Small Craft Advisory 475, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
454 PM AKDT WED JUN 3 2015

.ANALYSIS AND UPPER LEVELS...

THE UPPER LOW EXTENDS FROM THE CANADIAN ARCTIC TO THE UPPER LOW
SOUTH OF THE ALASKA PENINSULA AND ALEUTIANS. UNDERNEATH IS A WEAK
SURFACE LOW AND TROUGH THAT EXTENDS FROM THE WESTERN INTERIOR
ACROSS THE TANANA VALLEY. THIS BROAD UPPER TROUGH AND ASSOCIATED
JET MAX IS ADVECTING WARM AND MOIST SUBTROPICAL AIR INTO THE
SOUTHERN MAINLAND. AN INTENSIFYING SURFACE LOW IS WELL SOUTH OF
KODIAK ISLAND MOVING TOWARD THE NORTH GULF COAST. THIS WILL BECOME
THE DOMINANT LOW TONIGHT AND THURSDAY AS IT MOVES OVER THE
MAINLAND AND WILL BRING WIDESPREAD RAIN TO THE GULF COAST REGION
AND PRINCE WILLIAM SOUND. OVER THE NORTHERN GULF THE UPPER LEVEL
TROUGH HAS MOVED IN ALONG THE NORTH GULF COAST AND HAS PRODUCED
MODERATE TO HEAVY RAIN IN SOUTHEAST PRINCE WILLIAM SOUND.

THE AMPLIFYING RIDGE OF HIGH PRESSURE ALONG THE NORTH GULF COAST
BEHIND THIS TROUGH THAT HAS MOVED ONSHORE AND IN ADVANCE OF THE
INTENSIFYING LOW IS RESULTING IN INCREASED SOUTH WIND ACROSS THE
CHUGACH MOUNTAINS INTO THE COPPER RIVER BASIN AND STRONG SOUTH
WINDS HAVE BEGUN ALONG THE COPPER RIVER. THE SINKING AIR IS
MITIGATING RAINFALL ON THE LEE OF THE CHUGACH MOUNTAINS IN THE
COPPER RIVER BASIN.

OVER THE BERING SEA THE UPPER LEVEL RIDGE AND THE WEAK SURFACE
RIDGE OF HIGH PRESSURE IS OVER THE EXTREME WESTERN BERING SEA AND
ALEUTIANS.

&&

.MODEL DISCUSSION...
THE MODELS ARE IN EXCELLENT AGREEMENT WITH THE GENERAL LARGE
SCALE PATTERN HEADING INTO THE WEEKEND. THEY ALSO REMAIN IN GOOD
AGREEMENT WITH THE SMALLER SCALE FEATURES THROUGH THE SHORT TERM
FORECAST. THE PRIMARY DIFFERENCES ARE WITH RESPECT TO THE QPF OVER
THE COOK INLET REGION AND THE GFS IS THE MODEL OF CHOICE FOR THIS.

A SIGNIFICANT DIFFERENCE FROM THE PREVIOUS MODEL RUNS IS THE
MOVEMENT OF THE SURFACE AND UPPER LOW ACROSS THE COASTAL
MOUNTAINS INTO THE INTERIOR THROUGH FRIDAY. THE PREVIOUSLY
EXPECTED DOWN SLOPE ACROSS THE CHUGACH MOUNTAINS WILL NOT BE AS
SIGNIFICANT AND RAIN WILL BE MORE WIDESPREAD AND PERSISTENT OVER
THE COOK INLET REGION AND SUSITNA VALLEY. AFTER THE LOW MOVES
THROUGH THE FLOW AT ALL LEVELS WILL BE SOUTHERLY AND MOIST ACROSS
KODIAK ISLAND AND SOUTH CENTRAL ALASKA THROUGH THE WEEKEND.

THE MODELS ARE IN GOOD AGREEMENT WITH THE INSTABILITY OVER
SOUTHWEST ALASKA KEEPING THE KILBUCK MOUNTAINS AND THE LOWER
KUSKOKWIM VALLEY IN THE REGION WITH THE HIGHER THREAT FOR
THUNDERSTORM ACTIVITY TODAY.

OVER THE BERING SEA THE MODELS AGREE ON THE UPPER LEVEL AND
SURFACE RIDGE PASSING THROUGH THE CENTRAL BERING SEA AND CENTRAL
ALEUTIANS FRIDAY AND BRINGING THE SURFACE AND UPPER LOW INTO THE
EXTREME WESTERN ALEUTIANS.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (DAYS 1 AND 2)...
OVER THE NORTHERN SUSITNA VALLEY AND COPPER RIVER BASIN A
SHORTWAVE MOVING AHEAD OF THE APPROACHING GULF LOW WILL BRING
SHOWERS AND ISOLATED THUNDERSTORMS THROUGH THIS EVENING. THE
FORECAST ATTENTION THEN QUICKLY SHIFTS TO THE THE UPPER LEVEL LOW
CURRENTLY MOVING INTO THE GULF. THIS SYSTEM WILL USHER IN A PERIOD
OF WET AND CLOUDY WEATHER THAT IS EXPECTED TO REMAIN OVER THE
SOUTHERN MAINLAND THROUGH THE WORK WEEK. GALE TO STORM FORCE WINDS
AND HEAVY RAIN ARE STILL EXPECTED ALONG THE NORTH GULF COAST LATE
THIS EVENING AND PERSISTING INTO THURSDAY MORNING FROM THIS
SYSTEM. AS THIS LOW PROGRESSES TOWARDS SOUTHERN COOK INLET LATE
TONIGHT...THERE IS NOW VERY GOOD AGREEMENT THAT THIS LOW WILL MOVE
UP THE INLET TOWARDS ANCHORAGE DURING THE DAY THURSDAY. THIS IS A
CHANGE IN THE MODELS SINCE YESTERDAY...BUT BECAUSE OF IT THERE IS
NOW GOOD CONFIDENCE THAT STEADY RAIN WILL FALL AROUND MUCH OF
SOUTHCENTRAL FOR MOST OF DAY THURSDAY THIS SHOULD OCCUR EVEN WITH
STRONG WINDS ALONG TURNAGAIN ARM AND THE ANCHORAGE HILLSIDE...AS
MOISTURE AND INSTABILITY FROM THIS LOW SHOULD BE STRONG ENOUGH TO
OVERCOME ANY DOWNSLOPE DRYING. THE UPPER LEVEL ASSOCIATED WITH
THIS SYSTEM IS THEN EXPECTED TO STALL OVER KODIAK ISLAND...KEEPING
MUCH OF THE REGION CLOUDY AND RAINY...ESPECIALLY FOR THE GULF
COAST...THROUGH FRIDAY.

&&
.SHORT TERM FORECAST SOUTHWEST ALASKA (DAYS 1 AND 2)...
THE SHOWERY PATTERN WILL CONTINUE ACROSS THE SOUTHWEST MAINLAND
THROUGH FRIDAY AS A BROAD UPPER LEVEL TROUGH REMAINS CENTERED
OVER WESTERN ALASKA...WITH ISOLATED THUNDERSTORMS DEVELOPING
DURING THE AFTERNOON/EVENING AS DAYTIME HEATING GENERATES MODEST
INSTABILITY. THUNDERSTORM ACTIVITY WILL SHIFT EASTWARD ON
THURSDAY AND FRIDAY FROM THE KUSKOKWIM DELTA INTO THE KUSKOKWIM
VALLEY AND INTERIOR BRISTOL BAY AS AS THE TROUGH AXIS SHIFTS
SLIGHTLY EASTWARD.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (DAYS 1 AND 2)...
SHOWER ACTIVITY OVER THE BERING WILL GENERALLY BE LIMITED TO THE
ALASKA PENINSULA AND THE EASTERN ALEUTIANS THROUGH FRIDAY AS WEAK
DISTURBANCES MOVE ALONG THE WESTERN PERIPHERY OF THE MAINLAND
TROUGH. OTHERWISE...STRATUS AND FOG WILL DOMINATE MUCH OF THE
REGION AS HIGH PRESSURE BUILDS OVER THE WESTERN BERING.

&&

.LONG TERM FORECAST (DAYS 3 THROUGH 7)...
A LONGWAVE TROUGH WILL REMAIN OVER MUCH OF THE SOUTHERN MAINLAND
THROUGH SATURDAY AFTERNOON...KEEPING ONSHORE FLOW AND SHOWERY
CONDITIONS OVER THE AREA INTO THE WEEKEND. A RIDGE OF HIGH
PRESSURE FROM OVER THE WESTERN BERING/ALEUTIANS WILL PUSH FURTHER
EASTWARD THIS WEEKEND...EVENTUALLY SPLITTING THE TROUGH INTO AN
UPPER LOW THAT STAYS QUASI-STATIONARY NEAR EASTERN SIBERIA AND AN
UPPER LOW OVER THE GULF. THIS WILL ESSENTIALLY END THE SHOWERY
CONDITIONS OVER THE SOUTHWEST MAINLAND...AND KEEP SOME SHOWERY
CONDITIONS OVER SOUTHCENTRAL THROUGH MONDAY. THE RIDGE WILL THEN
AMPLIFY OVER THE GULF AND SOUTHCENTRAL THROUGH THE EARLY PORTION
OF NEXT WEEK...REDUCING SOME OF THE SHOWERY CONDITIONS OVER
SOUTHCENTRAL THROUGH TUESDAY. WITH MODELS STILL DEPICTING COLDER
AIR ALOFT WITH A SLACKENING FLOW OVER THE SOUTHCENTRAL
AREA...THERE REMAINS THE POTENTIAL OF SNEAKING IN A NICE DAY EARLY
NEXT WEEK. FURTHER WEST...ANOTHER SYSTEM WILL DEVELOP JUST SOUTH
OF THE WESTERN ALEUTIANS SATURDAY...AND DEEPEN AS IT TRACKS
EASTWARD OVER THE CENTRAL ALEUTIANS/BERING SUNDAY INTO MONDAY.
THIS SPREAD WET AND WINDY WEATHER FROM WEST TO EAST THROUGH THE
ALEUTIANS AND BERING THROUGH EARLY NEXT WEEK. AS THIS SYSTEM
WEAKENS AND COMBINES WITH ANOTHER SYSTEM OFF EASTERN SIBERIA
TUESDAY...BROAD CYCLONIC FLOW WILL BLANKET MUCH OF THE BERING
THROUGH WEDNESDAY MORNING.

MODELS START OFF IN FAIR AGREEMENT IN THE LONG TERM WITH THE
OVERALL SYNOPTIC PATTERN THROUGH SATURDAY AFTERNOON...THEN BEGIN
TO DIVERGE FROM A MORE COMMON SOLUTION. FOR THIS FORECAST PACKAGE
THE MOST REASONABLE SOLUTION WAS A MORE MIDDLE GROUND APPROACH
BETWEEN THE MODELS. THE ECMWF AND CANADIAN MODELS SEEMED TO DEPICT
THIS MIDDLE GROUND SOLUTION BEST...WITH THE GFS BEING A MORE
BULLISH OUTLIER WITH THE SYSTEM TRACKING THROUGH THE GULF OVER THE
WEEKEND. HOWEVER GIVEN THE CURRENT MODEL DISCREPANCIES...THERE
REMAINS LOWER FORECAST CONFIDENCE IN THE LONG TERM FORECAST.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...STORM 119.
         GALES 120 125 130 139.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...SA
SOUTHCENTRAL ALASKA...DEK
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...CB
LONG TERM...TP