Small Craft Advisory 140, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
1250 PM AKST TUE FEB 12 2013

.ANALYSIS AND UPPER LEVELS...A CLOSED UPPER LOW WHICH HAD BEEN
STATIONARY OVER THE NORTHERN BERING IS BEGINNING TO MOVE EASTWARD
TOWARD THE WEST COAST OF ALASKA. A VERY SHARP SHORT-WAVE TROUGH ON
THE SOUTH SIDE OF THIS LOW IS CROSSING THE EASTERN BERING HEADED
TOWARD SOUTHWEST ALASKA. A VERY DEEP AND COLD ARCTIC AIRMASS IS
ACCOMPANYING THIS TROUGH AS INDICATED BY THE 12Z SOUNDING FROM
SAINT PAUL (-22 DEGREES CELSIUS AT 850MB AND -39 DEGREES AT 500MB).
THIS IS CREATING SOME STEEP LAPSE RATES AND CONTRIBUTING TO STRONGER
VERTICAL MOTION OUT AHEAD OF THE TROUGH. OBSERVATIONS FROM ACROSS
THE EASTERN BERING SHOW MUCH STRONGER WINDS THAN THE MODELS...
INDICATIVE OF AN UNSTABLE ATMOSPHERE HELPING TO MIX STRONGER WINDS
FROM ALOFT DOWN TO THE SURFACE. THE RESULT OF ALL THIS IS LOCALLY
HEAVY SNOW AND STRONG WINDS ACROSS THE REGION.

DOWNSTREAM OF THIS TROUGH IS ANOTHER SHORT-WAVE OVER THE SOUTHWESTERN
GULF LIFTING NORTHEASTWARD TOWARD SOUTHCENTRAL ALASKA. BASED ON THE
SLOWER MOVEMENT OF THE GULF TROUGH EXPECT THE BERING TROUGH TO CATCH
UP AND PHASE INTO A SINGLE TROUGH. A SURFACE LOW HAS STRENGTHENED
JUST EAST OF KODIAK ISLAND THIS MORNING AND LOOKS NOTABLY STRONGER
THAN FORECAST BY THE MODELS. THE FEW SURFACE OBSERVATIONS IN THE
AREA...ESPECIALLY AUGUSTINE ISLAND WHERE WINDS HAVE REACHED GALE
FORCE)...ARE INDICATIVE OF A STRONGER LOW. THERE ARE SOME SHOWERS
CURRENTLY OVER SOUTHCENTRAL ASSOCIATED WITH WEAKER WAVES OUT AHEAD
OF THE MAIN TROUGH. STEADY PRECIP OUT AHEAD OF THE LOW IS JUST
BEGINNING TO SHOW UP ON MIDDLETON ISLAND RADAR.

FINALLY...A STRONG LOW HAS DEVELOPED ALONG THE SUBTROPICAL JET
SOUTH OF THE ALEUTIANS. THIS SYSTEM...OR ONES THAT DEVELOP FROM IT
WILL BE THE MAIN FOCUS OF THE FORECAST OVER THE NEXT FEW DAYS.

.MODEL DISCUSSION...MODELS ARE REALLY STRUGGLING WITH A FEW KEY
ASPECTS OF ONGOING WEATHER THIS MORNING. FIRST...THEY ARE ALL TOO
WEAK AND TOO FAR NORTHWEST WITH THE LOW EAST OF KODIAK ISLAND.
SECOND...QPF AND WINDS ARE UNDERDONE ACROSS THE EASTERN BERING
AND SOUTHWEST ALASKA. WILL MAKE HEAVY USE OF SURFACE OBSERVATIONS
AND SATELLITE/RADAR IMAGERY IN PREPARING THE FORECAST FOR TONIGHT.

MODELS HAVE COME IN TO BETTER AGREEMENT WITH THE BASIC EVOLUTION
OF THE SYSTEM OVER THE NORTH PACIFIC...ESPECIALLY WITH DEVELOPMENT
AND TRACK OF A TRIPLE POINT LOW TONIGHT AND WEDNESDAY. SOLUTIONS
DIVERGE A LITTLE MORE WITH A SEPARATE PACIFIC LOW THAT DEVELOPS AND
MOVES UP INTO THE WESTERN GULF ON THURSDAY. GFS SEEMS TO BE AN
OUTLIER WITH A SLOWER TRACK OF THIS LATTER SYSTEM...SO WILL TREND
TOWARD THE FASTER SOLUTION USING THE NAM MODEL.

.SHORT TERM FORECAST...

SOUTHCENTRAL ALASKA...THE MORE EASTERLY POSITION OF THE SURFACE
LOW NEAR KODIAK ISLAND MEANS LOW WILL LIKELY TRACK TOWARD THE
EASTERN KENAI PENINSULA WITH THE BULK OF PRECIP FALLING OVER THE
PRINCE WILLIAM SOUND REGION TONIGHT. THERE WILL ALSO BE PRECIP
FARTHER WEST AS GULF TROUGH AND TROUGH CURRENTLY CROSSING THE
EASTERN BERING PHASE. HOWEVER...EASTERLY FLOW AHEAD OF SURFACE LOW
WILL INITIALLY MAINTAIN MOSTLY DRY CONDITIONS IN THE LEE OF THE
CHUGACH AND KENAI MOUNTAINS. THE PHASED TROUGH WILL CROSS
SOUTHCENTRAL LATE TONIGHT THROUGH WEDNESDAY MORNING AND BRING SOME
SNOW TO MOST AREAS. EVEN WITH A PHASED TROUGH THE VERTICAL MOTION
DOES NOT LOOK ALL THAT IMPRESSIVE...SO GENERALLY EXPECT LIGHT
ACCUMULATION FROM THE WESTERN KENAI TO ANCHORAGE AND THE MAT-SU.

THE NEXT SYSTEM WILL MOVE UP FROM THE PACIFIC INTO THE GULF ON
WEDNESDAY WITH ANOTHER STORM TO QUICKLY FOLLOW ON THURSDAY. BASED
ON A TRACK ACROSS THE WESTERN GULF THESE WILL BE CHARACTERIZED BY
THE VERY TYPICAL STRONG EASTERLY FLOW ACROSS SOUTHCENTRAL...MEANING
THE POPULATION CENTERS WILL OBSERVE DOWNSLOPE DRYING AND THE BULK
OF PRECIP WILL FALL ALONG THE COAST AND THE WEST SIDE OF COOK
INLET/SUSITNA VALLEY. THERE IS GOOD POTENTIAL FOR SNOW IN THESE
OTHER AREAS ON THE BACK SIDE OF THIS SYSTEM LATE THIS WEEK.

SOUTHWEST ALASKA...EXPECT A BAND OF LOCALLY HEAVY SNOW TO MOVE
INSHORE OF THE KUSKOKWIM DELTA AND WESTERN CAPES OF BRISTOL BAY
THIS AFTERNOON AND EVENING AS BERING TROUGH ARRIVES. AS THE SOUTH
END OF THE TROUGH ROTATES INTO BRISTOL BAY EXPECT SNOW CURRENTLY
FALLING ON THE WEST SIDE OF THE GULF LOW FROM KING SALMON NORTH AND
EAST TO INTENSIFY AND MOVE NORTHWARD TOWARD THE SOUTHERN KUSKOKWIM
VALLEY. VERY COLD AIR MOVING IN BEHIND THE TROUGH WILL INITIALLY
CAUSE SOME STRONG GUSTY WINDS...PRIMARILY ACROSS THE KUSKOKWIM DELTA
WHERE ADVECTION WILL BE STRONGEST. AS WINDS DIE DOWN TEMPERATURES
WILL FALL. THE TROUGH WILL EXIT TO THE NORTH EARLY WEDNESDAY MORNING
ALLOWING THINGS TO DRY OUT AND WINDS TO SLACKEN. HOWEVER...
OVERRUNNING OUT AHEAD OF THE PACIFIC SYSTEM COMBINED WITH ANOTHER
UPPER TROUGH CROSSING THE BERING WILL BRING SNOW BACK TO THE REGION
BY THURSDAY. THE LATEST MODELS INDICATE A FURTHER EAST TRACK OF
PACIFIC SYSTEMS INTO THE WESTERN GULF...SO THIS SHOULD KEEP THE
KUSKOKWIM DELTA COLD AND DRY. THERE WILL LIKELY BE SOME STRONG
NORTHERLY WINDS ALONG THE DELTA AS THE NEXT BERING TROUGH ARRIVES
THURSDAY AND WILL HAVE TO CONSIDER ISSUING A WIND CHILL ADVISORY.

BERING SEA/ALEUTIANS...THE ALEUTIANS AND ALASKA PENINSULA WILL BE
ALONG THE NORTHERN PERIPHERY OF THE NORTH PACIFIC SYSTEM. EXPECT
SOME SNOW AND STRONG WINDS TO MOVE IN AHEAD OF THE OCCLUDED FRONT
ON WEDNESDAY...BUT STEADY PRECIP SHOULD BE SHORT-LIVED AS THE FRONT
QUICKLY DISSIPATES. THE STORM TRACK WILL THEN SHIFT FURTHER EASTWARD
TOWARD THE GULF. ARCTIC AIR WILL PLUNGE SOUTHWARD INTO THE PACIFIC
ON THE BACK SIDE OF THESE SYSTEMS. THIS WILL LEAVE THE BERING AND
ALEUTIANS IN A SHOWERY PATTERN WITH STRONG GUSTY WINDS OVER THE
EASTERN PORTION OF THE AREA.

.LONG TERM FORECAST...MODELS ARE CONSISTENT IN INDICATING A BLOCKING
RIDGE SETTING UP FROM THE BERING TO SIBERIA ON SATURDAY. THIS WILL
MAINTAIN A STORM TRACK ALONG THE SOUTHERN TIER OF THE FORECAST
AREA...ALONG OR SOUTH OF THE ALEUTIANS AND INTO THE GULF OF ALASKA.
ARCTIC AIR WILL BECOME FIRMLY ENTRENCHED ON THE EASTERN SIDE OF THE
RIDGE FROM THE EASTERN BERING TO MAINLAND ALASKA. THEREFORE...AS
SYSTEMS MOVE INTO THE GULF PRECIP-TYPE OVER SOUTHCENTRAL WILL BE
MAINLY IN THE FORM OF SNOW.

.AER/ALU...WATCH/WARNING SUMMARY
PUBLIC...NONE.
MARINE...GALE WARNING...120 130 132 138 150 155 172 175.
FIRE WEATHER...NONE.

SEB FEB 13