Small Craft Advisory 377, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
105 PM AKDT MON APR 29 2013

.ANALYSIS AND UPPER LEVELS...A POLAR VORTEX REMAINS NEARLY STATIONARY
OVER THE CANADIAN ARCTIC. ITS ASSOCIATED TROUGH ENCOMPASSES MAINLAND
ALASKA WITH A STEADY SUPPLY OF ARCTIC AIR MAINTAINING BELOW NORMAL
TEMPERATURES ACROSS THE REGION. SATELLITE IMAGERY SHOWS A SHORT-WAVE
TROUGH OVER NORTHWEST ALASKA TRACKING SOUTHEASTWARD DOWN THE BACK
SIDE OF THE TROUGH. LIGHT SNOW IS BEING OBSERVED OVER PORTIONS OF
INTERIOR WESTERN ALASKA. CLOUDS ARE INCREASING OUT AHEAD OF THE
SHORT-WAVE FROM SOUTHWEST ALASKA TO SOUTHCENTRAL ALASKA...BUT SO
FAR THERE ARE NO INDICATIONS OF SNOW REACHING THE GROUND. THE AIR
MASS OVER MOST OF THE SOUTHERN MAINLAND IS QUITE DRY AS SEEN IN AREA
SOUNDINGS (MCGRATH/KING SALMON/ANCHORAGE). THUS IT WILL TAKE A WHILE
TO SATURATE THE COLUMN AND FOR SNOW TO REACH THE SURFACE. SINCE THE
SHORT-WAVE IS NOT THAT STRONG TO BEGIN WITH...ANY SNOW THAT DOES
REACH THE SURFACE WILL BE LIGHT.

MEANWHILE A REX BLOCK IS IN PLACE TO THE WEST WITH A CLOSED HIGH
OVER THE BERING SEA AND A MULTI-CENTERED CLOSED LOW OVER THE
NORTH-CENTRAL PACIFIC. THE HIGH HAS BEGUN TO RETROGRADE NORTHWESTWARD
TOWARD RUSSIA ALLOWING THE ENERGY ROTATING AROUND THE LOW TO PUSH
NORTHWARD TOWARD THE ALEUTIANS. THE BERING SEA...ALEUTIANS...AND
SOUTHWEST COAST ARE MOSTLY COVERED IN STRATUS THANKS TO A STEEP
LOW LEVEL SUBSIDENCE INVERSION. THE INVERSION IS HIGH ENOUGH THAT
THERE HAS BEEN LITTLE OR NO FOG OBSERVED ACROSS THE REGION.

.MODEL DISCUSSION...MODEL RELATIVE HUMIDITY FIELDS APPEAR TOO HIGH
THIS MORNING ACROSS SOUTHCENTRAL ALASKA. THEY INDICATE THERE SHOULD
BE FAIRLY WIDESPREAD MID LEVEL CLOUDS MOVING INTO THE REGION WHEN
IN FACT OBSERVATIONS SHOW PRIMARILY CIRRUS (WITH THE EXCEPTION OF
THE NORTHERN SUSITNA VALLEY). MODELS TEND TO BE TOO FAST IN LOWERING
CEILINGS AND BRINGING IN PRECIPITATION WHEN THE ANTECEDENT AIRMASS
IS AS DRY AS IT IS THIS MORNING. WILL FACTOR THIS INTO THE NEW
FORECASTS.

BASED SOLELY ON SATELLITE IMAGERY...ALL MODELS APPEAR TO HAVE
INITIALIZED FAIRLY WELL WITH COMPLEX SERIES OF SURFACE LOWS/WAVES
ROTATING AROUND THE NORTH PACIFIC. THE MOST SIGNIFICANT MODEL ISSUE
THIS MORNING IS THE TRACK OF A LOW WHICH IS PROGGED TO FORM TUESDAY
AFTERNOON SOUTH OF KODIAK ISLAND AND TRACK ACROSS THE GULF TUESDAY
NIGHT AND WEDNESDAY. THE MODELS SHOW POOR RUN-TO-RUN CONSISTENCY
WITH THE TRACK OF THIS LOW...ALTHOUGH THERE IS A DEFINITIVE TREND
TOWARD A MORE EASTWARD TRACK IN RECENT RUNS. THE ECMWF/GEMGLB/NAM
ARE ALL GROUPED TOGETHER WITH THE EASTERN-MOST TRACK AND WILL BE
USED IN THE LATEST FORECAST.

LASTLY...MODEL SOLUTIONS VARY ON THE NORTHWARD EXTENT OF PRECIP
ASSOCIATED WITH SYSTEMS ROTATING AROUND THE PRIMARY UPPER LEVEL
LOW OVER THE NORTH PACIFIC. THE NAM IN PARTICULAR SEEMS TO KEEP
PRECIP FARTHER SOUTH. WILL TREND TOWARD THE WETTER SOLUTION AS
INDICATED BY A MAJORITY OF MODELS.

.SHORT TERM FORECAST...

SOUTHCENTRAL ALASKA...BASED ON THE LITTLE SNOW BEING OBSERVED WITH
THE NORTHWEST ALASKA SHORT-WAVE AND THE DRY AIRMASS IN PLACE (AS
DISCUSSED ABOVE) EXPECT SNOW TO INITIALLY OVER INTERIOR SOUTHWEST
ALASKA AND SOUTHCENTRAL ALASKA TO BE MOSTLY CONFINED TO THE MOUNTAINS
AS THE FIRST SHORT-WAVE MOVES THROUGH THIS AFTERNOON AND EVENING.
THIS MAY HELP MOISTEN UP THE LOW LEVELS ENOUGH SO THAT WHEN THE
SLIGHTLY MORE IMPRESSIVE VORTICITY MAXIMUM MOVES THROUGH LATE TONIGHT
THROUGH TUESDAY PRECIPITATION WILL HAVE A BETTER SHOT OF REACHING
THE GROUND OVER THE LOW-LANDS. THE AIRMASS WILL BE COLD ENOUGH TO
SUPPORT ALL SNOW TONIGHT AND TUESDAY MORNING. TEMPERATURES NEAR THE
SURFACE MAY WARM UP JUST ENOUGH TUESDAY AFTERNOON TO CAUSE A MIX
WITH RAIN IN SOME LOCATIONS. WITH PREDOMINANTLY LIGHT SNOW EXPECT
LITTLE OR NO ACCUMULATION AT SEA LEVEL. THERE IS A BETTER CHANCE
OF SOME MINOR ACCUMULATION AS YOU GO UP IN ELEVATION. THE ONE BIG
UNCERTAINTY IN ALL OF THIS IS EXACTLY WHEN THE AIR MASS BECOMES
SATURATED FOR SNOW TO REACH THE GROUND. IF IT OCCURS EARLIER THAN
EXPECTED THEN THINGS WILL BE A LITTLE SNOWIER THAN FORECAST.

THE NEXT STORM SYSTEM TO DEAL WITH WILL BE THE LOW CROSSING THE
GULF ON TUESDAY NIGHT AND WEDNESDAY. BASED ON THE EASTWARD TREND
IN THIS TRACK MENTIONED IN THE MODEL DISCUSSION ABOVE ITS LOOKING
DRIER OVER SOUTHCENTRAL ALASKA. THE BEST SHOT OF PRECIP WITH THIS
SYSTEM LOOKS TO BE FROM PRINCE WILLIAM SOUND TO THE EASTERN COPPER
BASIN. AS THIS SYSTEM CROSS THE GULF THE UPPER TROUGH WILL DIG
SOUTHWARD ACROSS SOUTHWEST ALASKA AND THE EASTERN GULF BRINGING A
REINFORCING SHOT OF ARCTIC AIR AND MAINTAINING BELOW NORMAL
TEMPERATURES ACROSS THE REGION.

SOUTHWEST ALASKA...THE SOUTHERN KUSKOKWIM VALLEY MAY SEE A COUPLE
PERIODS OF LIGHT SNOW AS THE UPSTREAM SHORT-WAVE AND VORTICITY
MAXIMUM MOVE THROUGH THIS AFTERNOON THROUGH TUESDAY MORNING. THIS
IS AN UNFAVORABLE DIRECTION TO RECEIVE MUCH PRECIP. HOWEVER...WITH
LOW CLOUDS CURRENTLY IN PLACE THERE IS A POSSIBILITY THAT A SEEDER
-FEEDER MECHANISM COULD BE INITIATED...WITH ICE CRYSTALS FALLING
OUT OF HIGHER LEVEL CLOUDS AND GENERATING PRECIP WHEN THEY FALL
INTO THE LOWER LEVEL CLOUDS. IN ANY CASE ANY SNOW THAT DOES FALL
WILL BE VERY LIGHT WITH LITTLE OR NO ACCUMULATION EXPECTED IN THE
VALLEY. STRATUS OVER THE KUSKOKWIM DELTA WILL REMAIN IN PLACE UNTIL
LOW LEVEL FLOW SHIFTS FROM ONSHORE TO OFFSHORE TONIGHT INTO TUESDAY.
THE UPPER LEVEL TROUGH OVER THE MAINLAND WILL DIG SOUTHWARD OVER
WESTERN ALASKA TUESDAY THROUGH THURSDAY BRINGING ANOTHER SHOT OF
ARCTIC AIR. THERE WILL BE SOME WEAK LIFT ON THE FRONT SIDE OF THIS
TROUGH OVER EASTERN PORTIONS OF SOUTHWEST ALASKA...RESULTING IN
CLOUDS AND A CHANCE OF SNOW.

BERING SEA/ALEUTIANS...STRONGER WINDS AND PRECIP WILL GRADUALLY
SPREAD TO THE ALEUTIANS AND SOUTHERN ALASKA TONIGHT AND TUESDAY
ALONG THE NORTHERN PERIPHERY OF STORMS PASSING TO THE SOUTH. THE
UPPER LEVEL TROUGH DIGGING DOWN ACROSS WESTERN ALASKA AND THE
EASTERN BERING MID WEEK WILL THEN PUSH EVERYTHING BACK SOUTHWARD.
ACCOMPANYING COLD ADVECTION WILL HELP CLEAR OUT THE LOW CLOUDS
MID WEEK...PRIMARILY FROM THE EASTERN TO CENTRAL BERING.

.LONG TERM FORECAST...TROUGH WHICH DIGS DOWN ACROSS WESTERN ALASKA
AND THE EASTERN BERING MID WEEK WILL BECOME SETTLE OVER MAINLAND
ALASKA THIS WEEKEND WITH HIGH CONFIDENCE IN TEMPERATURES REMAINING
BELOW NORMAL OVER MOST OF THE MAINLAND. THERE IS MUCH LOWER
CONFIDENCE WITH THE TRACK OF STORM SYSTEMS UP THE FRONT SIDE OF THE
TROUGH. THE EXACT PLACEMENT OF THE TROUGH WILL DETERMINE WHETHER
LOWS TRACK TO SOUTHCENTRAL OR MORE TOWARD THE NORTHEAST GULF AND
SOUTHEAST ALASKA. TEMPERATURES WILL ALSO BE SIGNIFICANTLY WARMER
FROM THE LOW CENTER EASTWARD. THE LATEST SOLUTIONS INDICATE THAT
THERE LIKELY WILL BE SOME IMPACT FROM THESE SYSTEMS OVER EASTERN
PORTIONS OF SOUTHCENTRAL...NAMELY EASTERN PRINCE WILLIAM SOUND AND
THE COPPER BASIN. THE BERING SEA AND ALEUTIANS LOOK TO REMAIN QUIET
AS ANOTHER RIDGE BUILDS IN LATER THIS WEEK.

CONFIDENCE REALLY FALLS OFF AS WE HEAD INTO EARLY NEXT WEEK WITH
SIGNIFICANTLY DIFFERENT SOLUTIONS INDICATED BY THE MEDIUM RANGE
MODELS.

.AER/ALU...WATCH/WARNING SUMMARY
PUBLIC...NONE.
MARINE...GALE WARNING...170 172 175.
FIRE WEATHER...NONE.

SEB APR 13