Gale Warning 168, NWS Anchorage, AK

AREA FORECAST DISCUSSION
SOUTHCENTRAL AND SOUTHWEST ALASKA
500 PM AKST SAT FEB 7 2009

.ANALYSIS AND UPPER LEVELS...

ALOFT...A POWERFUL UPPER VORTEX OVER THE ARCTIC SEA CENTERED NEAR
80N 160W AND AN AMPLIFIED RIDGE BUILDING OVER THE WESTERN BERING
SEA ARE THE KEY FEATURES AFFECTING THE WEATHER OVER THE STATE FOR
THE NEXT FEW DAYS. THE HEIGHT GRADIENT BETWEEN THE AMPLIFYING RIDGE
AND A STRONG SHORTWAVE MOVING INTO SOUTHWEST ALASKA IS INTENSIFYING
A POWERFUL NORTHWESTERLY JET STREAK RESPONSIBLE FOR BRINGING BLIZZARD
CONDITIONS CURRENTLY TO THE SOUTHERN ALASKA PENINSULA AS WELL AS THE
KUSKOKWIM DELTA. A BROADER SET OF BAGGY SHORTWAVES IS CENTERED
OVER SOUTHCENTRAL AND IS RESPONSIBLE FOR MOSTLY CLEAR SKIES AROUND
ANCHORAGE AND THE MAT-SU VALLEY OUTSIDE OF SOME PATCHY FOG.  OUT
WEST...ANOTHER STRONG UPPER LOW JUST TO THE WEST OF THE AMPLIFYING
RIDGE IS SUPPORTING STRONG WINDS TO HURRICANE FORCE IN THE WESTERN
ALEUTIANS.

AT THE SURFACE...A TROUGH CURRENTLY EXTENDS NORTH AND WESTWARD FROM
THE 982 MB LOW SOUTH OF YAKUTAT THROUGH THE WESTERN GULF AND ACROSS
THE INTERIOR OF SOUTHWEST ALASKA. ON THE WEST AND SOUTH SIDE OF THIS
FRONT STRONG COLD ADVECTION IS RESPONSIBLE FOR BRINGING WIND GUSTS
OF UP TO 50 MPH AND BITTER COLD TEMPERATURES ACROSS THE ALASKA
PENINSULA AND KUSKOKWIM DELTA REGIONS...WHILE SURFACE CONVERGENCE
WITH THE TROUGH IS FUNNELING COLD AIR SOUTHWARD FROM THE INTERIOR
INTO THE SUSITNA VALLEY TO THE NORTH OF THE FRONT. STRONG RIDGING
CONTINUES TO BUILD ACROSS THE CENTRAL ALEUTIANS AND THE WESTERN AND
CENTRAL BERING SEA. A POWERFUL 965MB LOW SOUTH OF SHEMYA IS BRINGING
HURRICANE FORCE WINDS TO THE WESTERN ALEUTIANS.

.MODEL DISCUSSION...THE GFS WAS USED FOR THIS FORECAST PACKAGE
MAINLY FOR ITS CONSISTENCY WITH PREVIOUS FORECASTS ...THOUGH IT
WAS NOT MUCH DIFFERENT THAN THE ECMWF AND THE NAM IN THE NEAR TERM.
DIFFERENCES BEGIN TO EMERGE ABOUT FOUR DAYS OUT...SO PERSISTENCE
FROM PREVIOUS FORECASTS WAS USED FROM THAT POINT ONWARDS THROUGH
THE EXTENDED. THE BIG DIFFERENCE OUT AT THAT TIME FRAME IS THE GFS
IS MUCH STRONGER THAN THE ECMWF WITH THE UPPER LOW (CURRENTLY WEST
OF SHEMYA) MOVING TOWARDS THE NORTHERN GULF TOWARDS THE MIDDLE PART
OF NEXT WEEK.

.SHORT TERM FORECAST...A LOW AMPLITUDE PROGRESSIVE SOUTHERN STREAM
PATTERN WILL DOMINATE THROUGH ABOUT MID WEEK RESULTING IN A FAIRLY
RAPID STORM TRACK FROM WEST TO EAST ACROSS THE FORECAST AREA.
MEANWHILE...A HEMISPHERIC VORTEX OVER THE BEAUFORT SEA WILL SINK
SOUTHWARD TOWARD MAINLAND ALASKA HELPING TO MAINTAIN THE ARCTIC AIR
MASS OVER THE NORTHERN HALF OF THE STATE. THE SOUTHERN STREAM TROUGH
CURRENTLY CENTERED OVER MAINLAND ALASKA AND THE GULF WILL AMPLIFY
TONIGHT AND SUNDAY AS SHORT-WAVES ALONG THE BACK SIDE OF THE TROUGH
DROP SOUTHWARD...WHILE THE TROUGH ITSELF PROGRESSES EASTWARD.
MODERATE TO STRONG COLD ADVECTION WILL SHIFT EASTWARD TOWARD THE
NORTHERN ALASKA PENINSULA AND KODIAK ISLAND WATERS PRODUCING
WIDESPREAD GALE FORCE WINDS WITH STORM FORCE GUSTS THROUGH THE
USUAL GAPS IN THE ALASKA PENINSULA. BLIZZARD CONDITIONS ARE ONGOING
ALONG THE KUSKOKWIM DELTA COAST AND SOUTHERN ALASKA PENINSULA...BUT
AS AREA OF STRONGEST COLD ADVECTION SHIFTS EASTWARD THIS EVENING
SNOW SHOWERS AND WINDS SHOULD BOTH BEGIN TO DIMINISH...SO BLIZZARD
CONDITIONS SHOULD END BY THIS EVENING. CURRENT WARNINGS EXTEND ONLY
THROUGH THIS AFTERNOON SO MAY HAVE TO EXTEND THESE INTO THE EARLY
EVENING WITH THE AFTERNOON FORECAST PACKAGE. IN ADDITION TO COLD
ADVECTION SNOW SHOWERS UPPER LEVEL SHORT-WAVES WILL TRACK
SOUTHEASTWARD ACROSS SOUTHWEST ALASKA AND PASS SOUTH OF KODIAK
ISLAND TONIGHT AND SUNDAY...GENERATING AREAS OF LIGHT SNOW ACROSS
SOUTHWEST ALASKA. MEANWHILE...COLD AIR WILL BEGIN TO FILTER INTO
SOUTHCENTRAL TONIGHT AND HELP TO MAINTAIN A WEAK SURFACE LOW OVER
THE NORTHEASTERN GULF. THE COLD ADVECTION COMBINED WITH A BIT OF
PRESSURE GRADIENT WILL PRODUCE GUSTY GAP WINDS ALONG THE ALASKA
RANGE AS WELL AS THE COASTAL RANGES THROUGH SUNDAY MORNING. WITH
VERY COLD TEMPERATURES ALREADY IN PLACE OVER THE NORTHERN SUSITNA
VALLEY WILL LIKELY NEED TO ISSUE A WIND CHILL ADVISORY BEGINNING
THIS EVENING AS WINDS INCREASE THROUGH BROAD PASS. WINDS WILL
REMAIN LIGHT IN ANCHORAGE AND VICINITY...SO FOG WILL CONTINUE TO
FLOAT AROUND THE AREA. WINDS WILL QUICKLY DIMINISH BEHIND THE
UPPER TROUGH LATE SUNDAY INTO MONDAY LEAVING THE SOUTHERN MAINLAND
COLD AND DRY.

THE STRONG LOW SOUTHWEST OF THE ALEUTIANS WILL TRACK NORTHWARD INTO
THE WESTERN BERING WHILE THE SOUTHERN PORTION OF THE FRONT IS PUSHED
EASTWARD ACROSS THE ALEUTIANS AND NORTHERN PACIFIC. THIS WILL HELP
TO ELONGATE AND WEAKEN THE FRONT WITH TIME. THERE APPEARS TO BE
ENOUGH WARM AIR ALONG THE FRONT TO PREVENT BLIZZARD CONDITIONS
ALONG THE ALEUTIANS AND SOUTHERN ALASKA PENINSULA. HOWEVER...BLIZZARD
CONDITIONS LOOKS LIKE A STRONG POSSIBILITY FOR THE PRIBILOF ISLANDS
BEGINNING LATE SUNDAY AFTERNOON AS THE FRONT APPROACHES FROM THE
SOUTH. THE MODELS INDICATE THE FRONT WILL NOT ACTUALLY REACH THE
PRIBILOFS...BUT PRECIPITATION AHEAD OF THE FRONT SHOULD EASILY MAKE
IT THERE. THEREFORE...WILL ISSUE A BLIZZARD WATCH WITH THE AFTERNOON
PACKAGE. A TRIPLE POINT LOW WILL FROM SOUTH OF THE EASTERN ALEUTIANS
SUNDAY AFTERNOON AND THEN QUICKLY TRACK SOUTH OF THE ALASKA PENINSULA
AND GULF OF ALASKA. DEPENDING ON THE EXACT TRACK THERE COULD BE A
PERIOD OF INTENSE SNOW AND LOW VISIBILITY ALONG THE SOUTHERN ALASKA
PENINSULA ON THE BACK SIDE OF THE LOW...BUT IF THIS DOES OCCUR IT
SHOULD EB SHORT-LIVED. ANOTHER STRONG LOW WILL WILL DEVELOP OVER
THE NORTHWEST PACIFIC AND TRACK TO THE WESTERN ALEUTIANS MONDAY
BRINING ANOTHER ROUND OF WIND AND PRECIP. AT THIS POINT IT LOOKS
LIKE THIS LOW WILL BE WEAKER THAN THE CURRENT SYSTEM. WITH A LESS
AMPLIFIED RIDGE OUT AHEAD OF IT THE FRONT WILL BE ABLE TO PROGRESS
ALTHOUGH THE WAY TO SOUTHWEST ALASKA BY TUESDAY.

.LONG TERM FORECAST...THERE IS VERY LOW CONFIDENCE IN PLACEMENT OF
STORM SYSTEMS MID TO LATE WEEK. HOWEVER...MODELS AGREE ON THE
LONG-WAVE PATTERN WITH TROUGH CENTERED OVER THE BERING AND NORTHWEST
PACIFIC AND A RIDGE BUILDING OVER THE EAST PACIFIC UP TOWARD THE GULF
OF ALASKA. THUS...EXPECT AN ACTIVE STORM TRACK ACROSS THE BERING SEA
AND ALEUTIANS TO MAINLAND ALASKA. AT THIS POINT IT LOOKS LIKE THE
LONG-WAVE PATTERN OVER ALASKA WILL REMAIN LOW AMPLITUDE...SO THE BULK
OF THE WARM AIR SHOULD REMAIN TO THE SOUTH OVER THE PACIFIC AND ANY
THE DOMINANT PRECIPITATION TYPE ACROSS THE FORECAST AREA SHOULD BE
SNOW.

.AER/ALU...WATCH/WARNING SUMMARY
PUBLIC...HIGH WIND WARNING 191. BLIZZARD WARNING 181.
         BLIZZARD WATCH 195.
MARINE...HURRICANE FORCE WIND WARNING...176.
         STORM WARNING...172 175.
         GALE WARNING...121 129 130 132 150 155 160 165 170 171 179
         180 185.
         HEAVY FREEZING SPRAY...121 129 130 138 150 155 160 165
         179.
FIRE WEATHER...NONE.

CC/SEB FEB 09