Prognostic Meteorological Discussion
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FXAK02 KWNH 051843
PMDAK

ALASKA EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
242 PM EDT TUE MAY 05 2015

VALID 12Z SAT MAY 09 2015 - 12Z WED MAY 13 2015

THE MOST COMMON FEATURES SEEN IN THE LATEST GUIDANCE ARE A RIDGE
OVER WRN CANADA... TROUGH JUST OFF THE SRN COAST/PANHANDLE... AND
A TROUGH HEADING INTO THE WRN-CNTRL PACIFIC.  SOME OF THE WRN
CANADA RIDGE MAY EXTEND INTO THE MAINLAND BY THE LATTER HALF OF
THE PERIOD.  DETAILS FROM OPERATIONAL SOLNS BEGIN TO DIVERGE
MEANINGFULLY FAIRLY EARLY IN THE FCST AND CONTINUITY HAS GENERALLY
BEEN BETTER IN RECENT ENSEMBLE MEAN RUNS VERSUS THE OPERATIONAL
MODELS.  THUS OVERALL PREFS LEAN RATHER STRONGLY TO THE ENSEMBLE
MEANS EXCEPT FOR NEAR THE START OF THE PERIOD 12Z SAT WHEN
CLUSTERING IS STILL SOMEWHAT ACCEPTABLE.

THE GREATEST SPREAD TODAY APPEARS TO BE WITH A NRN PAC SYSTEM BTWN
AN INITIAL LOW JUST SE OF KODIAK ISLAND AND A VIGOROUS
LATER-PERIOD SYSTEM LIKELY TRACKING TOWARD THE ALEUTIANS.  IN
VARIOUS WAYS THE LATEST GFS RUNS PULL THE SYSTEM INTO THE BERING
SEA... AS DOES THE 00Z CMC ON A MORE WRN TRACK.  ON THE OTHER HAND
THE 00Z ECMWF WEAKENS IT TO A MERE FRONT WITH GREATER EMPHASIS ON
ANOTHER WAVE FARTHER S WHILE THE 00Z UKMET HAS AN EXTREME NWRN LOW
AND ANOTHER WAVE WELL S OF THE ALEUTIANS.  CONTINUITY WAS MORE
SUPPRESSED AND GIVEN CURRENT SPLIT IN GUIDANCE AND ENSMEANS
REMAINING ON THE SUPPRESSED SIDE... THE BEST OPTION SEEMS TO LIE
WITH THE ENSEMBLE MEANS.  CONFIDENCE IS LOW GIVEN IMPORTANCE OF
SUBTLE MID LVL DETAILS THAT ARE TYPICALLY DIFFICULT TO RESOLVE IN
THE MEDIUM RANGE TIME FRAME.

UPSTREAM EVOLUTION WILL DEPEND ON THE INTERACTION OF LEADING WRN
PAC ENERGY AND DEEPER TRAILING DYNAMICS.  STRENGTH OF THE ENERGY
ALOFT MAY WELL SUPPORT A STRONGER SFC SYSTEM THAN SEEN IN THE
ENSEMBLE MEANS.  A MODEL/ENSMEAN COMPROMISE MIGHT BE IDEAL THOUGH
A MODESTLY DEEPER MANUAL ADJUSTMENT WILL HAVE TO SUFFICE GIVEN
THAT CONFIDENCE IN OPERATIONAL MODELS DOWNSTREAM IS TOO LOW TO
INCLUDE FOR A FCST BLEND.

OVER THE NERN PAC/MAINLAND... EARLY IN THE PERIOD THE OPERATIONAL
MODELS MAY BE BETTER THAN THE ENSMEANS IN DEPICTING A STRONGER
SYSTEM INITIALLY TO THE SE OF KODIAK ISLAND.  THEN LATER IN THE
PERIOD TELECONNECTIONS RELATIVE TO CNTRL CANADA RIDGING AND
TROUGHING DOWNSTREAM IN D+8 MULTI-DAY MEANS OFFER DECENT SUPPORT
FOR SOME RIDGING OVER THE MAINLAND AS SEEN IN THE ENSMEANS.

THE PREFERRED BLEND LEANS A LITTLE MORE TO THE 00Z GFS/ECMWF
VERSUS THE 06Z GEFS/00Z ECMWF MEANS AT THE START OF THE PERIOD 12Z
SAT TO REPRESENT BETTER DETAIL FOR BOTH SYSTEMS OF INTEREST.  THEN
THE BLEND QUICKLY TRANSITIONS TO GREATER ENSMEAN WEIGHTING
THEREAFTER... REACHING A STABLE 70/30 ECMWF MEAN/GEFS MEAN SOLN
FROM 00Z MON ONWARD.

RAUSCH

$$




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