Small Craft Advisory 650, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
432 PM AKDT TUE AUG 18 2015

.ANALYSIS AND UPPER LEVELS...

THE LAST VESTIGES OF THE RAINY PATTERN OF RECENT DAYS IS BLASTING
OFF TO THE EAST...DRIVEN BY A FAIRLY STRONG AND MOSTLY ZONAL JET
STREAM SOUTH OF MAINLAND ALASKA. AS EXPECTED...A LOW DEVELOPED
RAPIDLY OVERNIGHT NEAR PRINCE WILLIAM SOUND AND TRACKED INTO THE
COPPER RIVER BASIN. STRONG MID LEVEL DEFORMATION LED TO A FAIRLY
BROAD BAND OF RAIN FROM THE MOUNTAINS EAST OF ANCHORAGE INTO THE
NORTHEAST COPPER RIVER BASIN...WITH GENERALLY 1 TO 2 INCH RAIN
AMOUNTS IN THE LAST 24 HOURS IN THESE AREAS. MEANWHILE...EASTERN
PRINCE WILLIAM SOUND AND THE NORTH GULF COAST RECEIVED HIGHER
AMOUNTS...GENERALLY 2 TO 4 INCHES OF RAIN...AS THE DEEP
ATMOSPHERIC MOISTURE SLAMMED INTO THE COASTAL MOUNTAINS AND WAS
WRUNG OUT LIKE A SPONGE. STEADY RAIN IS CONTINUING OVER EASTERN
PORTIONS OF THE COPPER RIVER BASIN AS OF MIDDAY...BUT WILL BE SOON
COMING TO AN END AS MORE WESTERLY/ZONAL FLOW MOVING IN FROM THE
WEST HELPS TO DRY OUT BOTH THE UPPER AND LOWER LEVELS OF THE
ATMOSPHERE. SOME LIMITED CONVECTION HAS BROKEN OUT IN THE CLEARER
SKIES BEHIND THE DEPARTING STORM...FUELED BY COOLING TEMPERATURES
ALOFT THAT ARE DESTABILIZING THE ATMOSPHERE. A FEW THUNDERSTORMS
REMAIN POSSIBLE THROUGH THIS EVENING...BUT ARE NOT EXPECTED TO BE
A WIDESPREAD EVENT BY ANY MEANS. GUSTY WINDS THROUGH THE FAVORED
GAPS OF TURNAGAIN ARM AND THE KENAI PENINSULA HAVE BEGUN TO BLOW
OUT OF THE NORTH AND WEST...WITH "OUTFLOW" WINDS EXPECTED TO
SLOWLY SPREAD TO THE OTHER GAPS ALONG THE NORTH GULF COAST AND
PRINCE WILLIAM SOUND OVERNIGHT AND INTO TOMORROW.

OUT WEST...DEEP-LAYER WESTERLY FLOW IS IN PLACE ACROSS VIRTUALLY
ALL OF THE BERING/ALEUTIAN REGION AND THE SOUTHWEST MAINLAND. A
BROAD SHIELD OF CLOUDINESS OVER THE BERING SEA IS BEING PUSHED
INLAND ACROSS THE KUSKOKWIM DELTA AND PARTS OF BRISTOL BAY. THE
STRONGEST GAP WINDS ARE BLOWING FROM THE NORTHWEST THROUGH THE
ALASKA PENINSULA IN RESPONSE TO RISING PRESSURES IN THE BERING
SEA.

&&

.MODEL DISCUSSION...

MODELS ARE IN OUTSTANDING AGREEMENT THROUGH THE FIRST FEW DAYS OF
THE FORECAST PERIOD...DEPICTING WESTERLY/ZONAL FLOW ACROSS MOST
OF THE REGION. THIS WILL LEAVE THE AREA VULNERABLE TO FAST-MOVING
BUT RELATIVELY WEAK SHORTWAVES...WITH THE NEXT SUCH FEATURE
EXPECTED TO MOVE THROUGH THE SOUTHER MAINLAND DURING THE DAY
WEDNESDAY. BY THE END OF THE WEEK THE ZONAL JET WILL AGAIN
AMPLIFY/BUCKLE IN A MORE SIGNIFICANT WAY...WHICH MODELS ALL AGREE
WILL LEAD TO ANOTHER LOW PRESSURE SYSTEM WORKING INTO THE GULF.
THE BIG QUESTION AT THIS POINT REMAINS HOW FAR NORTH AND WEST RAIN
WILL BE SPREAD FROM THIS LOW...WITH THE NAM KEEPING INLAND AREAS
LIKE ANCHORAGE TOTALLY DRY...BUT THE GLOBAL MODELS BRINGING
ANOTHER ROUND OF RAIN INTO THE POPULATION CENTERS. WILL SPLIT THE
DIFFERENCE RIGHT NOW WITH THE GLOBAL MODELS...PENDING BETTER
MODEL AGREEMENT...AND BRING STEADY RAIN NO FURTHER NORTH OR WEST
THAN THE EASTERN KENAI PENINSULA ALIGNED MOST CLOSELY WITH THE
ECMWF.

&&

.FIRE WEATHER...
WITH ALL THE RECENT RAINFALL ACROSS THE AREA EVEN CLEARING SKIES
ARE NOT EXPECTED TO TAKE RH VALUES DOWN NEAR CRITICAL VALUES OVER
THE NEXT FEW DAYS. THE STRONGEST WINDS WILL BE FOUND THROUGH THE
USUAL TERRAIN GAPS ALONG THE GULF COASTLINE...WITH LIGHTER AND GENERALLY
NORTH TO WEST WINDS INLAND. AS MENTIONED ABOVE...A FEW
THUNDERSTORMS ARE EXPECTED TO DEVELOP TODAY IN THE UNSTABLE FLOW
SOUTH AND EAST OF THE ALASKA RANGE BEHIND THE DEPARTING SYSTEM.
ANOTHER WEAK SHORTWAVE MOVING THROUGH TOMORROW MAY PROVIDE ENOUGH
OF A DYNAMIC LIFT TO TAKE ADVANTAGE OF CONTINUING INSTABILITY. IN
ANY EVENT...NO SIGNIFICANT OR WIDESPREAD THUNDERSTORM OUTBREAKS
ARE EXPECTED.

&&

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

RAIN ACTIVITY ACROSS SOUTHCENTRAL ALASKA HAS TAPERED OFF AS
EXPECTED THIS AFTERNOON BUT NOT BEFORE SOME LOCATIONS ACROSS THE
COPPER RIVER BASIN AND NORTHERN GULF COAST RECORDED PRETTY
SIGNIFICANT RAINFALL. SHOWER ACTIVITY ASSOCIATED WITH NORTHWEST
FLOW ON THE BACKSIDE OF THE TROUGH PASSAGE CONTINUES THIS
AFTERNOON BUT HAS GENERALLY BEEN DISSIPATING AS THE MAJORITY OF
THE UPPER LEVEL ENERGY IS NOW PUSHING EAST OF THE AREA. THE WAVE
THAT WAS EXPECTED TO HELP SPARK SOME THUNDERSTORM ACTIVITY HAS
PUSHED THROUGH ON CUE...WITH THUNDERSTORMS DEVELOPING ACROSS THE
MATANUSKA AND SUSITNA VALLEYS. SOME OF THESE STORMS WILL CONTAIN
SMALL HAIL AS THE SFC LAYER HAS DESTABILIZED UNDER MOSTLY CLEAR
SKIES THROUGHOUT THE DAY AND COLD AIR ALOFT HAS CREATED QUITE THE
UNSTABLE ENVIRONMENT. THESE SHOWERS AND THUNDERSTORMS WILL SLOWLY
DIMINISH OVERNIGHT WITH THE LOSS OF DIURNAL HEATING...BUT A FEW
MAY LINGER INTO THE LATER EVENING AS THE COLD AIR ALOFT MAY BE
ENOUGH TO SUPPORT THE STORMS A TAD LONGER.

WEDNESDAY WILL BE DRIER BUT SOME SCATTERED SHOWERS AND ISOLATED
THUNDERSTORMS WILL ONCE AGAIN BE POSSIBLE. WHILE MODELS HAVE BEEN
STRUGGLING TO PLACE A SHORTWAVE TRAVERSING THE NEAR ZONAL
FLOW...THEY HAVE NOW DECIDED ON A SOLUTION THAT PUSHES THIS WAVE
ACROSS SOUTHCENTRAL ALASKA AROUND PEAK HEATING ON WEDNESDAY. THIS
COUPLED WITH MODEST MID-LEVEL LAPSE RATES SHOULD BE ENOUGH TO
SUPPORT A FEW ELEVATED THUNDERSTORMS. CONDITIONS ONLY IMPROVE
HEADING INTO THURSDAY WHEN SUNNY SKIES SHOULD BE OBSERVED ACROSS
ALL OF SOUTHCENTRAL ALASKA AHEAD OF THE NEXT SYSTEM LOOKING TO
IMPACT THE REGION HEADING INTO THE WEEKEND.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (DAYS 1 AND 2)...
ZONAL FLOW ALOFT IS BEGINNING TO TAKE HOLD AS HIGH PRESSURE BUILDS
IN AT THE SURFACE. DIURNAL SHOWERS ARE POSSIBLE OVER THE NEXT
COUPLE OF AFTERNOONS HOWEVER COVERAGE WILL BE ISOLATED TO
SCATTERED...MAINLY OVER THE KUSKOKWIM DELTA AND VALLEY. PERSISTENT
WESTERLY FLOW IN THE LOW LEVELS WILL KEEP LOW CLOUDS AROUND THE
DELTA. OTHERWISE GENERALLY BENIGN WEATHER CONDITIONS WILL PERSIST
OVER THE MAINLAND THROUGH THURSDAY.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (DAYS 1 AND 2)...
WEAK HIGH PRESSURE SITS OVER THE SOUTHERN BERING SEA MAKING PATCHY
FOG POSSIBLE OVERNIGHT. A FRONT AND UPPER LEVEL TROUGH ENTERS THE
WESTERN BERING TONIGHT BRINGING RAIN AND GUSTY SOUTHERLY WINDS.
ANOTHER WEAKER FRONT AND DISTURBANCE WILL SKIRT THE EASTERN
ALEUTIANS AND ALASKA PENINSULA ON WEDNESDAY NIGHT BRINGING A SHOT
OF RAIN.

&&

.LONG TERM FORECAST (DAYS 3 THROUGH 7..FRIDAY THROUGH
TUESDAY)...
MODELS CONTINUE TO STRUGGLE GREATLY IN THIS TIME PERIOD...SO
FORECAST CONFIDENCE IS LOW TO MODERATE. FIRST ITEM IS A DEVELOPING
SYSTEM EAST OF KODIAK ON FRIDAY. MODELS DID NOT RESOLVE THIS
FEATURE MUCH AT ALL YESTERDAY. TODAY THE NAM HAS IT...BUT MUCH
FURTHER EAST IN THE GULF THAN THE GLOBAL MODELS. SO FAR IN FACT
THAT IT ACTUALLY KEEPS MOST OF THE PRECIPITATION OFFSHORE. THE
GLOBAL MODELS CLIP THE COOK INLET WITH SOME LIGHT RAIN AND BRING
ANOTHER ROUND OF HEAVY RAIN TO THE PRINCE WILLIAM SOUND LATE
FRIDAY INTO EARLY SATURDAY.

GOING INTO THE WEEKEND...THE GENERAL PATTERN IS STILL TRYING TO
SHOW A SHIFT TOWARDS RIDGING BUILDING OVER THE EASTERN BERING.
THIS WOULD BRING A DRYING NORTHWESTERLY FLOW TO MUCH OF SOUTH
CENTRAL. HOWEVER THE OTHER NEW DEVELOPMENT TODAY IS THE COLDER
UPPER LEVEL LOW OVER THE ARCTIC IS TRYING TO PUSH FARTHER SOUTH
NEAR THE ALASKA RANGE SUNDAY INTO MONDAY. THIS WOULD ALLOW SOME
STRONGER SYSTEMS TO RIDE DOWN FROM THE YUKON DELTA AND BRING
SHOWERS TO MUCH OF THE MAINLAND. THIS WILL BE VERY INTERESTING TO
WATCH HOW MODELS HANDLE IT OVER THE NEXT SEVERAL DAYS. IT WILL BE
THE NORTH PACIFIC RIDGE...WHICH HAS CERTAINLY BEEN DOMINANT THIS
SUMMER...VS THE ARCTIC LOW DROPPING SOUTH...WHICH HAS CLIMATOLOGY
ON ITS SIDE AS WE MOVE INTO LATE AUGUST.

BY TUESDAY...MODELS STILL AGREE THAT THE PATTERN WILL BECOME MORE
AMPLIFIED ACROSS THE DOMAIN. THE NORTH PACIFIC RIDGE STARTS TO
ASSERT ITSELF MORE TUESDAY INTO WEDNESDAY. THIS SHOULD BRING A FEW
DAYS OF UNSEASONABLY SUNNY AND WARM CONDITIONS ACROSS MUCH OF THE
MAINLAND. MEANWHILE...THINGS WILL START TO BECOME MORE ACTIVE OVER
THE ALEUTIANS. TYPHOON REMNANTS WILL BEGIN TO PHASE WITH COLDER
AIR DROPPING SOUTH FROM EASTERN RUSSIA. THIS BAROCLINIC
ENVIRONMENT COULD BE THE START OF SOME DECENT LOW PRESSURE
SYSTEMS...BUT WE ARE NOT BITING OFF ON ANY ONE SOLUTION AT THIS
TIME.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE
MARINE...GALE WARNING 127 131
FIRE WEATHER...NONE

&&

SYNOPSIS AND MODEL DISCUSSION...AD
SOUTHCENTRAL ALASKA...MMC
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...MTL
LONG TERM...MSO