Small Craft Advisory 242, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
1230 PM AKDT FRI APR 1 2011

.ANALYSIS AND UPPER LEVELS...A RIDGE IS BUILDING OUT AHEAD OF AN
INTENSIFYING LOW OVER THE NORTH CENTRAL PACIFIC. THIS IN TURN IS
FORCING DOWNSTREAM SHORT-WAVES TO TRACK NORTHEASTWARD TOWARD
MAINLAND ALASKA. THE FIRST OF THESE IS CROSSING KODIAK ISLAND
AND SOUTHWEST ALASKA LATE THIS MORNING. SOUTHWEST ALASKA IS
OBSERVING NOTHING MORE THAN A FEW HIGH CLOUDS. HOWEVER...OVER
SOUTHCENTRAL ALASKA THERE IS REMNANT LOW LEVEL MOISTURE ASSOCIATED
WITH A WEAK SURFACE LOW OVER THE NORTHERN GULF. VERY LIGHT SNOW
HAS FORMED OUT AHEAD OF THE TROUGH OVER SOUTHERN COOK INLET AND
THE WESTERN KENAI PENINSULA. CONDITIONS FURTHER NORTH APPEAR MORE
UNSTABLE...ESPECIALLY IN THE ANCHORAGE AREA WHERE SKIES ARE
CURRENTLY PARTLY SUNNY. SO EXPECT A FEW SNOW SHOWERS TO DEVELOP
THIS AFTERNOON AS THE TROUGH CONTINUES TO PROGRESS INLAND. ANOTHER
SHORT-WAVE TROUGH IS FOLLOWING 6 TO 8 HOURS BEHIND THE FIRST ONE
AND APPEARS TO BE OF SIMILAR STRENGTH...SO THE THREAT FOR LIGHT
SNOW OR SNOW SHOWERS WILL CONTINUE ACROSS SOUTHCENTRAL THROUGH
TONIGHT.

LOOKING FURTHER WEST A COUPLE SYSTEMS ARE EXITING ASIA AND MOVING
INTO THE BACK SIDE OF THE LONG-WAVE TROUGH WHICH ENCOMPASSES ALL
OF ALASKA. THE FIRST ONE JUST EAST OF KAMCHATKA IS FAIRLY WEAK.
THE SECOND SYSTEM FURTHER SOUTH AND WEST HAS UPPER JET SUPPORT
AND WILL STRENGTHEN AS IT TRACKS EASTWARD THE NEXT COUPLE DAYS.

.MODEL DISCUSSION...THE 12Z RUNS OF THE GFS/ECMWF/UKMET HAVE COME
INTO GOOD AGREEMENT WITH TRACK OF STRONG NORTH PACIFIC LOW AS IT
TRACKS SOUTH OF THE ALASKA PENINSULA SATURDAY NIGHT AND OVER TO
THE BRITISH COLUMBIA COAST MONDAY NIGHT. THE NAM IS A SIGNIFICANT
OUTLIER ON THIS. WILL UPDATE FORECASTS USING THE GFS.
SIMILARLY...ECMWF/GFS ARE MUCH CLOSER IN HANDLING OF LOWS MOVING
ACROSS THE BERING SEA SUNDAY AND MONDAY SO WILL TREND HEAVILY
TOWARD THE GFS ON THIS AS WELL.

.SHORT TERM FORECAST...

SOUTHCENTRAL ALASKA...SCATTERED SNOW SHOWERS CAN BE EXPECTED MOST
AREAS AS TWO UPPER TROUGHS MOVE ACROSS THE REGION THIS AFTERNOON
THROUGH TONIGHT. SOME WEAK ISENTROPIC LIFT OUT AHEAD OF PACIFIC
LOW WILL FOLLOW RIGHT BEHIND SO DON'T EXPECT A BREAK IN THE CLOUDS
BEHIND SECOND TROUGH. THE PACIFIC LOW WILL REMAIN WELL SOUTH OF
SOUTHCENTRAL THE NEXT COUPLE DAYS AS UPPER JET MOVES ON BY LEAVING
THE LOW TO CUT-OFF AND THEN PROGRESS EASTWARD WITH THE UPPER WAVE
PATTERN. THIS MEANS THE FRONTAL SYSTEM WILL CROSS THE GULF AND THEN
STALL AND WEAKEN AS IT REACHES THE NORTH GULF COAST SUNDAY. STRONG
EASTERLY FLOW AHEAD OF THE FRONT WILL PRODUCE DOWNSLOPE DRYING IN
THE USUAL SPOTS AND ENSURE THAT THE BULK OF PRECIPITATION REMAINS
ALONG THE GULF COAST AND PRINCE WILLIAM SOUND AREAS. THE STORM WILL
EXIT SUNDAY NIGHT AND MONDAY BUT IT AGAIN LOOKS LIKE THERE WILL BE
LITTLE OR NO BREAK BEFORE ANOTHER SYSTEM ARRIVES FROM THE BERING
SEA.

SOUTHWEST ALASKA...QUIET CONDITIONS WILL QUICKLY COME TO AN END
DURING THE DAY SATURDAY AS FRONTAL SYSTEM FROM PACIFIC LOW MOVES
NORTHWARD ACROSS SOUTHWEST ALASKA. PRECIPITATION WILL START AS ALL
SNOW BUT MIX WITH OR CHANGE TO RAIN AS WARMER AIR MOVES INTO THE
BRISTOL BAY ZONE LATE SATURDAY AND SATURDAY NIGHT. STRONG EASTERLY
FLOW ALONG AND AHEAD OF THE FRONT WILL ALSO PRODUCE DOWNSLOPE DRYING
AND WARMING. THE KUSKOKWIM DELTA COAST ON THE OTHER HAND WILL REMAIN
COLD UNDER STRONG NORTH TO NORTHEAST WINDS AHEAD OF THE FRONTAL
SYSTEM. THEREFORE...THERE IS POTENTIAL FOR A BLIZZARD AS THE SNOW
ARRIVES SATURDAY AFTERNOON AND THEN CONTINUING INTO SUNDAY. PLAN
TO ISSUE A BLIZZARD WATCH WITH THE AFTERNOON PACKAGE. A WEAK UPPER
TROUGH WILL APPROACH FROM THE WEST JUST AS THIS SYSTEM BEGINS TO
EXIT TO THE EAST...SO DON'T EXPECT MUCH OF A BREAK IN CLOUDS AND
PRECIPITATION. IMMEDIATELY FOLLOWING THE TROUGH WILL BE ANOTHER
STORM SYSTEM WHICH IS EXPECTED TO AFFECT SOUTHWEST ALASKA BY THE
BEGINNING OF NEXT WEEK.

BERING SEA/ALEUTIANS...THERE WILL BE A CORRIDOR OF SNOW AND STRONG
WINDS ON THE BACK SIDE OF THE PACIFIC LOW THIS WEEKEND AS A TROUGH
CURRENTLY OVER THE WESTERN BERING DIGS SOUTH OF THE LOW FEEDING
ARCTIC AIR INTO THE SYSTEM AND STRENGTHENING THE OCCLUDED FRONT.
THE CHALLENGE IS DETERMINING EXACTLY WHERE THIS BACK EDGE WILL LINE
UP. RIGHT NOW IT LOOKS LIKE IT WILL EXTEND FROM JUST EAST OF THE
PRIBILOFS SOUTHWARD TO THE EASTERN ALEUTIANS. GUIDANCE INDICATES
SURFACE TEMPERATURES WILL BE TOO WARM IN THE UNALASKA AREA TO
PRODUCE BLIZZARD CONDITIONS. SO FOR THE TIME BEING WON'T ISSUE
ANYTHING BUT WILL MAKE MENTION OF BLOWING SNOW IN THE EASTERN
ALEUTIANS PUBLIC FORECAST. THE NEXT SYSTEM CURRENTLY EXITING ASIA
WILL MARCH QUICKLY EASTWARD ACROSS THE BERING AND ALEUTIANS SUNDAY
NIGHT AND MONDAY UNDER INCREASINGLY PROGRESSIVE UPPER FLOW.

.LONG TERM FORECAST...UPPER LEVEL FLOW LOOKS LIKE IT WILL REMAIN
VERY PROGRESSIVE THROUGH THE EXTENDED FORECAST PERIOD. THUS...AFTER
THE STORM WHICH MOVES OVER MAINLAND ALASKA EARLY NEXT WEEK EXITS
EXPECT ANOTHER MAJOR STORM TO TRACK FROM THE NORTH PACIFIC ACROSS
THE BERING SEA TO MAINLAND ALASKA DURING THE MIDDLE TO LATE PART
OF NEXT WEEK.

.AER/ALU...WATCH/WARNING SUMMARY
PUBLIC...NONE.
MARINE...STORM WARNING...120 130 132 138 150 155 170 171 172.
         GALE WARNING...125 140 160 165 176 179 180.
FIRE WEATHER...NONE.

SEB APR 11