Small Craft Advisory 752, NWS Anchorage, AK

AREA FORECAST DISCUSSION
SOUTHCENTRAL AND SOUTHWEST ALASKA
500 PM AKDT SUN SEP 20 2009

.MODEL DISCUSSION...

THE GFS...GEM...UKMET...SREF...AND EUROPEAN ARE ALL IN GOOD AGREEMENT
IN TIMING AND PLACEMENT OF THE LOW PRESSURE MOVING ONSHORE OF THE
GULF MONDAY MORNING...WITH THE NAM CONTINUING TO BE A SLOW OUTLIER
AND GENERALLY DISCARDED...LIKELY DUE TO UNREASONABLE AMPLIFICATION OF
A SYSTEM DOWNSTREAM OVER THE NORTH AMERICAN CONTINENT.

ON THE WEST SIDE...ATTENTION IS TURNED TOWARDS A STRONG STORM-FORCE
LOW THAT WAS ONCE SUPER TYPHOON CHOI-WAN. MODELS SEEM TO BE DOING A
DECENT JOB WITH TIMING AND PLACEMENT OF THIS SYSTEM...WITH A 12Z GFS
AND 00Z EUROPEAN BLEND USED THROUGH THE PERIOD FOR ITS SLIGHTLY
FARTHER SOUTH TRACK FROM THE ALEUTIAN CHAIN. AFTER THAT...DETAILS
BECOMING A LITTLE LESS CERTAIN. HOWEVER...WITH STRONG COLD ADVECTION
IN NORTHERLY FLOW ACROSS THE BERING THE CURRENT THINKING IS THAT THIS
LOW WILL BE DRAWN NORTHWARD INTO BRISTOL BAY LATE THURSDAY OR FRIDAY
AS AN UPPER LOW DIGS AND STRENGTHENS TO ITS IMMEDIATE NORTHWEST.

.ANALYSIS AND UPPER LEVELS...

A 978 MB LOW LOCATED UNDER A POTENT MID-LEVEL LOW IS CONTINUING TO
MOVE NORTH TOWARDS EVENTUAL LANDFALL LATE MONDAY MORNING NEAR SEWARD
ON THE KENAI PENINSULA. MEANWHILE...A RAPIDLY MOVING 530 DM UPPER LOW
IS HEADING RAPIDLY FOR THE CHUKCHI SEA FROM THE ARCTIC OCEAN ON ITS
EVENTUAL DESTINATION TO SOUTHWEST AND THEN SOUTHCENTRAL ALASKA OVER
THE COMING DAYS. A SERIES OF SHORTWAVES IS ROTATING AROUND THE
SOUTHWEST SIDE OF THE MAIN UPPER LOW ASSOCIATED WITH THE GULF
LOW...BUT THESE WILL ROTATE AROUND IN TO SOUTHEAST ALASKA AND WILL
NOT PLAY A MAJOR ROLE ON THE WEATHER EXCEPT IN OUR OFFSHORE GULF
WATERS. THE OTHER SIGNIFICANT FEATURE TO MENTION IS THE LOW THAT WAS
ONCE SUPER TYPHOON CHOI-WAN...AND THIS FEATURE IS STILL SEVERAL
HUNDRED MILES SOUTHWEST OF THE WESTERN ALEUTIANS. IT WILL BEGIN TO
AFFECT THE WESTERN ALEUTIANS DURING THE DAY MONDAY WITH STRONG WINDS
AS A HIGH PRESSURE RIDGES DOWN FROM THE ARCTIC.

.SHORT TERM FORECAST...

SOUTH AND EAST OF THE ALASKA RANGE...THE BIGGEST PROBLEM IN THE SHORT
TERM WILL BE DETERMINING JUST HOW STRONG THE BARRIER JET WILL BE THAT
WILL AFFECT THE NORTHERN GULF COAST. CURRENT THINKING RIGHT NOW IS
THAT STRONGEST WINDS ASSOCIATED WITH THIS FEATURE WILL OCCUR EARLY
MONDAY WITH WINDS AT STRONG GALE FORCE WITH GUSTS TO STORM FORCE
CERTAINLY NOT OUT OF THE REALM OF POSSIBILITY. MOST OF THIS REGION
WILL BEGIN TO EXPERIENCE RAINFALL BY LATE THIS AFTERNOON...INCLUDING
THE SUSITNA VALLEY AND COPPER RIVER BASIN. HEAVIEST RAINFALL WILL BE
FROM THE EASTERN KENAI PENINSULA INTO PRINCE WILLIAM SOUND...WHERE
TOPOGRAPHY AND MOISTURE ADVECTION WILL INDUCE FAIRLY HEAVY
PRECIPITATION. LESSER AMOUNTS OF RAIN WILL FALL ON THE LEE SIDE OF
THE MOUNTAINS...SUCH AS IN ANCHORAGE...AT LEAST UNTIL THE UPPER
TROUGH THAT IS DIVING SOUTH FROM THE ARCTIC OCEAN TOWARDS SOUTHWEST
ALASKA IS ABLE TO INDUCE LIFT FROM A SHORTWAVE ROUNDING THE BASE OF
THE TROUGH MONDAY EVENING. IT APPEARS THE FIRST SNOWFALL OF THE
SEASON WILL OCCUR SOMETIME TUESDAY MORNING AS STRONG COLD ADVECTION
COMBINES WITH MORE THAN ADEQUATE LIFT FROM ANOTHER DISTURBANCE
ROUNDING THE BASE OF THE TROUGH AND UPSLOPE FLOW TO PRODUCE SNOW AND
SNOW SHOWERS. WITH 1000-850 MB THICKNESSES FORECASTED TO CRASH INTO
THE 1290S BY 18Z TUESDAY AS DECENT FORCING FOR ASCENT IS MOVING
ACROSS THE AREA...CONFIDENCE IS FAIRLY HIGH THAT TERMINATION DUST
WILL FALL...WITH SNOW SHOWERS LIKELY EVEN DOWN TO SEA LEVEL FOR A
BRIEF PERIOD OF TIME BEFORE PRECIPITATION LIFTS OUT TO THE NORTH. BY
TUESDAY NIGHT AND WEDNESDAY WEAK HIGH PRESSURE BUILDS ACROSS THE
REGION BRINGING A PERIOD OF TRANQUIL WEATHER BEFORE THE NEXT SYSTEM
BEGINS TO APPROACH FROM THE SOUTH AND WEST ON THURSDAY.

SOUTH AND WEST OF THE ALASKA RANGE...SHOWERS WILL CONTINUE ALONG THE
ALASKA RANGE...WITH RELATIVELY BENIGN WEATHER UNTIL MONDAY MORNING.
THE EXCEPTION TO THIS MAY BE ALONG THE BORDER OF THE KUSKOKWIM DELTA
ZONE AND THE BRISTOL BAY ZONE...WHERE THERE SHOULD BE JUST ENOUGH
INSTABILITY AND LIFT FROM MID LEVEL DIVERGENCE TO GENERATE SOME
ISOLATED THUNDERSTORMS THIS EVENING. POP CHANCES SHOULD INCREASE
SIGNIFICANTLY TUESDAY WITH THE FIRST OF SEVERAL SHORTWAVES ASSOCIATED
WITH THE DIGGING UPPER LOW MOVING FROM THE NORTH INTO SOUTHWEST
ALASKA. THIS SHOULD RESULT IN PRECIPITATION BREAKING OUT FROM
NORTHWEST TO SOUTHEAST...WITH MOSTLY SNOW IN THE HIGHER ELEVATIONS
AND NORTHERN PARTS OF THE KUSKOKWIM DELTA REGION TRANSITIONING TO
RAIN ELSEWHERE. BY MONDAY NIGHT...HOWEVER...ENOUGH COLD AIR SHOULD
SPILL ACROSS THE REGION THAT TEMPERATURES WILL BE COLD ENOUGH TO
SUPPORT SNOW SHOWERS THROUGH THE ENTIRE MAINLAND REGION AT LEAST
UNTIL LATE TUESDAY AFTERNOON OR EVENING.

BERING SEA/ALEUTIANS...THE REMNANTS OF SUPER TYPHOON CHOI-WAN WILL
MOVE IN TO THE AREA OF RESPONSIBILITY DURING THE DAY
MONDAY...BRINGING GALE TO STORM FORCE WINDS AND RAIN TO THE WESTERN
ALEUTIANS THROUGH MONDAY NIGHT. THIS SYSTEM WILL THEN STAY A COUPLE
OF HUNDRED MILES SOUTH OF THE ALEUTIANS AS IT HEADS EASTWARD TO A
LOCATION WELL SOUTH OF UNALASKA ISLAND BY WEDNESDAY AFTERNOON...BUT
WITH HIGH PRESSURE RIDGING FROM THE NORTH AND THEN WEST CREATING A
FAIRLY STRONG PRESSURE GRADIENT...GALE FORCE WINDS ARE NEARLY CERTAIN
ON THE NORTH AND THEN WEST SIDE OF THIS LOW IN THE ALEUTIANS AND
SOUTHERN BERING SEA THROUGH MIDWEEK.

.LONG TERM FORECAST...

CONFIDENCE IS INCREASING SOMEWHAT THAT THE LOW AFFECTING THE
ALEUTIANS THIS WEEK WILL BE DRAWN NORTHWARD INTO BRISTOL BAY LATE
THURSDAY OR FRIDAY BY A DEEPENING UPPER LOW TO THE NORTHWEST THAT
WILL BE USHERING COLD AIR ON ITS BACKSIDE DOWN THROUGH THE BERING
SEA. ATTENTION WILL THEN LIKELY NEED TO BE FOCUSED ON THE GULF FOR
THE POSSIBILITY OF ANOTHER STRONG FRONT MOVING NORTH TOWARDS THE
COAST FOR SOMETIME NEXT WEEKEND. MEANWHILE...THE NEXT IN A SEEMINGLY
NEVERENDING PARADE OF STORMS APPEARS POSSIBLE FOR THE BERING SEA AND
ALEUTIANS BY NEXT WEEKEND.

.AER/ALU...WATCH/WARNING SUMMARY
PUBLIC...NONE.
MARINE...GALE WARNING...120 125 130 132 172 175
         STORM WARNING...176
FIRE WEATHER...NONE.

CC SEP 09