Prognostic Meteorological Discussion
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FXAK02 KWNH 211940
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ALASKA EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
239 PM EST TUE NOV 21 2017

VALID 12Z SAT NOV 25 2017 - 12Z WED NOV 29 2017

THE MEDIUM RANGE FORECAST PERIOD OVER ALASKA BEGINS WITH
RELATIVELY GOOD MODEL CONSENSUS AND ABOVE AVERAGE PREDICTABILITY.
ON DAY 4 (SAT), MODELS SHOW AVERAGE TO SLIGHTLY ABOVE AVERAGE
CONSENSUS WITH RESPECT TO AN ONGOING LOW PRESSURE SYSTEM IN THE
GULF OF AK.  AN ADDITIONAL SHORTWAVE IS FORECAST TO CROSS THE
ALEUTIANS DURING THIS PERIOD, WITH A SURFACE LOW PERHAPS
DEVELOPING ACROSS THE MIDDLE/EASTERN ALEUTIANS SAT-MON. A BLEND OF
THE DETERMINISTIC 00Z ECMWF/GFS/CMC (MOST WEIGHT PLACED TOWARD THE
ECMWF) SERVED AS A GOOD STARTING POINT DURING DAYS 3-4.

FORECAST CONFIDENCE DEGRADES RATHER QUICKLY THROUGH TIME, HOWEVER,
AND PREDICTABILITY IS VERY LOW BY THE END OF THE PERIOD. A LARGE
PORTION OF THE RESPONSIBILITY FOR THE INCREASING CHAOS IN THE FLOW
PATTERN SEEMS TO BE TIED TO THE BREAKDOWN OF A REX BLOCK ACROSS
THE CENTRAL PACIFIC, THE NORTHERN PORTION (ANOMALOUS RIDGE) OF
WHICH HAS BEEN PERSISTENT ACROSS THE ALEUTIANS/BERING SEA FOR
WEEKS NOW. MODEL CONSENSUS IS GOOD THAT THIS BLOCK WILL BREAK DOWN
BY LATE IN THE SHORT RANGE OR EARLY IN THE MEDIUM RANGE, AT WHICH
POINT A HIGHLY PROGRESSIVE FLOW PATTERN IS EXPECTED TO SET UP
ACROSS ALASKA. WITH THE BREAKDOWN OF THE RIDGE, AN AREA OF
PERSISTENT NEGATIVE HEIGHT ANOMALIES ACROSS EASTERN RUSSIA (-2
STANDARD DEVIATIONS) WILL HAVE INCREASED INFLUENCE ON THE
DOWNSTREAM PATTERN ACROSS AK. A SERIES OF SHORTWAVES EJECTING FROM
THIS FEATURE WILL QUICKLY MOVE EAST ACROSS THE ALEUTIANS.
DIFFERENCES IN TIMING AND AMPLITUDE OF THESE FEATURES, AND THEIR
INTERACTIONS INCREASE THROUGH TIME, AND ENSEMBLE SPREAD BY DAYS
7-8 (NEXT TUE-WED) IS QUITE LARGE. THE 12Z ECMWF RAPIDLY DEVELOPS
A DEEP CYCLONE (955 HPA) IN THE GULF OF AK BY 12Z TUE, BUT
ENSEMBLE SPREAD IS SO LARGE THAT THE ECENS MEAN ONLY SHOWS A VERY
WEAK LOW IN THE VICINITY. DETERMINISTIC SOLUTIONS ARE IN GENERAL
AGREEMENT ON ANOTHER DEEP CYCLONE SOUTH OF THE ALEUTIANS BY THAT
TIME, BUT AGAIN SPREAD IS LARGE. SURFACE LOW ENSEMBLE SCATTER
PLOTS AT THE AFOREMENTIONED TIME SHOW A SMATTERING OF SURFACE LOWS
ACROSS HUNDREDS OF MILES AMONG THE ECENS/GEFS/CMCE ENSEMBLE
MEMBERS. THUS, A GRADUAL TREND TOWARD HEAVIER ENSEMBLE MEAN
WEIGHTING WAS SHOWN FROM DAY 5 ONWARD. CONFIDENCE IN THE SPECIFICS
IS SO LOW DURING DAYS 6-7 THAT THE FORECAST WAS TRENDED ALMOST
ENTIRELY TOWARD ENSEMBLE MEANS.

RYAN

$$





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