Prognostic Meteorological Discussion
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ALASKA EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
307 PM EDT MON SEP 25 2017

VALID 12Z FRI SEP 29 2017 - 12Z TUE OCT 03 2017

WHILE THE UPPER PATTERN WILL BE MORE BLOCKED IN NATURE ACROSS THE
POLAR LATITUDES...IT WILL BE QUITE THE OPPOSITE TO THE SOUTH AS A
HOST OF SYSTEMS TRACK FROM THE WESTERN PACIFIC TOWARD THE BERING
SEA/ALEUTIANS REGION. MULTIPLE PERTURBATIONS WITHIN THE LONGWAVE
PATTERN MAY AMPLIFY LEADING TO FAIRLY INTENSE SURFACE CYCLONES.
BEING ABLE TO FORECAST SUCH INSTANCES WILL BE A CHALLENGE GIVEN
POOR RUN-TO-RUN CONTINUITY AND DIFFERENCES AMONG THE MODEL SUITES.
ACROSS THE GULF OF AK AND THE SURROUNDING WATERS...THERE IS DECENT
SYNOPTIC-SCALE AGREEMENT WITH AN INITIAL SYSTEM MOVING TOWARD THE
BRITISH COLUMBIA COAST ON FRIDAY. FARTHER UPSTREAM...A MORE POTENT
SHORTWAVE TRACKS ACROSS THE NORTHEAST PACIFIC WITH THE GUIDANCE IN
HALF DECENT AGREEMENT OVER THE WEEKEND.

CONSIDERING THE REGION OF COMPLEXITY SPANNING FROM THE WESTERN
PACIFIC TOWARD THE BERING SEA...MODELS DISPLAY A BROAD SPECTRUM OF
CYCLONE DEPTHS WITH THE WAVE TRACKING TOWARD THE WESTERN BERING
SEA ON FRIDAY. THE PAST FOUR RUNS OF THE ECMWF SHOW SURFACE
INTENSITIES AROUND 20 TO POSSIBLY 25 MB STRONGER THAN RECENT
CYCLES OF THE GFS. THE 00Z CMC SIDES SLIGHTLY TOWARD THE ECMWF
ALTHOUGH THE LARGE DIFFERENCE BETWEEN THE GFS/ECMWF IS SOMEWHAT
CONCERNING. EVENTUALLY ANOTHER POTENT WAVE BEGINS TO TRACK TOWARD
THE ALEUTIANS ON SUNDAY/MONDAY WITH GLOBAL MODELS SCATTERED ALL
ABOUT WITH INTENSITY AND TIMING. THE 00Z ECMWF/18Z GFS SHOW SOME
OF THE MORE AGGRESSIVE FORECASTS BUT THIS IS A DAY 7 FORECAST AND
MODELS SHOW A VARIETY OF SOLUTIONS AT THIS JUNCTURE.
ELSEWHERE...THE AFOREMENTIONED NEGATIVE HEIGHT ANOMALY
ENCOMPASSING THE NORTHEAST PACIFIC IS THE MOST AGREEMENT UPON
SECTOR OF THE MAP. DURING THE WEEKEND INTO EARLY NEXT WEEK...A
FAIRLY PRONOUNCED SYSTEM BEGINS TO AFFECT THE BRITISH COLUMBIA
COAST WITH SOME NORTH/SOUTH VARIABILITY AMONG THE OPERATIONAL
SOLUTIONS. ENSEMBLE MEANS ARE WELL CLUSTERED ALTHOUGH THE 00Z
ECMWF ENSEMBLE MEAN IS ACTUALLY 6 HOURS AHEAD OF THE REST OF THE
PACK WHILE THE PREVIOUS 12Z ENSEMBLE IS MORE FITTING.

GIVEN THE UNCERTAINTIES WITH THE STORM TRACK MOVING INTO THE
BERING SEA...HAD TO INCORPORATE MOSTLY ENSEMBLE GUIDANCE EXCEPT
FOR DAYS 4/5...FRIDAY/SATURDAY TO GET SOME DEPTH ON RELEVANT
FEATURES. KEPT ANY WHERE FROM 20 TO 40 PERCENT OF THE 00Z
GFS/ECMWF THROUGH DAY 5 BEFORE UTILIZING SOLELY ENSEMBLE MEANS.
FOR THE DAY 6-8 PERIOD...OCTOBER 1-3...WENT WITH 40 PERCENT EACH
OF THE 00Z GEFS/PREVIOUS 12Z ECMWF ENSEMBLE MEANS WHILE 20 PERCENT
WAS DEDICATED TO THE 00Z NAEFS ENSEMBLE MEAN.

REGARDING THE UPCOMING SENSIBLE WEATHER...HIGH TEMPERATURES ACROSS
MUCH OF THE STATE WILL RESIDE IN THE 40S TO PERHAPS LOWER 50S
EXCEPT FOR THE NORTHERN EXTENT OF AK AS READINGS STAY IN THE 30S.
OVER INTERIOR AK AND ACROSS THE HIGHER TERRAIN...OVERNIGHT
TEMPERATURES WILL GO BELOW FREEZING ALLOWING FOR ANY PRECIPITATION
TO FALL AS SNOW. WHILE MODELS ARE NOT OVERWHELMING IN TERMS OF
PRECIPITATION AMOUNTS...AN ACTIVE STORM TRACK SHOULD KEEP THE
THREAT FOR RAIN/SNOW IN THE PICTURE. ALSO GUSTY WINDS ARE LIKELY
WITH ANY SYSTEM THAT INTENSIFIES TO THE LIKES OF THE 00Z ECMWF.
HOWEVER...THIS IS NOT DEPICTED EXPLICITLY IN THE GRIDS GIVEN THE
HEAVIER USE OF ENSEMBLE MEANS.


RUBIN-OSTER


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