Small Craft Advisory 744, NWS Anchorage, AK

AREA FORECAST DISCUSSION
SOUTHCENTRAL AND SOUTHWEST ALASKA
100 PM AKDT FRI SEP 18 2009

.ANALYSIS AND UPPER LEVELS...LONG-WAVE TROUGH REMAINS CENTERED
OVER THE ALASKA REGION WITH A BLOCKING RIDGE JUST UPSTREAM OVER
EASTERN SIBERIA. THERE ARE TWO WEAK UPPER CENTERS EMBEDDED WITHIN
THE TROUGH. ONE LOW JUST SOUTH OF THE PRIBILOF ISLANDS IS SINKING
SOUTHWARD AS A SHORT-WAVE DROPPING DOWN THE EAST SIDE OF THE RIDGE
AMPLIFIES THE TROUGH SOUTHWARD ACROSS THE ALEUTIANS INTO THE NORTH
PACIFIC. THIS AMPLIFICATION IS ONE KEY TO EVENTUAL BOMBING OF
PACIFIC LOW WHICH IS CENTERED NEAR 44N 173W THIS MORNING...AND
IS RIDING THE UPPER JET EASTWARD. THE SECOND UPPER CENTER IS OVER
THE SOUTHERN GULF TRACKING EAST TOWARD SOUTHEAST ALASKA. SHOWERS
ARE FOCUSED IN THE VICINITY OF THESE LOWS. LARGE BREAKS IN THE
CLOUDS HAVE DEVELOPED OFF TO THE NORTH OVER MUCH OF SOUTHCENTRAL
AND INTERIOR SOUTHWEST ALASKA. ANOTHER NOTABLE FEATURE IS A
SHORT-WAVE OVER INTERIOR ALASKA DROPPING SOUTHWARD TOWARD
SOUTHCENTRAL...PRODUCING MAINLY HIGH CLOUDS OVER THE NORTHERN
TIER OF SOUTHCENTRAL RIGHT NOW.

.MODEL DISCUSSION...MODELS ARE IN FAIRLY GOOD AGREEMENT IN THE
SHORT-TERM...WITH THE EXCEPTION OF HANDLING OF INTERIOR SHORT-WAVE.
THE NAM IS AN OUTLIER IN AGRESSIVELY MOVING THIS SOUTHWARD INTO
SOUTHCENTRAL ON SATURDAY. FOR NOW WILL STICK WITH THE MODEL
CONSENSUS WHICH SWINGS THIS OFF TO THE EAST OVER THE YUKON.
THE NAM GFS/NAM/ECMWF SOLUTIONS ARE IN VERY GOOD AGREEMENT WITH
HANDLING OF STRONG PACIFIC LOW MOVING INTO THE GULF SUNDAY AND
INLAND OVER SOUTHCENTRAL MONDAY...SO CONFIDENCE IS MUCH HIGHER
THAN YESTERDAY AND MORE DETAIL WILL NOW BE ADDED TO THE FORECASTS.
CONFIDENCE REMAINS LOW WITH HANDLING OF REMNANTS OF TYPHOON
CHO-WAN AS IT TRACKS SOUTH OF ALEUTIANS EARLY TO MID WEEK AND UP
TOWARD ALASKA PENINSULA TOWARD THE END OF THE WEEK. ALTHOUGH
RUN-TO-RUN CONSISTENCY IS GETTING BETTER...THERE IS STILL NOT
ENOUGH CONSISTENCY IN TRACK OR STRENGTH TO BUY INTO ANY PARTICULAR
SOLUTION. SO WILL ONCE AGAIN STICK WITH CURRENT FORECAST WHICH
INDICATES PRESENCE OF ANOTHER STORM SYSTEM...BUT NOTHING NEAR
THE STRENGTH INDICATED BY LATEST MODEL SOLUTIONS.

.SHORT TERM FORECAST...

SOUTHCENTRAL ALASKA...THE UPPER LEVEL SHORT-WAVE OVER INTERIOR
ALASKA WILL MAINTAIN EXTENSIVE CLOUDS AND SHOWERS ALONG THE
ALASKA RANGE AS IT SKIRTS THE REGION ON ITS WAY TO THE YUKON.
THE REST OF SOUTHCENTRAL SHOULD REMAIN TRANQUIL THROUGH SATURDAY.
AS MENTIONED ABOVE...THE NAM SOLUTION TRACKS THIS FEATURE SOUTHWARD
INTO SOUTHCENTRAL SATURDAY...WHICH WOULD RESULT IN A CLOUDIER AND
WETTER FORECAST FOR THE REST OF SOUTHCENTRAL...SO WILL NEED TO
KEEP A CLOSE EYE ON THIS TO MAKE SURE THE NAM IS NOT CORRECT. A
MAJOR PACIFIC STORM WILL BEGIN TO IMPACT THE GULF SATURDAY NIGHT
AND THE REST OF SOUTHCENTRAL SUNDAY. BASED ON GOOD MODEL AGREEMENT
ON STRENGTH OF THIS SYSTEM...FORECAST WINDS WILL HAVE TO BE UPPED.
ALSO...LOOKS LIKE A GOOD SET-UP FOR A BARRIER JET AS THE OCCLUDED
FRONT MOVES TO THE NORTH GULF COAST. A GOOD SOUTHERLY JET WILL ALSO
HELP THIS FRONT MOVE INLAND...SO EXPECT GALE FORCE WINDS TO EXTEND
FROM CORDOVA THROUGH PRINCE WILLIAM SOUND AND PASSAGE CANAL TO
PORTAGE VALLEY. THIS WILL INITIALLY BE A GEOSTROPHIC WIND EVENT...SO
EXPECT MUCH LESS WIND ALONG WESTERN TURNAGAIN ARM. WITH STRONG
DYNAMICS AND A LOT OF TROPICAL MOISTURE ACCOMPANYING THIS
SYSTEM...RAINFALL WILL BE HEAVY AT TIMES ALONG THE GULF COAST.
INLAND AREAS WILL INITIALLY REMAIN MOSTLY DRY DUE TO
DOWNSLOPING...BUT AS THE LOW TRACKS INLAND MONDAY AND MONDAY NIGHT
EVERYWHERE SHOULD SEE SOME RAIN. LARGE PRESSURE RISES BEHIND
THE LOW WILL ALSO EXTEND GUSTY WINDS INLAND MONDAY NIGHT.

SOUTHWEST ALASKA...AS UPPER LOW OVER EASTERN BERING SINKS SOUTHWARD
TONIGHT REMAINING SHOWERS ALONG THE SOUTHWEST COAST WILL COME TO
AN END AND MOST AREAS WILL SEE AT LEAST PARTIAL CLEARING. AS STRONG
LOW MOVES INTO THE WESTERN GULF LATE SATURDAY AND SUNDAY NORTHERLY
WINDS WILL PICK UP ACROSS SOUTHWEST ALASKA AND PRECIPITATION WILL
BE MAINLY CONFINED TO EASTERN AREAS ALONG THE ALASKA AND ALEUTIAN
RANGES. AS THE GULF LOW MOVES INLAND OVER SOUTHCENTRAL A LARGE
UPPER LOW WILL DROP SOUTHWARD OUT OF THE ARCTIC...ADVECTING MUCH
COLDER AIR SOUTHWARD. MODELS HAVE BEEN INCONSISTENT IN DETAILS OF
WHERE THIS COLD AIR WILL GO...BUT CONFIDENCE IS GROWING IN THE
GENERAL IDEA. SO COLDER TEMPERATURES AND THE FIRST SNOW OF THE
YEAR ARE POSSIBLE OVER PORTIONS OF SOUTHWEST HEADING THROUGH THE
BEGINNING OF NEXT WEEK.

BERING SEA/ALEUTIANS...A BAND OF GUSTY NORTHERLIES WILL PERSIST
ACROSS THE CENTRAL BERING THROUGH SATURDAY NIGHT...WITH TIGHT
PRESSURE GRADIENT BETWEEN EASTERN BERING LOW AND SIBERIAN HIGH
AS WELL AS CONTRIBUTIONS FROM UPPER SHORT-WAVES DROPPING SOUTHWARD
ALONG THE EDGE OF THE RIDGE AND ASSOCIATED COLD ADVECTION. NORTHERLY
FLOW WILL LIGHTEN UP A BIT SUNDAY AND MONDAY AS UPPER TROUGH
CONTINUES TO DIG SOUTHWARD INTO THE PACIFIC. REMNANTS OF TYPHOON
CHO-WAN LOOK LIKE THEY WONT HAVE MUCH IMPACT ON ALEUTIANS AS IT
PASSES TO THE SOUTH MONDAY AND TUESDAY...BUT LATEST MODELS INDICATE
A QUICK NORTHWARD CURVATURE TOWARD THE ALASKA PENINSULA MID WEEK.
CONFIDENCE IN TRACK OF THIS IS LOW...SO WILL HAVE TO CONTINUE TO
MONITOR FUTURE MODEL RUNS.

.LONG TERM FORECAST...CONFIDENCE REMAINS LOW IN GENERAL THROUGH
THE EXTENDED FORECAST PERIOD. ONE OF THE BIG QUESTIONS IS EVOLUTION
OF LARGE UPPER LOW DROPPING OUT OF ARCTIC. LATEST MODEL RUNS ARE
DEPICTING ADVECTION OF SIGNIFICANTLY COLDER AIR INTO SOUTHWEST
ALASKA MONDAY NIGHT AND TUESDAY AND EASTWARD TO SOUTHCENTRAL TUESDAY
AS UPPER CENTER SETS UP OVER SOUTHWEST ALASKA...WHICH IF ACCURATE
COULD PRODUCE THE FIRST FLAKES OF THE SEASON ACROSS MUCH OF THE
REGION. UNCERTAINTY GROWS TOWARD THE END OF THE WEEK WITH HANDLING
OF THE STORM WHICH EVOLVES OUT OF THE REMNANTS OF TYPHOON CHO-WAN.
LATEST MODELS INDICATE THIS COULD BE A MAJOR STORM.

.AER/ALU...WATCH/WARNING SUMMARY
PUBLIC...NONE.
MARINE...GALE WARNING...132 150 155 185.
FIRE WEATHER...NONE.

SEB SEP 09