Small Craft Advisory 644, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
458 AM AKDT MON AUG 17 2015

.ANALYSIS AND UPPER LEVELS...

A WELL-DEFINED PROGRESSIVE TROUGH-RIDGE PATTERN IS IN PLACE ACROSS
SOUTHERN ALASKA. AN UPPER LEVEL LOW IS OVER THE NORTHEASTERN
BERING CENTERED OFF THE COAST OF THE YUKON DELTA. A POTENT UPPER
LEVEL TROUGH EXTENDING FROM THE LOW HAS AN AXIS THAT IS NEARLY
MERIDIONAL ALONG 170W. POLAR AIR FROM SIBERIA IS STREAMING
SOUTHEAST OVER THE BERING HELPING BOTH DEEPEN THE UPPER LEVEL LOW
AND HELP THE NEUTRALLY TILTED TROUGH DIG MUCH FURTHER
SOUTH...SOUTH OF 50N. THE DIGGING TROUGH BY COMPARISON IS AIDING
IN THE AMPLIFICATION OF THE RIDGE TO THE EAST OF IT...WITH ITS
AXIS OVER THE SOUTHEAST ALASKA PANHANDLE. THE REMNANTS OF TROPICAL
STORM MOLAVE ARE CURRENTLY MOVING INTO THE SOUTHERN ALASKA
PENINSULA AS EVIDENCED BY WATER VAPOR IMAGERY. THE BLENDED TPW
PRODUCT SHOWS MAXIMUM PWAT VALUES ASSOCIATED WITH IT OF 2.5
INCHES SOUTH OF DUTCH HARBOR. MOLAVES REMNANTS ARE PROVIDING BOTH
A STRONG MOISTURE SOURCE FOR HEAVY RAIN...AND INCREASING THE
TEMPERATURE GRADIENT BETWEEN IT AND THE POLAR AIR DIVING SOUTH
OUT OF RUSSIA. BETWEEN THESE TWO FEATURES IS A VERY STRONG JET
STREAK FOR THIS TIME OF YEAR...PEAKING AT 140 KTS AT 225 MB ON
THE 12Z ANCHORAGE SOUNDING...WHOSE AXIS EXTENDS FROM NORTHERN
BRISTOL BAY ACROSS THE KUSKOKWIM AND SUSITNA VALLEYS INTO THE
COPPER RIVER BASIN. THIS SHIFT MUCH FURTHER NORTH OF THE AXIS OF
THE JET AS COMPARED WITH PREVIOUS FORECASTS WAS THE RESULT OF THE
BERING TROUGH DIGGING MUCH FURTHER SOUTH THAN EXPECTED FROM
PREVIOUS DAYS...LEADING TO A MUCH MORE AMPLIFIED RIDGE...WHICH IN
TURN PUSHED THE JET STREAK FURTHER NORTH.

&&

.MODEL DISCUSSION...

REGARDING THE REMNANTS OF MOLAVE...THE MODELS ARE ALL IN GENERAL
AGREEMENT AS TO THE PATH OF THE STORM...THOUGH THAT AGREEMENT IN
TRACK IS FURTHER NORTH THAN THE AGREEMENT OF THE PAST COUPLE
NIGHTS...OWING TO THE JETS SHIFTING NORTH. THUS THE AGREED UPON
TRACK FROM ITS CURRENT LOCATION NEAR COLD BAY AND SAND POINT IS TO
MOVE THROUGH SHELIKOF STRAIT INTO PRINCE WILLIAM SOUND TONIGHT.
THE NAM IS BOTH THE FASTEST OF THE MODELS AND THE DEEPEST
CONCERNING THE MSLP OF THE STORM...THOUGH IN TERMS OF TOTAL
PRECIPITATION EXPECTED...THE DIFFERENCE IS INSIGNIFICANT. WITH THE
HEAVIEST PRECIPITATION EXPECTED ON THE SOUTH SIDE OF THE LOW...THE
MODELS AGREE NOW THAT THE HEAVIEST RAINS WILL HIT KODIAK AND TRACK
ALONG THE GULF COAST FROM SEWARD OVER TO THE PRINCE WILLIAM SOUND.
THE SAVING GRACE FOR THESE AREAS FROM SEEING PRODIGIOUS RAINFALL
TOTALS IS THE FAST SPEED OF THE STORM...MOVING NORTHEAST AT 35
MPH.

BEHIND MOLAVES REMNANTS...UPPER LEVEL SHORTWAVES WILL HELP
CONTINUE FORCING SOME UNSETTLED WEATHER TUESDAY FROM THE WEST
SLOPES OF THE ALASKA RANGE EAST. A DRYING TREND WILL BEGIN FROM
THE KUSKOKWIM VALLEY AND BRISTOL BAY WEST ON TUESDAY. THE USUAL
MODEL DISCREPANCIES IN THIS SCENARIO PERTAIN TO THE TIMING AND
INTENSITY OF THOSE FEATURES AND THEIR ABILITY TO USE THE AVAILABLE
MOISTURE TO PRODUCE MORE RAINFALL. THE MODELS AGREE IN A ZONAL
PATTERN SETTING UP OVER SOUTHERN ALASKA THROUGH THURSDAY WITH THE
JET ROUGHLY FOLLOWING THE 58TH PARALLEL BEFORE THE NEXT TROUGH
MOVES IN BRINGING MORE UNSETTLED WEATHER.

&&

.FIRE WEATHER...

WIDESPREAD WETTING RAINS THAT MAY BE HEAVY AT TIMES IN SOME AREAS TODAY
OVER NEARLY ALL OF SOUTHERN ALASKA AND INTO TUESDAY FOR
SOUTHCENTRAL WILL KEEP FIRE WEATHER CONCERNS AT NIL. A DRYING
TREND REMAINS IN THE FORECAST FROM WEDNESDAY ON...THOUGH THAT
PRIMARILY PERTAINS TO FEWER AREAS EXPECTED TO SEE RAINFALL ON
THOSE DAYS RATHER THAN THE EXPECTATION OF LOW RH VALUES. TERRAIN
FORCING...COLD AIR ALOFT...NUMEROUS UPPER LEVEL WAVE
PASSAGES...AND ABUNDANT MOISTURE SPELL MANY OF THE INGREDIENTS
NECESSARY FOR WET THUNDERSTORMS ALONG THE MOUNTAINS OF
SOUTHCENTRAL TUESDAY AND WEDNESDAY. RIDGING INCREASES SUBSIDENCE
AND REDUCES THE THUNDERSTORM THREAT FOR THURSDAY.

&&

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

AS EXPECTED RAIN IS OCCURRING OVER JUST ABOUT ALL OF SOUTHCENTRAL
ALASKA. WHAT IS DIFFERENT IS THAT THE JET STREAM PUSHED FARTHER TO
THE NORTH WHICH IS BRINGING MORE OF THE MOISTURE INTO
SOUTHCENTRAL. THIS WILL BRING HIGHER RAINFALL AMOUNTS TO MOST
AREAS AS WELL AS STRONGER WINDS TO MANY OF THE MARINE AREAS AROUND
THE GULF OF ALASKA.
AS THE UPPER LEVEL LOW MOVES EASTWARD TOMORROW THE COOLER AIR
ALOFT WILL DESTABILIZE THE ATMOSPHERE FROM THE KENAI PENINSULA
NORTHWARD. THIS MAY PRODUCE SOME THUNDERSTORMS IN THESE AREAS
TOMORROW AFTERNOON INTO EVENING. THE BIG LIMITING FACTOR WILL BE
THE LOW LEVEL HEATING WITH SIGNIFICANT CLOUD COVER STILL EXPECTED
ESPECIALLY TO THE NORTH. AREAS THAT DO GET SOME BREAKS IN THE
CLOUDS AND SOME DAYTIME HEATING MAY BE ABLE TO PRODUCE SOME
THUNDERSTORMS TOMORROW.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (DAYS 1 AND 2)...
RAINFALL IS EXPECTED TO BEGIN TO TAPER OFF OVER THE SOUTHWEST
MAINLAND THROUGH TUESDAY AS THE AXIS OF THE UPPER LEVEL TROUGH
THAT HAS BEEN CENTERED OVER THE BERING STRAIT WILL MOVE EASTWARD.
TODAY...THE MAIN FORECAST CHALLENGE IS THE INTERACTION BETWEEN
THE UPPER TROUGH AND THE RAPIDLY DEVELOPING SURFACE LOW SOUTH OF
THE ALASKA PENINSULA THAT WILL MOVE EASTWARD TODAY. AS NOTED
ABOVE...THIS LOW HAS EXCELLENT JET SUPPORT AND IS EMBEDDED IN A
VERY MOIST TROPICAL AIRMASS...THUS WIDESPREAD RAINFALL IS EXPECTED
ALONG THE PATH OF THE LOW. THE EXACT TRACK OF THE LOW AND THE
SPEED OF THE UPPER TROUGH WILL CHANGE THE LOCATION...
INTENSITY...AND TIMING OF PRECIPITATION OVER MOST OF THE AREA
THIS MORNING AS THE SURFACE LOW MOVES EASTWARD. BY THIS
AFTERNOON...THE LOW IS EXPECTED TO BE EAST OF KODIAK ISLAND WITH
THE AXIS OF THE TROUGH STARTING TO PUSH INTO THE AREA...THUS
SERVING AS THE FOCAL POINT FOR THE SECOND ROUND OF RAINFALL
THROUGH EARLY TOMORROW MORNING. MODELS HAVE CONSISTENTLY BEEN
SLOW ON TIMING WITH THIS SYSTEM...THUS CONFIDENCE IN THE EXACT
LOCATION AND TIMING OF PRECIPITATION TODAY IS BELOW AVERAGE.

ON TUESDAY...RAINFALL IS EXPECTED MAINLY IN UPSLOPED AREAS AS THE
ATMOSPHERE WILL REMAIN UNSTABLE BEHIND THE TROUGH AND THE FLOW
WILL TURN MORE NORTHWESTERLY. RAINFALL WILL TAPER OFF THROUGH
TUESDAY EVENING AS DRIER AIR MOVES INTO THE SOUTHWEST MAINLAND.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (DAYS 1 AND 2)...
GALE FORCE WINDS AND RAPIDLY BUILDING SEAS ARE EXPECTED IN THE
MARINE AREAS SOUTH OF THE ALASKA PENINSULA TODAY AS A RAPIDLY
DEVELOPING LOW MOVES EASTWARD TODAY. AS NOTED YESTERDAY...THIS LOW
IS NOT EXPECTED TO BE ESPECIALLY STRONG BUT THE PRESSURE GRADIENT
WILL REMAIN TIGHT AS THE HIGH IN THE NORTH PACIFIC REMAINS FIRMLY
IN PLACE. THE LONG FETCH AND FAIRLY LINEAR TRACK OF THIS LOW WILL
ALLOW FOR HIGHER SIGNIFICANT WAVE HEIGHTS THAN WOULD NORMALLY BE
EXPECTED WITH A LOW OF THIS PRESSURE. WINDS ARE EXPECTED TO
DIMINISH LATE TONIGHT TO BELOW GALE FORCE AS THIS LOW MOVES INTO
THE NORTHERN GULF OF ALASKA. WIDESPREAD RAIN IS EXPECTED EAST OF
DUTCH HARBOR TODAY WITH THIS SYSTEM

FOR THE REMAINDER OF THE BERING SEA...WINDS WILL GRADUALLY
DIMINISH AS HIGH PRESSURE BUILDS ALONG THE ALEUTIAN ISLANDS
THROUGH TUESDAY.

&&

.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...GALE 119 120 131 132 150 155.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...JW
SOUTHCENTRAL ALASKA...EZ
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...RF
LONG TERM...MO