Small Craft Advisory 379, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
1221 PM AKDT SUN MAY 11 2014

.ANALYSIS AND UPPER LEVELS...REX BLOCK HAS SLOWLY BUILT NORTH AND
EASTWARD THE PAST 24 HOURS WITH THE CENTER NOW LOCATED OVER
INTERIOR SOUTHWEST ALASKA. MEANWHILE EASTERLY LOW LEVEL FLOW OVER
THE GULF HAS ADVECTED THE MARINE LAYER FROM THE EASTERN GULF ALL
THE WAY WESTWARD TO KODIAK ISLAND. THE PRESSURE GRADIENT ALONG THE
NORTH GULF COAST WAS EXPECTED TO REMAIN IN THE OFFSHORE DIRECTION.
HOWEVER A WEAK ONSHORE FLOW DEVELOPED OVERNIGHT HELPING TO PULL
FOG AND STRATUS NORTHWARD INTO PRINCE WILLIAM SOUND AND GULF
COASTAL COMMUNITIES (SEWARD AND CORDOVA). THIS WILL RESULT IN
MUCH COOLER AND MOISTER CONDITIONS ALONG THE GULF COAST COMPARED
TO THE LAST COUPLE DAYS WHERE STRONG OFFSHORE FLOW AND SUNSHINE
DOMINATED. A MARINE LAYER HAS ALSO DEVELOPED BENEATH THE RIDGE
ALONG PORTIONS OF THE ALASKA PENINSULA...EASTERN BERING...AND
WESTERN ALASKA COAST. AMPLE SUNSHINE WILL HELP BURN AT LEAST SOME
OF THIS OFF OVER LAND. UPSTREAM OF THE REX BLOCK IS A LARGE HIGH
AMPLITUDE LOW OVER THE NORTHWEST PACIFIC WHICH IS SLOWLY LOFTING
NORTHWARD TOWARD THE WESTERN ALEUTIANS.

&&

.MODEL DISCUSSION...MODELS HAVE COME INTO MUCH BETTER AGREEMENT ON
THE PRIMARY FEATURE OF CONCERN IN THE SHORT TERM...A POTENT ARCTIC
TROUGH BARRELING ACROSS MAINLAND ALASKA MONDAY NIGHT THROUGH
WEDNESDAY MORNING. THERE ARE SLIGHT DIFFERENCES IN SPEED AND MINOR
DIFFERENCES IN THE SOUTHERN EXTENT OF THE SHORT-WAVE. THERE IS
HIGH CONFIDENCE IN THE EXPECTED WEATHER WITH THIS TROUGH
PASSAGE...WITH SOME UNCERTAINTY IN EXACT TIMING. THE BIGGEST
CONCERNS WILL BE FIRE WEATHER CONDITIONS AND THE EXACT TIME OF
DAY OF TROUGH PASSAGE CAN GREATLY AFFECT THE RESULTANT RELATIVE
HUMIDITIES.

AS FAR THE MARINE LAYER GOES...THE NAM BOUNDARY LAYER RELATIVE
HUMIDITY IS VERIFYING EXTREMELY WELL OVER THE GULF. WILL MAKE
HEAVY USE OF THIS GUIDANCE FOR PROGRESSION OF MARINE LAYER
TONIGHT AND MONDAY. ALSO OF NOTE...THE GOES SATELLITE PROBABILITY
OF MVFR/IFR/LIFR PRODUCTS ARE DOING A DECENT JOB IN REFLECTING
THE LOWEST CEILINGS/VISIBILITIES. BASED ON THESE PRODUCTS THE
LOWEST CONDITIONS ARE ON THE FRONT EDGE OF THE MARINE LAYER...WITH
IMPROVING VISIBILITIES AND A HIGHER STRATUS DECK ONCE YOU GET A
LITTLE FURTHER INTO THE MARINE LAYER.

&&

.SHORT TERM FORECAST...

SOUTHCENTRAL/SOUTHWEST ALASKA...EXPECT A SLIGHTLY STRONGER THERMAL
TROUGH TO DEVELOP OVER SOUTHWEST ALASKA AND INTO INTERIOR
SOUTHCENTRAL ALASKA THIS AFTERNOON/EVENING. THIS WILL LEAD TO
DEVELOPMENT OF SEA BREEZES ALONG THE GULF COAST AND UP INLET FLOW
UP COOK INLET. THUS EXPECT THE MARINE LAYER TO MAKE MORE PROGRESS
INLAND ALONG THE GULF COAST/KODIAK ISLAND AND COOK INLET REGIONS
TONIGHT INTO MONDAY MORNING. UP INLET FLOW WILL BE WEAK SO ITS A
TOUGH CALL ON WHETHER THE FOG/STRATUS WILL MAKE IT ALL THE WAY
INTO ANCHORAGE. IN ANY CASE SUNSHINE WILL HELP BURN OFF MUCH OF
THIS DURING THE DAY. IT ALSO LOOKS LIKE LOW LEVEL OFFSHORE FLOW
WILL DEVELOP ALONG THE NORTHERN GULF COAST MONDAY OUT AHEAD OF A
FRONTAL BOUNDARY MOVING UP FROM THE NORTHEAST PACIFIC.
THEREFORE...MARINE LAYER TUESDAY NIGHT WILL BE CONFINED TO THE
WESTERN GULF AND COOK INLET. THE MARINE LAYER WILL ALSO REMAIN IN
PLACE ALONG PORTIONS OF THE ALASKA PENINSULA/EASTERN BERING/WEST
COAST OF ALASKA AS A REINFORCING RIDGE BUILDS AHEAD OF THE SYSTEM
SOUTH OF THE WESTERN ALEUTIANS.

A BRIEF BREAK IN THE CLEAR AND DRY CONDITIONS WILL COME MONDAY
NIGHT AND TUESDAY AS A STRONG UPPER LEVEL WAVE CROSSES THE
MAINLAND. THE STRONGEST LIFT WILL BE OVER THE NORTHERN HALF OF
THE STATE...BUT THERE WILL BE ENOUGH LIFT TO THE SOUTH TO
GENERATE SOME CLOUDS AND HIGHER ELEVATION SHOWERS. THERE WILL BE
VERY LITTLE CHANCE OF PRECIP OVER THE LOWLANDS WITH THIS TROUGH AS
LOW LEVELS REMAIN QUITE DRY. THE BIGGEST CONCERN WILL BE STRONG
SUBSIDENCE ON THE BACK SIDE OF THE TROUGH AS A STRONG UPPER RIDGE
RAPIDLY BUILDS IN. EXPECT GUSTY WINDS AND DRIER AIR TO MIX DOWN
FROM ALOFT CREATING CRITICAL FIRE WEATHER CONDITIONS. WILL ISSUE A
FIRE WEATHER WATCH FOR INTERIOR SOUTHWEST ALASKA SINCE ALL MODELS
TRACK TROUGH THROUGH TUESDAY AFTERNOON INTO EVENING DURING TIME OF
MAX HEATING. TIMING OVER SOUTHCENTRAL IS A LITTLE TOUGHER WITH THE
TROUGH LIKELY CROSSING THROUGH TUESDAY EVENING AND OVERNIGHT
TUESDAY. EVEN WITH A NIGHT-TIME PASSAGE STILL EXPECT WINDS AND
DRIER AIR TO MIX DOWN...POSSIBLY REACHING RED FLAG CONDITIONS. BY
WEDNESDAY CONFIDENCE INCREASES IN WIDESPREAD GUSTY OFFSHORE FLOW
AND VERY DRY CONDITIONS ACROSS SOUTHCENTRAL. ANOTHER KEY POINT IS
THAT WINDY CONDITIONS TUESDAY NIGHT WILL PREVENT MUCH RECOVERY IN
RELATIVE HUMIDITY ACROSS THE SOUTHERN MAINLAND.
THEREFORE...WEDNESDAY COULD TURN OUT TO BE THE DRIEST DAY OF THE
WEEK. THE TROUGH PASSAGE WILL ALSO QUICKLY ERODE ANY REMAINING
MARINE LAYER OVER THE GULF AND BRING A RETURN TO VERY WARM
TEMPERATURES ALONG THE GULF COAST.

THE RIDGE WILL CONTINUE TO BUILD OVER THE SOUTHERN MAINLAND
THROUGH THE REMAINDER OF THE WEEK. THIS WILL BE A STRONGER RIDGE
THAN THE CURRENT ONE. SUSPECT RECORD OR NEAR RECORD TEMPERATURES
ARE A GOOD POSSIBILITY ACROSS THE REGION.

BERING SEA/ALEUTIANS...THE SYSTEM SOUTH OF THE WESTERN ALEUTIANS
WILL CONTINUE TO LIFT NORTHWARD AS DOWNSTREAM RIDGE AMPLIFIES AND
STRENGTHENS IN RESPONSE TO ENERGY MOVING THROUGH THE BASE OF THE
TROUGH. THIS WILL CONFINE PRECIP TO THE WESTERN TO CENTRAL BERING
AND ALEUTIANS OVER THE NEXT FEW DAYS. FURTHER EAST UNDERNEATH THE
RIDGE STRATUS AND FOG WILL DOMINATE...WITH BREAKS OF SUN OVER LAND
DURING THE AFTERNOON AND EVENING HOURS.

&&

.LONG TERM FORECAST...A STRONG BLOCKING RIDGE WILL DOMINATE THE
WEATHER THROUGH MOST OF THE DAY 4 THROUGH 7 TIME PERIOD. AS IS
NEARLY ALWAYS THE CASE MODELS STRUGGLE WITH THE BREAKDOWN OF SUCH
STRONG RIDGES...TENDING TO BE TOO QUICK TO DO SO. PREFER THE GFS
SOLUTION IN HOLDING STRONG WITH THE RIDGE THROUGH NEXT
WEEKEND...ALTHOUGH SHIFTING A BIT FURTHER SOUTH AND WEST WITH TIME
(TOWARD THE SOUTHERN ALASKA PENINSULA) AS SHORT-WAVE MOVING OVER
TOP OF THE RIDGE TRY AND MAKE PROGRESS SOUTHWARD ACROSS
SOUTHCENTRAL. BASED ON A MEAN RIDGE CENTERED OVER SOUTHWEST ALASKA
IT LOOKS LIKE OFFSHORE FLOW MAY REMAIN STRONG ENOUGH TO PREVENT
SEA BREEZE FORMATION OVER SOUTHCENTRAL. THUS...TEMPERATURES COULD
BE QUITE STEAMY FOR BOTH INTERIOR AND COASTAL LOCATIONS.

OUT WEST...AS THE ALEUTIAN SYSTEM EXITS TO THE NORTH MID WEEK A
SPOKE OF THE UPPER RIDGE WILL BUILD IN BEHIND IT. AT THAT POINT IN
TIME NEARLY ALL OF THE FORECAST AREA FROM THE BERING SEA TO
SOUTHERN MAINLAND WILL BE UNDER THE INFLUENCE OF THE RIDGE.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...NONE.
FIRE WEATHER...NONE.
&&

SEB MAY 14