Prognostic Meteorological Discussion
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FXAK02 KWNH 301917
PMDAK

ALASKA EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
316 PM EDT SAT APR 30 2016

VALID 12Z WED MAY 04 2016 - 12Z SUN MAY 08 2016

THE GENERAL THEME FOR THE MEDIUM RANGE PERIOD IS --- TRANSITION.
WHAT HAS BEEN A RATHER STABLE AND PERSISTENT UPPER-LEVEL RIDGE
PATTERN ALONG THE WESTERN MOUNTAINS OF CANADA --- SLOWLY ERODES
THROUGH THE EARLY PORTION OF THIS FORECAST PERIOD. IN TURN...
CYCLONIC ACTIVITY BEGINS TO EDGE EASTWARD INTO THE SOUTHERN BERING
AND NORTHERN GULF OF ALASKA. THEN LATE IN THE PERIOD ... THE RIDGE
ATTEMPTS TO REBUILD OVER NORTHERN ALASKA AND NORTHWEST CANADA. IN
RESPONSE... CYCLONIC ACTIVITY AND STORM TRACK BEGINS TO SHIFT
SOUTHWARD AND PACIFIC ENERGY IS DIVERTED AWAY FROM THE INTERIOR.

UNFORTUNATELY... THE PATTERN SELDOM BEHAVES ACCORDING TO A
`PERFECT PROG` PLAN . ALTHOUGH THE DAY 7-8 SURFACE GRAPHICS/500MB
PROGS DO REBUILD A WELL-DEFINED RIDGE AXIS DOWNSTREAM OF ALASKA
---THE ENSEMBLE MEANS ESTABLISH A MEAN LONG WAVE TROUGH ALONG 150W
--- JUST BEYOND DAY 8. THIS LONGITUDE IS NOT QUITE FAR ENOUGH
DOWNSTREAM FROM THE GULF OF ALASKA --- TO COMPLETE THE
`TRANSITION` BACK TO WHAT HAD BEEN THE PERSISTENCE `SPRING
PATTERN`. THERE WILL BE RESIDUAL TROUGHING ALOFT ALONG 160W --- TO
PERHAPS --- KEEP THE GULF `ACTIVE` EVEN WHEN THERE IS MORE
STABILITY OVER THE INTERIOR. SO THE FORECAST DOES CARRY SOME
`UNCERTAINTY` AFTER DAY 7 --- AND CONSIDERED THE THREE MEANS ---
THE `BEST AVAILABLE` BLEND FOR THE GRIDDED FILES.

THE 30/00Z ECENS...GEFS...NAEFS MEANS AND 30/00Z DETERMINISTIC
ECMWF/GFS COMPRISED THE EARLY PORTION OF THIS MEDIUM RANGE PERIOD.
AFTER 5/00Z HOWEVER...ONLY THE 30/00Z GFS SEEMED WORTH `HOLDING
ONTO` --- GIVEN THE MULTIPLE SHORTWAVE SOLUTIONS THAT WILL TEST
THE DOWNSTREAM RIDGE IN THE COMING DAYS.

OVERALL... THE MEANS WILL LIKELY CAPTURE THE SUBTLE CHANGES IN THE
STRUCTURE OF THIS PERSISTENT UPPER-LEVEL RIDGE AXIS. AND HOPEFULLY
... WILL ACCURATELY TIME THE `ENDING` OF AN `UNSETTLED WEATHER
PATTERN` STATEWIDE.

VOJTESAK

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