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Alaska Extended Forecast Discussion
 
(Latest Discussion - Issued 1841Z Apr 24, 2014)
 
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ALASKA EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
241 PM EDT THU APR 24 2014

VALID 12Z MON APR 28 2014 - 12Z FRI MAY 02 2014

MEDIUM RANGE FORECAST FOR ALASKA WILL TRANSITION FROM
NEGATIVE-TILT UPPER-LEVEL TROUGH ALONG 45-50N LATITUDE AND BROAD
RIDGE OVER THE INTERIOR...TO AN UPPER-LEVEL FLOW PATTERN THAT
FOSTERS DEEPER TROUGH ALONG THE DATELINE AND SOUTHWEST BERING SEA.
DOWNSTREAM...THE UPPER-LEVEL RIDGE RESHUFFLES INTO A CONFIGURATION
THAT SUPPORTS AMPLIFICATION WITH MORE OF A SOUTH-TO-NORTH
ORIENTATION WITH AN AXIS EMERGING ALONG 140W LONGITUDE.

GETTING THERE WILL BE THE FORECAST CHALLENGE. THE GUIDANCE SPREAD
IS MUCH  LARGER THAN NORMAL...WITH THE DETERMINISTIC 24/00Z
GFS/ECMWF AT ODDS WITH SEVERAL KEY ELEMENTS OF THE PATTERN SHIFT.

IN PARTICULAR...THE 'RE-CONFIGURATION' OF THE UPPER-LEVEL RIDGE
HAS A DIRECT IMPACT ON THE SENSIBLE WEATHER DETAILS FOR THE
PANHANDLE. HERE...THE ISSUE IS WITH THE JET STREAM AROUND DAY 5
(29/12Z) AND WHETHER OR NOT...THE NOSE OF THE JET SERVES AS THE
FOCAL POINT FOR MOVING THE HIGH ARCTIC PORTION OF THE UPPER-LEVEL
RIDGE EAST OF THE CANADIAN ROCKIES. THE GFS SAYS YES...THE ECMWF
SAYS NO. A 'YES 'SOLUTION MEANS WETTER AND MILDER PATTERN FOR THE
PANHANDLE WITH IMPULSES GLIDING NORTHEASTWARD FROM THE CENTRAL
PACIFIC. A 'NO SOLUTION' MEANS COLD...CANADIAN OFFSHORE FLOW...AND
MOISTURE DIRECTED INTO THE SOUTHWEST COAST...YUKON DELTA AND
ALEUTIANS.

A SECONDARY BUT IMPORTANT KEY FEATURE OF THE GFS VS ECMWF
SOLUTION...IS THE STRENGTH OF A SERIES OF SHORTWAVES THAT ROTATES
NORTHWARD AT THE BASE OF THE 'FORMING' UPPER-LEVEL TROUGH ALONG
THE DATELINE. THE 24/00Z DETERMINISTIC GFS AND ECMWF ARE 15
DEGREES OF LONGITUDE APART BY DAY 6...WITH THE DEVELOPMENT OF A
STRONG SURFACE WAVE THAT TRACKS NORTHEASTWARD ACROSS THE SOUTHERN
BERING SEA.

THE OVERALL SYNOPTIC-SCALE PATTERN IN THE CENTRAL PACIFIC FAVORS
SOMETHING LESS RADICAL THAN THE 24/00Z GFS BUT SOMETHING A LITTLE
FASTER THAN THE DETERMINISTIC 24/00Z ECMWF. FOR THE WPC MEDIUM
RANGE FORECAST PACKAGE CONCERNING ALASKA...I GAVE SOME WEIGHT TO
THE DETERMINISTIC ECMWF THROUGH DAY 5...THEN TAPERED AWAY FROM ANY
DETERMINISTIC GUIDANCE...AND GAVE MORE WEIGHT TO THE ECENS MEANS
970 PCT) OVER THE GEFS MEANS (30 PCT) AFTER DAY 5. THIS STILL
MAKES FOR A VERY DRAB AND BENIGN SURFACE PRESSURE GRADIENT OVER
MUCH OF INTERIOR ALASKA. I CAN SEE A GENERAL WARM UP DEVELOPING
INLAND...BUT AM NOT PLEASED WITH THE LACK OF LOCAL WIND GENERATION
THAT SHOULD ACCOMPANY THE CHANGE IN THE FLOW PATTERN FOR THE HIGH
LATITUDE PACIFIC.

VOJTESAK