Gale Warning 568, NWS Anchorage, AK

AREA FORECAST DISCUSSION
SOUTHCENTRAL AND SOUTHWEST ALASKA
145 PM AKDT WED SEP 2 2009

.ANALYSIS AND UPPER LEVELS...LARGE CLOSED LOW CENTERED OFF THE
KUSKOKWIM DELTA COAST THIS MORNING IS DIVING SOUTHEAST TOWARD
THE GULF/NORTHEAST PACIFIC. THIS MOTION IS PRIMARILY DRIVEN BY
A STRONG TROPICAL JET (ESTIMATED MAX SPEED AROUND 120 KT) WHICH
EXTENDS FROM THE WESTERN ALEUTIANS SOUTHEASTWARD INTO THE BASE
OF THE TROUGH...ACTUALLY PRODUCING STRONG DIGGING OF A SLIGHTLY
OUT-OF PHASE SHORT-WAVE AT THE BASE OF THE TROUGH. A SURFACE LOW
HAS NOW DEVELOPED JUST AHEAD OF THIS SHORT-WAVE. THIS JET WILL
SOON COMBINE WITH A MUCH WEAKER JET ON THE WEST SIDE OF THE UPPER
LOW AND PUSH THE ENTIRE SYSTEM DOWN INTO THE NORTHEAST PACIFIC.

SYNOPTIC LIFT OUT AHEAD OF THE UPPER TROUGH CONTINUES TO PRODUCE
AREAS OF RAIN ACROSS THE SOUTHERN MAINLAND. A SURFACE RIDGE
EXTENDING FROM SOUTHEAST ALASKA ALONG THE NORTH GULF COAST HAS
PRODUCED STRONG GRADIENT WINDS WITH DOWNSLOPING ON THE LEE SIDE
OF THE CHUGACH AND KENAI MOUNTAINS. HOWEVER...THE RIDGE IS NOW
BEING PUSHED INLAND AS THE EASTERN BERING TROUGH BEGINS TO
TRANSITION TO THE GULF AND WINDS ARE DIMINISHING...SO THAT
LIGHT RAIN IS AGAIN FALLING ON THE LEE SIDE OF THE MOUNTAINS.

A TIGHT GRADIENT ON THE BACK SIDE OF THE EASTERN BERING LOW
ALONG WITH DECENT JET SUPPORT ALOFT AND LOW LEVEL COLD ADVECTION
ARE WORKING TO PRODUCE GUSTY WINDS FROM THE CENTRAL BERING DOWN
TO THE EASTERN ALEUTIANS AND SOUTHERN ALASKA PENINSULA. A SHORT-WAVE
RIDGE DOMINATES WEST OF HERE...ALTHOUGH A SHORT-WAVE EMBEDDED WITHIN
THE WESTERLIES IS AIDING IN DEVELOPMENT OF A SURFACE LOW SOUTHWEST
OF THE WESTERN ALEUTIANS.

.MODEL DISCUSSION...MODELS ARE IN GENERAL AGREEMENT THROUGH THE
FIRST FEW DAYS OF THE FORECAST. MINOR DIFFERENCES CONTINUE IN
HANDLING OF EASTERN BERING SURFACE LOW AS IT TRANSITIONS TO THE
GULF AND ITS INTERACTION WITH THE NEWLY FORMED LOW NEAR THE BASE
OF THE TROUGH. THE GFS/NAM SWING THE NEW LOW TOWARD THE QUEEN
CHARLOTTE ISLANDS AND ABSORB THE BERING LOW INTO THIS NEW LOW.
THE ECMWF/UKMET QUICKLY MERGE THE TWO LOWS BACK TO THE WEST
SOUTHEAST OF KODIAK ISLAND. A RUN-TO-RUN COMPARISON SHOWS EACH
MODEL TO BE CONSISTENT WITH ITSELF. CURRENT FORECAST IS NOT TOO
FAR FROM THE GFS/NAM SOLUTION SO PLAN TO STICK WITH THAT FOR NOW.
THE GREATEST IMPACT FROM THIS WOULD BE FOR THE GULF AND KODIAK
ISLAND.

MODELS HAVE A MORE SIMILAR HANDLING OF THE LOW MOVING TOWARD THE
WESTERN ALEUTIANS...ALTHOUGH THERE IS A DEFINITE TREND TOWARD
SLOWER EASTWARD PROGRESSION WHICH WILL BE REFLECTED IN THE NEW
FORECAST.

THERE HAS BEEN A CHANGE IN THE HANDLING OF THE RIDGE BUILDING
OVER MAINLAND ALASKA THIS WEEKEND. THE LATEST MODEL SOLUTIONS
KEEP THE RIDGE CENTERED FROM INTERIOR ALASKA TO SOUTHWEST
ALASKA...WHICH ALLOWS A SHORT-WAVE TO DROP SOUTHWESTWARD FROM
THE YUKON TO ANCHORAGE ON SATURDAY. PRIOR TO THIS MODELS HAD
CONSISTENTLY SHOWED THIS PASSING TO THE EAST ACROSS THE EASTERN
COPPER RIVER BASIN. SINCE THIS WOULD BE A DRAMATIC CHANGE AM
NOT READY TO BUY INTO IT YET...BUT WITH AGREEMENT AMONG MODELS
WILL TREND TOWARD MORE CLOUDS AND INTRODUCE A CHANCE OF RAIN AS
FAR WEST AS ANCHORAGE.

.SHORT TERM FORECAST...

SOUTHCENTRAL ALASKA...AS THE EASTERN BERING LOW TRANSITIONS
TO THE GULF TONIGHT FLOW WILL GRADUALLY BACK TO THE EAST AND
EVENTUALLY NORTH...RESULTING IN DRYING OVER INTERIOR SOUTHCENTRAL
TONIGHT AND SPREADING TOWARD THE COAST THURSDAY MORNING. THIS
PROCESS WILL CONTINUE ON THURSDAY AS THE UPPER LEVEL LOW MOVES
TO A POSITION SOUTH OF THE GULF...SO THAT ALL OF SOUTHCENTRAL
(INCLUDING KODIAK ISLAND) WILL DRY OUT BY THURSDAY NIGHT. FRIDAY
LOOKS LIKE A NICE DAY ACROSS THE REGION AS HIGH PRESSURE WILL
BUILD FROM THE BERING SEA TO INTERIOR ALASKA. HOWEVER...MODELS
NOW INDICATE THAT A SHORT-WAVE WILL NOW TRACK BETWEEN THE INTERIOR
RIDGE AND THE NORTHEAST PACIFIC LOW...HEADING FROM THE YUKON
SOUTHWESTWARD ACROSS THE COPPER BASIN TO ANCHORAGE AND THE KENAI
PENINSULA. IF SO...SATURDAY WILL BE A RAINY DAY. HOWEVER...CONFIDENCE
IN THE TRACKING OF THIS FEATURE IS LOW...SO WILL WILL HAVE TO
REEVALUATE THIS WITH THE NEXT FORECAST TONIGHT. EVEN IF IT DOES
MATERIALIZE IT LOOKS LIKE IT SHOULD CLEAR BACK OUT BY SUNDAY.

SOUTHWEST ALASKA...AREAS OF RAIN WILL CONTINUE THROUGH TONIGHT
AS THE LOW PASSES ACROSS BRISTOL BAY AND ACROSS THE ALASKA PENINSULA.
RAIN SHOWERS WILL THEN END FROM NORTH TO SOUTH ON THURSDAY AS THE
SYSTEM MOVES DOWN INTO THE GULF AND NORTHEAST PACIFIC. HIGH PRESSURE
WILL THEN BUILD IN AND PROVIDE A NICE WEEKEND. AN ALEUTIAN LOW WILL
APPROACH THE REGION BY MONDAY...BRINGING A RETURN TO CLOUDS AND RAIN.

BERING SEA/ALEUTIANS...THE LOW FORMING SOUTHWEST OF THE ALEUTIANS
HAS GOOD UPPER SUPPORT AND WILL CONTINUE TO STRENGTHEN AS IT
APPROACHES THE WESTERN ALEUTIANS TONIGHT AND THURSDAY. WITH A RIDGE
TO THE EAST THE SURFACE PRESSURE GRADIENT WILL BE TIGHT...SO EXPECT
GALE FORCE WINDS TO ACCOMPANY THE STORM. THE LOW WILL QUICKLY MATURE
INTO A VERTICALLY STACKED CUT-OFF LOW WITH LITTLE IN THE WAY OF
STEERING FLOW. THUS..THE LOW WILL STALL SOUTH OF SHEMYA THURSDAY
NIGHT AND THEN BEGIN A SLOW EASTWARD TRACK ALONG THE ALEUTIANS
THIS WEEKEND AS THE FLOW BECOMES A LITTLE MORE PROGRESSIVE.

.LONG TERM FORECAST...CONFIDENCE IN THE MEDIUM RANGE IS QUITE
LOW DUE TO LARGE MODEL DIFFERENCES AND RUN-TO-RUN INCONSISTENCIES.
AN ENSEMBLE APPROACH IS POINTING TOWARD A HIGH AMPLITUDE TROUGH
CENTERED AROUND 160W. THIS WOULD FOCUS STORM SYSTEMS SOUTH OF THE
ALASKA PENINSULA AND INTO THE GULF...PRODUCING WET AND COOL
CONDITIONS OVER MAINLAND ALASKA.

.AER/ALU...WATCH/WARNING SUMMARY
PUBLIC...NONE.
MARINE...GALE WARNING...175 176.
FIRE WEATHER...NONE.

SEB SEP 09