Small Craft Advisory 139, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
1140 AM AKST SUN FEB 20 2011

.ANALYSIS AND UPPER LEVELS...ALASKA IS SITUATED IN BETWEEN A
STRONG NORTH PACIFIC RIDGE AND A DEEP LOW OVER EASTERN RUSSIA.
A ZONAL 145 KT JET EXTENDS ACROSS THE BERING SEA AND SOUTHERN
MAINLAND. AT THE SURFACE A DISSIPATING FRONT IS MOVING FROM
SOUTHWEST ALASKA TO SOUTHCENTRAL ALASKA AND THE GULF. WITH BROAD
WESTERLY LOW LEVEL FLOW THE LOW LEVEL MOISTURE WITH THE FRONT
IS BANKED UP ALONG THE WEST SIDE OF THE ALASKA AND ALEUTIAN RANGE.
THE COMBINATION OF THE WESTERLY FLOW WITH A WEAKENING FRONT MEANS
THERE WILL NOT BE MUCH IMPACT OVER SOUTHCENTRAL ALASKA. COLD
ADVECTION BEHIND THE FRONT IS PRODUCING THE USUAL SNOW SHOWERS
AND GUSTY WINDS ACROSS THE BERING SEA AND INTO SOUTHWEST ALASKA.

.MODEL DISCUSSION...MODELS ARE IN VERY GOOD AGREEMENT FOR THE
NEXT FEW DAYS FOR THE BULK OF THE FORECAST AREA. THE ONE PROBLEM
AREA IS OVER THE WESTERN BERING/ALEUTIANS BEGINNING TUESDAY. A
STATIONARY FRONT SETS UP IN THIS CORRIDOR AND MODELS INDICATE
LOWS WILL FORM ALONG THE FRONT AND TRACK NORTHWARD. THE RUN-TO-RUN
CONSISTENCY OF LOW POSITIONS AND STRENGTH IS QUITE POOR. FOR NOW
PLAN TO MAKE USE OF THE GFS SOLUTION BECAUSE IT IS NOT VERY
DIFFERENT FROM THE EXISTING FORECAST AND WILL RESULT IN MINIMAL
CHANGES UNTIL MODELS GAIN A BETTER HANDLE ON THE SITUATION.

.SHORT TERM FORECAST...

SOUTHCENTRAL ALASKA...MODELS INDICATE VERY WEAK UPWARD MOTION AS
THE REMNANTS OF THE FRONT MOVE ACROSS SOUTHCENTRAL THIS AFTERNOON
AND EVENING. THIS COMBINED WITH WESTERLY UPSLOPE FLOW AND COLD
ADVECTION SHOULD BE JUST ENOUGH TO PRODUCE SOME LIGHT SNOW ON THE
WEST SIDE OF THE KENAI/CHUGACH/TALKEETNA MOUNTAINS...AND SIMILARLY
ALONG THE WRANGELL MOUNTAINS AND PORTIONS OF THE ALASKA RANGE.
ACCUMULATION WILL BE VERY LIGHT WITH A DUSTING UP TO PERHAPS AND
INCH AT HIGHER ELEVATIONS. HIGH PRESSURE WILL BUILD IN FROM THE
WEST BEHIND THE FRONT RESULTING IN A TIGHTENING PRESSURE GRADIENT
BETWEEN THE GULF AND MAINLAND. COLD AIR ADVECTION AND JET ALOFT
WILL COMBINE WITH THIS PRESSURE GRADIENT TO PRODUCE ANOTHER ROUND
OF STRONG WINDS THROUGH CHANNELED AREAS ALONG THE NORTH TO WEST
GULF COAST. THE UPPER LEVEL RIDGE WILL AMPLIFY OVER MAINLAND ALASKA
AS WE HEAD THROUGH THE WEEK SO COLD AND DRY CONDITIONS WILL PERSIST.

SOUTHWEST ALASKA...A FEW SNOW SHOWERS WILL CONTINUE TO MOVE IN
FROM THE BERING SEA THROUGH MONDAY NIGHT...BUT GENERALLY EXPECT
DRY CONDITIONS ACROSS SOUTHWEST ALASKA.  WESTERLY WINDS WILL
REMAIN GUSTY THROUGH TONIGHT WHICH SHOULD MINIMIZE FOG FORMATION.
HOWEVER...AS HIGH PRESSURE BUILDS OVERHEAD LATE MONDAY WINDS
WILL BEGIN TO LIGHTEN AND FOG WILL BECOME A GREATER THREAT. HIGH
PRESSURE WILL DOMINATE THROUGH MID WEEK...ALTHOUGH THERE MIGHT BE
A LITTLE WIND AS FRONTS CROSSING THE BERING NEAR THE WESTERN ALASKA
COAST BEGINNING TUESDAY.

BERING SEA/ALEUTIANS...A TROUGH WILL DROP OUT OF RUSSIA AND INTO
THE FAR WESTERN BERING SEA ON TUESDAY. THIS WILL LEAD TO
AMPLIFICATION OF THE DOWNSTREAM RIDGE OVER THE EASTERN BERING SEA
AND INTO WESTERN ALASKA. THUS...A SOUTH TO NORTH STORM TRACK WILL
SET UP OVER THE WESTERN ALEUTIANS AND BERING. AS WE HEAD THROUGH
THE WEEK THIS FRONTAL CORRIDOR WILL PROGRESS EASTWARD AND WEAKEN
AS THE UPPER TROUGH IS FORCED OVER THE RIDGE.

.LONG TERM FORECAST...THE REMNANTS OF THE FRONT MOVING ACROSS THE
BERING THIS WEEK SHOULD REACH THE MAINLAND BY WEEK'S END...ALTHOUGH
IN A MUCH WEAKENED STATE. MODELS INDICATE HIGH PRESSURE WILL BUILD
RIGHT BACK IN BEHIND THE FRONT. SO GENERALLY LOOKS LIKE THE DRY AND
COLD CONDITIONS WILL CONTINUE TO DOMINATE THE SOUTHERN MAINLAND OF
ALASKA. THIS WILL ONCE AGAIN SHIFT THE ACTIVITY BACK TO THE WESTERN
BERING NEXT WEEKEND...SIMILAR TO WHAT IS EXPECTED TO EVOLVE OVER
THE NEXT COUPLE OF DAYS.

.AER/ALU...WATCH/WARNING SUMMARY
PUBLIC...NONE.
MARINE...GALE WARNING...120 125 130 132 150 180 185.
         HEAVY FREEZING SPRAY WARNING...185.
FIRE WEATHER...NONE.

SEB FEB 11