Small Craft Advisory 642, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
404 PM AKDT SUN AUG 16 2015

.ANALYSIS AND UPPER LEVELS...
A LARGE UPPER LEVEL LOW IS CENTERED OVER THE NORTHERN BERING SEA
WITH A HIGH AMPLITUDE SHORT-WAVE TROUGH DIGGING SOUTHEASTWARD
ACROSS THE CENTRAL BERING AND ALEUTIANS. THIS TROUGH IS NOW
BEGINNING TO INTERACT WITH THE REMNANTS OF TROPICAL STORM MOLAVE
OVER THE NORTH CENTRAL PACIFIC. THE SATELLITE DERIVED BLENDED
TOTAL PRECIPITABLE WATER (TPW) PRODUCT INDICATES TPW VALUES OF 2
INCHES OR MORE BEGINNING TO STREAM NORTHEASTWARD OUT AHEAD OF THE
BERING TROUGH. A WEAK ELONGATED SURFACE LOW HAS DEVELOPED AHEAD OF
THE PRIMARY NORTH PACIFIC LOW. THE DEEPEST MOISTURE WILL ACCOMPANY
THIS LOW NORTHEASTWARD THE NEXT COUPLE DAYS...SO THE TRACK OF THIS
LOW WILL BE KEY TO WHERE THE HEAVIEST RAIN FALLS.

MEANWHILE WEAKER UPPER LEVEL WAVES EMBEDDED IN DEEP SOUTH TO
SOUTHWEST FLOW ARE MOVING ACROSS THE ALASKA PENINSULA AND
SOUTHERN MAINLAND OF ALASKA THIS MORNING PRODUCING AREAS OF RAIN.
ALTHOUGH NOT NEARLY AS MOIST AS THE AIRMASS OVER THE NORTH
PACIFIC...MORNING SOUNDINGS FROM ACROSS THE REGION SHOW TPW
VALUES GENERALLY BETWEEN 0.75 AND 1.00 INCH. A 100 KT+ SUBTROPICAL
JET EXTENDING FROM THE CENTRAL ALEUTIANS NORTHEAST TO THE KENAI
PENINSULA IS PROVIDING SOME UPPER LEVEL SUPPORT.

&&

.MODEL DISCUSSION...
THERE CONTINUES TO BE SMALL BUT KEY DIFFERENCES AMONG THE NUMERICAL
GUIDANCE WHICH AFFECT TIMING AND DISTRIBUTION OF RAINFALL OVER
THE SOUTHERN MAINLAND OF ALASKA FOR THE NEXT COUPLE DAYS. OF MOST
SIGNIFICANCE IS THE EVOLUTION OF THE CENTRAL BERING/ALEUTIANS
TROUGH AS IT SWINGS EASTWARD TO THE GULF ALONG WITH THE RESULTANT
TRACK OF THE SURFACE LOW. THERE HAS BEEN A UNIFIED NORTHWARD TREND
TO THE TRACK OF THIS LOW NOTED IN THE 12Z MODEL CYCLE. THE GEM IS
AN OUTLIER WITH A TRACK OVER THE KENAI PENINSULA. PREFER A
COMPROMISE OF THE REMAINING SOLUTIONS WHICH KEEP IT JUST SOUTH OF
THE KENAI PENINSULA AND INTO PRINCE WILLIAM SOUND. BASED ON THIS
TRACK THE HEAVIEST RAIN WILL FALL IN A CORRIDOR FROM THE SOUTH
SIDE OF THE ALASKA PENINSULA TO KODIAK ISLAND TO EASTERN PRINCE
WILLIAM SOUND AND INTO PORTIONS OF THE COPPER RIVER BASIN. IF THE
LOW WERE TO TRACK JUST A LITTLE BIT FARTHER NORTH AND WEST THEN
THE POPULATION CENTERS OF SOUTHCENTRAL WOULD SEE MUCH HEAVIER
RAINFALL AMOUNTS MONDAY AFTERNOON THROUGH MONDAY NIGHT.

FORECAST CONFIDENCE IS GENERALLY HIGH WITH THE OVERALL PATTERN
AND RAINY FORECAST. FORECAST CONFIDENCE IN TIMING OF EACH BATCH
OF RAIN AS WELL AS RAINFALL AMOUNTS IS MUCH LOWER...ESPECIALLY
OVER SOUTHCENTRAL ALASKA MONDAY THROUGH TUESDAY.

&&

.FIRE WEATHER...
FIRE WEATHER CONCERNS REMAIN LOW THE NEXT COUPLE DAYS WITH
WIDESPREAD WETTING RAINS ACROSS THE SOUTHERN MAINLAND OF ALASKA.
THERE WILL BE A DRYING TREND OVER SOUTHWEST ALASKA TUESDAY
SPREADING EASTWARD TO SOUTHCENTRAL TUESDAY NIGHT AND WEDNESDAY...
THOUGH GENERALLY COOL LOW LEVEL AIR COMBINED WITH SHORTER DAYS
MEANS RELATIVE HUMIDITIES WILL REMAIN FAIRLY HIGH.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (DAYS 1 AND 2)...

A COMPLEX AND ACTIVE PATTERN CONTINUES ACROSS SOUTHCENTRAL AS
SOUTHWEST FLOW AHEAD OF A STRONGER LOW OVER THE NORTH PACIFIC
FEEDS MOISTURE INTO THE REGION. THE FIRST SHORTWAVE TROUGH HAS
ALREADY LIFTED TOWARD THE ALASKA RANGE AND WEAKENED...BUT
CONTINUED UPLIFT BEHIND THIS SYSTEM HAS SUPPORTED ALL DAY RAIN
ACROSS NEARLY ALL OF SOUTHCENTRAL. ANOTHER SHORTWAVE TROUGH
CURRENTLY SEEN ON SATELLITE OVER SOUTHWEST ALASKA WILL MOVE INTO
SOUTHCENTRAL OVERNIGHT...BRINGING ANOTHER PERIOD OF RAIN TO MOST
OF THE REGION INTO MONDAY MORNING. THE MAIN LOW WILL TRACK INTO
SOUTHCENTRAL MONDAY AFTERNOON AND EVENING...BRINGING PERIODIC
HEAVY RAIN TO THE ENTIRE REGION...INCLUDING THE COPPER BASIN. THIS
WILL PERSIST INTO TUESDAY MORNING BEFORE THE LOW MOVES EAST. RAIN
SHOWERS WILL REMAIN IN ITS WAKE TUESDAY...ALONG WITH INCREASINGLY
GUSTY NORTH TO WEST GAP WINDS.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (DAYS 1 AND 2)...
ANOTHER ROUND TAPERS OFF TO SPOTTY SHOWERS OVERNIGHT TONIGHT OVER
THE MAINLAND...HOWEVER THE ACTIVE MOIST PATTERN CONTINUES ON
MONDAY. DRIVEN BY THE UPPER LEVEL JET...A SURFACE LOW PRESSURE
WILL RIDE A STALLED FRONTAL BOUNDARY JUST SOUTH OF THE ALASKA
PENINSULA. THE HEAVIEST AND STEADIER RAIN WILL STAY CONFINED TO
THE ALASKA PENINSULA WHILE INTERIOR AREAS WILL REMAIN UNDER A MORE
SHOWERY REGIME ASSOCIATED WITH THE COLD UPPER TROUGH ALOFT. RAIN
AND SHOWERS WILL STAY IN THE FORECAST ON TUESDAY...MAINLY ALONG
WINDWARD SIDES OF THE ALASKA/ALEUTIAN RANGE AND KUSKOKWIM
MOUNTAINS. SHOWER ACTIVITY WILL DECREASE ON TUESDAY BUT WILL STILL
BE POSSIBLE AS THE UPPER TROUGH HANGS OVERHEAD. WINDS WILL BACK
AROUND TO THE WEST ON TUESDAY BRINGING IN A SLIGHTLY COOLER
AIRMASS.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (DAYS 1 AND 2)...
A FRONTAL BOUNDARY WILL JUST CLIP THE ALASKA PENINSULA TONIGHT
BEFORE MOVING INTO THE GULF. SCATTERED RAIN SHOWERS WILL AFFECT
THE EASTERN BERING SEA WHILE STILL UNDER THE UPPER TROUGH. MAINLY
DRY CONDITIONS WILL REMAIN THROUGH TUESDAY AS HIGH PRESSURE BUILDS
IN FROM THE WESTERN BERING SEA.

&&

.LONG TERM FORECAST (DAYS 3 THROUGH 7...WED THROUGH SUN)...AN
ACTIVE PROGRESSIVE PATTERN WILL CONTINUE FROM THE MIDDLE TO THE
END OF THE WEEK BEFORE A DRYING TREND SETS UP FOR THE WEEKEND.
THE JET STREAM WILL CONTINUE TO DIVE DOWN THROUGH THE KAMCHATKA
PENINSULA TOWARD THE WESTERN ALEUTIANS BEFORE MAKING A FAIRLY
SHARP TURN TO THE EAST. IT WILL THEN RIDE THROUGH THE NORTHERN
GULF OF ALASKA AT AROUND 90 KTS. THIS WILL ALLOW SEVERAL WEAKER
SYSTEMS TO PASS FROM BRISTOL BAY TOWARDS THE COPPER RIVER BASIN
AND KEEP GUSTY WESTERLY WINDS BLOWING THROUGH FAVORED LOCATIONS. THESE
SYSTEMS WILL NOT HAVE THE SAME SUBTROPICAL MOISTURE SOURCE AS THE
ONES EARLY IN THE WEEK. THUS...WE ARE NOT LIKELY TO SEE ANY BIG
STRATIFORM RAIN EVENT BUT SHOWERS WILL STILL BE WIDESPREAD. THE
BEST CHANCE FOR SHOWERS LOOKS TO BE ALONG HIGHER TERRAIN EAST OF
THE SUSITNA VALLEY ON WEDNESDAY AND THEN RIDING A WAVE DOWN FROM
THE YK-DELTA THROUGH THE NORTHEASTERN GULF LATE FRIDAY INTO EARLY
SATURDAY. IF ENOUGH SUNSHINE CAN BREAK OUT IN THE COPPER RIVER
BASIN ON WEDNESDAY...WE MAY BE ABLE TO BUILD ENOUGH INSTABILITY TO
TRIGGER SOME ISOLATED THUNDERSTORMS ALONG THE HIGHER TERRAIN IN
THE AFTERNOON HOURS.

AS WE MOVE INTO THE WEEKEND...THE PATTERN BEGINS TO SHIFT AND
BRING SOME DRIER NORTHWEST OFFSHORE FLOW THROUGH THE AREA. MOST
POPULATION CENTERS WILL SEE DRY CONDITIONS DEVELOPING SATURDAY
AND PERSISTING THROUGH SUNDAY. AS LOW PRESSURE SLIDES OFF TO THE
EASTERN GULF AND HIGHER PRESSURE STARTS TO BUILD IN OVER THE
ALASKA PENINSULA...WE COULD SOME SOME MORE GUSTY WEST TO NORTHWEST
WINDS. THIS WILL FAVOR AREAS LIKE KAMISHAK GAP AND POSSIBLY
WHITTIER AND PASSAGE CANAL. MODEL DIFFERENCES START TO BECOME
QUITE DRASTIC BY EARLY NEXT WEEK. THE GFS CONTINUES TO ADVERTISE
SOME STRONG PHASING OVER THE BERING WHILE THE EC KEEPS BUILDING A
RIDGE FROM THE NORTH PACIFIC UNDER WARM AIR ADVECTION. THIS
FORECAST PACKAGE LEANED MAINLY TOWARD THE EC ENSEMBLE MEAN.

&&

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

&&

SYNOPSIS AND MODEL DISCUSSION...SB
SOUTHCENTRAL ALASKA...JA
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...ML
LONG TERM...MO