Small Craft Advisory 687, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
400 PM AKDT THU SEP 27 2012

.ANALYSIS AND UPPER LEVELS...THE BIG STORY ON THE CURRENT WEATHER MAP
IS THE LARGE LOW IN THE NORTHWEST GULF OF ALASKA. THE LOW DEEPENED TO
950 MB WEDNESDAY MORNING AND HAS NOW WEAKENED TO 970 MB THIS
AFTERNOON. A TROUGH EXTENDS NORTHWARD OUT OF THIS LOW INTO THE
INTERIOR OF THE STATE WHICH IS KEEPING MOST OF SOUTH CENTRAL ALASKA
OUT OF THE STRONG WINDS. THE STRONG PRESSURE GRADIENT AROUND THIS LOW
ENCOMPASSES SOUTHWEST ALASKA AND AND THE ALASKA PENINSULA AND EXTENDS
WELL INTO THE BERING SEA.
IN THE UPPER LEVELS A LONG WAVE TROUGH EXTENDS OUT OF THE ARCTIC OVER
THE MAINLAND PART OF THE STATE WITH A CUT OFF LOW VERTICALLY STACKED
OVER THE SURFACE LOW IN THE GULF OF ALASKA. ANOTHER UPPER CUT OFF LOW
IS DEVELOPING IN WESTERN ALASKA THAT WILL BRING COLDER AIR INTO
SOUTHWEST ALASKA TOMORROW AND THEN TO SOUTH CENTRAL ALASKA BY
SATURDAY NIGHT.

.MODEL DISCUSSION...MODELS ARE DOING WELL WITH THE HANDLING OF THE
LARGE LOW IN THE GULF. THEY ALL ALSO DEVELOP A WEAK LOW IN THE
WESTERN GULF SATURDAY BUT THERE IS NOT A COMMON SOLUTION AS TO
EXACTLY WHERE THAT ONE WILL GO. MOST LIKELY TRACK KEEPS IN OFFSHORE
IN THE NORTHERN GULF LATE IN THE WEEKEND.
LATE IN THE WEEKEND A LOW COMPLEX SOUTH OF THE WESTERN ALEUTIANS IS
ALSO CAUSING DIFFICULTIES WITH THE MODELS. SINCE MOST OF THIS AREA
IMPACTED BY THESE LOWS WILL BE SOUTH OF THE FORECAST ARE IT SHOULD
HAVE MINIMAL IMPACT ON FORECASTS REGARDLESS OF HOW THAT SYSTEM PANS
OUT.

.SHORT TERM FORECAST...
SOUTH CENTRAL...MODERATE TO HEAVY RAIN SHOWERS CONTINUE TO MOVE ON
SHORE ALONG THE NORTH GULF COAST. MOST OF SOUTH CENTRAL ALASKA INLAND
REMAINS DOWNSLOPED OUT. AS THE LOW MOVE NORTHWARD FRIDAY IT WILL CUT
OFF THE DOWNSLOPE AND SHIFT THE PRECIPITATION AREAS FARTHER NORTH AND
INLAND. HIGHER ELEVATIONS WILL SEE SOME SNOW SHOWERS OVER THE NEXT
DAY OR SO WITH SNOW LEVELS DROPPING INTO THE WEEKEND AS THE COLD AIR
IN SOUTHWEST ALASKA MOVES INTO SOUTH CENTRAL ALASKA. BY SATURDAY
NIGHT THERE IS A CHANCE TO SEE SNOW MIXED DOWN TO SEA LEVEL...HOWEVER
THE AMOUNT OF PRECIPITATION LEFT IN THE AREA WILL BE GREATLY
DIMINISHED BY THEN.
SOUTHWEST...THE STRONG PRESSURE GRADIENT AROUND THE LOW IN THE GULF
OF ALASKA WILL CONTINUE TO BRING GUSTY NORTHWEST WINDS. SOME RAIN AND
SNOW SHOWERS WILL ALSO OCCUR IN THE COLD NORTHWEST FLOW. BOTH WINDS
AND AREAS OF PRECIPITATION WILL DIMINISH TOMORROW. RAIN SHOWERS WILL
PERSIST ALONG THE ALEUTIAN ISLANDS AND ALASKA PENINSULA WITH THE
ONGOING NORTHWEST FLOW.

.LONG TERM FORECAST...COLD AIR AND BUILDING HIGH PRESSURE BEHIND THE
LOW CURRENTLY IN THE GULF OF ALASKA WILL LIKELY RESULT IN SOME OF THE
COLDEST TEMPERATURES OF THE SEASON...ESPECIALLY ACROSS SHELTERED
VALLEY LOCATIONS. MINIMUM TEMPERATURES WILL EASILY FALL WELL BELOW
FREEZING EARLY WEEK...INCLUDING ANCHORAGE AND THE MATANUSKA VALLEY. A
HIGHLY AMPLIFIED PATTERN IS SHAPING FOR THE END OF NEXT WEEK...WITH A
RETURN TO WARMER SOUTHERLY FLOW AND INCREASED CHANCES FOR RAINFALL.

.AER/ALU...WATCH/WARNING SUMMARY
PUBLIC...FLOOD WARNING 121 125.
MARINE...GALE 120 1255 130 150 155 160 165 180.
FIRE WEATHER...NONE.

SEP 12