Prognostic Meteorological Discussion
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FXAK02 KWNH 171830
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ALASKA EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
229 PM EDT FRI APR 17 2015

VALID 12Z TUE APR 21 2015 - 12Z SAT APR 25 2015

GUIDANCE STILL SHOWS THE MOST AGREEMENT WITH A MEAN TROUGH JUST W
OF THE MAINLAND AND SOME RIDGING DOWNSTREAM OVER WRN CANADA.  SOME
DETAILS REMAIN IN DOUBT THOUGH... WITH MEANINGFUL CONTINUITY
CHANGES ALSO NOTED.

ONE SIGNIFICANT ADJUSTMENT FROM 24 HRS AGO IS A FASTER TREND WITH
THE PARENT LOW ASSOC WITH A SYSTEM TRACKING EWD FROM JUST W OF
KAMCHATKA... AS MOST GUIDANCE HAD PREVIOUSLY INDICATED HOLDING
MORE ENERGY BACK WHILE ONLY A WEAK/WEAKENING LEADING FRONT WOULD
CONTINUE WELL EWD.  THE 00Z ECMWF/ECMWF MEAN ARE THE MOST
ENTHUSIASTIC WITH THE FASTER TREND INTO THU BUT LATEST GEFS/CMC
MEANS DO EVENTUALLY CATCH UP.  THE OTHER NOTICEABLE CONTINUITY
CHANGE IS AHEAD OF THIS SYSTEM AS RECENT TRENDS HAVE BEEN TOWARD
GREATER SEPARATION OF FLOW BTWN THE WESTERLIES AND A MID-LATITUDE
CNTRL PAC UPR LOW.  THIS GREATER SEPARATION LEADS TO A SLOWER
PROGRESSION OF CORRESPONDING LOW PRES... WITH 00Z/06Z GFS RUNS ON
THE FAST EDGE OF THE ENSEMBLE SPREAD.  THE 12Z GFS HAS ADJUSTED
SLOWER WITH THIS WAVE TO YIELD A SOLN MORE SIMILAR IN PRINCIPLE TO
THE 00Z ECMWF WHICH LIFTS IT NWD INTO THE BERING SEA AHEAD OF THE
KAMCHATKA SYSTEM.  THE QUESTION OF HOW NRN PAC FLOW EVOLVES AND
PSBLY INTERACTS WITH LEADING ENERGY IS AN ONGOING POINT OF
UNCERTAINTY.  SO FAR CONFIDENCE IS TOO LOW TO LATCH ONTO AN
OPERATIONAL SCENARIO AFTER DAY 5 WED.

FARTHER EWD FROM TUE INTO THU... A MODERATELY SLOW SOLN APPEARS
MOST REASONABLE FOR THE UPR LOW EXPECTED TO BE JUST W OF THE
MAINLAND AS OF TUE AND WEAKEN/LIFT AWAY WITH TIME.  MEANWHILE
ECMWF-BASED 00Z GUIDANCE AND THE 00Z NAEFS MEAN OFFER THE BEST
INTERMEDIATE SOLN FOR A WEAK SFC TROUGH/EMBEDDED WAVE THAT MAY
EXTEND OVER PARTS OF THE NERN PAC WED-THU.

LATER IN THE PERIOD SOLNS INCREASINGLY DIFFER WITH THEIR FLOW OVER
THE MAINLAND.  IN VARYING WAYS THE 00Z ECMWF/12Z GFS BECOME RATHER
STRONG WITH RIDGING THAT BUILDS OVER PORTIONS OF THE MAINLAND.  IN
ONE WAY OR ANOTHER MOST REMAINING GUIDANCE SUGGESTS THAT ANY
RIDGING SHOULD BE WEAKER.. MORE OPEN.. AND/OR MORE TRANSIENT.

BASED ON EARLY PERIOD PREFS THE 00Z ECMWF/ECMWF MEAN/NAEFS MEAN
COMPRISE THE STARTING BLEND FOR THE DAYS 4-5 TUE-WED FCST... WITH
70 PCT WEIGHTING OF THE 00Z ECMWF TUE DECREASING TO 30-40 PCT WED.
 DAYS 6-8 THU-SAT START WITH A 70/30 BLEND OF THE 00Z ECMWF
MEAN/NAEFS MEAN RESPECTIVELY.  PROGRESSIVE NRN PAC FLOW AND RECENT
TRENDS... AS WELL AS BETTER DEFINITION OF THE SFC SYSTEM... SEEM
TO FAVOR GREATER WEIGHTING OF THE FASTER/FARTHER SWD ECMWF MEAN
FOR THE KAMCHATKA SYSTEM AS IT CONTINUES EWD.  DIFFS IN DETAILS OF
EVOLUTION ALOFT EVENTUALLY LEAD TO A FARTHER S TRACK IN THE NAEFS
MEAN BY FRI BUT THE ECMWF MEAN ENDS UP FAVORABLY NEAR THE MIDDLE
OF THE SOLN SPREAD BY NEXT SAT.

RAUSCH

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