Small Craft Advisory 666, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
426 PM AKDT MON AUG 24 2015

.ANALYSIS AND UPPER LEVELS...
AN AMPLIFIED RIDGE STRETCHING FROM THE WESTERN BERING SEA THROUGH
CENTRAL ALASKA IS THE DOMINANT FEATURE IN THE CURRENT WEATHER
PATTERN. A RAPIDLY WEAKENING LOW IN THE GULF OF ALASKA THAT WAS
RESPONSIBLE FOR A RAINY PATTERN THE LAST FEW DAYS IN THE PRINCE
WILLIAM SOUND AND COPPER RIVER BASIN REGION HAS EXITED THE GULF
TO THE SOUTHEAST. RISING HEIGHTS AND OFFSHORE FLOW ACROSS THE
SOUTHERN MAINLAND IN ADVANCE OF THE UPSTREAM RIDGE ARE PRODUCING
MOSTLY CLEAR SKIES FROM THE ALASKA RANGE EAST THROUGH THE
WRANGELLS...WITH THE EXCEPTION OF A FEW BANDS OF HIGH CLOUDS
SPILLING OVER THE RIDGE AXIS.

FURTHER WEST...A SUBTROPICAL MOISTURE STREAM ASSOCIATED WITH AN
ELONGATED LOW OFF THE KAMCHATKA COAST IS VISIBLE ON SATELLITE WITH
A DEFORMATION ZONE OVER THE NORTHERN BERING SEA. THIS STREAM IS BRINGING
SHOWERS TO THE WESTERN ALEUTIANS AND OVERALL CLOUDY CONDITIONS TO
THE WESTERN MAINLAND AS THE SUBTROPICAL MOISTURE WRAPS AROUND THE
RIDGE AXIS. OTHERWISE...THE CENTRAL ALEUTIANS AND THE ALASKA PENINSULA
ARE UNDER THE INFLUENCE OF STRONG HIGH PRESSURE WITH A STABLE MARINE
LAYER NEAR THE SURFACE KEEPING THE AREA ENTRENCHED IN LOW LEVEL CLOUD
COVER.

&&

.MODEL DISCUSSION...
MODELS ARE IN FAIRLY GOOD AGREEMENT SYNOPTICALLY...HOWEVER THERE
ARE SOME DISCREPANCIES WITH THE TIMING OF THE PRECIPITATION MOVING
INTO THE MAINLAND IN THE MIDDLE OF THE WEEK. THE EC HAS BEEN THE
SLOWER SOLUTION...BUT THE 12Z RUN HAS COME INTO BETTER AGREEMENT
WITH THE SLIGHTLY FASTER GFS/NAM SOLUTIONS. THE NEW RUN IS SHOWING
A MORE ZONAL FLOW PATTERN OVER THE BERING SEA THAN THE PREVIOUS
RUNS...WHICH WOULD BRING THE RAIN INTO THE MAINLAND SLIGHTLY
FASTER THAN THE PREVIOUS MODEL RUNS INDICATED. WITH THE LATEST
TRENDS TOWARDS A FASTER SOLUTION AND A STRONG ZONAL SUBTROPICAL
JET SUPPORTING THE RAPID TRANSPORT OF MOISTURE EASTWARD OVER THE
BERING...THE NAM WAS THE PREFERRED MODEL FOR THIS SYSTEM.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (DAYS 1 AND 2)...
INCREASING CLOUDS WILL BE SEEN ACROSS SOUTHCENTRAL ALASKA OVERNIGHT
TONIGHT AS A WEAK SHORTWAVE PROPAGATES ALONG THE EASTERN PERIPHERY
OF THE BUILDING HIGH PRESSURE ACROSS THE AREA. WATER VAPOR IMAGERY
SHOWS THIS FEATURE WELL THIS AFTERNOON AND BASED ON SATELLITE
TRENDS...CLOUDS HAVE ALREADY ENCOMPASSED MUCH OF SOUTHWEST ALASKA.
THIS WILL LEAD TO A SLIGHTLY WARMER OVERNIGHT PERIOD AS CLOUDS
PREVENT SIGNIFICANT RADIATIONAL COOLING. THE AFOREMENTIONED RIDGE
WILL SKIRT ACROSS THE SOUTHERN HALF OF SOUTHCENTRAL ALASKA ON
TUESDAY AND BRING YET ANOTHER NICE AFTERNOON. HOWEVER...THE SAME
CAN NOT BE SAID ABOUT THE COPPER RIVER BASIN AND SUSITNA VALLEY AS
A WEAK WAVE ON THE BACK SIDE OF THE QUICKLY DEPARTING RIDGE LOOKS
TO KEEP CLOUDS AND THE POSSIBILITY OF RAIN OVER THESE LOCATIONS.

HEADING INTO TUESDAY NIGHT IS WHERE THE FORECAST GETS A BIT
COMPLICATED. MODELS HAVE RECENTLY COME INTO BETTER AGREEMENT WITH
THE RIDGE OVER THE BERING SEA/SOUTHERN ALASKA MAINLAND FLATTENING
OUT. THIS CREATES QUITE THE ABRUPT TRANSITION FROM HIGH PRESSURE
IN THE NORTH PACIFIC AND ARCTIC LOW PRESSURE OVER THE INTERIOR OF
ALASKA. A ZONAL JET WILL DEVELOP BETWEEN THESE TWO FEATURES THAT
WILL ALLOW MULTIPLE UPPER LEVEL DISTURBANCES TO MOVE ACROSS
SOUTHCENTRAL ALASKA AS EARLY AS EARLY WEDNESDAY MORNING. BY MIDDAY
ON WEDNESDAY...IT APPEARS RAIN WILL HAVE PUSHED INTO THE ANCHORAGE
AREA AND CONTINUE TO OVERSPREAD SOUTHCENTRAL ALASKA. AS MENTIONED
EARLIER...THERE ARE SOME MODEL DISCREPANCIES ON THE TIMING OF THE
RIDGE FLATTENING OUT...WHICH WILL CHANGE THE ONSET TIME OF
PRECIPITATION. NONETHELESS...WEDNESDAY AND WEDNESDAY NIGHT APPEAR
TO BE QUITE RAINY ACROSS SOUTHCENTRAL ALASKA.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (DAYS 1 AND 2)...
A STREAM OF MOISTURE RICH AIR WILL MOVE AROUND THE BERING RIDGE
AND BRING SHOWERS TO THE AREA TONIGHT. SHOWERS WILL BEGIN TO
CHANGE TO STEADY RAIN FOR THE DELTA ON TUE...AND WILL THEN SPREAD
TO THE ALASKA AND ALEUTIAN RANGES TUE NIGHT INTO WED. RAIN WILL BE
ACCOMPANIED BY WEST WIND...GUSTY AT TIMES.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (DAYS 1 AND 2)...
AS THE RIDGE OVER THE BERING MIGRATES TO THE GULF...THE STREAM OF
RAIN AND GALE FORCE WINDS OVER THE CENTRAL ALEUTIANS BRIEFLY
WEAKENS LATER TONIGHT INTO TUE. THE SHORT WAVE SWEEPING ACROSS THE
NORTHERN BERING WILL ENHANCE THE WINDS AND RAIN TUE AFTERNOON
THROUGH WED AS IT MOVES ACROSS THE BERING AND PRIBILOFS. THE
EASTERN ALEUTIANS WILL EXPERIENCE THE RIDGE WITH PARTIAL CLEARING
DOWNWIND OF THE ISLANDS. BECAUSE OF THE INTENSE WARM ADVECTION
OVER THE BERING WATERS...WIDESPREAD FOG IS EXPECTED OVER MUCH OF
THE CHAIN AND BERING THROUGH WED.

&&

.LONG TERM FORECAST (DAYS 3 THROUGH 7)...
BEGINNING THE EXTENDED PERIOD WEDNESDAY EVENING...SOUTHERN ALASKA
AND THE BERING SEA WILL BE SANDWICHED IN BETWEEN A VERY STRONG
NORTH CENTRAL PACIFIC RIDGE AND AN ARCTIC TROUGH OVER THE BEAUFORT
SEA. THE RIDGE WILL BE ENHANCED BY THE LEFTOVER REMNANTS OF FORMER
TYPHOON ATSANI. THE COMBINATION OF THESE TWO FEATURES WILL RESULT
IN AN ENHANCED BAROCLINIC ZONE AND ASSOCIATED UPPER LEVEL JET
STREAK. THE FIRST OF THESE SHORTWAVE TROUGHS WILL BE MOVING EAST
TOWARD THE YUKON WEDNESDAY EVENING. WHILE THERE REMAINS SOME
UNCERTAINTY ON TIMING...ALL THE GLOBAL DETERMINISTIC MODELS ARE
STRONGLY SUGGESTING THE DEVELOPMENT OF TWO MORE ROBUST SHORTWAVE
TROUGHS IN NORTHWEST FLOW RAPIDLY MOVING THROUGH SOUTHERN ALASKA
INTO THE GULF OF ALASKA. THESE WILL BE AIDED BY THE AFOREMENTIONED
ARCTIC TROUGH...AND THIS WILL LEAD TO THE FIRST ROBUST OUTFLOW
WIND EVENT OF THE EARLY FALL SEASON THURSDAY INTO FRIDAY. SHOWERS
AND RAIN WILL GRADUALLY END THURSDAY NIGHT AS THE
OUTFLOW/DOWNSLOPE DRYING COMMENCES IN EARNEST AS THE GULF LOW
STRENGTHENS. UNCERTAINTY REGARDING THE BREAKDOWN OF THE UPPER
LEVEL RIDGE GROWS BY EARLY WEEKEND AS BOTH THE GFS AND ECMWF HINT
AT ANOTHER STRONG SHORTWAVE TROUGH TRACKING THROUGH SOUTHCENTRAL.
THIS WOULD BRING AN END TO THE DRY OFFSHORE FLOW SOMETIME SATURDAY
OR SUNDAY...BUT RIGHT NOW THERE IS LOWER CONFIDENCE ON TIMING OF
THAT SYSTEM.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...GALE WARNING 175 177 179 181 185.
FIRE WEATHER...NONE.

&&

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