Prognostic Meteorological Discussion
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FXAK02 KWNH 251847
PMDAK

ALASKA EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
246 PM EDT SUN JUN 25 2017

VALID 12Z THU JUN 29 2017 - 12Z MON JUL 03 2017

TODAY`S FCST SHOWS DECENT CONTINUITY/CONSENSUS FOR THE LARGE SCALE
PATTERN EVOLUTION WHILE REFINING NERN PACIFIC/GULF OF ALASKA
DETAILS TO REFLECT IMPROVING CLUSTERING OF GUIDANCE.

CONTINUE TO EXPECT HGT FALLS TO PROCEED INTO THE EXTREME NERN
PAC/GULF OF ALASKA WITH THE ARRIVAL OF LEADING ENERGY NEAR THE
START OF THE PERIOD AND THEN THE EWD PROGRESSION OF UPSTREAM
ENERGY FROM THE ALEUTIANS AND VICINITY.  BY NEXT SAT-MON THIS
SHOULD LEAD TO A CLOSED LOW ALOFT ALONG OR JUST S OF A PATH BTWN
KODIAK ISLAND AND THE SRN PANHANDLE.  MEANWHILE WRN CANADA MEAN
RIDGING SHOULD BUILD A RIDGE AXIS STEADILY MORE WWD ACROSS THE
MAINLAND.  SOME GUIDANCE HAS BEEN SHOWING POTENTIAL FOR AN UPR
HIGH TO CLOSE OFF OVER OR NEAR THE CNTRL-NERN MAINLAND/NRN YUKON
BY LATE IN THE PERIOD.  IN THIS LATER TIME FRAME RECENT GFS RUNS
HAVE GENERALLY BEEN A LITTLE CLOSER TO AN AVG OF THE ENSEMBLE
MEANS FOR THE UPR RIDGE/HIGH POSN THAN THE 00Z ECMWF THAT IS
FARTHER SW.

GUIDANCE SPREAD HAS NARROWED FOR WHAT HAS BEEN A DIFFICULT FCST
FOR ONE OR MORE CNTRL PAC INTO NERN PAC/GULF OF ALASKA WAVES, BUT
PLENTY OF UNCERTAINTY REMAINS.  CURRENT MAJORITY SCENARIO HAS A
LEADING SYSTEM THAT IS SOMEWHAT SLOWER THAN YDAY`S FAST GFS/UKMET
SOLNS, FOLLOWED BY ANOTHER WAVE (SUPPORTED BY AN UPR LOW NOW N OF
JAPAN) AND THEN ALEUTIANS ENERGY THAT MERGE WITH/MAINTAIN THE
LEADING SYSTEM.  THE END RESULT IS LOW PRESSURE THAT SHOULD DRIFT
TOWARD KODIAK ISLAND, WITH MODELS/MEANS KEEPING THE FEATURE WELL
DEFINED FOR A LONGER TIME THAN PREVIOUS FCST.  THERE ARE STILL
OTHER SOLNS THAT KEEP FEATURES MORE SEPARATE OR INVOLVE DIFFERING
INTERACTION.

FROM DAY 4 THU INTO DAY 5 FRI THE 06Z GFS/00Z ECMWF PROVIDED THE
BEST TEMPLATE WITH SUPPORT IN PRINCIPLE FROM THE ENSEMBLE MEANS.
THEN THE 00Z ECMWF BECAME MORE PROGRESSIVE THAN THE MEANS AND
00Z-06Z GFS/00Z CMC RUNS.  THUS BY SAT PREFERRED TO REDUCE ECMWF
WEIGHT MORE QUICKLY THAN THE 06Z GFS WHILE INCREASING 06Z GEFS/00Z
ECMWF MEAN INPUT.  AMONG NEW 12Z GUIDANCE THERE IS STILL A DECENT
RANGE OF SOLNS FOR PROGRESSION/DETAILS.  THERE IS A HINT OF FASTER
TIMING IN GUIDANCE AS A WHOLE OVER THE PAST DAY BY THE LATTER HALF
OF THE PERIOD BUT THE FACT THAT AN UPR HIGH MAY CLOSE OFF TO THE N
SEEMED TO FAVOR STICKING WITH AN AVG OF THE MEANS FOR THE TIME
BEING.

UPSTREAM THERE IS STILL FAIR AGREEMENT ON THE DEVELOPMENT OF A
MEAN TROUGH FROM SIBERIA SWD THROUGH THE EXTREME WRN BERING SEA
AND WRN PAC.  HOWEVER BY MID-LATE PERIOD THERE IS WIDE
MODEL/ENSEMBLE SPREAD AND POOR RUN-RUN MODEL CONTINUITY FOR
DETAILS WITHIN AND AHEAD OF THE MEAN TROUGH, ULTIMATELY AFFECTING
THE FCST ACROSS MUCH OF THE ALEUTIANS/BERING SEA.  FOR NOW IT IS
DIFFICULT TO HAVE ANY CONFIDENCE IN A PARTICULAR MODEL SOLN SO
PREFER TO LEAN MORE TO THE ENSEMBLE MEANS WHILE AWAITING A BETTER
CONSENSUS.

BASED ON ABOVE FCST CONSIDERATIONS OVER THE COURSE OF THE PERIOD,
TODAY`S STARTING BLEND CONSISTED OF THE 06Z GFS/00Z ECMWF INTO DAY
5 FRI FOLLOWED BY GRADUALLY DECREASED OPERATIONAL RUN/INCREASED
ENSEMBLE MEAN WEIGHTING SUCH THAT DAY 8 MON USED THE MEANS
EXCLUSIVELY.

RAUSCH

$$




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