Prognostic Meteorological Discussion
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FXAK02 KWNH 241840
PMDAK

ALASKA EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
240 PM EDT SAT SEP 24 2016

VALID 12Z WED SEP 28 2016 - 12Z SUN OCT 02 2016

AFTER PASSAGE OF SHRTWV ENERGY ACROSS THE MAINLAND EARLY IN THE
PERIOD... GUIDANCE SHOWS A LARGE SCALE EVOLUTION DOMINATED BY AN
AMPLIFIED RIDGE BUILDING OVER THE MAINLAND DOWNSTREAM FROM A MEAN
CLOSED LOW THAT SHOULD BE OVER EXTREME ERN ASIA/JUST W OF THE
BERING SEA.  FROM LATE WEEK ONWARD THE MODELS AND INDIVIDUAL
ENSEMBLE MEMBERS SHOW INCREASING SPREAD REGARDING THE
STRENGTH/AMPLITUDE AND SHAPE OF THE RIDGE WITH CORRESPONDING DIFFS
FOR SURROUNDING SHRTWV ENERGY AND ASSOC SFC PATTERN.

THE FIRST POINT OF CONTENTION IN THE FCST IS WITHIN THE MEAN UPR
LOW.  DURING THE FIRST PART OF THE PERIOD THE ESTABLISHED
CONSENSUS SHOWS AN INTERACTION OF MID-LATITUDE AND ERN RUSSIA
FEATURES TO YIELD A RATHER STRONG SYSTEM NOT FAR FROM KAMCHATKA.
THE 00Z/06Z GFS RUNS ARE SOMEWHAT WEAKER/MORE DISJOINTED THAN THE
MAJORITY OF SOLNS.  THE 12Z GFS KEEPS THE TWO SEPARATE SO THAT
THERE IS A STRONG SYSTEM TRACKING INTO THE WRN ALEUTIANS THEN
CNTRL BERING SEA INSTEAD.  WHILE THIS IDEA DIFFERS SIGNIFICANTLY
FROM CONSENSUS... THE NEW UKMET HAS SOME SIMILARITIES TO THE 12Z
GFS SCENARIO SO CONFIDENCE IN A PARTICULAR SOLN IS DECREASING.

AS THE RIDGE TAKES SHAPE/BUILDS INTO THE MAINLAND BY DAYS 5-6
THU-FRI BOTH THE GEFS AND ECMWF ENSEMBLE MEANS HAVE TRENDED TOWARD
A STRONGER AND SHARPER RIDGE OVER THE PAST 1-2 DAYS.  DURING THE
SAME PERIOD OPERATIONAL ECMWF RUNS HAVE GENERALLY BEEN MORE STABLE
THAN GFS RUNS WITH THE LATTER QUITE PRONOUNCED WITH THE STRONGER
RIDGE STARTING WITH THE 12Z/23 RUN.  FROM THIS POINT ONWARD DIFFS
IN THE STRENGTH AND SHAPE OF THE RIDGE INFLUENCE SFC DETAILS
ACROSS THE NRN PAC/ALEUTIANS AND BERING SEA INTO THE WRN MAINLAND.
 GREATER STRENGTH/NWD AMPLITUDE OF THE RIDGE IN LATEST GFS AND TO
SOME DEGREE GEFS RUNS LEAD TO SLOWER TIMING OF THE APPROACHING
FRONTAL SYSTEM AND MORE WAVINESS NEAR THE ERN ALEUTIANS RELATIVE
TO MOST OTHER SOLNS.  THE 00Z-06Z GFS SCENARIO ULTIMATELY LEADS TO
THE CLOSING OF AN UPR HIGH BY LATE IN THE FCST WITH THE GEFS MEAN
JUST HINTING AT SUCH AN EVOLUTION.  OTHER SOLNS MAINTAIN AN OPEN
RIDGE.  THE STRENGTHENING TRENDS FOR THE RIDGE ALOFT SEEM TO
RECOMMEND HOLDING THE BERING SEA SFC FRONT FARTHER WWD THAN
CONTINUITY OR THE 00Z ECMWF... BUT TOO MUCH UNCERTAINTY EXISTS
LATE IN THE PERIOD TO ADJUST TO THE CLOSED RIDGE ALOFT IN SOME GFS
RUNS.  HOWEVER MEANS DO SUGGEST POTENTIAL FOR AT LEAST A WEAK NRN
PAC/ALEUTIANS WAVE ALONG THE SRN PART OF THE SFC FRONT THAT MAY
APPROACH THE WRN MAINLAND.

BASED ON CONSIDERATIONS NEAR KAMCHATKA ALONG WITH 00Z/06Z GFS RUNS
NOT COMPARING WELL TO CONSENSUS OFF THE NWRN COAST OF THE MAINLAND
EARLY IN THE PERIOD... DAYS 4-5 WED-THU ARE WEIGHTED MORE TOWARD
THE 00Z ECMWF WITH SMALL COMPONENTS OF ITS MEAN AND 06Z GEFS MEAN
ALSO INCLUDED.  AFTER A DAY 5 TRANSITION THE REST OF THE PERIOD
MAKES USE OF THE 00Z NAEFS/ECMWF MEANS AND 06Z GEFS MEAN TO
REFLECT LATEST TRENDS WITH THE RIDGE ALOFT WHILE MAINTAINING
BETTER CONTINUITY THAN WOULD BE ACHIEVED BY FOLLOWING ANY OF THE
INCREASINGLY DIVERSE OPERATIONAL MODEL RUNS.

RAUSCH

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