Prognostic Meteorological Discussion
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FXAK02 KWNH 231935
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ALASKA EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
335 PM EDT TUE AUG 23 2016

VALID 12Z SAT AUG 27 2016 - 12Z WED AUG 31 2016

GUIDANCE CONTINUES TO EXHIBIT SIGNIFICANT SPREAD AND RUN TO RUN
VARIABILITY WITH SYSTEMS... LEADING TO MORE EMPHASIS ON THE 06Z
GEFS AND 00Z ECMWF ENSEMBLE MEAN SOLNS.

THE MODELS CONTINUE TO FORECAST AN UPPER LOW WITH A SURFACE
REFLECTION TO MEANDER OVER THE NORTH PACIFIC ACROSS AREAS SOUTH OF
THE ALEUTIANS.  HOWEVER THERE IS STILL A LOT OF SPREAD FOR WHAT
HAPPENS WHERE THE LOW MERGES WITH TROUGH ENERGY OVER THE CNTRL
BERING SEA...WITH THE LOW POSSIBLY MOVING INTO THE ALEUTIANS OR
STALLING OUT IN THE VICINITY.
THE 12Z GFS TRENDED SEVERAL HOURS FASTER WITH THE NORTHWARD
PROGRESSION OF THE LOW...SO EVEN ALL FORECAST HAVE THE
FEATURE...THE TIMING AND PHASING REMAIN UP FOR DEBATE EARLY TO MID
WEEK NEXT.  THIS LEAD TO A BLEND OF THE 00Z ECMWF/06Z GEFS
ENSEMBLE MEANS.

OVER THE WEEKEND MODELS MAINTAIN A BROAD AREA OF LOW PRESSURE AT
THE SFC AND CYCLONIC FLOW ALOFT IN THE VICINITY OF THE
PANHANDLE/GULF OF ALASKA.  THE 12Z GFS TRENDED TOWARDS A DEEPER
LOW THAT RETROGRADES UNDERNEATH THE UPPER RIDGE WEST ACROSS
SOUTHERN  AK EARLY NEXT WEEK...WHICH APPEARS TO BE AN OUTLIER
SOLUTION.  THE 12Z ECMWF HAS THE 500 MB WAVE REMAINING PROGRESSIVE
AND MOVING DOWNSTREAM EVENTUALLY ONSHORE INTO BRITISH COLUMBIA.
THE PREFERENCE WAS TO USE THE 00Z ECMWF MEAN/06Z GEFS MEAN IDEA OF
THE MID UPPER RIDGE ACROSS WESTERN AK AND THE LACK OF CLOSED LOW
OVER SOUTHERN AK.

FURTHER UNCERTAINTY PERSISTS WITH THE DETAILS OF FLOW EJECTING
FROM THE INITIALLY AMPLIFIED/DEEP TROUGH OVER ERN ASIA AND
DOWNSTREAM RIDGE.  THE 12Z GFS LOW NEAR 58N 159E NEAR KAMCHATKA IS
A WHOPPING 700 NM SOUTH OF ITS 06Z RUN FORECAST POSITION 12Z MON.
THE CONTINUING TROUGH AMPLITUDE/TIMING/PHASING DIFFERENCES LEAD TO
LOW CONFIDENCE IN ANY ONE RUN SO THE GEFS MEAN WAS PROMINENTLY
USED IN THE FORECAST WITH NONE OF THE CURRENT GFS RUN BEYOND DAY
5.

THE ECMWF HAS SHOWN RUN TO RUN CONTINUITY OF TROUGH FRACTURE ONCE
SYSTEM MERGE EAST OF KAMCHATKA PLUS A CLOSED HIGH DEVELOPING NEAR
THE CHUKCHI PENINSULA ON MON 29 AUG.
THE ECMWF ENSEMBLE MEAN HAS THE RIDGE AXIS FURTHER EAST OVER THE
BERING STRAITS...WHICH IT HAD IN YESTERDAY`S RUN AS WELL.  THE
CONTINUING DISAGREEMENT BETWEEN THE OPERATIONAL AND MEAN RUNS
LEADS TO LOW CONFIDENCE HERE AS WELL. A BLEND OF THE 2 MEAN OF THE
ECMWF AND 06Z GEFS WAS USED TO MITIGATE THE CONTINUING
UNCERTAINTY.

MOST OF AK MINUS THE PANHANDLE AND LOWER ALEUTAINS HAVE FORECAST
HEIGHTS ABOVE NORMAL THROUGH THE WEEKEND INTO EARLY NEXT WEEK...SO
ODDS ARE GOOD TEMPERATURES WILL BE ABOVE NORMAL FOR AN EXTENDED
PERIOD FOR MAINLAND LOCATIONS.

PETERSEN

$$





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