Small Craft Advisory 384, NWS Anchorage, AK

AREA FORECAST DISCUSSION
SOUTHCENTRAL AND SOUTHWEST ALASKA
230 PM AKDT WED MAY 6 2009

.MODEL DISCUSSION...MODELS INITIALIZED WELL ON UPPER LOW CENTER
MOVING SOUTHEAST ACROSS THE KUSKOKWIM VALLEY THIS MORNING. FORECAST
TREND WITH THIS SYSTEM IS HANDLED WELL IN GENERAL BY THE MODELS.
MODEL INITIALIZATIONS AND FORECAST TREND AT THE SURFACE WERE IN GOOD
AGREEMENT THROUGH THE SHORT TERM(SAT).

.ANALYSIS AND UPPER LEVELS...A WEAK UPPER RIDGE (OMEGA BLOCK)
CONTINUES TO SLIDE EASTWARD OVER THE EASTERN BERING SEA EARLY THIS
AFTERNOON WITH A WEAK 1018 MB SURFACE HIGH NEAR THE PRIBILOFS.  THIS
IS ALLOWING FOR LOW STRATUS BENEATH THE SUBSIDENCE INVERSION OVER A
LARGE PART OF THE BERING SEA.  OFFSHORE FLOW ON THE DOWNSTREAM SIDE
OF THE RIDGE MEANWHILE HAS RESULTED IN FAIR SKIES FOR MOST OF BRISTOL
BAY BUT BECOMES ONSHORE FLOW ALONG THE SOUTHERN HALF OF THE ALASKA
PENINSULA.  OVER THE MAINLAND...AN UPPER LOW CONTINUES TO MOVE SOUTH
AND EAST TOWARDS THE GULF OF ALASKA FROM THE KUSKOKWIM
VALLEY...CURRENTLY CROSSING OVER THE NORTHERN ALEUTIAN CHAIN AND
LOWER COOK INLET.  A WEAK 1000 MB SURFACE LOW IS LOCATED IN THE
VICINITY OF THE EASTERN KENAI PENINSULA/WESTERN PRINCE WILLIAM SOUND
AREA...WITH A WEAK FRONT EXTENDING NORTH AND EASTWARD TO A 997 MB LOW
OVER THE SOUTHERN YUKON TERRITORY.  GEOSTROPHIC VEERING FLOW WITH
HEIGHT AND SUFFICIENT POSITIVE VORTICITY ADVECTION...AS WELL AS
ONSHORE FLOW AND LIFT DUE TO THE WEAK FRONT...ARE ALL HELPING TO
GENERATE LIGHT PRECIPITATION FROM SOUTHCENTRAL THROUGH PRINCE WILLIAM
SOUND AND INTO THE COPPER RIVER BASIN. IN HIGHER ELEVATIONS...THIS
LIGHT PRECIPITATION IS FALLING IN THE FORM OF LIGHT SNOW...EVEN AS
LOW AS THE HILLSIDE OF ANCHORAGE.

OTHERWISE...OUT WEST THERE IS A STRONG UPPER LOW WITH 995 MB SURFACE
LOW LOCATED JUST SOUTH AND WEST OF THE WESTERN ALEUTIANS...WITH AN
EMBEDDED SHORTWAVE MOVING NORTHWARD OVER THE BERING SEA ATTEMPTING TO
TRAVERSE THE RIDGE.

.SHORT TERM FORECAST...

SOUTH AND EAST OF THE ALASKA RANGE...THE UPPER LOW NOW OVER LOWER
COOK INLET WILL CONTINUE TO MOVE SOUTH AND EASTWARD TOWARDS THE
WESTERN GULF...DRAGGING THE WEAK SURFACE LOW SOUTHWARD INTO THE GULF
AS WELL.  THIS WILL DIMINISH PRECIPITATION WEST OF THE CHUGACH AND
KENAI RANGES...BUT WITH SOUTHEASTERLY ONSHORE FLOW...SOUTHEASTWARD
FACING SLOPES ALONG PRINCE WILLIAM SOUND WILL SEE THE HIGHEST
PRECIPITATION AMOUNTS.  PERSISTENT WEAK DEFORMATION MAINLY EAST OF
THE RICHARDSON HIGHWAY WILL KEEP RESPECTABLE POPS IN THE FORECAST
THROUGH TONIGHT...AND THEN THE DEVELOPMENT OF A WEAK THERMAL TROUGH
OVER THE COPPER RIVER BASIN COULD INITIATE A FEW SHOWERS THURSDAY
THROUGH SATURDAY...ESPECIALLY IN THE LATE AFTERNOON AND EVENING
HOURS.  BY SATURDAY...RIDGING ALOFT SHOULD EXTEND FAR ENOUGH EASTWARD
TO BRING FAIR WEATHER TO MOST OF SOUTHCENTRAL AND WARMER
TEMPERATURES.

SOUTHWEST ALASKA/BERING...THE MAIN PROBLEM IS STRATUS AND FOG FOR
THIS FORECAST PACKAGE ESPECIALLY OVER THE PRIBILOFS AND THE DELTA
REGION (WITH WEAK ONSHORE FLOW BEGINNING AS EARLY AS THURSDAY)...AS
WEAK UPPER RIDGING MOVES OVER SOUTHWEST ALASKA FROM THE BERING SEA
AND THEN STRENGTHENS BY FRIDAY AND EXPANDS ON SATURDAY.  THIS WILL
FORCE THE SYSTEM AFFECTING THE WESTERN ALEUTIANS TO MOVE SOUTH AND
EASTWARD INTO THE NORTH PACIFIC...WITH MINIMAL EFFECT ON THE CENTRAL
ALEUTIANS WITH RESPECT TO PRECIPITATION THROUGH FRIDAY AND SATURDAY.
IN ADDITION...FLOODING CONTINUES TO BE A CONCERN ALONG THE KUSKOKWIM
RIVER AND DELTA REGION.

.LONG TERM FORECAST...
UPPER LEVEL PATTERN FOR THE NEW WORKWEEK WILL BE CHARACTERIZED BY A
LARGE UPPER LOW CENTERED OVER THE NORTHWEST PACIFIC AND A WEAK LONG
WAVE RIDGE OVER THE CENTRAL AND WESTERN BERING SEA. PATTERN IS
CONDUCIVE TO INCREASED CLOUDINESS AND SLIGHTLY COOLER TEMPERATURES. A
TYPICAL SUMMERTIME PATTERN. POTENTIAL FOR AFTERNOON AND EVENING
CONVECTIVE DEVELOPMENT OVER INLAND AREAS.

.AER/ALU...WATCH/WARNING SUMMARY
PUBLIC...FLOOD WATCH...155
         FLOOD WARNING...141 155
MARINE...NONE.
FIRE WEATHER...NONE.

RMC/CC MAY 09