Prognostic Meteorological Discussion
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ALASKA EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
249 PM EDT MON SEP 26 2016

VALID 12Z FRI SEP 30 2016 - 12Z TUE OCT 04 2016

GUIDANCE CONTINUES TO SHOW DIFFERENCES IN HOW MID LVL ENERGY AND
ASSOC SFC SYSTEM INITIALLY NEAR THE ALEUTIANS INTERACTS WITH A
MEAN UPR LOW OVER/NEAR SIBERIA... WITH FURTHER QUESTION MARKS ON
HOW DOWNSTREAM MAINLAND RIDGING EVOLVES WITH TIME.

INDIVIDUAL MODELS/ENSEMBLE MEMBERS DISPLAY A FAIR AMOUNT OF SPREAD
WITH THE ULTIMATE TRACK OF ALEUTIANS/BERING SEA LOW PRESSURE
DURING THE FIRST HALF OF THE PERIOD.  OVER THE PAST COUPLE DAYS
OPERATIONAL RUNS HAVE APPEARED TO BE WELL AHEAD OF THE ENSEMBLE
MEANS IN DEPICTING THE STRENGTH OF THIS FEATURE AS ENSEMBLE MEMBER
SPREAD HAS BEEN DILUTING THE RESULTING MEANS.  AT LEAST THE LATEST
RUNS OF THE MEANS ARE TRENDING TOWARD BETTER DEFINITION AND OFFER
REASONABLE SUPPORT FOR STARTING WITH A 00Z GFS/ECMWF BLEND FOR
TRACK FROM DAY 4 FRI INTO DAY 5 SAT.

AS THE ALEUTIANS/BERING SEA SYSTEM HEADS INTO THE MAINLAND RIDGE
MOST GUIDANCE EXPECTS A SPLITTING OF ENERGY/MSTR WITH SOME LIFTING
BACK NWWD TOWARD THE MEAN UPR LOW OVER/NEAR SIBERIA AND THE REST
BEING STRUNG OUT ACROSS THE SWRN COAST OF THE MAINLAND/ALASKA
PENINSULA AND INTO NERN PAC/GULF OF ALASKA.  THE 06Z GFS BRINGS
MORE MID LVL ENERGY/MSTR INTO AND ACROSS THE MAINLAND AND IS NOT
FAVORED GIVEN THE EXPECTED STRENGTH OF THE DOWNSTREAM RIDGE.  THE
12Z GFS HAS ADJUSTED SOMEWHAT AWAY FROM THE 06Z RUN.  MOST
OPERATIONAL MODEL RUNS OVER THE PAST COUPLE DAYS HAVE BEEN
SIGNALING AN UPR HIGH CLOSING OFF OVER THE MAINLAND AROUND SAT
WITH INCREASINGLY VARIED SOLNS ON THE PATH OF THIS HIGH LATER IN
THE PERIOD.  UP UNTIL THE PAST 1-2 RUNS THE GEFS MEANS HAVE BEEN
AHEAD OF THE ECMWF MEAN IN HINTING AT OPERATIONAL SIGNALS THAT THE
RIDGE WOULD CLOSE OFF.  NOW THE 00Z ECMWF MEAN HOLDS ONTO A
STRONGER-NWD RIDGE/HIGH THAN THE 06Z GEFS MEAN THOUGH THE 00Z GEFS
MEAN WAS CLOSER TO THE ECMWF MEAN.  THUS AFTER A MODEL/ENSEMBLE
MEAN TRANSITION LATE SAT INTO SUN... PREFS LEAN TOWARD MAJORITY
WEIGHTING OF THE 00Z ECMWF MEAN RELATIVE TO THE 00Z GEFS/NAEFS
MEANS.  GUIDANCE TENDS TO HAVE DIFFICULTY IN FORECASTING THE PATH
OF CLOSED HIGHS VERY FAR INTO THE FUTURE SO CONFIDENCE DECLINES
RAPIDLY AFTER SAT-SUN.

ELSEWHERE... THERE IS REASONABLE AGREEMENT THAT SHRTWV ENERGY
DIVING DOWN THE ERN SIDE OF THE MAINLAND RIDGE WILL PASS OVER THE
PANHANDLE NEAR THE START OF THE PERIOD FOLLOWED BY A GENERAL
WEAKNESS WITH LOW CONFIDENCE ON DETAILS.  MEANWHILE PROGRESSIVE
FLOW FROM ASIA ACROSS THE NRN PAC/ALEUTIANS/BERING SEA MAY CARRY
ONE OR MORE WEAK FEATURES MID-LATE PERIOD... AGAIN WITH FAIRLY LOW
CONFIDENCE GIVEN FEATURE SCALE AND FAST FLOW.  IN BOTH CASES THE
PREFERRED INCREASING EMPHASIS ON ENSEMBLES WITH TIME PROVIDES A
GOOD CONSERVATIVE STARTING POINT WHILE AWAITING BETTER AGREEMENT
FOR SPECIFICS.

RAUSCH

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