Small Craft Advisory 653, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
433 PM AKDT WED AUG 19 2015

.ANALYSIS AND UPPER LEVELS...

MOSTLY ZONAL FLOW CONTINUES ACROSS THE FORECAST AREA THIS MORNING.
A WEAK SHORTWAVE TROUGH IS EVIDENT ON WATER VAPOR MOVING THROUGH
UPPER COOK INLET...SUPPORTING A FEW SHOWERS AROUND ANCHORAGE AND
THE MAT-SU VALLEYS AT MIDDAY. THE 12Z SOUNDING FROM ANCHORAGE
SHOWED A DECREASE IN LAPSE RATES (MORE STABLE) COMPARED TO LAST
EVENING...WHEN UNUSUALLY STRONG THUNDERSTORMS DEVELOPED AROUND THE
REGION. THE MOST UNSTABLE CONDITIONS TODAY ARE CONFINED MORE TO
THE COPPER RIVER BASIN AREA...AND AS A RESULT IS THE MOST LIKELY
LOCATION TO SEE SOME THUNDERSTORMS DEVELOP LATER THIS AFTERNOON.

SHORTWAVE RIDGING IS PRESENT OVER SOUTHWEST ALASKA AND IS
COMBINING WITH VERY STABLE MARINE AIR MOVING ONSHORE FROM THE
BERING SEA TO PRODUCE A COOL AND CLOUDY DAY TODAY IN MOST PLACES.

WAY OUT WEST...A DIGGING UPPER LEVEL TROUGH OVER THE NORTHWEST
BERING IS MOVING SOUTHEASTWARD WITH AN ASSOCIATED 1006 MB SURFACE
LOW. A SMALL AREA OF GALE FORCE WINDS WRAPS AROUND THE LOW CENTER
ITSELF AS THE SYSTEM MOVES TOWARD THE PRIBILOF ISLANDS.

&&

.MODEL DISCUSSION...

THE MODELS REMAIN IN GOOD SYNOPTIC AGREEMENT THROUGH FRIDAY
AFTERNOON AS THE AMPLIFYING UPPER TROUGH IN THE BERING SEA
DEVELOPS INTO A CLOSED LOW AND MOVES INTO THE GULF OF ALASKA BY
THE START OF THE WEEKEND. THE ASSOCIATED SURFACE LOW WILL WEAKEN
AND DISSIPATE SOUTHEAST OF THE PRIBILOF ISLANDS...BUT THE EXACT
TRACK OF THE SURFACE LOW WILL HAVE AN IMPACT PRIMARILY ON WIND
DIRECTIONS IN THE PRIBILOF ISLANDS. THE OVERALL MODEL TREND HAS
BEEN EASTWARD...WITH THE GFS A GOOD BALANCE OF CONTINUITY AND
MODEL TRENDS...SO WILL LEAN HEAVILY THAT DIRECTION WITH THE
FORECAST. LATER...AS A NEW SURFACE LOW DEVELOPS IN THE GULF ON
FRIDAY...THE NAM IS A BIT OF AN OUTLIER BEING TOO FAR SOUTH WITH
THE LOW CENTER. PREFER TO LEAN TOWARD A GFS/EC TRACK THAT BRINGS
PRECIP TO THE NORTH GULF COAST BUT NOT INLAND.

&&

.FIRE WEATHER...

SUNNIER AND WARMER CONDITIONS WILL MOVE INTO THE SOUTHCENTRAL AREA
FOR THURSDAY AND FRIDAY. RH VALUES WILL LIKELY DROP INTO THE 30S
BOTH DAYS IN THE COPPER RIVER BASIN AND THE SUSITNA VALLEY. WINDS
ARE EXPECTED TO BE LIGHT ON THURSDAY...BUT ON FRIDAY GUSTY
NORTHWEST WINDS WILL DEVELOP IN THE SUSITNA VALLEY IN RESPONSE TO
THE LOW DEVELOPING IN THE GULF. THIS MAY PRODUCE CONDITIONS THAT
APPROACH RED FLAG CRITERIA...AND WILL BE MONITORED CLOSELY OVER
THE NEXT 24 HOURS.

WEST OF THE ALASKA RANGE...A HEAVILY MARITIME-INFLUENCED AIRMASS
WILL REMAIN IN PLACE. THUS...EVEN WITH SUNSHINE BREAKING OUT FOR
MANY INLAND AREAS THE RH VALUES SHOULD REMAIN WELL ABOVE CRITICAL
LEVELS.

&&

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

SHOWERS AND THUNDERSTORMS HAVE DEVELOPED ACROSS THE SUSITNA VALLEY
ALL THE WAY TO THE SOUTHEAST PORTIONS OF PRINCE WILLIAM SOUND.
ANCHORAGE AND THE KENAI PENINSULA SEEM TO HAVE MISSED OUT ON THE
ACTION WITH THE BEST UPPER LEVEL SUPPORT JUST NORTH TOWARD THE
MAT-SU VALLEYS. THESE SHOWERS AND THUNDERSTORMS WILL QUICKLY
DISSIPATE AS THE EVENING PROGRESSES AS THE UPPER LEVEL DISTURBANCE
ASSOCIATED WITH THESE SHOWERS QUICKLY PROPAGATES EASTWARD AND A
WEAK UPPER LEVEL OF RIDGE BRINGS DRIER AIR BACK TO THE REGION.
COME MIDNIGHT...MUCH OF SOUTHCENTRAL ALASKA WILL SEE CLEAR SKIES WITH
THE EXCEPTION OF THE COPPER RIVER BASIN WHICH MAY TAKE JUST A
LITTLE LONGER TO CLEAR OUT. ALTHOUGH SKIES ARE EXPECTED TO CLEAR
AFTER THE DECENT RAINFALL SEEN OVER THE PAST SEVERAL DAYS...FOG IS
NOT EXPECTED AT THIS TIME. NORTHERLY WINDS USHERING IN DRIER AIR
ARE PRODUCING UNFAVORABLE HYDROLAPSES (HUMIDITY CHANGES WITH
HEIGHT) WHICH WILL PREVENT THE FORMATION OF FOG...BUT AREAS ALONG
BODIES OF WATER AND WHERE SOME OF THE HEAVIER RAIN RECENTLY MIGHT
SEE SOME VERY LOCALIZED PATCHY FOG. HAVE OPTED TO OMIT FROM THE
FORECAST FOR THE TIME BEING WITH LOW COVERAGE AND LOW CERTAINTY OF
ANY DEVELOPMENT. THURSDAY SHOULD BE A BEAUTIFUL DAY AS SUNNY SKIES
WILL BE SEEN ACROSS SOUTHCENTRAL ALASKA. SOME GUSTY OUTFLOW WINDS
WILL DEVELOP THIS EVENING THROUGH VALDEZ...THOMPSON PASS...THE
COPPER RIVER DELTA AND WHITTIER WHICH WILL BE THE RESULT FROM THE
INITIAL PRESSURE RISES (ISALLOBARIC WINDS) WITH HIGH PRESSURE
BUILDING ACROSS THE REGION. THESE WIND ARE EXPECTED TO DIMINISH
THURSDAY AFTERNOON AS HIGH PRESSURE CONTINUES TO ENCOMPASS THE
AREA AND THE GRADIENT BECOMES LESS FAVORABLE FOR AGEOSTROPHIC FLOW
THROUGH THE AFOREMENTIONED GAPS. CLOUDS WILL BE ON THE RISE
THURSDAY EVENING INTO FRIDAY AHEAD OF THE NEXT SYSTEM MOVING INTO
THE GULF OF ALASKA...BUT IT LOOKS TO STAY OFFSHORE AND MUCH OF
SOUTHCENTRAL ALASKA SHOULD STAY ON THE DRY SIDE.

&&

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

ONSHORE FLOW CONTINUES TO SPREAD MARINE CLOUDS OVER MUCH OF THE
REGION THIS EVENING. THIS IS HELPING TO KEEP TEMPERATURES
COOLER AND AFTERNOON INSTABILITY TO A MINIMUM. ISOLATED SHOWERS
WILL MAINLY REMAIN OVER THE KUSKOKWIM DELTA AND LOWER KUSKOKWIM
VALLEY TONIGHT. A TROUGH OVER THE BERING WILL MAKE EASTWARD
PROGRESS THURSDAY...WITH INCREASING SHOWER CHANCES REGION WIDE
THURSDAY NIGHT AND FRIDAY.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (DAYS 1 AND 2)...

A TROUGH OVER THE NORTHWEST BERING WILL MOVE EAST THROUGH THURSDAY
WITH HIGH PRESSURE BUILDING IN BEHIND IT. RAIN WILL IMPACT THE
PRIBILOF ISLANDS BEFORE THE FRONT WEAKENS AND FALLS APART AS IT
MOVES OVER THE EASTERN ALEUTIANS. THIS WILL SUPPORT SHOWERY
ACTIVITY AND INCREASING NORTH TO NORTHWEST WINDS THURSDAY NIGHT
AND FRIDAY. HIGH PRESSURE BUILDING IN OVER THE WESTERN BERING AND
ALEUTIANS WILL SUPPORT WIDESPREAD FOG AND STRATUS.

&&

.LONG TERM FORECAST (DAYS 3 THROUGH 7...SAT THROUGH WED)...MODELS
CONTINUE TO "HUNT AND PECK" IN TRYING TO FIND THE ANSWERS IN THIS
TIME PERIOD. LETS START WITH WHERE THIS IS SOME MORE
CERTAINTY...THE WEEKEND. WITH BETTER MODEL AGREEMENT...IT NOW
LOOKS LIKE THE PRECIPITATION ASSOCIATED WITH THE LOW IN THE GULF
WILL STAY PRIMARILY OFFSHORE. AS THE LOW PULLS AWAY TOWARD THE
PANHANDLE...RIDGING WILL START TO BUILD IN FROM THE NORTH
PACIFIC/EASTERN BERING. THIS COULD KEEP SOME FAIRLY GUSTY
NORTHWEST FLOW THROUGH FAVORED AREAS SUCH AS WHITTIER AND KAMISHAK
GAP. BUT THAT NORTHWEST FLOW WILL ALSO USHER IN SOME DRIER AIR
ACROSS MUCH OF THE MAINLAND. THE EXCEPTION TO THIS WILL BE A FEW
WEAK WAVES COMING ACROSS THE ALASKA RANGE WHICH COULD TRIGGER SOME
AFTERNOON/EVENING SHOWERS ON HIGHER TERRAIN. WINDS SHOULD START
TO RELAX SLOWLY ON SUNDAY AND MOSTLY DRY CONDITIONS WILL PERSIST.

AS WE MOVE INTO EARLY NEXT WEEK...THERE IS STILL A FAIR AMOUNT OF
MODEL UNCERTAINTY WHICH KEEPS FORECAST CONFIDENCE AT A MODERATE
LEVEL. ALL MODELS ARE NOW SUGGESTING THAT THE NORTHERN ENERGY
STREAM WILL PUSH SLIGHTLY FARTHER SOUTH OVER THE MAINLAND MON INTO
TUE. THE GFS SWEEPS THE SHORTWAVES ASSOCIATED WITH IT THROUGH THE
INTERIOR WHILE THE EC MOVES THEM AS FAR SOUTH AS THE ALASKA RANGE.
EVEN THOUGH SOME DIFFERENCES REMAIN...IT DOES LOOK LIKE MUCH OF
THE MAINLAND WILL SEE A BROAD WARM FRONT MOVE THROUGH TUE AS FLOW
SWITCHES TO SOUTHWESTERLY. THE PATTERN CONTINUES TO SHOW
AMPLIFICATION THROUGH THE END OF NEXT WEEK. REMNANTS OF TYPHOON
ATSANI CREEP NEAR THE WESTERN ALEUTIANS AND DEEPEN AS AN EXTRA-
TROPICAL SYSTEM. MODELS VARY WILDLY ON STRENGTH AND
PLACEMENT...BUT THE GENERAL IDEA IS THERE.

&&

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

&&

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