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Alaska Extended Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 1852Z Apr 08, 2014)
 
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ALASKA EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
252 PM EDT TUE APR 08 2014

VALID 12Z SAT APR 12 2014 - 12Z WED APR 16 2014

THE 08/00Z GFS/ECMWF FORECAST CYCLE CONTINUED THE THEME OF A
PERSISTENT...QUASI-STATIONARY UPPER-LEVEL RIDGE OVER THE WESTERN
MOUNTAINS OF NORTH AMERICA...NESTLED BETWEEN A BROAD TROUGH OVER
HUDSON BAY AND A SECOND BROAD TROUGH EAST OF THE DATELINE BETWEEN
KAMCHATKA AND NORTHERN JAPAN. THE CHALLENGE WITH THIS MEDIUM RANGE
FORECAST WILL BE TO ACCURATELY DEPICT THE UPSTREAM TROUGH
MIGRATION ACROSS THE NORTH PACIFIC THROUGH TIME.--FROM THE
SOUTHWEST BERING SEA-KAMCHATKA PENINSULA (ON DAY 4) TO THE WEST
CENTRAL ALEUTIANS (ON DAY 8).

SENSIBLE WEATHER IMPACTS LIE AHEAD FOR SOUTHWEST ALASKA...AND THEN
FOR THE SOUTH COAST INTO THE FAR NORTHERN PANHANDLE. BUT THEIR
EFFECTS...HINGE UPON A SHORTWAVE ON DAY 4 ROTATING NORTHWESTWARD
THROUGH THE BASE OF THE UPPER TROUGH. THIS TROUGH AXIS (POSITIONED
EAST OF THE DATELINE) SHEARS DOWNSTREAM INVOF 55N 160W. AT WHAT
LATITUDE WILL THERE BE A WEAKNESS IN THE RIDGE AXIS AS THE
SHORTWAVE SHEARS DOWNSTREAM...IS WHERE THE MORE PROGRESSIVE
GFS/GEFS DIFFERS FROM THE SLOWER...STRONGER ECENS/ECMWF.

THE PERSISTENCE AND FREQUENCY OF ADDITIONAL SHORTWAVE ENERGY
(BEYOND DAY 4) BECOMING ENTRAINED INTO THE BASE OF THE TROUGH
AXIS...HAVE A DIRECT IMPACT ON THE ALEUTIANS AND SOME OF THE
SOUTHWEST COAST WITH RESPECT TO CLOUD COVER AND TEMPERATURES. AND
ELSEWHERE...PRIMARILY A CLOUD COVER ISSUE...AND POSSIBLE TIMING OF
BRIEF MID-LEVEL WARM ADVECTION PRECIPITATION EPISODES FOR THE GULF
OF ALASKA PROPER AND THE WEST CENTRAL COAST-YUKON DELTA...SOUTH
COAST AND INTERIOR.

THE IMPACT UPON THE DOWNSTREAM RIDGE LODGED OVER THE ALASKAN
INTERIOR AND ALONG THE WEST COAST OF NORTH AMERICA REMAINS A
LONGER-TERM CONCERN AND IS DIRECTLY TIED TO THE EXTENT AND OUTCOME
OF RIDGE 'WEAKNESS'. BUT UNLIKE PREVIOUS RUNS OF THE GLOBAL
GUIDANCE---THAT SUGGESTED A VERY AMPLIFIED UPPER-LEVEL FLOW ALONG
THE WESTERN SHORES OF NORTH AMERICAN--THE FLOW PATTERN WILL
TRANSITION TO SOMEWHAT DIFFERENT THAN THE STRONG 'BLOCK' USUALLY
ASSOCIATED WITH HIGHLY-AMPLIFIED FLOW IN THE NORTHEAST PACIFIC.

THE 8/00Z GEFS/ECENS MEANS GENERALLY CAPTURED THE PRIMARY
MECHANISMS FOR SUBTLE FLOW PATTERN CHANGE...AND THOUGHT A 40/60
BLEND OF THE MEANS RESPECTIVELY...WILL BE SUFFICIENT TO DEPICT THE
PRIMARY SURFACE AND 500MB FEATURES. A LACK-OF-DETAIL IN THE 
DETERMINISTIC GUIDANCE HOPEFULLY CAN BECOME RESOLVED THROUGH
TIME...WITH RESPECT TO THE INDIVIDUAL SHORTWAVE TRACK AND
INTENSITY.

VOJTESAK