Small Craft Advisory 228, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION...CORRECTED
NATIONAL WEATHER SERVICE ANCHORAGE AK
439 PM AKDT FRI MAR 13 2015

.ANALYSIS AND UPPER LEVELS...
AN UPPER LEVEL SHORT-WAVE TROUGH OVER THE GULF IS LIFTING NORTHWARD
ACROSS THE COPPER RIVER BASIN AND INTO THE YUKON...BRINGING AN END
TO STEADY STRATIFORM SNOW IN THE REGION. CYCLONIC FLOW PERSISTS
BEHIND THIS WAVE AROUND AN ELONGATED LOW CENTER FROM THE KENAI
PENINSULA NORTHWARD TO INTERIOR ALASKA. MORE IMPORTANTLY VERY COLD
AIR ALOFT IS PRODUCING UNSTABLE CONDITIONS OVER THE NORTHERN
GULF/EASTERN PRINCE WILLIAM SOUND/EASTERN COPPER RIVER BASIN...WHICH
IS LEADING TO SNOW SHOWERS. RADAR IMAGERY SHOWS THE HEAVIEST SHOWERS
ARE FORMING OVER THE RELATIVELY WARM WATERS OF THE GULF AND ROTATING
AROUND THE EAST SIDE OF A SURFACE LOW INTO EASTERN PRINCE WILLIAM
SOUND. STRONG GAP WINDS...SNOW SHOWERS...AND HEAVY FREEZING SPRAY
CONTINUE OVER THE WESTERN GULF/ALASKA PENINSULA. COLD ADVECTION IS
COMING TO AN END AND PRESSURE GRADIENTS ARE BEGINNING TO SLACKEN
SO EXPECT WINDS TO BEGIN TO SLOWLY DIMINISH.

A LARGE UPPER LEVEL LOW IS CENTERED OVER THE FAR WESTERN BERING
SEA. A SERIES OF SHORTWAVES ARE DROPPING DOWN THE BACK SIDE AND
THEN BEING DRIVEN EASTWARD ACROSS THE ALEUTIANS/SOUTHERN BERING
BY A ZONAL 180KT SUBTROPICAL JET OVER THE NORTH PACIFIC. THE
STRONGEST OF THE CURRENT SHORT-WAVES IS MOVING INTO THE CENTRAL
BERING SEA HELPING TO REINFORCE A FRONTAL SYSTEM OVER THE REGION.
THE STRONG UPPER LEVEL WESTERLY FLOW ALONG THE ALEUTIANS IS DRIVING
THE SOUTHERN END OF THE FRONT EASTWARD...WHILE THE PORTION OVER
THE BERING SEA IS PROGRESSING NORTH AND EASTWARD MUCH MORE SLOWLY.
THUS...THE FRONT WILL BEGIN TO ELONGATE AND WEAKEN WITH TIME.
A DEEP COLD AIR MASS IN PLACE OUT AHEAD OF THE FRONT IS PRODUCING
MAINLY SNOW...BUT WARMER AIR BEHIND IT WILL LEAD TO A MIX OF RAIN
AND SNOW SHOWERS. WITH PRIMARILY SOUTHEASTERLY WINDS AT THE
SURFACE TEMPERATURES ARE AROUND FREEZING WHICH WILL PRECLUDE ANY
MAJOR BLOWING SNOW ISSUES.

&&

.MODEL DISCUSSION...
MODELS REMAIN IN GOOD AGREEMENT WITH MAJOR FEATURES EARLY ON.
DIFFERENCES ARISE WITH DEVELOPMENT OF A LARGE STORM SYSTEM SOUTH
OF THE EASTERN ALEUTIANS/SOUTHERN ALASKA PENINSULA SATURDAY AND
PROGRESSION TOWARD THE SOUTHERN MAINLAND SATURDAY NIGHT AND
SUNDAY. SOLUTIONS ARE MUCH CLOSER WITH INITIAL POSITION AND TRACK
OF LOW. HOWEVER...SIGNIFICANT DIFFERENCES ARISE WITH NORTHWARD
PROGRESSION OF THE LOW CENTER AND FRONTAL SYSTEM ALONG WITH
HANDLING OF SHORT-WAVES EJECTING OUT OF THE TROUGH INTO THE GULF
OF ALASKA. INITIALLY PREFER THE ECMWF SOLUTION...BUT ON SUNDAY IT
BECOMES AN OUTLIER WITH LOW AND FRONTAL POSITIONS...SO THEN PREFER
NAM/GFS. FORECAST CONFIDENCE IS ABOVE AVERAGE THROUGH TOMORROW BUT
QUICKLY DROPS TO BELOW AVERAGE SUNDAY...ESPECIALLY WITH THE
DETAILS OF WIND AND PRECIP OVER THE SOUTHERN MAINLAND.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (DAYS 1 AND 2)...
THE UPPER LEVEL TROUGH WILL CONTINUE TO MOVE EASTWARD. THIS WILL
RESULT IN THE SURFACE LOW NEAR CORDOVA TO MOVE INLAND AND
DISSIPATE THIS EVENING. SNOW SHOWERS WILL DIMINISH ACROSS THE GULF
THIS EVENING BY CONTINUE INTO THE MORNING HOURS OVER THE COPPER
RIVER BASIN. THE HEAVIEST SNOWFALL IN THE COPPER RIVER BASIN IS
IN THE SOUTHERN AREA EXTENDING INTO THE YUKON TERRITORY. THE REST
OF SOUTHCENTRAL WILL CONTINUE WITH COLD AND DRY CONDITIONS WITH
STRONG GAP WINDS FROM THE BARREN ISLANDS NORTHWARD INTO PRINCE
WILLIAM SOUND. THE GAP WINDS WILL DIMINISH BY SATURDAY WITH THE
UPPER LEVEL TROUGH EXITING THE REGION AND COINCIDING WITH THE
WEAKENING OF THE PRESSURE GRADIENTS. BY SUNDAY MORNING A WEATHER
FRONT WILL BE MOVING INTO THE SOUTHWESTERN GULF REGION. THIS WILL
BRING STORM FORCE WINDS INTO THE SHELIKOF STRAIT...BARREN ISLANDS
AND THE SOUTHERN COOK INLET REGION. THIS WILL BRING A LARGE SWATH
OF MOISTURE TO THE SOUTHCENTRAL ALASKA REGION. THIS IS SETTING UP
FOR THE POTENTIAL FOR SIGNIFICANT SNOWFALL TO THE EASTERN KENAI
PENINSULA...ESPECIALLY THE INLAND AREAS. THE ANCHORAGE BOWL AND
THE WESTERN KENAI PENINSULA WILL DOWN SLOPE OUT WITH SOME MOISTURE
MAKING INTO THE AREA BY SUNDAY NIGHT WITH THE FLOW IN THE LOWER
LEVELS STARTING TO BECOME MORE SOUTHERLY.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (DAYS 1 AND 2)...
CLEAR AND COLD CONDITIONS WILL LINGER THROUGH SATURDAY AS THE
UPPER TROUGH REMAINS OVER THE MAINLAND. THE PATTERN WILL THEN
BEGIN TO CHANGE ON SATURDAY NIGHT AS SNOW MOVES INTO THE BRISTOL
BAY REGION WITH THE ARRIVAL OF THE BERING FRONT ALONG THE COAST.
SNOW WILL THEN SPREAD NORTHWARD INTO THE KUSKOKWIM DELTA AND
VALLEY ON SUNDAY. SNOWFALL IS EXPECTED TO BE RELATIVELY LIGHT WITH
ONLY MINOR ACCUMULATIONS...BUT THE POTENTIAL FOR BLOWING SNOW
WILL HAVE TO BE MONITORED FOR PORTIONS OF BRISTOL BAY AS WINDS
BEGIN TO INCREASE AHEAD OF THE FRONT.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (DAYS 1 AND 2)...
THE FRONT OVER THE BERING WILL BRING SNOW AND BLOWING SNOW TO THE
PRIBILOF ISLANDS THROUGH SATURDAY MORNING AS IT PIVOTS NORTHWARD
AND STRETCHES OUT. DESPITE TEMPERATURES HOVERING NEAR FREEZING
AND A RELATIVE LACK OF TRANSPORTABLE SNOW...VISIBILITIES ARE
STILL EXPECTED TO BE REDUCED TO A HALF MILE AT TIMES AS SNOW
BECOMES HEAVIER OVERNIGHT WITH STRONG EASTERLY WINDS REMAINING IN
PLACE. THEREFORE...A BLOWING SNOW ADVISORY HAS BEEN ISSUED THROUGH
EARLY SATURDAY MORNING. MEANWHILE...A SURFACE LOW WILL RAPIDLY
INTENSIFY LATE TONIGHT SOUTH OF DUTCH HARBOR AND MOVE SLOWLY
TOWARDS THE ALASKA PENINSULA THROUGH SUNDAY...KEEPING GUSTY WINDS
AND A MIX OF RAIN AND SNOW OVER THE SOUTHEASTERN BERING THROUGH
THE WEEKEND. SHOWERY CONDITIONS WILL ALSO PERSIST OVER THE CENTRAL
AND WESTERN PORTIONS OF THE ALEUTIAN CHAIN BENEATH BROAD CYCLONIC
FLOW OVER THE BERING SEA.

&&

.LONG TERM FORECAST (DAYS 3 THROUGH 7)...
A LOW SOUTH OF THE ALASKA PENINSULA WILL PUSH A FRONT THROUGH THE
GULF AND INTO THE SOUTHCENTRAL ON SUNDAY. THE MAJORITY OF THE
PRECIPITATION WILL FALL ALONG THE GULF COAST AND IN PRINCE
WILLIAM SOUND. WITH OVERALL SOUTHEASTERLY FLOW MANY OF THE AREAS
ON THE WESTERN SIDE OF THE KENAI PENINSULA WILL SEE DOWNSLOPING.
HOWEVER...THE CORE OF THE WINDS WILL BE ABOVE MOUNTAIN TOP SO SOME
SPILL OVER IS EXPECTED. THE PRECIPITATION WILL PRIMARILY START AS
SNOW BUT WILL TRANSITION TO RAIN FROM THE SOUTH AS MORE WARM AIR
IS BROUGHT IN. WARM SOUTHERLY FLOW WILL CONTINUE AFTER THE FIRST
SYSTEM AND BY MID WEEK ANOTHER LOW WILL APPROACH BRINGING MORE
WARM CONDITIONS.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...WINTER WEATHER ADVISORY FOR BLOWING SNOW 195.
MARINE...GALES 120 121 127 129 130 131 138 139 150 155 160 165 170
172 179 180 185 351 352 411 412 413 414.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...SB
SOUTHCENTRAL ALASKA...PD
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...CB
LONG TERM...DK