Small Craft Advisory 804, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
432 PM AKDT WED OCT 14 2015

.ANALYSIS AND UPPER LEVELS...
A HIGH AMPLITUDE DYNAMIC PATTERN HAS DEVELOPED OVER THE NORTH
PACIFIC. THE SOUTHERN TIER OF ALASKA...FROM THE ALEUTIANS EASTWARD
TO THE GULF OF ALASKA...IS SITTING ALONG THE NORTHERN TIER OF THE
MOST ACTIVE WEATHER. A LOOK AT SATELLITE IMAGERY SHOWS A DEEP
CLOSED LOW/TROUGH OVER THE NORTHWEST PACIFIC...A RIDGE OVER THE
CENTRAL ALEUTIANS AND BERING...ANOTHER CLOSED LOW AND DIGGING
TROUGH OVER THE NORTHEAST PACIFIC...AND A BUILDING RIDGE OVER
THE FAR EAST PACIFIC NUDGING UP INTO THE GULF OF ALASKA.

AN OCCLUDED FRONT SPREADING OUT FROM THE NORTHWEST PACIFIC LOW IS
PUSHING A GALE FORCE OCCLUDED FRONT TOWARD THE ALEUTIANS. A SERIES
OF NOTABLY WEAKER FEATURES ARE AFFECTING THE REST OF THE FORECAST
AREA TODAY...WITH GENERALLY WEAK FLOW ALOFT AWAY FROM THE MAIN JET
DYNAMICS. A NEARLY STATIONARY UPPER LEVEL SHORT-WAVE OVER THE
EASTERN BERING IS OVERHEAD OF AN INVERTED TROUGH BETWEEN COLD BAY
AND UNALASKA. A FAIRLY NARROW BAND OF RAIN IS FOCUSED ALONG THIS
TROUGH AXIS. A WEAK WARM FRONT HAS MOVED UP FROM THE PACIFIC AND
EXTENDS FROM KODIAK ISLAND TO PRINCE WILLIAM SOUND WITH STEADY
RAIN SPREADING NORTH AND WEST WITH THE FRONT. OUT AHEAD OF THIS A
SHORT-WAVE TROUGH IS LIFTING NORTHWARD ACROSS INTERIOR SOUTHWEST
ALASKA AND WESTERN SOUTHCENTRAL PRODUCING SCATTERED SHOWERS.

&&

.MODEL DISCUSSION...
MODELS REMAIN IN EXCELLENT AGREEMENT WITH SYNOPTIC SCALE FEATURES.
HOWEVER THERE ARE SOME KEY DIFFERENCES IN THE MESOSCALE TO LOCAL
SCALE OVER THE NEXT COUPLE DAYS WHICH LOWER FORECAST CONFIDENCE
TO AVERAGE TO BELOW AVERAGE. THE MOST IMPACTFUL DIFFERENCE TO THE
FORECAST IS DIFFERENCES IN SPEED OF THE WARM FRONT MOVING UP FROM
THE EAST PACIFIC AND ACROSS THE GULF OF ALASKA TO THE SOUTHERN
MAINLAND...ALONG WITH DIFFERENCES IN LOCATION AND EXTENT OF TRIPLE
POINT DEVELOPMENT SOUTH OF KODIAK ISLAND ON THURSDAY OR THURSDAY
NIGHT. THERE IS A DEFINITE TREND IN ALL MODEL SOLUTIONS TOWARD
FASTER MOVEMENT OF THE WARM FRONT WESTWARD TOWARD KODIAK ISLAND
AND BRISTOL BAY. THIS MAKES A LOT OF SENSE BASED ON STRENGTH AND
ORIENTATION OF THE UPPER JET. BASED ON THIS WILL MOVE UP ARRIVAL
OF RAIN AND STRONG WINDS. THE PRESENCE OF A TRIPLE POINT LOW (OR
LACK THERE OF) WILL AFFECT THE STRENGTH OF WINDS ACROSS KODIAK
ISLAND/WESTERN GULF. FOR NOW HAVE DOWNPLAYED THE PRESENCE OF A
LOW SOUTH OF KODIAK...SO WILL NEED TO CONTINUE TO MONITOR THIS AND
BE PREPARED FOR STRONGER WINDS IF A LOW DOES DEVELOP.

MEANWHILE...OUT WEST MODELS ARE ALL OVER THE PLACE WITH LOCATION
OF TRIPLE POINT DEVELOPMENT IN THE VICINITY OF THE CENTRAL ALEUTIANS.
HAVE TRENDED TOWARD THE ECMWF WHICH REPRESENTS A MIDDLE GROUND OF
ALL SOLUTIONS...BUT CONFIDENCE IS LOW. THE LOCATION OF THE TRIPLE
POINT LOW WILL AFFECT MOVEMENT OF THE FRONTAL SYSTEM ACROSS THE
CENTRAL BERING AND ALEUTIANS AND THUS THE LOCATION OF STRONGEST
WINDS AND RAINFALL.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (DAYS 1 AND 2)...
THE WARM FRONT PUSHING INTO THE NORTH GULF COAST WILL CONTINUE TO
BRING RAIN TO THE AREA THIS EVENING BUT WILL DIMINISH OVERNIGHT AS
THE FRONT FALLS APART. THE WARM FRONT COMBINED WITH THE SHORT WAVE
MOVING THROUGH THE WESTERN PORTION OF THE SOUTHCENTRAL HAS HELPED
PRODUCE RAIN IN AREAS AROUND NORTHERN COOK INLET EXTENDING NORTH
INTO THE MATANUSKA VALLEY. THIS RAIN WILL BE ON AND OFF IN THE
EVENING BEFORE DIMINISHING OVERNIGHT. THURSDAY WILL BE DRY FOR THE
SOUTHERN MAINLAND BUT KODIAK WILL START TO SEE THE EFFECTS OF THE
FRONT MOVING THROUGH THE GULF. THE FRONT WILL MAKE IT TO THE
MAINLAND LATE THURSDAY NIGHT BRINGING BACK RAIN THROUGH FRIDAY
WITH THE HEAVIEST AMOUNTS AROUND PRINCE WILLIAM SOUND.

THURSDAY NIGHT BEFORE THE WARM FRONT MAKES IT TO THE AREA
TEMPERATURES IN THE COPPER RIVER BASIN WILL DROP BELOW FREEZING.
AS THE FRONT MOVES IN IT WILL BE WARM ENOUGH ALOFT FOR RAIN. THAT
WILL LEAD TO THE POTENTIAL FOR LIGHT FREEZING RAIN IN THE COPPER
RIVER BASIN LATE THURSDAY NIGHT INTO FRIDAY MORNING.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (DAYS 1 AND 2)...
A WEAK WARM FRONT APPROACHING THE ALEUTIAN RANGE THIS EVENING
WILL BE THE FIRST IN A SERIES OF WARM FRONTS TO BRING IN WARMER
TEMPERATURES AND STEADIER RAINFALL TO THE AREA THROUGH THE END OF
THE WEEK. IT SHOULD HAVE ENOUGH EAST TO SOUTHEAST FLOW IN THE LOW
TO MID LEVELS TO DOWNSLOPE SOME OF THE AREAS CLOSER TO THE
MOUNTAINS...BUT EXPECT PRECIPITATION TO REDEVELOP NEAR DILLINGHAM.
SOME OF THIS FLOW COULD CREATE A FEW GUSTS AT HIGHER
ELEVATIONS...BUT WINDS SHOULD STAY ELEVATED WITH WARM AIR
ADVECTION. THE STRONGER WARM FRONT WILL START TO PUSH INTO THE
BRISTOL BAY AREA FROM THE GULF THURSDAY AFTERNOON...BRINGING
ANOTHER ROUND OF SLIGHTLY STRONGER OFFSHORE FLOW AND SHOULD ALSO
LEAD TO SOME DECENT DRYING WITH A FEW BREAKS IN THE CLOUDS.
HOWEVER...THAT WILL BE SHORT LIVED AS THE RAIN ASSOCIATED WITH
THAT FRONT TRACKS ACROSS THE ALEUTIAN RANGE BY FRIDAY MORNING.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (DAYS 1 AND 2)...
AN INVERTED TROUGH OVER THE EASTERN BERING/ALEUTIANS WILL CONTINUE
TO PIVOT TO THE WEST THIS EVENING WHILE A FRONTAL SYSTEM OVER THE
WESTERN ALEUTIANS MAKES ITS WAY INTO THE CENTRAL BERING/ALEUTIANS
BY THURSDAY MORNING. GUSTY WINDS AND RAINFALL ASSOCIATED WITH
THIS FRONTAL SYSTEM WILL BE STRONGEST OVER THE WESTERN ALEUTIANS
TONIGHT AND SLOWLY DIMINISH AS IT STARTS TO OUTRUN ITS UPPER
LEVEL SUPPORT AND STALLS OUT OVER THE CENTRAL ALEUTIANS THURSDAY
EVENING. A 130 KT JET JUST SOUTH OF THE WESTERN ALEUTIANS WILL
SUPPORT THIS SYSTEM MOVING EASTWARD AT A STEADY PACE...AS WELL AS
MIX DOWN SOME STRONGER WINDS THIS EVENING AS WINDS SWITCH FROM
SOUTHEASTERLY TO SOUTHWESTERLY. AS THE PARENT LOW MOVES INTO THE
WESTERN ALEUTIANS FRIDAY MORNING RAIN WILL BECOME MORE WIDESPREAD
OVER THE AREA.

&&

.LONG TERM FORECAST (DAYS 3 THROUGH 7)...
A FRONTAL SYSTEM ASSOCIATED WITH A WEAKENING LOW CENTERED NEAR
KODIAK ISLAND WILL BRING CONTINUED RAIN FROM THE GULF COAST INTO
BRISTOL BAY INTO SATURDAY MORNING. THIS RAINFALL WILL THEN BEGIN
TO LIFT INTO SOUTHCENTRAL AND THE INTERIOR SOUTHWEST MAINLAND ON
SATURDAY AS THE UPPER LEVEL TROUGH BEGINS TO LIFT INTO THE
MAINLAND. MEANWHILE OVER THE WESTERN ALEUTIANS A WEAK LOW WILL
BEGIN TO ENTER THE AREA ON SATURDAY MORNING...AND MERGE WITH THE
GULF LOW AS IT MOVES INTO THE BERING ON SUNDAY. THIS WILL PRODUCE
A BROAD TROUGH CENTERED OVER THE EASTERN BERING THAT WILL PERSIST
INTO EARLY NEXT WEEK...KEEPING THE JET AND ANY MAJOR WEATHER
SYSTEMS SOUTH OF THE AREA.

BY NEXT TUESDAY SIGNIFICANT MODEL DISAGREEMENT DEVELOPS WITH THE
PLACEMENT OF A LOW NEAR THE GULF. THE GFS HAS CONSISTENTLY BEEN AN
OUTLIER WITH THIS FEATURE BY BRINGING IT AS FAR NORTH AS KODIAK
ISLAND...WHILE THE GEM/EC HAVE BEEN EQUALLY CONSISTENT IN A MUCH
FURTHER SOUTH SOLUTION. FOR NOW THE CURRENT FORECAST HEAVILY
USES ENSEMBLES TO BRING A MUCH WEAKER LOW INTO THE GULF BY MID
WEEK...BUT FURTHER MODEL RUNS WILL NEED TO BE MONITORED TO
DETERMINE THE EXACT TIMING OF THIS SYSTEM. AS A RESULT FORECAST
CONFIDENCE IS LOW BEYOND EARLY NEXT WEEK...ESPECIALLY FOR THE
SOUTHERN MAINLAND AND THE GULF.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...GALE 119 120 130-139 150 177 178.

&&

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