Small Craft Advisory 18, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
420 PM AKST WED JAN 7 2015

.ANALYSIS AND UPPER LEVELS...
A SIGNIFICANT PATTERN CHANGE IS WELL UNDER WAY FOR THE SOUTHERN
MAINLAND OF ALASKA. A DIGGING TROUGH OVER THE NORTHEAST PACIFIC
IS CAUSING AMPLIFICATION OF A DOWNSTREAM RIDGE FROM THE NORTHWEST
US TO THE EASTERN GULF OF ALASKA AND INTO SOUTHCENTRAL ALASKA. A
GLANCE AT WATER VAPOR IMAGERY TELLS THE STORY...WITH DEEP MOISTURE
TRANSPORT FROM THE EAST PACIFIC TROPICS NORTHWESTWARD ACROSS THE
GULF OF ALASKA AND KODIAK ISLAND AND INTO BRISTOL BAY. SIGNIFICANT
LOW LEVEL WARMING HAS BEEN OBSERVED ON KODIAK ISLAND AND ACROSS
THE ALASKA PENINSULA...WITH WARM AIR CONTINUING TO ADVECT
NORTHWESTWARD ACROSS BRISTOL BAY. FREEZING RAIN WAS BRIEFLY
OBSERVED IN KING SALMON OVERNIGHT BUT SURFACE TEMPERATURES QUICKLY
WARMED. THE ARCTIC AIR MASS IS DEEPER AS YOU HEAD NORTH AND WEST
TO THE REST OF SOUTHWEST ALASKA. OFFSHORE FLOW IN THEE AREAS WILL
ALSO HELP HOLD IN SUB-FREEZING TEMPERATURES AT THE SURFACE. THIS
IS THE PERFECT RECIPE FOR FREEZING RAIN. FREEZING RAIN ADVISORIES
ARE IN EFFECT FOR BRISTOL BAY AND THE KUSKOKWIM DELTA.

THE UPPER RIDGE IS MAINTAINING DRY AND COLD AIR OVER SOUTHCENTRAL
ALASKA FOR THE MOMENT...BUT THAT WILL RAPIDLY CHANGE AS THE RIDGE
AXIS SHIFTS NORTHWARD TONIGHT. MEANWHILE A DEEP CLOSED LOW IS
NEARLY STATIONARY OVER THE BERING SEA...PRODUCING SNOW OR SNOW
SHOWERS OVER THE EASTERN BERING AND PRIBILOF ISLANDS. THE DEEPER
MOISTURE AND WARMER AIR MOVING ACROSS SOUTHWEST ALASKA IS ABOUT
TO START WRAPPING AROUND THE NORTH SIDE OF THIS SYSTEM. THIS WILL
INCREASE BAROCLINICITY AND PRODUCE STEADIER AND HEAVIER
PRECIPITATION.

OUT WEST A WEAKENING SHORT-WAVE TROUGH IS LIFTING NORTHWARD ACROSS
THE WESTERN BERING. A SHORT-WAVE RIDGE IS BUILDING IN BEHIND IT
WITH BENIGN CONDITIONS SETTLING IN ACROSS THE ALEUTIAN CHAIN.

&&

.MODEL DISCUSSION...
ALL MODELS REFLECT THE SAME GENERAL IDEA OVER THE NEXT COUPLE
DAYS...BUT THERE ARE SOME KEY DIFFERENCES IN SURFACE LOW POSITIONS
AS WELL AS TIMING OF UPPER WAVES EMBEDDED WITHIN THE LONG FETCH OF
SOUTHERLY FLOW FROM THE EAST PACIFIC. MODELS HAVE CONVERGED ON
DEVELOPMENT OF A DEEPER SURFACE LOW OVER THE NORTHEAST PACIFIC
TODAY AND TONIGHT. ALL SOLUTIONS EXCEPT FOR THE NAM TRACK THIS
LOW TO JUST SOUTH OF KODIAK ISLAND WHERE IT WEAKENS. BASED ON A
STRONGER STORM SYSTEM WILL BUMP UP WINDS AND QPF ACROSS THE
GULF...SOUTHCENTRAL...AND BRISTOL BAY.

THERE ARE SOME MINOR DIFFERENCES IN POSITION OF LOW OVER THE
EASTERN BERING. THIS WILL AFFECT WHERE THE HEAVIEST BAND OF SNOW
SETS UP ON THE NORTH TO WEST SIDE OF THE LOW CENTER. RIGHT NOW IT
APPEARS FAVORABLE FOR THE BAND TO BE IN THE VICINITY OF THE
PRIBILOF ISLANDS...SO WILL INCREASE SNOW AMOUNTS.

OVERALL FORECAST CONFIDENCE IS ABOUT AVERAGE...WHILE CONFIDENCE
IN THE DETAILS OF PRECIPITATION TIMING AND PRECIPITATION TYPE IS
LOWER. THE MOST SIGNIFICANT CHALLENGE IS DETERMINING WHERE AND
WHEN FREEZING RAIN WILL FALL. THERE IS CERTAINLY POTENTIAL FOR
FREEZING RAIN ACROSS A SIGNIFICANT PORTION OF SOUTHWEST ALASKA
AND SOUTHCENTRAL ALASKA. RELATED TO THE PRECIP CHALLENGE...STEADY
WARM AIR ADVECTION AND EROSION OF THE COLD DOME OVER THE SOUTHERN
MAINLAND ALSO MAKES FOR A TRICKY TEMPERATURE FORECAST.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (DAYS 1 AND 2)...
THE DEVELOPING STORM SYSTEM OVER THE NORTHEAST PACIFIC WILL SEND A
STRONG WARM FRONT TOWARD SOUTHCENTRAL TONIGHT INTO THURSDAY...WITH
PRECIP AND WARMER AIR RAPIDLY MOVING INTO THE REGION. THE SOUTHWEST
TO NORTHEAST ORIENTATION OF THE FRONT IS FAVORABLE FOR MAINTAINING
OFFSHORE FLOW ACROSS SOUTHCENTRAL AS IT APPROACHES....WHICH WILL
HOLD IN COLD AIR AT THE SURFACE WHILE ABOVE FREEZING AIR MOVES IN
ALOFT. THE HIGHEST PROBABILITY OF FREEZING RAIN WILL BE THE EASTERN
INTERIOR OF THE KENAI PENINSULA (INCLUDING THE SEWARD HIGHWAY
CORRIDOR) ALONG WITH THE WESTERN PORTION OF THE WESTERN KENAI
(ALONG COOK INLET).

STRONG CROSS BARRIER FLOW ACROSS THE KENAI AND CHUGACH MOUNTAINS
WILL LIMIT PRECIP FOR THE WESTERN INTERIOR OF THE KENAI ON UP TO
ANCHORAGE...THE MAT VALLEY...AND THE EASTERN SUSITNA VALLEY. THERE
ARE SOME FAVORABLE FACTORS FOR AT LEAST A BRIEF PERIOD OF PRECIP
IN THE TYPICAL DOWNSLOPE AREAS. THE STRONGEST WINDS ARE ABOVE
MOUNTAIN TOP ABOVE STEEP SURFACE-BASED INVERSIONS. UNTIL SOME OF
THE COLD AIR IS ERODED DOWNSLOPE WILL BE LESS EFFECTIVE AND THERE
SHOULD BE AT LEAST A LITTLE SLOP-OVER OF PRECIP ACROSS THE
MOUNTAINS. DEEP TROPICAL MOISTURE TRANSPORT ALONG WITH ARRIVAL OF
AN UPPER LEVEL WAVE EARLY TOMORROW MORNING ALSO FAVOR PRECIP.
THEREFORE...WILL ISSUE FREEZING RAIN ADVISORIES FROM THE KENAI
PENINSULA UP TO ANCHORAGE AND PORTIONS OF THE MAT-SU. THERE IS
POTENTIAL FOR A COUPLE TENTHS OF ICE ACCUMULATION ON THE KENAI
WHILE AREAS FURTHER NORTH SHOULD SEE MUCH LESS QPF WITH VERY
LITTLE ICE ACCUMULATION.

WINDS THROUGH THOMPSON PASS WILL DIMINISH AS LOW LEVEL FLOW SHIFTS
TO SOUTHEAST. THE BIG QUESTION IS WHETHER THERE IS ENOUGH WIND
LEFT AS SNOW ARRIVES EARLY THURSDAY MORNING TO CAUSE SIGNIFICANTLY
REDUCED VISIBILITIES IN BLOWING SNOW. REPORTS FROM THE PASS INDICATE
THE SNOW PACK IS WIND BLOWN AND NOT TRANSPORTABLE...THUS THE ONLY
THREAT FOR BLOWING SNOW WILL COME FROM WHAT IS FALLING. THINK
THERE WILL STILL BE PLENTY OF WIND AT THE TOP OF THE PASS...SO IT
MAY ONLY BE A FAIRLY SMALL PORTION OF THE HIGHWAY WITH THE WORST
CONDITIONS. BASED ON THE FACT THAT THESE TYPE OF CONDITIONS REALLY
HAVEN'T HAPPENED YET THIS WINTER WILL LIKELY GO AHEAD AND ISSUE
A BLIZZARD WARNING.

WARMER AND WARMER AIR WILL CONTINUE TO FLOW INTO THE REGION
THURSDAY AND THURSDAY NIGHT WITH TEMPERATURES RISING ABOVE
FREEZING FOR ALL BUT INTERIOR SECTIONS OF SOUTHCENTRAL...ESPECIALLY
IN THE COPPER RIVER BASIN WHERE ARCTIC AIR WILL REMAIN FIRMLY
ENTRENCHED. THUS...THE THREAT OF FREEZING RAIN WILL DIMINISH
AS HEAD THROUGH THE DAY THURSDAY. ALSO...AS COLD AIR IS ERODED
DOWNSLOPE DRYING WILL INCREASE...SO AFTER A QUICK SHOT OF PRECIP
TOMORROW MORNING EXPECT THE LEE OF THE KENAI AND CHUGACH TO REMAIN
MOSTLY DRY. THE STORM SYSTEM WILL WEAK OVER THE GULF ON FRIDAY
WITH DIMINISHING PRECIP AND WINDS. THE BULK OF REMAINING PRECIP
SHOULD BE CONFINED TO THE GULF COAST AND WILL MOST DEFINITELY BE
IN THE FORM OF RAIN WITH SURFACE TEMPERATURES WELL ABOVE FREEZING.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (DAYS 1 AND 2)...
AS WARM AIR CONTINUES TO SPREAD NORTH AND WEST TONIGHT AND
THURSDAY A PRECIP-TYPE TRANSITION TO FREEZING RAIN WILL FOLLOW.
AS IS TYPICAL...DILLINGHAM AND NORTH WILL BE SLOW TO WARM AT
THE SURFACE THANKS TO STRONG NORTHERLY FLOW AT THE SURFACE.
THEREFORE...IT COULD BE A FAIRLY PROLONGED PERIOD OF FREEZING
RAIN IN THAT AREA. CONFIDENCE IN EXTENT OF FREEZING RAIN ACROSS
THE KUSKOKWIM DELTA IS LOWER. GUSTY NORTHEAST FLOW AT THE SURFACE
WILL PERSIST THROUGH THURSDAY MAINTAINING COLD LOW LEVEL AIR. THE
WARMING ALOFT WON'T BE NEARLY AS IMPRESSIVE AS AREAS FARTHER SOUTH
AND EAST. THE BEST CHANCE FOR FREEZING RAIN SEEMS TO BE FROM
AROUND BETHEL SOUTHWARD (ALONG THE KUSKOKWIM BAY). THIS AREA MAY
SEE SOME ADDITIONAL WARMING FROM DOWNSLOPE FLOW OFF THE KILBUCK
MOUNTAINS. FOR NOW WILL EXTEND THE FREEZING RAIN ADVISORY FOR THE
KUSKOKWIM DELTA THROUGH MID-MORNING TOMORROW...RECOGNIZING THAT
THERE IS POTENTIAL FOR IT TO LAST LONGER IN THE SOUTHERN PORTION
OF THE ZONE. AS YOU HEAD NORTHWEST FROM BETHEL MODEL LOW LEVEL
THICKNESSES INDICATE PRECIP SHOULD STAY ALL SNOW. FINALLY...WILL
ALSO ISSUE A FREEZING RAIN ADVISORY FOR THE SOUTHERN KUSKOKWIM
VALLEY. PRECIP HERE SHOULD BE FAIRLY SHORT-LIVED AND COINCIDENT
WITH UPPER LEVEL WAVES MOVING OVERHEAD...SO EXPECT LITTLE ICE
ACCUMULATION.

THE UPPER LEVEL TROUGH AND LOW OVER THE EASTERN BERING WILL LIFT
NORTHWARD THURSDAY NIGHT AND FRIDAY WITH COLDER AIR GRADUALLY
MOVING INTO SOUTHWEST ALASKA FROM THE SOUTH AND WEST. THUS EXPECT
A TRANSITION IN PRECIP-TYPE BACK TOWARD SNOW AS PRECIP DIMINISHES
ACROSS THE REGION.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (DAYS 1 AND 2)...
THE MAIN FEATURE OF CONCERN FOR THE NEXT COUPLE DAYS WILL BE
THE LOW OVER THE EASTERN BERING SEA. AS MENTIONED IN THE MODEL
DISCUSSION ABOVE IT LOOKS LIKE THE PRIBILOF ISLANDS ARE IN A
FAVORABLE SPOT (RIGHT ON THE WEST SIDE OF THE LOW CENTER) FOR
STEADY SNOW. THUS...EXPECT THERE COULD BE SEVERAL INCHES OF
ACCUMULATION OVER THE NEXT 24 TO 36 HOURS BEFORE THE LOW EXITS
TO THE NORTH. THE REMAINDER OF THE BERING SEA AND ALEUTIANS WILL
REMAIN QUIET UNDER THE INFLUENCE OF AN UPPER LEVEL RIDGE.

&&

.LONG TERM FORECAST (DAYS 3 THROUGH 7)...
A WARMER AND MORE UNSETTLED PATTERN IS IN STORE FOR SOUTHCENTRAL
ALASKA FROM THIS WEEKEND INTO THE LATTER HALF OF NEXT WEEK AS A
LONGWAVE RIDGE REMAINS ENTRENCHED OVER THE WEST COAST OF NORTH
AMERICA AND EXTENDS OVER THE INTERIOR MAINLAND TO NEAR THE BERING
STRAIT. THIS WILL ALLOW PERSISTENT SOUTHERLY FLOW TO BECOME
ESTABLISHED OVER THE GULF...YIELDING ABOVE NORMAL TEMPERATURES AS
VARIOUS DISTURBANCES ROTATE NORTHWARD TOWARDS THE MAINLAND. THE
DETAILS OF THESE FEATURES REMAIN DIFFICULT TO NAIL DOWN AT THIS
POINT IN TIME...BUT SHOWER ACTIVITY WILL GENERALLY BE FOCUSED
ALONG THE GULF COAST AND OCCASIONALLY SPREAD INLAND AS
DISTURBANCES MOVE THROUGH.

MEANWHILE...SHOWER ACTIVITY WILL BE ON A DOWNWARD TREND ACROSS
MUCH OF THE BERING THIS WEEKEND AS ONE LOW DEPARTS TO THE NORTH
BUT WILL QUICKLY SPREAD BACK OVER THE ALEUTIANS AS A DEEP NORTH
PACIFIC LOW RETROGRADES WESTWARD ALONG THE SOUTHERN SIDE OF THE
CHAIN EARLY IN THE WORK WEEK. THE BROAD LONGWAVE TROUGH CENTERED
OVER THE NORTH PACIFIC WILL THEN KEEP SHOWERY CONDITIONS OVER THE
REGION THROUGH MUCH OF NEXT WEEK.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...BLIZZARD WARNING 131.
         FREEZING RAIN ADVISORY 101 111 121 125 145 152 155 161.
MARINE...GALE 119 120 127 130-139 150 160 180
		 HEAVY FREEZING SPRAY 181.
FIRE WEATHER...NONE.
&&

JAN 15 SEB