THE FCST FOR LOW PRESSURE IN
PARTICULAR IS VERY SENSITIVE TO THE RELATIVE INFLUENCE OF
PROGRESSIVE NRN PAC MEAN FLOW AND THE MORE STAGNANT/BLOCKY REGIME
IN PLACE OVER HIGHER LATITUDES.  OVERALL THERE IS SOMEWHAT BETTER
CONFIDENCE IN A FAIRLY PROGRESSIVE FRONT EXTENDING OUT FROM THE
PARENT LOW.  DURING RECENT DAYS THE ECMWF MEAN HAS BEEN THE MOST
STABLE OVERALL AND LIES BTWN THE FASTER 00Z CMC MEAN AND SLOWER
GEFS MEANS/GFS RUNS/00Z ECMWF-CMC AT SOME FCST TIMES.  HOWEVER THE
ECMWF MEAN HAS NUDGED A LITTLE SLOWER AFTER JUMPING AHEAD A BIT IN
YDAY`S 00Z/22 RUN SO A COMPROMISE BTWN THE ECMWF MEAN AND SLOWER
SOLNS MAY BE THE BEST OPTION AT THIS TIME.

MEANWHILE THERE IS STILL SPREAD FOR ONE OR MORE NERN PAC SFC LOWS
THAT MAY AFFECT THE PANHANDLE BUT SOME GENERAL THEMES ARE
EMERGING.  THERE IS RELATIVE AGREEMENT ON ONE SYSTEM NEARING THE
SRN PANHANDLE AT THE START OF THE PERIOD THU, WITH A 12Z GFS/00Z
ECMWF COMPROMISE OFFERING THE BEST ACCOUNT FOR CURRENT SPREAD.
THE NEW 12Z UKMET/CMC/GEFS MEAN HAVE COME IN CLOSER TO THE 12Z
GFS.  THE NEXT SYSTEM WITH A DECENT SIGNAL EMERGES FROM LOWER
LATITUDES EARLIER IN THE PERIOD AND APPROACHES THE SRN PANHANDLE
DURING THE WEEKEND.  THE 12Z GFS IS ON THE DEEP EDGE OF GUIDANCE
BUT COMPARABLE IN TRACK TO THE 00Z ECMWF WHILE THE 06Z GEFS/00Z
ECMWF MEANS DEFLECTED THE WAVE FARTHER WWD.  AT LEAST THROUGH 12Z
SAT THE NEW 12Z SOLNS SO FAR ARE WEAKER THAN THE 12Z GFS BUT WITH
SIMILAR TRACKS.

FCST PREFS WERE BEST REFLECTED BY A BLEND OF MODEL (00Z ECMWF/12Z
GFS) AND ENSEMBLE MEAN (00Z ECMWF MEAN; 00Z NAEFS THROUGH DAY 7
SUN THEN 06Z GEFS MEAN DAY 8 MON) GUIDANCE WITH MORE WEIGHT ON THE
MODELS EARLY AND ENSEMBLE MEANS LATE.  THE 00Z ECMWF HAD SOMEWHAT
HIGHER WEIGHT THAN THE 12Z GFS WITH A SMALL ECMWF COMPONENT
MAINTAINED THROUGH DAY 8.  THE ECMWF MEAN HAD HIGHER WEIGHT THAN
OTHER MEANS.  THE ENSEMBLE MEAN SWITCH LATE IN THE FCST WAS DUE TO
THE NAEFS BECOMING WASHED OUT FOR THE ALEUTIANS SYSTEM WHILE THE
GEFS MEAN CAUGHT UP TO THE ECMWF MEAN FOR TIMING.  WHILE THE NAEFS
WAS DEFINED FOR THE ALEUTIANS/BERING SYSTEM IT HAD GREATER
INFLUENCE FROM THE SLOWER GEFS VS THE FASTER CMC ENSEMBLES.

RAUSCH

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