Small Craft Advisory 649, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
200 PM AKDT TUE AUG 27 2013

.ANALYSIS AND UPPER LEVELS...
AN UPPER LEVEL LOW IS CENTERED OVER THE BERING STRAIT WITH AN UPPER
LEVEL RIDGE OVER THE NORTH CENTRAL PACIFIC. BETWEEN THESE FEATURES
IS VERY FAST FLOW ALOFT RACING ACROSS THE ALEUTIANS INTO SOUTHERN
ALASKA. A RIDGE ALONG THE ALASKA/CANADA BORDER THAT HAS BEEN
BLOCKING THE FLOW INTO SOUTH CENTRAL ALASKA IS MOVING TO THE EAST.

.MODEL DISCUSSION...
THE NUMERICAL GUIDANCE IS IN FAIRLY GOOD AGREEMENT THROUGH 84
HOURS...ESPECIALLY CONSIDERING THE FAST FLOW OVER THE AREA. IN
PREVIOUS RUNS THERE HAS BEEN SOME DISAGREEMENT ON HOW FAR NORTH THE
SURFACE LOW GO ONCE AN UPPER LOW MOVES INTO THE GULF OF ALASKA ON
WEDNESDAY AND THURSDAY. THE 12Z RUN OF THE NAM PUT THE LOW FURTHER
NORTH...FOLLOWING THE TREND OF THE 06Z GFS. HOWEVER...THE 12Z GFS IS
TRENDING TOWARD THE ECMWF SOLUTION AND PUTTING THE LOW FURTHER
SOUTH. WITH THE GFS AND ECMWF FAIRLY CLOSE (AND THE GEM AS WELL)
THIS MAKES THE NAM THE OUTLIER SOLUTION AND THUS THE AFTERNOON
FORECASTS PREFER THE GFS AND ECMWF.

.SHORT TERM FORECAST...
A SHORT WAVE TROUGH ASSOCIATED WITH THE BERING STRAIT LOW IS OVER
SOUTHWEST ALASKA AND WILL MOVE ACROSS SOUTH CENTRAL ALASKA TONIGHT.
THIS IS A FAIRLY WEAK SYSTEM BUT WILL HELP TO PUSH THE RIDGE INTO
NORTHWEST CANADA AND OPEN UP SOUTHWESTERLY FLOW OVER SOUTHERN
ALASKA. NOT A LOW OF DYNAMICS WITH THIS SYSTEM...BUT SOME
PRECIPITATION IS EXPECTED OVER SOME AREAS WITH THE UP-SLOPE
COMPONENT FROM THE WESTERLY FLOW.

A STRONGER SYSTEM IS SPINNING UP OVER THE WESTERN AND CENTRAL
ALEUTIANS. THIS SYSTEM WILL PUSH TO THE SOUTHWEST COAST EARLY
WEDNESDAY MORNING. IT WILL THEN BEGIN TO "PHASE UP" WITH A TROUGH
DIVING OUT OF THE BERING STRAIT LOW AS IT MOVES INTO SOUTHERN ALASKA
DURING THE DAY WEDNESDAY. THE NORTH PACIFIC RIDGE WILL BUILD NORTH
INTO THE BERING SEA BEHIND THIS SYSTEM. WIDESPREAD PRECIPITATION
WILL MOVE INTO SOUTH CENTRAL ALASKA LATE WEDNESDAY INTO THURSDAY AS
THE UPPER LOW DEEPENS AND MOVES INTO THE GULF. ON THURSDAY NIGHT
THIS SYSTEM WILL DROP SOUTH AND LEAVE SOUTH CENTRAL IN A "COL"
BETWEEN THE SYSTEM NOW IN THE GULF AND THE BERING LOW.

A STRONG FRONTAL SYSTEM WILL MOVE INTO THE WESTERN BERING SEA AND
WESTERN ALEUTIANS ON THURSDAY AND PUSH ACROSS MOST OF THE BERING SEA
AND ALEUTIANS BY FRIDAY AFTERNOON. ON FRIDAY THE BERING STRAIT LOW
AND BERING SEA HIGH BOTH PUSH EAST AHEAD OF THE BERING/ALEUTIAN
SYSTEM AND WESTERLY FLOW ALOFT WILL DEVELOP AGAIN OVER SOUTHERN
ALASKA.

.AER/ALU...WATCH/WARNING SUMMARY
PUBLIC...NONE.
MARINE...GALE WARNING 150 155 170 172.
FIRE WEATHER...NONE.

LUDWIG AUG 13