Small Craft Advisory 674, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
513 AM AKDT THU AUG 27 2015

.ANALYSIS AND UPPER LEVELS...

A 150 KT JET STREAK IN THE SUBTROPICAL JET IS NEARLY ZONAL ACROSS
THE NORTHERN BERING AND NOSING INTO SOUTHERN ALASKA THIS MORNING
WITH THE POLAR JET MIGRATING INTO THE BERING STRAIT. ABUNDANT
SUBTROPICAL MOISTURE ADVECTED OVER SOUTHCENTRAL AND THE GULF
DURING THE LAST 24 HOURS WITH THE LARGE PLUME OF MOISTURE EVIDENT
ON IR SATELLITE IMAGERY. A SHORTWAVE MOVING OUT AHEAD OF THE JET
STREAK IS BRINGING SHOWER ACTIVITY TO THE COPPER RIVER BASIN AND
ACROSS THE GULF OF ALASKA.

OVER THE BERING...THE STRONG VERTICALLY STACKED SUBTROPICAL HIGH
REMAINS STATIONARY OVER THE CENTRAL AND EASTERN ALEUTIANS. WATER
VAPOR IMAGERY SHOWS VERY DRY AIR ASSOCIATED WITH THE HIGH OVER THE
BERING MOVING EAST INTO SOUTHWEST ALASKA. COLD BAYS SOUNDING FROM
YESTERDAY EVENING AND THIS MORNING SAMPLED THE WARM AND DRY AIR
FROM ABOUT 800 TO 400 MB. TO PUT THE BREADTH OF THE WARM AIR INTO
PERSPECTIVE... THE FREEZING LEVEL ON THE COLD BAY SOUNDING THIS
MORNING WAS OVER 16700 FEET. THAT IS HIGHER THAN MIAMI
FLORIDA...PHOENIX ARIZONA...AND EVEN HILO HAWAIIS SOUNDINGS FROM
THIS MORNING. ALONG THE WESTERN ALEUTIANS...A STRONG FRONT MOVED
INTO SHEMYA WITH A TROPICAL MOISTURE FEED STREAMING IN FROM THE
REMNANTS OF TYPHOON ATSANI. THE STRONG SOUTHERLY WINDS MOVING OVER
THE RELATIVELY COOLER WATERS IS CAUSING WARM ADVECTION FOG OVER
THE WESTERN ALEUTIANS REDUCING VISIBILITIES TO ONE QUARTER MILE AT
TIMES.

ONE LAST NOTE THIS MORNING ON THE POLAR JET AS IT WILL BE A
SIGNIFICANT PATTERN TO WATCH THROUGH THE REMAINDER OF THE
WEEK...THIS JET BEING DRAWN SOUTHWARD IS MOVING A POLAR AIR MASS
FROM THE ARCTIC OCEAN. WATER VAPOR IMAGERY SHOWS A STRONG SHORTWAVE
NOW MOVING INTO THE SEWARD PENINSULA AND REPRESENTS THE LEADING
EDGE OF A JET STREAK. WHEN THIS ENERGY REACHES SOUTHCENTRAL THIS
AFTERNOON...AN ACTIVE WEATHER CHANGE BEGINS AS THE COLD AIR MERGES
WITH THE WARM AND MOIST SUBTROPICAL AIR MASS CURRENTLY OVER
SOUTHCENTRAL.

&&

.MODEL DISCUSSION...

THE MODELS ARE IN CLOSE AGREEMENT ON THE LATEST EVOLUTION
REGARDING THE ANTICIPATED WIND EVENT ACROSS SOUTHCENTRAL TODAY
THROUGH FRIDAY. THE NAMS HIGH RESOLUTION MADE IT THE MODEL OF
CHOICE IN DETERMINING THE MAGNITUDE AND LOCATION OF THE STRONGEST
WINDS. THE NEXT CHALLENGE IN THE FORECAST IS ON SATURDAY AS THE
NEXT SHORTWAVE ROUNDS THE FLOW OF A PARENT LOW DIVING FROM THE
NORTHWEST COAST. MODELS ARE STRUGGLING WITH THE STRENGTH AND TRACK
OF THIS NEXT SYSTEM AS THE FIRST CLOSED LOW SLOWLY DRIFTS SOUTH
ACROSS THE EASTERN GULF. THE GFS IS THE CURRENT OUTLIER SHOWING
STRONGER PRECIPITATION INTENSITY...BUT TIMING IT ABOUT 6 HOURS
SLOWER THAN THE NAM AND EC.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (DAYS 1 AND 2)...
A COUPLE OF UPPER LEVEL SYSTEMS WILL CROSS THE AREA TODAY. THE
FIRST IS A STRONG VORTICITY MAXIMUM THAT WILL PUSH QUICKLY
THROUGH SOUTH CENTRAL ALASKA THIS MORNING. DOWN-SLOPING OFF OF
THE ALASKA RANGE WILL KEEP PRECIPITATION AT A MINIMUM AROUND COOK
INLET AND THE SUSITNA VALLEY...WITH POPS RANGING FROM ZERO UP TO
JUST SCATTERED SHOWERS. FURTHER EAST WILL HAVE MUCH BETTER CHANCES
OF PRECIPITATION...WITH LIKELY TO CATEGORICAL POPS OVER THE COPPER
RIVER BASIN THROUGH THE EASTERN PRINCE WILLIAM SOUND AREA. AS THE
VORT MAX SLIDES TOWARD THE CANADIAN BORDER THIS AFTERNOON A
SURFACE LOW WILL DEVELOP OVER THE NORTHEAST GULF OF ALASKA. THE
NEXT VORT MAX WILL PUSH INTO SOUTH CENTRAL THIS AFTERNOON AS THE
FLOW ALOFT TURNS MORE NORTHERLY. THIS SYSTEM WILL PRODUCE LITTLE
OR NO PRECIPITATION BUT BRINGS WITH IT A DECENT COLD FRONT. 850 MB
TEMPERATURES WILL DROP BELOW ZERO OVER MOST OF SOUTH CENTRAL
TONIGHT AS THE COLD FRONT PUSHES THROUGH. THE SURFACE GRADIENT
WILL ALSO INCREASE DRAMATICALLY THURSDAY NIGHT AS THE GULF LOW
DEEPENS AND HIGH PRESSURE BUILDS INTO CENTRAL ALASKA BEHIND THE
FRONT. THE SURFACE GRADIENT WILL SLACKEN FRIDAY AND FRIDAY NIGHT
AS THE GULF LOW SLIDES SOUTHEAST AND THE CENTRAL ALASKA SURFACE
HIGH WEAKENS AHEAD OF A SYSTEM MOVING INTO WESTERN ALASKA.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (DAYS 1 AND 2)...
WIDESPREAD SHOWER ACTIVITY WILL CONTINUE INTO THIS AFTERNOON
ACROSS THE SOUTHWEST MAINLAND...WITH STEADIER RAIN ACROSS THE
KUSKOKWIM VALLEY AND THE WESTERN SLOPES OF THE ALASKA RANGE AS A
PAIR OF VIGOROUS SHORTWAVES MOVES OVERHEAD IN RAPID SUCCESSION.
PRECIPITATION WILL THEN QUICKLY DIMINISH THIS EVENING AS THE
SECOND OF THESE WAVES DEPARTS INTO THE GULF OF ALASKA AND USHERS
IN BRISK NORTHWEST FLOW ON THE BACKSIDE OF THE DIGGING TROUGH.
THIS WILL PRODUCE GUSTY NORTHWEST WINDS AND ALSO CONTRIBUTE TO
COLDER TEMPERATURES TONIGHT AS OVERNIGHT LOWS DIP INTO THE 30S
ACROSS MUCH OF THE AREA BENEATH DECREASING CLOUD COVER. THE PERIOD
OF DRY WEATHER WILL BE SHORT LIVED AS ANOTHER WAVE MOVES OVER THE
TOP OF THE BERING RIDGE AND BRINGS ANOTHER ROUND OF RAINFALL TO
THE SOUTHWEST MAINLAND BY FRIDAY AFTERNOON.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (DAYS 1 AND 2)...
ASIDE FROM A FEW SHOWERS IN NORTHWEST FLOW ACROSS THE ALASKA
PENINSULA...THE BULK OF THE RAINFALL WILL REMAIN CONFINED TO THE
FAR WESTERN PARTS OF THE ALEUTIAN CHAIN THROUGH TONIGHT AS THE
NORTH PACIFIC RIDGE IS RE-ESTABLISHED OVER THE CENTRAL BERING SEA
YIELDING WIDESPREAD STRATUS AND FOG. RAIN WILL THEN GRADUALLY
SPREAD TO THE NORTHERN AND EASTERN BERING FRIDAY AND FRIDAY NIGHT
AS AN UPPER WAVE ROUNDS THE TOP OF THE RIDGE...WITH RAIN RETURNING
TO THE REST OF THE ALEUTIAN CHAIN BY THE WEEKEND AS THE RIDGE
BREAKS DOWN.

&&

.LONG TERM FORECAST (DAYS 3 THROUGH 7)...

BEGINNING THE EXTENDED PERIOD FRIDAY NIGHT...THE REGION WILL BE
UNDER THE INFLUENCE OF AN ELONGATED TROUGH EXTENDING FROM THE
NORTH SLOPE OF ALASKA TO THE COAST OF BRITISH COLUMBIA. THE
OUTFLOW WIND EVENT WILL BE WINDING DOWN FRIDAY NIGHT AS THE GULF
LOW CONTINUES TO WEAKEN AND DRIFT SOUTHEAST. EVENTUALLY WITH
TIME...A SHORTWAVE TROUGH IN NORTHWEST FLOW ORIGINATING FROM
EASTERN SIBERIA WILL MERGE WITH THE AFOREMENTIONED ARCTIC TROUGH
OVER THE NORTH SLOPE SATURDAY AND SUNDAY. THERE UNFORTUNATELY
REMAINS MUCH UNCERTAINTY WITH THE DEVELOPMENT OF THIS LOW AS IT
MOVES FROM THE NORTHERN BERING SEA TO THE GULF OF ALASKA. THE
UNCERTAINTY SEEMS TO LIE IN HOW MUCH SUBTROPICAL ENERGY OVER THE
WESTERN BERING MERGES WITH THE DIGGING TROUGH. THE GFS FAVORS A
STRONGER SOLUTION AND MORE INTACT MERGER...WHICH BRINGS MORE
PRECIPITATION INTO SOUTHWEST ALASKA FRIDAY NIGHT INTO
SATURDAY...AND EVENTUALLY INTO SOUTHCENTRAL SATURDAY AFTERNOON AND
EVENING. THE WEAKER SOLUTIONS SUCH AS THE ECMWF AND CANADIAN GDPS
FAVOR A FARTHER SOUTH TRACK OF THE LOW...AND WEAKER DEVELOPMENT IN
THE GULF OF ALASKA. FOR THIS FORECAST...A CONTINUED CONSERVATIVE
FORECAST WAS TAKEN FAVORING A WPC ENSEMBLE BLEND ALONG WITH SOME
ECMWF. THIS WILL KEEP LOWER END CHANCES FOR PRECIPITATION OVER
MAINLAND LOCATIONS...WITH HIGHER CHANCES REMAINING LARGELY OVER
THE BERING AND GULF OF ALASKA AS THE LOW SKIRTS THE REGION.
THEREAFTER...THERE IS HIGHER CONFIDENCE LEADING INTO MIDWEEK AS
SOUTHCENTRAL REMAINS UNDER THE INFLUENCE OF DRY AND COOLER
OFFSHORE FLOW...WITH A SLOWLY DEVELOPING FLAT RIDGE OVER THE
BERING. PRECIPITATION CHANCES INCREASE AGAIN TOWARDS THE MIDDLE TO
END OF NEXT WEEK...WITH A SLOW REBOUND IN TEMPERATURES AFTER THE
COOL PERIOD THIS WEEKEND AND EARLY NEXT WEEK.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...HIGH WIND WATCH 131.
MARINE...GALE WARNING 119 120 121 127 128 129 130 131 132 136 137
 150.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...JW/KH
SOUTHCENTRAL ALASKA...BL
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...CB
LONG TERM...JA

SPECIAL WEATHER STATEMENT
NATIONAL WEATHER SERVICE ANCHORAGE AK
1150 AM AKDT THU AUG 27 2015

AKZ171-281800-
KODIAK ISLAND-
INCLUDING THE CITY OF...KODIAK CITY
1150 AM AKDT THU AUG 27 2015

RE-SUSPENSION OF ASH FROM THE KATMAI VOLCANO POSSIBLE BY THURSDAY
EVENING THROUGH FRIDAY MORNING. NORTH TO NORTHWEST WINDS WILL
INCREASE TO 45 TO 60 MPH BY THIS EVENING AND COULD RE-SUSPEND ASH
AND BLOW IT INTO PARTS OF KODIAK ISLAND. COMMUNITIES ALONG
SHELIKOF STRAIT ARE THE MOST LIKELY TO BE AFFECTED...BUT ASH
COULD MAKE IT AS FAR AS AKHIOK. THE BIGGEST IMPACTS WILL BE
REDUCED VISIBILITIES AND POOR AIR QUALITY. THOSE WHO ARE MORE
SENSITIVE TO POOR AIR QUALITY SHOULD EXERCISE CAUTION AND CONSIDER
STAYING INDOORS.

MSO

SPECIAL WEATHER STATEMENT...CORRECTED
NATIONAL WEATHER SERVICE ANCHORAGE AK
1150 AM AKDT THU AUG 27 2015

CORRECTED TYPO

AKZ171-281800-
KODIAK ISLAND-
INCLUDING THE CITY OF...KODIAK CITY
1150 AM AKDT THU AUG 27 2015

RESUSPENSION OF ASH FROM THE KATMAI VOLCANO POSSIBLE BY THURSDAY
EVENING THROUGH FRIDAY MORNING. NORTH TO NORTHWEST WINDS WILL
INCREASE TO 45 TO 60 MPH BY THIS EVENING AND COULD RESUSPEND ASH
AND BLOW IT INTO PARTS OF KODIAK ISLAND. COMMUNITIES ALONG
SHELIKOF STRAIT ARE THE MOST LIKELY TO BE AFFECTED...BUT ASH COULD
MAKE IT AS FAR AS AKHIOK. THE BIGGEST IMPACTS WILL BE REDUCED
VISIBILITIES AND POOR AIR QUALITY. THOSE WHO ARE MORE SENSITIVE TO
POOR AIR QUALITY SHOULD EXERCISE CAUTION AND CONSIDER STAYING
INDOORS.

MSO

SPECIAL WEATHER STATEMENT...CORRECTED
NATIONAL WEATHER SERVICE ANCHORAGE AK
1150 AM AKDT THU AUG 27 2015

CORRECTED HEADLINE

AKZ171-281800-
KODIAK ISLAND-
INCLUDING THE CITY OF...KODIAK CITY
1150 AM AKDT THU AUG 27 2015

...ASH BEING RESUSPENDED FROM THE 1912 ERUPTION OF KATMAI VOLCANO...

NORTH TO NORTHWEST WINDS WILL INCREASE TO 45 TO 60 MPH BY THIS
EVENING. ASH FROM THE 1912 ERUPTION OF KATMAI VOLCANO HAS BEGUN TO
BLOW ACROSS SHELIKOF STRAIT AND ONTO PARTS OF KODIAK...AND WILL
CONTINUE THROUGH FRIDAY MORNING. COMMUNITIES ALONG SHELIKOF STRAIT
ARE THE MOST LIKELY TO BE AFFECTED...BUT ASH COULD MAKE IT AS FAR
AS AKHIOK. THE BIGGEST IMPACTS WILL BE REDUCED VISIBILITIES AND
POOR AIR QUALITY. THOSE WHO ARE MORE SENSITIVE TO POOR AIR QUALITY
SHOULD EXERCISE CAUTION AND CONSIDER STAYING INDOORS.

MSO/AMD