Prognostic Meteorological Discussion
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FXAK02 KWNH 261859
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ALASKA EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
259 PM EDT WED JUL 26 2017

VALID 12Z SUN JUL 30 2017 - 12Z THU AUG 03 2017

THE MAJORITY CLUSTER OF GUIDANCE MAINTAINS THE SHORT RANGE MEAN
PATTERN INTO THE EXTENDED TIME FRAME.  THIS WOULD HAVE PERIODIC
BUNDLES OF NRN STREAM ENERGY FLOWING AROUND THE PERIPHERY OF A
STRONG CNTRL PACIFIC RIDGE AND FEEDING INTO A MEAN TROUGH/EMBEDDED
CLOSED LOW OVER THE NERN PAC/GULF OF ALASKA.  MEANWHILE RIDGING
WOULD PERSIST ACROSS NWRN CANADA INTO MAINLAND ALASKA.

IN THE FIRST HALF OF THE PERIOD ONE OF THE PRIMARY UNCERTAINTIES
IS THE PATH OF THE INITIAL UPR LOW OVER THE GULF OF ALASKA.  WHILE
THE 00Z UKMET ALSO SHOWED FAIRLY QUICK EJECTION ACROSS THE
PANHANDLE, A MAJORITY OF OTHER SOLNS INCLUDING THE 00Z ECMWF MEAN
RECOMMENDED HOLDING THE UPR LOW OVER THE GULF FOR A LONGER TIME.
THIS IDEA IS ALSO CLOSER TO CONTINUITY ALBEIT WITH A SLIGHTLY
WEAKER TREND COMPARED TO 24 HRS AGO.  MOST NEW 12Z GUIDANCE SEEMS
TO SUPPORT LEANING AWAY FROM THE 00Z ECMWF SCENARIO BUT THE 12Z
ECMWF STILL SIDES WITH RAPID EJECTION.

BY THE LATTER HALF OF THE PERIOD GUIDANCE DIVERGES REGARDING THE
RELATIVE STRENGTH OF NRN STREAM BERING SEA ENERGY FEEDING INTO THE
NERN PAC MEAN TROUGH VS THE CNTRL PAC RIDGE.  RECENT CMC/GEM RUNS
HAVE BEEN QUITE STRONG WITH THE RIDGE, DEFLECTING ENERGY WELL NWD
VS OTHER SOLNS.  LATEST GEFS MEANS HAVE BEEN HOLDING ONTO HIGHER
HGTS ALOFT THAN OPERATIONAL GFS RUNS/00Z ECMWF-EC MEAN THOUGH.
THUS FAR THERE HAVE BEEN RUN TO RUN ADJUSTMENTS BUT WITH NO
PRONOUNCED LONGER TERM TREND OVER THE PAST COUPLE DAYS, FAVORING
AN INTERMEDIATE SOLN UNTIL GUIDANCE SPREAD NARROWS.  GIVEN THIS
MID-LATE PERIOD SPREAD THERE IS ALSO A FAIR DEGREE OF UNCERTAINTY
OVER WHAT ULTIMATELY HAPPENS TO THE INITIAL GULF LOW, RANGING BTWN
EJECTING INLAND OR IN SOME WAY MERGING WITH APPROACHING ENERGY.
IN THE INTEREST OF CONTINUITY PREFERENCE LEANS TOWARD A MORE
STEADY-STATE FEATURE WHILE ACKNOWLEDGING THAT EVENTUAL EJECTION IS
QUITE PLAUSIBLE GIVEN THAT HAS HAPPENED WITH SOME PRECEDING
FEATURES.

SOME GUIDANCE HAS BEEN SIGNALING POTENTIAL FOR SOME WAVINESS TO
COME INTO THE PICTURE NEAR THE FAR WRN ALEUTIANS BY AROUND DAY 7
WED.  SPREAD IS QUITE BROAD SO PREFER TO DEPICT ONLY A WEAK
FEATURE FOR NOW.

BASED ON OVERALL FCST PREFS THE STARTING BLEND USED 70 PCT
OPERATIONAL GUIDANCE (06Z GFS AND 00Z ECMWF/CMC-GEM) AND 30 PCT
ENSEMBLE MEANS (06Z GEFS/00Z ECMWF MEAN) FROM DAY 4 SUN INTO DAY 6
TUE, FOLLOWED BY EVEN MODEL-MEAN WEIGHTING FOR DAY 7 WED ON THE
WAY TO 70 PCT MEANS BY DAY 8 THU.  ECMWF WEIGHTING WAS LOW ENOUGH
TO DOWNPLAY ITS SOLN NEAR THE PANHANDLE WHILE THE BLEND HELPED TO
RESOLVE DIFFS IN POSN OF THE UPR TROUGH NEAR THE SWRN MAINLAND
LATER IN THE PERIOD.

RAUSCH

$$




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