Prognostic Meteorological Discussion
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FXAK02 KWNH 191846
PMDAK

ALASKA EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
146 PM EST SUN NOV 19 2017

VALID 12Z THU NOV 23 2017 - 12Z MON NOV 27 2017

THE RECENT TENDENCY FOR SYSTEMS TO ROUND A STRONG CENTRAL PACIFIC
RIDGE ALOFT AND THEN DROP INTO A NORTHEAST PACIFIC TROUGH WILL
CONTINUE INTO THE FIRST PART OF THE EXTENDED PERIOD AS A DEEP
BERING SEA SYSTEM NEARING THE SOUTHWESTERN MAINLAND BY 12Z THU
TRACKS SOUTHEASTWARD THEREAFTER.  BEHIND THE TRAILING UPPER RIDGE
THAT APPROACHES/REACHES THE WESTERN MAINLAND BY DAY 6 SAT, THERE
IS STILL DECENT AGREEMENT ON A FRONT PASSING THROUGH THE BERING
SEA/ALEUTIANS AROUND SAT AND THEN A SIGNIFICANT LOWERING OF
HEIGHTS ACROSS THE NORTH PACIFIC.  THUS FAR MODELS/ENSEMBLE
MEMBERS HAVE DIFFERED CONSIDERABLY ON DETAILS OF THIS LATE PERIOD
EVOLUTION AS WELL AS FOR FLOW DETAILS OVER THE MAINLAND IN THAT
TIME FRAME.

AS OF THE START OF THE PERIOD 12Z THU THERE IS STILL A FAIR AMOUNT
OF SPREAD FOR THE DEEP STORM NEAR THE SOUTHWEST MAINLAND AS OF THE
00Z-06Z CYCLES.  THE OVERALL ENVELOPE OF MODEL/ENSEMBLE GUIDANCE
HAS SHIFTED A BIT TO THE SOUTHEAST COMPARED TO 24 HOURS AGO WITH
THE 00Z ECMWF ON THE NORTH/NORTHWEST SIDE OF THE OPERATIONAL
SPREAD AND THE 06Z GFS ON THE SOUTHEAST SIDE.  THE 00Z CMC MEAN
LEANS CLOSER TO THE ECMWF/CMC WHILE THE GEFS/ECMWF MEANS SHOW A
TRACK A BIT TO THE EAST OF THEIR OPERATIONAL COUNTERPARTS.  AT
THIS TIME A COMPROMISE BETWEEN THE 00Z ECMWF AND 00Z UKMET/00Z GFS
BEST REPRESENTS THE MIDDLE OF THE ENSEMBLE SPREAD, WITH THE 12Z
GFS ALSO A REASONABLE COMPONENT GIVEN ITS ADJUSTMENT BACK CLOSE TO
THE 00Z RUN.  BY EARLY FRI THE ECMWF/GFS RUNS CLUSTER DECENTLY FOR
A 5-DAY FORECAST WHILE UKMET DETAILS BECOME QUESTIONABLE.
REMAINING 12Z GUIDANCE SEEMS TO BE GRAVITATING TOWARD THE GFS AS
OF 12Z THU.  FARTHER NORTH, GUIDANCE BEGINS TO DIVERGE LATE IN THE
SHORT RANGE FOR LOW PRESSURE REACHING THE CHUKCHI SEA.  THE 00Z
ECMWF APPEARS TO TAKE LOW PRESSURE TOO FAR NORTHWEST BUT SOME
ECMWF ENSEMBLE MEMBERS SHOW A SIMILAR IDEA.  ON THE OTHER HAND,
THE 12Z GFS ALONG THE NORTHERN COAST OF THE MAINLAND MAY BE
SOMEWHAT DEEP/CONCENTRATED AS OF EARLY THU.  GIVEN THE DETAIL
UNCERTAINTY, PREFER THE RESULT OF THE BLEND USED FOR THE SYSTEM TO
THE SOUTH-- A GENERAL AREA OF SURFACE TROUGHING.  THE 12Z
UKMET/CMC/GEFS MEAN SHOW INCREASING THE POTENTIAL IN A WEAKER
VERSION OF THE GFS.  FOR BOTH AFOREMENTIONED SYSTEMS THE 12Z ECMWF
HOLDS ONTO TRAITS FROM THE 00Z RUN.

UPSTREAM THERE ARE STILL HINTS OF POTENTIAL WAVINESS ALONG THE
FRONT CROSSING THE BERING SEA/ALEUTIANS AROUND SAT, ANCHORED BY
LOW PRESSURE TRACKING OVER/NEAR EASTERN SIBERIA.  ALSO IT IS STILL
THE CASE THAT ECMWF RUNS ARE MORE MUTED WITH SUCH WAVINESS THAN
GFS/CMC RUNS AND ENSEMBLE MEMBERS ARE DIVERSE ENOUGH TO KEEP THE
MEANS FROM DEPICTING ANY WELL DEFINED WAVES.  PREFER STICKING
CLOSER TO THE MEANS WITH A LESS WAVY FRONT UNTIL THERE IS BETTER
MODEL AGREEMENT.  BEHIND THIS FRONT, AS MORE PRONOUNCED HEIGHT
FALLS PROGRESS ACROSS THE NORTHWEST-NORTH PACIFIC, THE ENSEMBLE
MEANS ARE SHOWING SIMILAR IDEAS WITH A SURFACE LOW/FRONTAL SYSTEM
AFFECTING THE ALEUTIANS/BERING SEA AREA BY DAYS 7-8 SUN-MON.
ECMWF MEAN RUNS LATCHED ONTO THE SHAPE OF FLOW ALOFT BEFORE THE
GEFS MEAN BUT NOW THE 00Z/06Z GEFS MEANS SHOW A SOMEWHAT DEEPER
TROUGH/SURFACE LOW THAN THE 00Z ECMWF MEAN.  ASSOCIATED WITH THE
LATE PERIOD UPPER TROUGH, OPERATIONAL RUNS AND SOME ENSEMBLE
MEMBERS SUGGEST STRONG LOW PRESSURE COULD DEVELOP/TRACK ANYWHERE
BETWEEN THE NORTH-CENTRAL PACIFIC (00Z ECMWF) AND THE
ALEUTIANS/BERING SEA (00Z-06Z GFS/00Z CMC).  AS WITH THE PRECEDING
FRONT, PREFER THE MORE CONSISTENT/SIMILAR ENSEMBLE MEANS UNTIL
MODEL CLUSTERING IMPROVES.  THE ENSEMBLE MEAN FORECAST YIELDS A
PRIMARY LOW OVER THE WESTERN BERING WITH A LEADING FRONT EXTENDING
OUT ACROSS THE ALEUTIANS INTO NORTH PACIFIC.

THE ILL-DEFINED SOUTHWESTERLY FLOW ALOFT OF THE MEANS ACROSS THE
MAINLAND BY SUN-MON HELPS TO MANAGE THE FORECAST WHERE INDIVIDUAL
MODEL RUNS AND ENSEMBLE MEMBERS SHOW VERY DIFFERENT FLOW DETAILS.

BASED ON PREFERENCES ABOVE, THE OPERATIONAL MODEL EMPHASIS ON DAY
4 THU (00Z ECMWF/00Z-12Z GFS/00Z UKMET) GRADUALLY INCREASES 06Z
GEFS/00Z ECMWF MEAN WEIGHTING WITH TIME, ULTIMATELY LEADING TO
EXCLUSIVE USE OF THE MEANS BY DAY 8 MON.

RAUSCH

$$





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