Small Craft Advisory 595, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
200 PM AKDT THU AUG 8 2013

.MODEL DISCUSSION...
MODEL INITIALIZATIONS WERE REASONABLE THOUGH A LITTLE OFF ON THE
POSITIONING OF THE UPPER CENTER OVER WESTERN BRISTOL BAY THIS
MORNING. FORECAST TRENDS THROUGH THE SHORT TERM ARE IN GOOD
AGREEMENT. THE MODELS STRUGGLE THOUGH WITH THE POSITIONING OF A WAVE
APPROACHING THE WESTERN ALEUTIANS ON SUNDAY. THE ECMWF APPEARS TO
HAVE THE MORE CONSISTENT SOLUTION. ONE OF THE FORECAST CHALLENGES
WILL BE THE TIMING OF IMPULSES THAT RIPPLE NORTH ACROSS THE GULF AND
SOUTHCENTRAL OVER THE NEXT COUPLE OF DAYS.

.UPPER LEVELS...
.500 MB...
THIS MORNINGS WATER VAPOR IMAGERY SHOWS THE TROUGH LIFTING NORTH
ACROSS THE SOUTHCENTRAL ALASKA FOLLOWED BY A NARROW BAND OF DRYING.
FURTHER WEST A LOW IS VISIBLE OVER BRISTOL BAY WITH A SW SURGE OF
MOISTURE FROM SOUTH OF THE ALEUTIANS TO OVER COOK INLET.

THE TROUGH OVER SOUTHCENTRAL WILL CONTINUE TO MOVE NORTH AND WEAKEN.
THE LOW NEAR BRISTOL BAY WILL DRIFT NORTH AND WEST AND MAINTAIN A
MEAN POSITION NEAR NUNIVAK ISLAND THROUGH SAT. THE ASSOCIATED TROUGH
SHARPENS SOUTH OF THE AK PEN AND SETS UP RELATIVELY DEEP SOUTHERLY
FLOW OVER THE GULF AND SOUTHCENTRAL TONIGHT AND INTO SAT. THE MEAN
POSITION OF THE TROUGH AXIS WILL BE SLOW TO MOVE EAST AS SEVERAL
IMPULSES MOVE NORTH ALONG ITS BOUNDARY. THE TIMING OF THESE IMPULSES
WILL BE SOMEWHAT PROBLEMATIC.

.SURFACE...
A FRONTAL TROUGH MOVING NORTH ACROSS SOUTHCENTRAL WAS WEAKENING.
SCATTERED SHOWERS WITH SOME BREAKS ON ITS BACKSIDE WERE EVIDENT.
PRESSURE RISES ON THE BACK SIDE WERE PRODUCING GUSTY SOUTH WINDS
ACROSS THE COPPER RIVER BASIN. GUSTY GAP FLOWS ARE ALSO EVIDENT
ACROSS TURNAGAIN ARM AND FORECAST FOR THE KNIK VALLEY TODAY. FURTHER
WEST THE LOW OVER WESTERN BRISTOL BAY AND ASSOCIATED FRONT WERE
PRODUCING WIDESPREAD RAIN/SHOWERS ACROSS SW ALASKA. A WEAK TROUGH WAS
MOVING ACROSS THE WESTERN ALEUTIANS.

.SHORT TERM FORECAST...
.SOUTHCENTRAL ALASKA...
A FRONTAL TROUGH APPROACHES THE WESTERN GULF THIS EVENING. THE FRONT
WILL MAKE SLOW PROGRESS ACROSS THE GULF WITH GENERALLY SOUTHERLY FLOW
ALOFT. THE DEEPER MOISTURE FETCH WITH THIS FRONT WILL LIKELY PRODUCE
HIGHER PRECIPITATION VALUES ACROSS THE GULF AND SUSITNA VALLEY THEN
THE ONE CURRENTLY MOVING NORTH ACROSS THE MAINLAND. QPF VALUES OF A
COUPLE OF INCHES OR MORE ARE EXCEPTED ACROSS THE EAST KENAI AND
WESTERN PRINCE WILLIAM SOUND OVER THE NEXT 36 HRS. THE DEEP SOUTHERLY
FLOW ALOFT WILL ALSO ALLOW PRECIPITATION TO SPREAD INTO INLAND AREAS
WHICH ARE NORMALLY PROTECTED BY DOWNSLOPE CONDITIONS. PRESSURE
GRADIENTS WILL INCREASE ACROSS SOUTHCENTRAL AND PRODUCE STRONG GAP
FLOWS...ESPECIALLY TURNAGAIN ARM...TONIGHT AND FRI AS THE FRONT
PROGRESSES SLOWLY EAST.

.SOUTHWEST ALASKA...
THE PROXIMITY OF THE UPPER TROUGH AND LINGERING SURFACE CENTERS WILL
CONTINUE UNSETTLED WEATHER ACROSS THE REGION OVER THE NEXT COUPLE OF
DAYS. RAIN TODAY WILL GIVE WAY TO SHOWERS FRI AND SAT. STRONG GAP
FLOWS OVER ALONG THE ALEUTIAN RANGE WILL DIMINISH OVERNIGHT.

.BERING SEA AND ALEUTIANS...
THE STACKED LOW PRESSURE SYSTEM ENCOMPASSES MOST OF THE BERING SEA
AND WILL KEEP PERIODIC RAIN IN THE FORECAST THROUGH THE
WEEKEND...ESPECIALLY FROM THE PRIBILOF ISLANDS EASTWARD. HIGH
PRESSURE WILL SLIDE IN BEHIND THE SYSTEM OVER THE WEEKEND.

.LONG TERM FORECAST...
EXTENDED GUIDANCE SUGGESTS A BIT OF A PATTERN SWITCH FROM THE CURRENT
PATTERN WITH THE UPPER LOW BREAKING DOWN AND THE YUKON HIGH SPREADING
OVER THE MAINLAND TO SOME EXTEND. TROUGHING IN SOME SENSE WILL TRY TO
RE-FORM OVER THE BERING SEA WHICH IS CLIMATOLOGICALLY FAVORED.

.AER/ALU...WATCH/WARNING SUMMARY
PUBLIC...NONE.
MARINE...GALE 120 130 132 150
FIRE WEATHER...NONE.

RMC AUG 13