ALASKA EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 113 PM EST MON FEB 17 2014 VALID 12Z FRI FEB 21 2014 - 12Z TUE FEB 25 2014 THE 17/00Z ECENS AND 17/06Z GEFS MEANS COVERED THE SYNOPTIC-SCALE FLOW THROUGH DAY 7-8 WITHOUT MUCH FANFARE. FOR THIS FORECAST PERIOD AND PRIMARILY BELOW THE 500-MB LEVEL...THE DETAILS OF INDIVIDUAL SHORTWAVES ROTATING AROUND A BROAD BUT TRANSITORY UPPER-LEVEL LOW ARE LESS DEFINITIVE AFTER DAY 5. THE FACT THAT THE BROAD CYCLONIC FLOW IN THE BERING SEA AND OVER THE ALEUTIANS TRANSITIONS TO MORE OF A ZONAL-INFLUENCED PATTERN...ELONGATES AND ATTEMPTS TO BECOME A NEGATIVE-TILT TROUGH DOWNSTREAM DURING A 5-DAY PERIOD...IS NOT COMPLEX ENOUGH. ITS THAT THE ENTIRE TROUGH SUBTLY SHEARS SOUTHEASTWARD AND DRIFTS AWAY FROM SOUTHWEST ALASKA AND THE GULF OF ALASKA. WHAT SEEMS TO BE THE EASY PART OF THE FORECAST--(IF THERE IS ONE)-- THE MEAN SOLUTIONS ESTABLISH A TIGHT WESTERLY GRADIENT AT 500 MBS FROM THE EAST ASIAN COAST TO 140W ALONG 40 NORTH LATITUDE. THIS PURE ZONAL CONFIGURATION SUGGESTS LITTLE CHANCE FOR AMPLIFICATION OF THE PATTERN...SOMETHING THAT HAS PLAGUED THE WEST COAST OF NORTH AMERICA AND ALASKA FROM TIME TO TIME FOR THE PAST 6 MONTHS. SO EVEN THOUGH AN UPPER-LEVEL RIDGE AXIS ATTEMPTS TO ANCHOR ALONG/JUST WEST OF THE CANADIAN ROCKIES AND CASCADIA...THE TRAJECTORY OF THE UPPER-LEVEL FLOW EVENTUALLY TILTS THE SOUTHERN END OF THE RIDGE (THE CONCEPT OF A NEGATIVE-TILT TROUGH)...BY DEFAULT...EMERGES FROM THE PATTERN AT MID LATITUDES. THIS ALLOWS A WESTWARD SPILLAGE OF THE CANADIAN CONTINENTAL AIR MASS INTO THE PANHANDLE...THE INSIDE PASSAGE AND OUT INTO THE FAR EASTERN FRINGES OF THE GULF OF ALASKA. WHAT HAS BEEN MY EXPERIENCE PROGGING THESE 'UNDERCUTTING" AND "NEGATIVELY-TILTED" SCENARIOS IS ROOTED IN THE SPRING TRANSITION SEASON (GENERALLY MARCH-APRIL) AND WITH THE 700MB FLOW PATTERN. BELIEVE...BOTH ARE SUGGESTING THE TRUE NATURE OF THE PATTERN...EVEN WHEN THE 500MB PATTERN SUGGESTS A CLASSIC TROUGH-RIDGE-TROUGH FOR THE EASTERN PACIFIC TO THE WESTERN ATLANTIC. WHERE THE DETAILS FALTER...IS WHEN AND WHERE THE ZONAL FLOW (AND ESPECIALLY AT THE 700MB LEVEL) WHEN AND WHERE THE WEST-NORTHWESTERLY 700MB FLOW BEGINS TO GENERATE ENOUGH CYCLONIC CURVATURE...TO SET A WAVE INTO MOTION. AND UNTIL THE BROAD LOW DOES SHUFFLE GRADUALLY AWAY FROM THE CENTRAL ALEUTIANS AND SOUTHWEST COAST...THE FLOW IN THE LOWER LEVELS REMAINS BROAD EAST TO SOUTHEAST THROUGHOUT A MAJOR PORTION OF THE GULF OF ALASKA. AND IN THIS SLOW-DEVELOPING SITUATION...SEVERAL 700MB SHORTWAVES...INCLUDING A SUBTROPICAL MOISTURE PLUME...WILL HAVE AMPLE OPPORTUNITY TO BRUSH ACROSS THE PANHANDLE AND SHEAR NORTHWARD...IN A PREVAILING EASTERLY LOW-LEVEL FLOW PATTERN AND ROTATE BACK TOWARDS THE UPPER-LEVEL LOW AS A ZONE OF 850MB-700MB WARM ADVECTION. NEITHER THE GFS...CANADIAN OR ECMWF DETERMINISTIC RUNS HAVE LOCKED IN WITH ANY SUBSTANTIAL DETAIL WITH A WIDE SPECTRUM OF 700MB LOW SOLUTIONS. IN FACT...THEY ARE ALL OVER THE PLACE WITH RESPECT TO LATITUDE AND LONGITUDE. AND THE SAVING GRACE...IN ALL THIS...IS AND ALWAYS HAS BEEN...OUR SATELLITE NETWORK OVER THE PACIFIC. FOR NOW...THE ENSEMBLE MEANS ARE THE BEST GUIDANCE TOOL WITH THE ENTIRE FLOW PATTERN TRANSITION...AND INDIVIDUAL SHORTWAVE DEVELOPMENT...WILL BE A SHORT-TERM FORECAST ISSUE AND CHALLENGE. VOJTESAK