Small Craft Advisory 556, NWS Anchorage, AK

AREA FORECAST DISCUSSION
SOUTHCENTRAL AND SOUTHWEST ALASKA
200 PM AKDT WED JUL 16 2008

.ANALYSIS AND UPPER LEVELS...A LARGE CLOSED LOW CENTERED OVER
THE WESTERN BERING SEA HAS DEVELOPED A SECOND CENTER OVER THE
EASTERN BERING. WATER VAPOR IMAGERY SHOWS A STRONG VORT-MAX JUST
SOUTH OF THIS SECOND LOW ROTATING NORTHWARD AND CONTRIBUTING TO
THE STRENGTHENING OF A SURFACE LOW WHICH IS SPINNING SOUTH OF
NUNIVAK ISLAND. THIS LOW IS ALSO IN THE LEFT EXIT REGION OF A 130KT
ZONAL  JET AT 250MB RIDING OVER TOP OF THE ALEUTIANS. AN AREA OF
UPPER LEVEL DIFFLUENCE IS ALSO NOTED OVER BRISTOL BAY AND SOUTHWEST
ALASKA. ALL OF THESE FACTORS ARE LEADING TO BANDS OF MODERATE TO
HEAVY RAIN MOVING ACROSS COASTAL SOUTHWEST ALASKA. LIGHT RAIN AHEAD
OF THE SYSTEM IS ALSO MOVING INTO PORTIONS OF SOUTHCENTRAL.

.MODEL DISCUSSION...SURFACE OBSERVATIONS AND SCATTEROMETER WINDS
INDICATE A STRONGER SURFACE LOW THAN INDICATED BY MODELS WITH AN
ESTIMATED CENTRAL PRESSURE OF 995 MB AND MODEL PRESSURES AROUND
1000MB. OTHERWISE MODELS ARE VERIFYING WELL WITH SYNOPTIC FEATURES.
THE BIGGEST CONCERN IS THE NEXT LOW TRACKING ALONG THE ALEUTIANS IN
FAST WESTERLY FLOW THURSDAY AND FRIDAY. THE GFS IS FASTER THAN ALL
OTHER SOLUTIONS...WHICH MAKES SENSE WITH THE STRONG JET OVERHEAD.
HOWEVER...THE GFS DEEPENS THE LOW IN THE MID 980S AS IT MOVES INTO
THE GULF. RUN-TO-RUN CONSISTENCY IS VERY POOR AND OTHER SOLUTIONS
SHOW LITTLE OR NO DEEPENING. THEREFORE...WILL STICK CLOSE TO THE
GFS TRACK BUT GO WITH A WEAKER LOW FOR NOW.

.SHORT TERM FORECAST...

THE COLD FRONT EXTENDING FROM THE EASTERN BERING LOW WILL PUSH
ACROSS SOUTHWEST ALASKA THIS EVENING AND INTO SOUTHCENTRAL ALASKA
TONIGHT. GUSTY SOUTHEAST WINDS JUST AHEAD OF THE FRONT WILL AFFECT
THE USUAL AREAS. FLOW ALOFT WILL BE OUT OF THE SOUTHWEST...BRINGING
WIDESPREAD RAIN TO THE MOST OF THE SOUTHERN MAINLAND. VORT-MAXES
EMBEDDED WITHIN THE WESTERLY FLOW SOUTH OF THE LOW CENTER WILL ALSO
CONTINUE TO ROTATE OVER THE MAINLAND THROUGH THURSDAY. MODELS
INDICATE A STRONGER VORT-MAX HELPING TO INDUCE FORMATION OF A
SURFACE LOW IN THE VICINITY OF THE BARREN ISLANDS THURSDAY MORNING
AND THEN MOVING TO PRINCE WILLIAM SOUND. THIS WILL MAINTAIN STEADY
RAIN OVER MOST OF SOUTHCENTRAL THROUGH THURSDAY NIGHT. THE UPPER
LOW WILL GRADUALLY TRANSITION EASTWARD INTO THE GULF THROUGH
SATURDAY...WITH THE ALEUTIAN LOW BEING ABSORBED INTO THE FLOW.
A DRYING TREND WILL BEGIN OVER SOUTHWEST ALASKA AND THE ALASKA
PENINSULA FRIDAY NIGHT AND THEN ACROSS SOUTHCENTRAL ON SATURDAY.

AS MENTIONED ABOVE THERE IS A LOT OF UNCERTAINTY WITH LOW MOVING
ACROSS THE ALEUTIANS IN THE THURSDAY/FRIDAY TIME FRAME. IT WILL MOVE
QUICKLY AND WHEN IT REACHES THE GULF IT SHOULD STAY MAINLY SOUTH OF
SOUTHCENTRAL WITH UPPER CENTER TO THE NORTH.

ASHFALL ADVISORIES REMAIN OUT FOR THE EASTERN ALEUTIANS. OKMOK
VOLCANO ACTIVITY HAS DIMINISHED...BUT THERE WAS A REPORT OF VERY
LIGHT ASHFALL IN THE DUTCH HARBOR AREA THIS MORNING. ANY ASH THAT
MAY STILL BE COMING OUT OF THE VOLCANO IS TRACKING EASTWARD AND THAT
WILL REMAIN THE DOMINANT FLOW FOR THE NEAR FUTURE.

.LONG TERM FORECAST...MEAN TROUGH WILL REMAIN CENTERED OVER MAINLAND
ALASKA THROUGH THE EXTENDED PERIOD AS TROUGHS DROP DOWN FROM THE
ARCTIC. THUS...ASIDE FROM A BRIEF PERIOD OF NICE WEATHER IN BETWEEN
TROUGHS...THE WEATHER WILL REMAIN UNSETTLED WITH MOSTLY CLOUDY SKIES
ACROSS THE REGION.

.AER/ALU...WATCH/WARNING SUMMARY
PUBLIC...ASHFALL ADVISORY 185.
MARINE...ASHFALL ADVISORY 170 171. GALES 172 175 180.
FIRE WEATHER...NONE.

SEB JUL 08