Small Craft Advisory 594, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
403 PM AKDT FRI JUL 24 2015

.ANALYSIS AND UPPER LEVELS...
THE LOW AND FRONT IN THE NORTHERN TO EASTERN BERING SEA TO THE
ALASKA PENINSULA CONTINUES TO PUSH EAST. ANOTHER LOW SOUTH OF THE
WESTERN ALEUTIANS HAS GOOD JET SUPPORT WITH HIGH PRECIPITABLE
WATER AND IS CONVECTIVE AS EVIDENCED BY LIGHTNING STRIKES BEING
DETECTED IN IT. THIS SYSTEM WILL RAPIDLY MOVE TOWARD SOUTH CENTRAL
ALASKA IN THE STRONG ZONAL FLOW BRINGING WITH IT COOLER CLOUDY
AND WET CONDITIONS TO THE SOUTH CENTRAL REGION TOMORROW NIGHT AND
SUNDAY. THIS WILL RESULT IN A SIGNIFICANT CHANGE TO THE FORECAST
FOR KODIAK ISLAND AS WE ANTICIPATE WIDESPREAD RAIN WILL BE LIKELY
SAT NIGHT TRANSITIONING TO A SHOWERY REGIME SUNDAY.

CURRENTLY THE DRY OFFSHORE FLOW OVER THE SOUTHERN MAINLAND IS
RESULTING IN SUNNY WARM CONDITIONS ACROSS THE REGION. THE WATER
VAPOR LOOP DOES SHOW MOISTURE AVAILABLE OVER THE AREA AND THUS WE
SHOULD SEE THE SHOWERS AND THUNDERSTORMS DEVELOP THIS EVENING
ALONG THE MOUNTAINS AS ANTICIPATED.

&&

.MODEL DISCUSSION...
THE MODELS ARE DOING A GOOD JOB DEPICTING THE MOISTURE SURGE INTO
SOUTH CENTRAL ALASKA THROUGH SATURDAY. THE UPPER LOW PREVIOUSLY
DISCUSSED AND EXPECTED TO MOVE INTO THE BRISTOL BAY REGION SUNDAY
IS BEING TAKEN MORE INTO THE SOUTHWEST INTERIOR ON THE LATEST
MODEL RUN WITH A SECOND WEAKER UPPER LOW OVER THE COOK INLET
REGION. AT THE SURFACE THE LOW WILL RAPIDLY MOVE ACROSS THE BERING
SEA AND INLAND ON THE KUSKOKWIM DELTA THEN OPEN UP TO A TROUGH
FROM THE NORTHEAST INTERIOR TO THE GULF OF ALASKA SUNDAY. THE
MODELS ARE ALL IN GOOD AGREEMENT WITH THIS SOLUTION.

THE MODELS HAVE ALSO COME INTO AGREEMENT WITH A SOLUTION THAT
DOES NOT PRODUCE THE CROSS BARRIER DOWN SLOPE DRYING ON THE WEST
SIDE OF THE KENAI MOUNTAINS AND IN ANCHORAGE. THEREFORE WE ARE
MORE CONFIDENT FOR MEASURABLE RAIN AND SHOWERS ACROSS THE REGION
SATURDAY NIGHT AND SUNDAY.

&&

.FIRE WEATHER...

OVER SOUTH CENTRAL ALASKA THE OFFSHORE GAP WINDS AROUND PRINCE
WILLIAM SOUND ARE DIMINISHING AND WILL END TONIGHT. UNSTABLE
CONDITIONS ALONG THE TALKEETNA MOUNTAINS WILL FOCUS THE CONVECTIVE
ACTIVITY TO THAT REGION. ANY THUNDERSTORM ACTIVITY WILL BE
ISOLATED AND WET THIS EVENING AND TONIGHT. THE INCOMING MOISTURE
FROM THE BERING SEA LOW WILL BRING COOLER AND CLOUDIER CONDITIONS
TO THE SOUTHERN MAINLAND BY LATE SATURDAY AFTERNOON PERSISTING THROUGH
SUNDAY. THIS WILL KEEP FIRE CONCERNS MINIMAL THROUGH THE WEEKEND.

OVER SOUTHWEST ALASKA CLOUDY COOLER CONDITIONS ARE PUSHING IN
ALONG THE BERING SEA COAST. THE COOL MARINE LAYER HAS PERSISTED
ACROSS THE REGION TODAY AND IT IS HIGHLY UNLIKELY WE WILL SEE ANY
THUNDERSTORMS IN SOUTHWEST ALASKA TONIGHT. THE UPPER TROUGH
SWINGS THROUGH SOUTHWEST ALASKA TONIGHT TO BE FOLLOWED BY A BROAD
COOL MOIST SOUTHWEST FLOW INTO SOUTHWEST ALASKA THROUGH THE
WEEKEND.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (DAYS 1 AND 2)...
ENJOY THE BLUEBIRD EVENING BECAUSE THINGS GO QUICKLY DOWNHILL
SATURDAY AFTERNOON. ISOLATED SHOWERS AND THUNDERSTORMS REMAIN
POSSIBLE THIS EVENING OVER ELEVATED TERRAIN...BUT SHOULD STAY
ANCHORED THERE. LINGERING INSTABILITY SATURDAY AFTERNOON WILL BE
TAPPED BY AN INCOMING DISTURBANCE STARTING OFF SOME SHOWERS OFF
THE MOUNTAINS BEFORE A STRONGER TROUGH BRINGS MORE WIDESPREAD LIFT
AND STEADY RAIN IN THE LATE EVENING. THE UPPER LEVEL SYSTEM WILL
LINGER THROUGH THE WEEKEND...HOWEVER STEADY RAINFALL WILL TAPER TO
PERIODS OF RAIN OR SHOWERS LATE SUNDAY INTO MONDAY.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (TONIGHT THROUGH SUN)...

THE DISSIPATING AREA OF SFC LOW PRESSURE OVER THE ALASKA PENINSULA
CONTINUES TO SLIDE TO THE EAST THIS AFTERNOON. THE ASSOCIATED COLD
FRONT IS CURRENTLY OCCLUDING OVER THE TIP OF THE ALASKA PENINSULA
AND WILL SHIFT INTO THE GULF OF ALASKA OVERNIGHT MAINLY DUE TO AN
AREA OF POTENTIAL VORTICITY AT 500 MB DROPPING SOUTH INTO THE
GULF OF ALASKA. THIS SHOULD KEEP THE MAJORITY OF SOUTHWEST ALASKA
ON THE DRY SIDE THROUGH THE OVERNIGHT PERIOD AS AMPLE CLOUD COVER
HAS SUPPRESSED THE DEVELOPMENT OF INSTABILITY AND ALL UPPER LEVEL
FORCING RESIDES SOUTH AND WEST OF THE AREA. THINGS WILL CHANGE
QUITE SIGNIFICANTLY ON SATURDAY AS ANOTHER LOW TRACKS ACROSS THE
NORTHERN BERING SEA AND DRIVES YET ANOTHER FRONT TOWARD THE ALASKA
MAINLAND. RAINFALL WILL START ACROSS THE WEST COAST OF ALASKA MID
TO LATE SATURDAY MORNING BEFORE SPREADING INLAND SATURDAY AFTERNOON
AS BOTH THE SFC LOW AND FRONT PUSH INLAND. IN ADDITION...A STRONG
PUSH OF POTENTIAL VORTICITY ADVECTION WILL PROVIDE UPPER LEVEL
SUPPORT TO PRODUCE SOME HEAVY RAINFALL AT TIMES. SATELLITE IMAGERY
SHOWS THIS SYSTEM (CURRENTLY OVER THE WESTERN ALEUTIANS) HAS
PRECIPITABLE WATER VALUES WELL OVER 200% OF THE TYPICAL VALUES.
THIS MOISTURE LADEN AIR MASS WILL PUSH OVER SOUTHWEST ALASKA ON
SATURDAY AND WILL INHIBIT THUNDERSTORM DEVELOPMENT OVER THE
WEEKEND. MID-LEVEL LAPSE RATES WILL BE NEARLY MOIST ADIABATIC
WHICH TYPICALLY INDICATES A FAIRLY STABLE STRATIFORM PRECIPITATION
EVENT. HOWEVER...A TOUCH OF UPPER LEVEL ENERGY MAY BE TAPPED INTO
TO CREATE SOME CONVECTIVE SHOWERS EMBEDDED WITHIN THE STRATIFORM
RAINFALL. RAINFALL WILL GRADUALLY TAPER OFF FROM WEST TO EAST ON
SUNDAY AS THE LOW THAT MOVES INLAND ON SUNDAY MOVES INTO THE GULF
OF ALASKA/SOUTHCENTRAL ALASKA.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (TONIGHT THROUGH SUN)...

LIGHTNING STRIKES HAVE BEEN OBSERVED ACROSS THE NORTHERN PACIFIC
SOUTH OF THE ALEUTIAN ISLANDS THIS AFTERNOON. IT IS QUITE
INTERESTING THAT UPON ANALYSIS OF SFC TO 700 MB AND 700 MB TO 500
MB LAYER INSTABILITY THAT THE ATMOSPHERE IS ACTUALLY QUITE STABLE.
IT LOOKS LIKE THE LIGHTNING IS THE RESULT OF THE AGEOSTROPHIC
INFLUENCES BY THE UPPER LEVEL JET. THIS MAKE IT A BIT DIFFICULT TO
DETERMINE HOW LONG THE LIGHTNING WILL LAST AND IF TI WILL EVEN
MAKE IT INTO THE ALASKA COASTAL WATERS. THE MOST RECENT SATELLITE
IMAGERY SUGGESTS THE LIGHTNING ACTIVITY HAS LESSENED AND THE
FORWARD FLANK OF THE BAROCLINIC ZONE HAS OUTRUN THE UPPER LEVEL
JET. HOWEVER...MODEL DATA SUGGESTS THE JET STRENGTHENS OVER THE
CENTRAL ALEUTIANS THIS EVENING WHICH MAY BE JUST ENOUGH TO PRODUCE
A FEW LIGHTNING STRIKES.

LOOKING BACK AT THE BIG PICTURE...A COLD FRONT PUSHING ACROSS THE
ALASKA PENINSULA IS QUICKLY SHIFTING EASTWARD WHICH WILL HELP
BRING AN END TO PRECIPITATION ACROSS THE EASTERN ALEUTIANS
THROUGH THE OVERNIGHT PERIOD. HOWEVER...AN AREA OF LOW PRESSURE
OVER THE NORTHERN BERING SEA HAS BEEN SPARKING SHOWERS NEAR THE
PRIBILOF ISLANDS AND NORTHERN BERING SEA AND WILL CONTINUE TO DUE
SO THROUGH THE OVERNIGHT PERIOD. AT THE SAME TIME...AN UPPER LEVEL
DISTURBANCE WILL PUSH ACROSS THE BERING SEA THROUGH THE WEEKEND
AND BRING DECENT RAINFALL TO THE ALEUTIAN ISLANDS. THIS RAINFALL
WILL TAPER OFF FROM WEST TO EAST THROUGHOUT THE WEEKEND AS THE
DISTURBANCES PUSHES TOWARDS SOUTHWEST ALASKA.

&&

.LONG TERM FORECAST (DAYS 3 THROUGH 7)...
RAIN AND GUSTY WINDS ASSOCIATED WITH THE FRONTAL SYSTEM TRACKING
INTO THE NORTHERN GULF COAST WILL BEGIN TO TAPER OFF AND BECOME
MORE SHOWERY SUNDAY EVENING...AS THE FRONT WEAKENS AND STALLS
ALONG THE COAST AND THE VERTICALLY STACKED PARENT LOW TRACKS
THROUGH THE SOUTHWEST MAINLAND AND INTO THE GULF BY MONDAY
MORNING. MODELS ARE IN GOOD AGREEMENT HANDLING THIS SYSTEM...
HOWEVER THE EC SOLUTION WAS USED FOR UPDATES AS IT HAS PROVIDED
THE BEST MODEL RUN CONSISTENCY. AS THIS SYSTEM ENTERS THE GULF AND
RIDGING BUILDS OVER THE WESTERN ALEUTIANS/BERING...GUSTY
NORTHWESTERLY FLOW WILL DEVELOP OVER THE EASTERN ALEUTIANS AND
ALASKA PENINSULA OVER THE WEEKEND. THESE GUSTY WINDS WILL SPREAD
EAST INTO THE WESTERN GULF THROUGH EARLY TUESDAY MORNING...AS THE
LOW TRACKS FURTHER EASTWARD.

THE BERING AND ALEUTIANS WILL STAY IN A RELATIVELY QUIETER
PATTERN THROUGH TUESDAY. AN UPPER LEVEL RIDGE BUILDS ACROSS THE
WESTERN BERING WITH SCATTERED SHOWERS STILL POSSIBLE ACROSS
EASTERN AREAS DUE TO WEAK SHORTWAVES MOVING ALONG THE BACKSIDE OF
THE MAIN LOW MOVING INTO THE GULF. FORECAST CONFIDENCE BEGINS TO
DIP BELOW AVERAGE AS MODELS BEGIN TO STRUGGLE WITH AN UPPER LEVEL
LOW TRACKING THROUGH THE NORTHERN MAINLAND TUESDAY NIGHT AND INTO
THE EASTERN BERING BY THE END OF NEXT WEEK. THE ECMWF IS THE MOST
BULLISH OF THE MODELS WITH THIS FEATURE...WITH THE CANADIAN MODEL
IN MOST RECENT RUNS STARTING TO TREND TOWARD THE EC SOLUTION. THE
SURFACE REFLECTION OF THIS IS A SURFACE CIRCULATION TRACKING
THROUGH THE EASTERN ALEUTIANS BY NEXT WEEKEND...WITH AN
ASSOCIATED FRONTAL SYSTEM TRACKING INTO THE NORTHERN GULF.

&&

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

&&

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