Prognostic Meteorological Discussion
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000
FXAK02 KWNH 211841
PMDAK

ALASKA EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
240 PM EDT MON AUG 21 2017

VALID 12Z FRI AUG 25 2017 - 12Z TUE AUG 29 2017

DURING THE FIRST HALF OF THE PERIOD THE DOMINANT FOCUS OF THE FCST
WILL BE ON AN UPR LOW/SFC SYSTEM TRACKING OVER THE GULF OF ALASKA,
BRINGING HEAVY RAIN POTENTIAL TO THE SERN COAST/PANHANDLE REGION.
THE PRIMARY GUIDANCE CONSIDERATION INVOLVED THE 00Z/06Z GFS SFC
LOW TRACK BEING SOMEWHAT FARTHER N THAN THE MAJORITY OF OTHER
MODELS (INCLUDING THE 00Z UKMET/NAVGEM) AND ENSEMBLES.  THE 12Z
GFS HAS TRENDED TO THE CONSENSUS SCENARIO, ALLOWING FOR PRIMARY
EMPHASIS ON THE 00Z ECMWF/12Z GFS FOR THE DAYS 4-5 FRI-SAT PERIOD.
 THE SYSTEM SHOULD WEAKEN BY DAY 6 SUN.

THERE IS REASONABLE AGREEMENT IN PRINCIPLE THAT UPSTREAM FLOW
SHOULD BRING AN ELONGATED SYSTEM ACROSS THE SRN BERING
SEA/ALEUTIANS DURING THE WEEKEND/VERY EARLY NEXT WEEK.  GENERAL
TRENDS FOR THIS FEATURE OVER THE PAST DAY ARE MORE SUPPRESSED IN
RESPONSE TO A STRONGER RIDGE ALOFT OVER ERN SIBERIA.  THE 12Z GFS
REVERSED THIS TREND A BIT.  MOST GUIDANCE EXPECTS ENERGY WITHIN
THIS NRN PAC/ALEUTIANS FLOW AND PERHAPS WITH SOME CONTRIBUTION
FROM LINGERING ENERGY OVER THE WRN MAINLAND TO SUPPORT NRN PAC LOW
PRESSURE REACHING THE VICINITY OF KODIAK ISLAND BY DAY 8 TUE, WITH
THE ASSOC UPR LOW NOT TOO FAR FROM THE POSN OF THE UPR LOW AT THE
START OF THE PERIOD.  FOR THIS LATTER PERIOD EVOLUTION SFC/ALOFT,
THE ENSEMBLE MEANS ARE SIMILAR IN PRINCIPLE BUT FASTER TRENDS OVER
THE PAST DAY AND THE 00Z CMC MEAN SEEM TO FAVOR LEANING MORE
TOWARD THE ECMWF MEAN FOR THIS PART OF THE FCST RELATIVE TO THE
SLOWER GEFS MEAN.  THE 00Z ECMWF/CMC ARE THE CLOSEST OPERATIONAL
MODELS TO THE MEANS ALOFT WHILE THE 00Z CMC IS A LITTLE FARTHER
EWD AT THE SFC.  RECENT GFS RUNS SET UP THE UPR LOW/TROUGH AND SFC
SYSTEM A BIT ON THE WRN SIDE OF THE SPREAD.  OPERATIONAL RUNS ARE
STILL IN THE EARLY STAGES OF TRYING TO LATCH ONTO THIS SYSTEM SO
CONFIDENCE IN SPECIFICS IS NOT PARTICULARLY HIGH.  HOWEVER THEY DO
SIGNAL POTENTIAL FOR A FAIRLY STRONG STORM, WITH LOWER SFC
PRESSURES THAN FOR THE GULF OF ALASKA SYSTEM EARLIER IN THE PERIOD.

BASED ON ABOVE CONSIDERATIONS, THE 12Z GFS/00Z ECMWF PROVIDED MOST
OF THE INPUT FOR THE DAYS 4-5 FRI-SAT PERIOD WITH THE REST
CONSISTING OF THE 00Z ECMWF MEAN.  THEN THE FCST TRANSITIONED
TOWARD A MORE ENSEMBLE MEAN BASED BLEND BY DAYS 7-8 MON-TUE WITH
HALF 00Z ECMWF MEAN AND THE REST 06Z GEFS MEAN AND 00Z ECMWF.

RAUSCH

$$




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