Prognostic Meteorological Discussion
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FXAK02 KWNH 191838
PMDAK

ALASKA EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
238 PM EDT SUN APR 19 2015

VALID 12Z THU APR 23 2015 - 12Z MON APR 27 2015

ESPECIALLY IN THE ENSEMBLE MEANS AND TO A REASONABLE DEGREE IN
OPERATIONAL RUNS ON AVG... THERE REMAINS FAIRLY GOOD AGREEMENT AND
CONTINUITY WITH THE GENERAL IDEA OF A MEAN TROUGH EXTENDING FROM
SIBERIA SEWD ACROSS THE ERN BERING SEA AND THROUGH THE NERN
PACIFIC WHILE MEAN RIDGING PERSISTS OVER E-CNTRL CANADA.  TO THE S
OF THE MEAN TROUGH EXPECT A PROGRESSIVE NRN PACIFIC/SRN BERING SEA
STREAM TO CARRY A SERIES OF SYSTEMS SPACED 2-4 DAYS APART.  THERE
IS STILL A HINT OF A RIDGE BUILDING OVER THE MAINLAND LATE IN THE
PERIOD WITH TELECONS RELATIVE TO POSITIVE HGT ANOMALIES OVER
EXTREME NRN CANADA CONTINUING TO SUPPORT THE MODEST NATURE OF THIS
RIDGE AS DEPICTED IN THE ENSEMBLE MEANS.

GUIDANCE HAS HAD DIFFICULTY IN LOCKING ONTO AT LEAST SOME ASPECTS
OF BOTH SYSTEMS OF INTEREST AS OF THE START OF THE FCST PERIOD 12Z
THU... ONE OVER THE NERN PAC AND THE OTHER OVER THE BERING SEA.
00Z ENSEMBLES STILL SHOW A LOT OF NW-SE SPREAD FOR THE NERN PAC
SYSTEM BUT THE BEST CONSENSUS OF GFS/ECMWF RUNS AND GEFS/ECMWF/CMC
MEANS HAS ADJUSTED TO THE NRN SIDE OF THE FULL SPREAD THUS LEAVING
THE 00Z UKMET/CMC IN THE MORE SRN MINORITY.  BEST AGREEMENT IS
TOWARD QUICKER WEAKENING THAN FCST BY THE 00Z GFS RUN.  FOR THE
BERING SEA SYSTEM... TIMING SEEMS TO BE OSCILLATING SOMEWHAT ON A
DAY TO DAY BASIS WITH THE PAST 24 HRS TRENDING SOMEWHAT FASTER.
ESPECIALLY FOR THIS FEATURE A DECENT AMOUNT OF OPERATIONAL INPUT
IS DESIRABLE EARLY IN THE PERIOD AS BY 12Z THU THE ENSMEANS APPEAR
TO MERGE THE SEPARATE UPR SYSTEMS OVER THE BERING SEA AND ERN TIP
OF SIBERIA.  HOWEVER BY DAY 5 FRI THE 06Z GFS/00Z ECMWF AND 00Z
GEFS/ECMWF MEANS CLUSTER WELL.  THE 06Z GEFS MEAN IS FASTER AND
THE 00Z CMC MEAN SLOWER.  BY DAY 7 SUN THERE IS STILL GOOD
AGREEMENT RELATIVE TO THE FCST PROJECTION WITH A SFC LOW NEAR OR
JUST N OF 50N 140W.  BY THAT TIME THE CMC MEAN IS SIMILAR AS WELL.

THE NEXT SYSTEM COMING INTO THE PICTURE BY DAYS 7-8 SUN-MON
CURRENTLY SHOWS BETTER THAN AVG AGREEMENT FOR THAT DISTANT OF A
TIME FRAME.  IN PRINCIPLE AN AVG OF 00Z-06Z-12Z GFS/00Z ECMWF RUNS
AND THE 00Z GEFS/ECMWF MEANS SEEMS REASONABLE... THOUGH DETAILS
FARTHER EWD MAKE IT MORE PRACTICAL TO START WITH THE ENSEMBLE
MEANS AND ADJUST SLIGHTLY DEEPER.

WHERE DIFFS AMONG OPERATIONAL MODELS EXIST EARLY IN THE PERIOD THE
00Z ECMWF PROVIDES A SOMEWHAT BETTER MATCH TO CONSENSUS SO IT IS
GIVEN GREATEST EMPHASIS THU INTO FRI WITH A TRANSITION TO MORE
EVEN WEIGHTING RELATIVE TO THE 00Z GEFS/ECMWF MEANS AROUND THE
MIDDLE OF THE PERIOD.  THEN THE FAVORED BLEND USES THE MEANS
EXCLUSIVELY BY SUN-MON.

RAUSCH

$$





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