Small Craft Advisory 712, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
100 PM AKDT MON SEP 16 2013

.ANALYSIS AND UPPER LEVELS...THE PRIMARY FEATURE OF CONCERN THIS
MORNING IS AN UPPER LEVEL LOW OVER THE EASTERN GULF WHICH HAS
STARTED TO LIFT SLOWLY NORTHWARD UP THE WEST SIDE OF A HIGH
AMPLITUDE DOWNSTREAM RIDGE. THERE ARE MULTIPLE VORTICITY CENTERS
NOTED IN WATER VAPOR IMAGERY TRACKING AROUND THE MAIN CENTER...WITH
THOSE ON THE NORTH SIDE TRACKING WESTWARD TOWARD SOUTHCENTRAL ALASKA.
RADAR IMAGERY SHOWS SHOWERS OR LIGHT RAIN SPREADING WESTWARD ACROSS
THE NORTHEASTERN GULF AND INTO THE EASTERN COPPER BASIN. THERE ARE
MULTIPLE LOW CENTERS AT THE SURFACE..BUT THE PRIMARY IMPACT IS A
TIGHT PRESSURE GRADIENT BETWEEN THE LOW AND THE GULF AND HIGH
PRESSURE INLAND WHICH CONTINUES TO PRODUCE LOCALIZED GAP WINDS
ALONG THE GULF COAST.

MEANWHILE...A TROUGH OVER THE EASTERN BERING IS DIGGING
EAST-SOUTHEASTWARD TOWARD THE GULF. AREAS OF LIGHT RAIN MOVED
ONSHORE ALONG PORTIONS OF THE SOUTHWEST COAST OUT AHEAD OF THIS
TROUGH. THIS RAIN IS NOW STARTING TO DISSIPATE AS THE BULK OF
ENERGY WITH THE TROUGH EXITS TO THE SOUTHEAST. FURTHER UPSTREAM
A WEAK LOW PRESSURE SYSTEM CROSSING THE WESTERN ALEUTIANS IS
RAPIDLY WEAKENING AS IT GETS SWALLOWED BY A RIDGE BUILDING
JUST UPSTREAM. THIS RIDGE IS BUILDING IN RESPONSE TO AN FORMER
TROPICAL SYSTEM OVER THE FAR NORTHWEST PACIFIC WHICH IS TRACKING
QUICKLY NORTHWARD TOWARD EASTERN RUSSIA.

.MODEL DISCUSSION...MODELS ALL INITIALIZED REASONABLY WELL WITH
SYNOPTIC SCALE FEATURES. THE KEY MODEL DIFFERENCE FOR THE SHORT-TERM
FORECAST IS THE TRACK OF THE UPPER LEVEL LOW AS IT MOVES INLAND
ACROSS SOUTHCENTRAL. THE NAM IS AN OUTLIER WITH A MORE AGGRESSIVE
WESTWARD TRACK AND THEREFORE IS FASTER AND FURTHER SOUTH AND WEST
WITH PROGRESSION OF RAIN TONIGHT AND TUESDAY. PREFER AN AVERAGE
POSITION OF THE REST OF THE MODELS...TRACKING THE UPPER LOW CENTER
NORTHWARD THROUGH THE COPPER RIVER BASIN. THIS MAKES IT QUESTIONABLE
WHETHER RAIN WILL MAKE IT ALL THE WAY TO ANCHORAGE AND THE MAT-SU.
CERTAINLY THINK THERE WILL BE A FEW SHOWERS WHICH CROSS THE MOUNTAINS
FROM THE EAST...BUT NOT READY TO BUY INTO STEADY RAIN.

.SHORT TERM FORECAST...

SOUTHCENTRAL ALASKA...AFTER A COUPLE NIGHTS OF FREEZING TEMPERATURES
ACROSS PORTIONS OF SOUTHCENTRAL MINIMUM TEMPERATURES WILL BEGIN TO
MODERATE TONIGHT AS CLOUDS AND PRECIPITATION MOVE BACK INTO THE
REGION. TEMPERATURES WILL REMAIN COOL ENOUGH HOWEVER FOR SOME
MOUNTAIN SNOWS TONIGHT AND TUESDAY. IN PARTICULAR...COOLER AIR IN
THE EASTERN FOOTHILLS OF THE TALKEETNA MOUNTAINS (THE WESTERN COPPER
RIVER BASIN ZONE) COMBINED WITH ADDITIONAL COOLING AS FALLING SNOW
ALOFT MELTS WILL LIKELY COOL THE COLUMN ENOUGH TO PRODUCE A PERIOD
OF ALL SNOW LATE TONIGHT THROUGH TUESDAY MORNING. A SPECIAL WEATHER
STATEMENT HAS BEEN ISSUED TO HIGHLIGHT THE LIKELIHOOD OF FALLING
SNOW IN THIS AREA WHICH INCLUDES THE GLENN HIGHWAY NEAR EUREKA (AT
OVER 3000 FEET ABOVE SEA LEVEL). SURFACE TEMPERATURES ARE WARM AND
PRECIPITATION IS EXPECTED TO BE LIGHT...SO ANY ACCUMULATION WILL BE
LIGHT AND CONFINED TO COOLER NON-PAVED SURFACES. PRECIPITATION WILL
REMAIN RAIN IN THE LOWLANDS OF SOUTHCENTRAL. ALSO EXPECT PRECIP TO
BE INTERMITTENT FOR MOST AREAS AS VARIOUS WAVES ROTATING AROUND THE
UPPER CENTER MOVE THROUGH.

THE LIKELIHOOD OF RAIN FOR THE WESTERN KENAI TO ANCHORAGE AND THE
MAT-SU INCREASES WEDNESDAY AS A TROUGH DROPPING OUT OF THE ARCTIC
APPROACHES FROM THE WEST. ALL THE STRENGTH OF THIS FEATURES HAS
CHANGED FROM MODEL RUN TO MODEL RUN THE MODELS HAVE BEEN VERY
CONSISTENT ON THE TIMING OF THE TROUGH. THEREFORE...WILL UP TO
LIKELY POPS FOR THESE AREAS. THINGS SHOULD DRY OUT ON THURSDAY AS
THIS TROUGH EXITS TO THE NORTH AND FLOW SHIFTS TO OFFSHORE AHEAD
OF A DEEP LOW MOVING INTO THE NORTHEAST PACIFIC. THIS LOW WILL
PRODUCE ANOTHER GAP FLOW WIND EVENT ALONG THE GULF COAST AS WE
HEAD THROUGH THURSDAY AND FRIDAY.

SOUTHWEST ALASKA...THE UPPER TROUGH OVER THE EASTERN BERING WILL
QUICKLY EXIT LATER TODAY AND TONIGHT LEADING TO OFFSHORE FLOW AND
CLEARING SKIES ALONG THE SOUTHWEST COAST. ANOTHER TROUGH WILL DROP
DOWN OUT OF THE ARCTIC AND ACROSS SOUTHWEST ALASKA ON TUESDAY
BRINGING WITH IT MARGINALLY COLDER AIR. THIS WILL LEAD TO A COOLING
TREND...ESPECIALLY FOR INTERIOR SOUTHWEST ALASKA WHERE TEMPERATURES
WILL FALL BELOW FREEZING EACH NIGHT.

BERING SEA/ALEUTIANS...THE TROUGH OVER THE EASTERN BERING AS WELL
AS THE WEAK SYSTEM OVER THE WESTERN ALEUTIANS WILL QUICKLY EXIT TO
THE SOUTHEAST WITH A VERY HIGH AMPLITUDE RIDGE BUILDING IN BEHIND
IT. THE FORMER TROPICAL SYSTEM...NOW A VERY STRONG EXTRA-TROPICAL
LOW...WILL REMAIN WELL WEST OF THE FORECAST AREA. HOWEVER...A
FRONTAL SYSTEM WILL MARCH EASTWARD INTO THE WESTERN BERING AND
ALEUTIANS AND QUICKLY WEAKEN AS IT CONTINUES EASTWARD THROUGH
THE HEART OF THE RIDGE.

.LONG TERM FORECAST...BY THE END OF THE SHORT-TERM (WEDNESDAY) A
STRONG RIDGE WILL SET UP OVER THE NORTH CENTRAL PACIFIC WITH A
FALL-LIKE SUBTROPICAL JET STREAMING OVER-TOP OF IT RIGHT ACROSS
THE SOUTHERN BERING SEA AND THEN DIVING SOUTHWARD INTO A HIGH
AMPLITUDE TROUGH OVER THE GULF OF ALASKA AND NORTHEAST PACIFIC.
THIS PATTERN WILL PRODUCE AN ACTIVE STORM TRACK ALONG THE ALEUTIANS
AND INTO THE GULF. MODELS HAVE NOT BEEN CONSISTENT WITH THE TRACK
OF INDIVIDUAL SYSTEMS...BUT WHAT IS CONSISTENT IS FAVORABLE
CONDITIONS FOR DEVELOPMENT OF STRONG STORM SYSTEMS. WITH A COOLER
AIR MASS IN PLACE OVER MAINLAND ALASKA AHEAD OF THE FIRST
STORM...THIS MEANS MOUNTAIN SNOW IS A LIKELIHOOD...WHICH MATCHES
WELL WITH CLIMATOLOGY FOR LATE SEPTEMBER.

.AER/ALU...WATCH/WARNING SUMMARY
PUBLIC...NONE.
MARINE...NONE.
FIRE WEATHER...NONE.

SEB SEP 13

SPECIAL WEATHER STATEMENT
NATIONAL WEATHER SERVICE ANCHORAGE AK
1045 AM AKDT MON SEP 16 2013

AKZ141-170300-

...WET SNOW POSSIBLE FOR THE FOOTHILLS OF THE TALKEETNA MOUNTAINS
LATE TONIGHT THROUGH TUESDAY MORNING...

AN AREA OF LOW PRESSURE OVER THE NORTHEASTERN GULF OF ALASKA WILL
MOVE TO PRINCE WILLIAM SOUND ON TUESDAY. THIS WILL SPREAD
PRECIPITATION WESTWARD ACROSS THE COPPER RIVER BASIN OVERNIGHT
TONIGHT. PRECIPITATION WILL FALL AS RAIN ACROSS MOST OF THE REGION.
THE ONE EXCEPTION IS IN THE FOOTHILLS OF THE TALKEETNA
MOUNTAINS...INCLUDING THE GLENN HIGHWAY NEAR EUREKA. SNOW LEVELS
IN THIS AREA ARE EXPECTED TO DROP TO AROUND 2500 FEET BY TUESDAY
MORNING BEFORE RISING BACK UP TO AROUND 4000 FEET TUESDAY AFTERNOON.

PRECIPITATION IN THIS AREA WILL BEGIN AS RAIN DURING THE OVERNIGHT
HOURS AND CHANGE TO SNOW BY MORNING...THEN CHANGE BACK TO RAIN BY
TUESDAY AFTERNOON. LITTLE OR NO SNOW ACCUMULATION IS EXPECTED ON
THE GLENN HIGHWAY NEAR EUREKA AND OTHER PAVED SURFACES IN THE AREA.
HOWEVER...THERE MAY BE SOME WET AND SLUSHY SNOW ACCUMULATION ON
OTHER GROUND SURFACES IN THE REGION ON TUESDAY MORNING.

OUTDOOR ENTHUSIASTS IN THE FOOTHILLS OF THE TALKEETNA MOUNTAINS
ARE ADVISED TO PREPARE FOR MINOR SNOW ACCUMULATION. ALTHOUGH SNOW
IS NOT EXPECTED TO STICK TO THE GLENN HIGHWAY...THOSE TRAVELING
THE HIGHWAY NEAR EUREKA ARE ADVISED TO PREPARE FOR FALLING SNOW
LATE TONIGHT THROUGH TUESDAY MORNING.

FOR THE LATEST FORECASTS GO TO HTTP://WWW.WEATHER.GOV/ANCHORAGE.