Prognostic Meteorological Discussion
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FXAK02 KWNH 201941
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ALASKA EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
240 PM EST MON NOV 20 2017

VALID 12Z FRI NOV 24 2017 - 12Z TUE NOV 28 2017

DURING THE FIRST PART OF THE PERIOD THERE IS DECENT AGREEMENT AND
CONTINUITY WITH THE SYSTEM DRIFTING SOUTHEAST OVER THE
NORTHEASTERN PACIFIC AND THE UPPER RIDGE HEADING INTO THE MAINLAND
FROM THE BERING SEA.  JUST UPSTREAM FROM THE UPPER RIDGE THERE
WILL BE A WEAKENING SURFACE FRONT CROSSING THE BERING
SEA/ALEUTIANS.  WHAT SOLUTIONS HAD SUGGESTED POTENTIAL FOR A
PRONOUNCED WAVE ALONG THIS FRONT HAVE GENERALLY TRENDED WEAKER,
THE NEW 12Z CMC BY 00Z SUN BEING THE ONLY EXCEPTION AT THE MOMENT.
 IN ORDER TO REFLECT THE CURRENT CONSENSUS WITH THESE FEATURES,
THE DAYS 4-5 FRI-SAT BLEND EMPHASIZED RECENT OPERATIONAL GUIDANCE
(06Z-12Z GFS, 00Z ECMWF AND 00Z CMC).

THE ONE ONGOING CONTENTIOUS ASPECT OF THE EARLY PART OF THE
EXTENDED FORECAST INVOLVES A SHORT RANGE CHUKCHI SEA LOW WHICH
ECMWF RUNS AND A DECENT NUMBER OF ITS ENSEMBLES TRACK WELL
NORTHWEST OF OTHER SOLUTIONS.  EVEN AMONG NON-ECMWF MODELS/MEANS
THERE IS CONSIDERABLE SPREAD FOR EASTWARD PROGRESSION.  BASED ON
GUIDANCE THROUGH THE 06Z CYCLE AND 12Z GFS, THE AFOREMENTIONED
BLEND`S RESULT OF A WEAK WAVE OFF THE NORTHERN COAST REFLECTED A
REASONABLE INTERMEDIATE SOLUTION.  THE 12Z UKMET/CMC SUGGEST THE
FEATURE COULD HANG BACK FARTHER TO THE WEST.


THE MID-LATE PART OF THE FORECAST OFFERS MORE UNCERTAINTY FOR
SYNOPTIC SCALE FEATURES ACROSS THE NORTH PACIFIC AND
ALEUTIANS/BERING SEA/MAINLAND ALASKA.  MODELS/ENSEMBLE MEMBERS
DIVERGE SIGNIFICANTLY FOR EVOLUTION OF THE UPPER RIDGE BUILDING
INTO THE MAINLAND DURING THE WEEKEND, UPSTREAM SHORTWAVE ENERGY
SUPPORTING THE WEAKENING BERING/ALEUTIANS FRONT, POSSIBLE TRAILING
RIDGE, AND A POTENTIAL SERIES OF NORTH PACIFIC/ALEUTIANS SYSTEMS.

MULTI-DAY MEANS BY D+8 SHOW THE MOST PROMINENT HEIGHT ANOMALY
CENTERS BEING A NEGATIVE ONE JUST WEST OF KAMCHATKA AND A POSITIVE
ONE NEAR THE NORTHERN CANADIAN ARCHIPELAGO.  TELECONNECTIONS
RELATIVE TO THESE CENTERS BOTH FAVOR AN AXIS OF ABOVE NORMAL
HEIGHTS/POSSIBLE WEAK RIDGING OVER THE MAINLAND AND EXTENDING INTO
THE EASTERN ALEUTIANS, WITH SOME DEGREE OF SPLIT FLOW IN PLACE.
THIS RELATIONSHIP PROVIDES SUPPORT FOR INCLUDING SOME GEFS/NAEFS
GUIDANCE WHICH HAS HIGHER HEIGHTS OVER THE MAINLAND THAN THE
ECMWF/CMC-BASED MODELS AND ENSEMBLES LATE IN THE PERIOD, BUT NOT
THE 06Z/12Z GFS RUNS THAT BUILD A RIDGE WEST OF THE MAINLAND.  THE
POTENTIAL FOR A SPLIT FLOW PATTERN GIVES SOME ROOM FOR A NORTH
PACIFIC SYSTEM PASSING SOUTH OF THE ALEUTIANS AROUND SUN-MON.
LATEST GEFS MEANS ARE STARTING TO HINT AT THIS SYSTEM WHILE THE
00Z ECMWF MEAN SHOWS MINIMAL SURFACE REFLECTION UNTIL CATCHING UP
TO LEADING ENERGY.  SO FAR THERE APPEARS TO BE A MORE STABLE
SIGNAL FOR THE NEXT SYSTEM REACHING SOMEWHERE BETWEEN THE WESTERN
BERING SEA AND NORTH PACIFIC BY DAY 8 TUE, BUT AS EXPECTED A BROAD
SPREAD AMONG INDIVIDUAL MODELS/ENSEMBLE MEMBERS.

OVER THE NORTHEAST PACIFIC EXPECT STRONG BUT GRADUALLY WEAKENING
LOW PRESSURE EARLY IN THE PERIOD TO CONTINUE WEAKENING AND WOBBLE
SOUTHEAST AND THEN POSSIBLY EAST/NORTHEAST AS UPPER LEVEL ENERGY
BEGINS TO MOVE TOWARD WESTERN NORTH AMERICA.  AROUND THE SOUTHEAST
PERIPHERY OF THIS SYSTEM THE 06Z GFS/00Z CMC SHOW A PRONOUNCED
WAVE TRACKING NORTH NEAR HAIDA GWAII AND BEYOND AROUND DAY 6 SUN.
THIS IS A MINORITY SCENARIO AND THE 12Z GFS HAS TRENDED SOMEWHAT
WEAKER WITH THIS WAVE.

BASED ON THE LARGE SCALE CONSIDERATIONS, THE DAYS 6-8 SUN-TUE
PERIOD WAS WEIGHTED MORE TO THE 06Z GEFS/00Z NAEFS/00Z ECMWF MEANS
WITH ABOUT 1/4 OF THE 00Z ECMWF INCLUDED TO PROVIDE A LITTLE
OPERATIONAL INPUT ACROSS THE NORTH PACIFIC AND GIVEN ITS DECENT
ENSEMBLE MEAN SUPPORT OFF THE ALASKA PANHANDLE.  OVERALL
CONFIDENCE IN THIS TIME FRAME IS BELOW AVERAGE WITH SOME ASPECTS
OF THE FORECAST LIKELY SUBJECT TO SIGNIFICANT CHANGE IN FUTURE
CYCLES.

RAUSCH

$$





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