Small Craft Advisory 757, NWS Anchorage, AK

AREA FORECAST DISCUSSION
SOUTHCENTRAL AND SOUTHWEST ALASKA
140 PM AKDT MON SEP 21 2009

.ANALYSIS AND UPPER LEVELS...MAJOR FEATURES OF INTEREST TODAY
INCLUDE A DEEP LOW SOUTHWEST OF THE ALEUTIANS (FORMERLY TYPHOON
CHOI-WAN) WHICH IS TRACKING EASTWARD ALONG THE SUBTROPICAL JET.
AN 18Z QUIKSCAT WIND IMAGE SHOWS A BAND OF GALE TO STORM FORCE
WINDS NORTH OF THE LOW CENTER...WHICH WILL TRANSLATE EASTWARD
WITH THE LOW. JUST DOWNSTREAM FROM THIS STORM A SHORT-WAVE RIDGE
IS MAINTAINING TRANQUIL CONDITIONS OVER THE CENTRAL TO EASTERN
BERING AND ALEUTIANS. HEADING FURTHER DOWNSTREAM...A CLOSED LOW
IS DROPPING SOUTHWARD ACROSS WESTERN ALASKA WHILE A WEAKENING
LOW OVER THE NORTHERN GULF LIFTS NORTHWARD TOWARD SOUTHCENTRAL.
COLD ADVECTION ASSOCIATED WITH THE UPPER LOW IS HELPING TO KICK
UP WINDS ALONG THE SOUTHWEST COAST. SCATTERED SHOWERS ARE IN THE
FORM OF RAIN...ALTHOUGH THAT WILL CHANGE BY TONIGHT AS THE AIR
BECOMES COLD ENOUGH TO GENERATE SNOW. GALE FORCE WINDS CONTINUE
ON THE EAST SIDE OF THE NORTH GULF LOW...WITH SOME ENHANCEMENT
FROM A BARRIER JET ALONG THE IMMEDIATE COAST...BUT THESE WINDS
WILL DIMINISH QUICKLY AS THE LOW MOVES INLAND THIS EVENING AND
WINDS VEER TOWARD THE SOUTH.

.MODEL DISCUSSION...MODELS ARE IN GOOD AGREEMENT THROUGH WEDNESDAY
ACROSS THE ANCHORAGE FORECAST AREA. AFTER THAT SOLUTIONS BEGIN TO
DIVERGE WITH TRACK OF ALEUTIAN STORM AND DISPLAY POOR RUN-TO-RUN
CONSISTENCY. MODEL STORM TRACKS HAVE RANGED FROM BRISTOL BAY TO
THE WESTERN GULF...WHICH OF COURSE MAKES A LARGE DIFFERENCE IN
THE FORECAST. ALL OF THE 12Z MODELS DO INDICATE A TRACK SOUTH OF
THE ALASKA PENINSULA AND THERE IS SUPPORT FOR THIS FROM THE GFS
ENSEMBLES. THEREFORE...WILL LEAN TOWARD THIS MORE EASTERLY TRACK.

.SHORT TERM FORECAST...

SOUTHCENTRAL ALASKA...BASED ON CURRENT LOW POSITION WEST OF
MIDDLETON ISLAND AND NORTHWARD DRIFT...LOW WILL TRACK ACROSS
WESTERN PRINCE WILLIAM SOUND AND THEN INLAND OVERNIGHT.
MEANWHILE...AS THE UPPER LOW CONTINUES TO DROP SOUTHWARD INTO
SOUTHWEST ALASKA A SHORT-WAVE WILL ROTATE EASTWARD TOWARD
SOUTHCENTRAL...HELPING TO ENHANCE PRECIPITATION ACROSS THE
REGION. ADDITIONAL SHORT-WAVES TIGHT ON THE PERIPHERY OF THE
UPPER LOW WILL CONTINUE TO ROTATE ACROSS SOUTHCENTRAL ON TUESDAY
BEFORE HEADING INTO THE INTERIOR TUESDAY NIGHT INTO WEDNESDAY.
MEANWHILE...COLDER AIR WILL FILTER INTO SOUTHCENTRAL. THE PROBLEM
OF THE DAY IS DETERMINING PRECIPITATION TYPE AS THE COLDER AIR
MOVES IN. BASED ON MODEL 1000-850MB THICKNESSES...WHICH REMAIN
MOSTLY ABOVE 1300M ACROSS MOST OF SOUTHCENTRAL THROUGH TUESDAY
MORNING...PRECIPITATION WILL REMAIN RAIN NEAR SEA LEVEL. IT DOES
APPEAR THAT THE UPPER SHORT-WAVES WILL PRODUCE A SIGNIFICANT
AMOUNT OF DYNAMIC LIFT...SO THERE MAY BE BANDS OF HEAVY
PRECIPITATION. PRECIPITATION WILL ALSO BE ENHANCED ALONG THE
MOUNTAINS FROM ANCHORAGE UP TO THE NORTHERN SUSITNA VALLEY LATE
TONIGHT AND TUESDAY AS SOUTH TO SOUTHWEST LOW LEVEL FLOW DEVELOPS
BEHIND THE EXITING SURFACE LOW. HEAVIER PRECIPITATION WILL HELP
COOL THE COLUMN AND LOWER THE SNOW LEVEL. THEREFORE...MINOR SNOW
ACCUMULATION IS LIKELY OVER HIGHER ELEVATIONS OF SOUTHCENTRAL.
AREAS NEAR SEA LEVEL MAY SEE A PERIOD OF SNOW MIXED IN WITH THE
RAIN IN HEAVIER BANDS OF PRECIPITATION...BUT DONT EXPECT IT TO
CHANGE TO ALL SNOW. THE GREATEST POTENTIAL FOR ACCUMULATION IS THE
NORTHERN SUSITNA VALLEY WHICH WILL SEE THE COLDEST AIR AND WHERE
PRECIPITATION WILL LAST THE LONGEST. SHOWERS WILL LINGER ACROSS
THE REGION THROUGH TUESDAY NIGHT...AND WITH COLDER AIR CONTINUING
TO MOVE IN...SOME OF THESE COULD BE IN THE FORM OF SNOW. THERE WILL
THEN BE A BRIEF BREAK WEDNESDAY BEFORE THE ALEUTIAN SYSTEM BEGINS
TO AFFECT SOUTHCENTRAL...QUICKLY ADVECTING WARMER AIR BACK INTO
THE REGION.

SOUTHWEST ALASKA...AS THE UPPER LOW MOVES TO SOUTHWEST ALASKA
TONIGHT THE AIRMASS WILL CONTINUE TO COOL...CHANGING SHOWERS
FROM RAIN TO SNOW. THERE IS LESS MOISTURE TO WORK WITH OVER
SOUTHWEST ALASKA COMPARED WITH SOUTHCENTRAL...AND THE BEST
LIFT WILL BE DOWNSTREAM OF THE UPPER LOW...SO PRECIPITATION
WILL REMAIN PREDOMINANTLY SHOWERY. THERE WILL BE SOME ENHANCED
UPSLOPE SNOW IN WESTERLY FLOW ALONG THE ALASKA RANGE AND OTHER
SIMILARLY ORIENTED MOUNTAINS. THESE SHOWERS WILL CONTINUE TUESDAY
ALONG THE SOUTHERN PERIPHERY OF THE UPPER LOW...PRIMARILY FROM
THE KUSKOKWIM DELTA TO KUSKOKWIM VALLEY...AND THEN TAPER OFF
TUESDAY NIGHT AS THE UPPER LOW LIFTS BACK NORTHWARD. THERE IS
STILL SOME UNCERTAINTY IN THE TRACK OF THE ALEUTIAN LOW...BUT
AT THIS POINT IT LOOKS LIKE IT WILL TRACK JUST SOUTH OF THE
ALASKA PENINSULA...WHICH WOULD SPREAD RAIN AND WIND NORTH AND
WEST ACROSS SOUTHWEST ALASKA ON THURSDAY.

BERING SEA/ALEUTIANS...THE STRONG ALEUTIAN LOW WILL TRACK ALMOST
DUE EAST...REMAINING JUST SOUTH OF THE ALEUTIANS...BUT CLOSE
ENOUGH TO SPREAD GALES AND RAIN ALONG THE ENTIRE CHAIN ALONG
THE WAY. AS THE SYSTEM MOVES SOUTH OF THE ALASKA PENINSULA THURSDAY
A COUPLE SHORT-WAVES WILL DROP SOUTHWARD ACROSS THE BERING...HELPING
TO ADVECT COLDER AIR SOUTHWARD AND INTO THE SYSTEM AND MAINTAIN
STRONG AND GUSTY NORTHERLIES ACROSS THE CENTRAL TO EASTERN BERING.

.LONG TERM FORECAST...ASSUMING THE ALEUTIAN LOW DOES IN FACT TRACK
INTO THE GULF AT THE END OF THE WEEK SOUTHCENTRAL WILL REMAIN WET
THROUGH THE WEEKEND AS THIS SYSTEM DISSIPATES. THIS TRACK ALSO
MEANS THAT COLDER AIR WILL LIKELY MOVE BACK IN BEHIND IT SUNDAY
INTO MONDAY. MEANWHILE...ANOTHER STORM SYSTEM WILL MOVE TO THE
ALEUTIANS OVER THE WEEKEND...ALTHOUGH THE DETAILS ON POSITION
AND STRENGTH ARE VERY MUCH IN QUESTION AT THIS POINT IN TIME.

.AER/ALU...WATCH/WARNING SUMMARY
PUBLIC...NONE.
MARINE...STORM WARNING...176.
         GALE WARNING...170 172 175 180.
FIRE WEATHER...NONE.

SEB SEP 09