Small Craft Advisory 936, NWS Anchorage, AK

AREA FORECAST DISCUSSION
SOUTHCENTRAL AND SOUTHWEST ALASKA
1230 PM AKST WED NOV 26 2008

.ANALYSIS AND UPPER LEVELS...SATELLITE IMAGERY SHOWS TWO STRONG
LOWS OVER THE NORTH PACIFIC TRAVELING EASTWARD ALONG THE UPPER
JET THIS MORNING. ONE LOW IS SOUTH OF THE GULF TRACKING NORTHEAST
TOWARD SOUTHEAST ALASKA. THE SURFACE LOW...WHICH IS ON THE NORTH
SIDE OF THE UPPER CIRCULATION...HAS AN ESTIMATED CENTRAL PRESSURE
OF 979 MB AND IS BEGINNING TO SLOWLY WEAKEN. THE ARCTIC FRONT OVER
THE CENTRAL GULF HAS STARTED TO MOVE BACK NORTHWESTWARD AS A WARM
FRONT. OVERRUNNING PRECIPITATION IS ALREADY VISIBLE ON THE EASTERN
EDGE OF THE MIDDLETON ISLAND RADAR. A SECOND NEARLY VERTICALLY
STACKED LOW IS ABOUT 400 NM SOUTH OF ADAK IS TRACKING EAST-NORTHEAST.
ITS CENTRAL PRESSURE...WHICH IS STILL FALLING...IS APPROACHING
950 MB. MOISTURE AND WINDS ASSOCIATED WITH THIS SYSTEM ARE
PROGRESSING TOWARD THE ALEUTIANS. CONDITIONS BACK OVER MAINLAND
ALASKA ARE FAIRLY CALM THIS MORNING. THE UPPER TROUGH THAT MOVED
THROUGH SOUTHCENTRAL TUESDAY NIGHT IS NOW HEADED INTO INTERIOR
ALASKA. SNOW CONTINUES ALONG UPSLOPE AREAS OF THE ALASKA RANGE
WITH JUST A FEW SHOWERS LEFT OVER THE REST OF SOUTHCENTRAL.
FINALLY...A SURFACE LOW IS TRACKING EAST ACROSS THE NORTHERN
BERING SEA NEAR SAINT MATTHEW ISLAND THIS MORNING. THERE ARE NO
OBSERVATIONS AVAILABLE...BUT BASED ON SATELLITE IMAGERY AND MODEL
GUIDANCE EXPECT WINDS NEAR THE CENTER OF THE LOW RANGE FROM
25 TO 35 KT.

.MODEL DISCUSSION...MODELS ALL INITIALIZED WELL. IN THE SHORT-TERM
CONTINUE TO FAVOR THE NAM...ESPECIALLY WITH THE DETAILS OVER
SOUTHCENTRAL. THERE ARE SOME MINOR DIFFERENCES WITH TRACK OF LOW
MOVING INTO THE GULF TONIGHT. SINCE NAM IS A BIT FARTHER SOUTH
THAN THE REST OF THE SOLUTIONS WILL BLEND WITH THE GFS ON THIS
POSITION. THIS WILL HAVE LITTLE IMPACT ON THE FORECAST FARTHER
NORTH. ALL OF THE MODELS HAVE TRENDED SOUTHEAST WITH THE TRACK
OF THE MAJOR LOW CURRENTLY SOUTH OF THE ALEUTIANS...NOW BRINING
IT TO THE PRINCE WILLIAM SOUND REGION INSTEAD OF SOUTHERN COOK
INLET. BASED ON RECENT STORM TRACKS THIS TRACK MAKES MORE SENSE.
SO WILL UPDATE THE FORECAST TO REFLECT THIS CHANGE. MODEL AGREEMENT
AND CONSISTENCY HEADS QUICKLY DOWNHILL AFTER THIS SYSTEM MOVES
THROUGH. GFS TRACKS A STRONG LOW ACROSS THE BERING FRIDAY NIGHT
THROUGH SUNDAY WHILE THE UKMET/ECMWF INDICATE THAT THIS LOW WILL
DISSIPATE OVER THE WESTERN ALEUTIANS FRIDAY NIGHT. WITH THE NEXT
PACIFIC LOW TRACKING SOUTH OF ALEUTIANS THIS WEEKEND THE 12Z ECMWF
HAS COME IN WITH A RADICALLY DIFFERENT SOLUTION WHICH TRACKS THE
LOW NORTHWARD TOWARD THE CENTRAL ALEUTIANS. PLAN TO STICK CLOSE
TO THE CURRENT FORECAST WHICH WAS BASED OFF OF THE 00Z ECMWF.

.SHORT TERM FORECAST...

SOUTHCENTRAL ALASKA...SNOW ALONG THE ALASKA RANGE WILL TAPER OFF
THIS AFTERNOON AS LOW LEVEL FLOW GRADUALLY SHIFTS AROUND TO THE
NORTH AHEAD OF THE NEXT SYSTEM. MEANWHILE...OVERRUNNING PRECIPITATION
WILL SPREAD NORTH AND WEST ACROSS SOUTHCENTRAL ALASKA THIS AFTERNOON
THROUGH TONIGHT. A WEAK UPPER TROUGH OVER SOUTHWEST ALASKA WILL
ALSO SWING NORTHEASTWARD AND WEAKEN AS IT CROSSES SOUTHCENTRAL
LATE TONIGHT. THIS WILL PROVIDE SOME ADDITIONAL LIFT AND HELP TO
ENHANCE PRECIPITATION INTENSITY...ESPECIALLY OVER THE PRINCE WILLIAM
SOUND REGION. THERE WILL BE A SHARP DELINEATION OF THE WESTERN EDGE
OF SNOW...MAKING A TOUGH CALL FOR ANCHORAGE AND THE MATANUSKA VALLEY.
AT THIS POINT IT LOOKS LIKE THE MOISTURE WILL JUST MAKE IT OVER
THE CHUGACH AND THE UPPER TROUGH WILL PROVIDE SOME LIFT TO PRODUCE
A PERIOD OF SNOW WITH LIGHT ACCUMULATION. STEADY SNOW OVER PRINCE
WILLIAM SOUND AND THE COPPER BASIN WILL PRODUCE HIGHER SNOW AMOUNTS
IN THE 3 TO 6 INCH RANGE THROUGH THURSDAY MORNING. WARM AIR ADVECTION
WILL CHANGE PRECIPITATION TO RAIN ACROSS SOUTHEAST PRINCE WILLIAM
SOUND...BUT ALL OTHER AREAS SHOULD REMAIN SNOW. THE GULF LOW WILL
DISSIPATE BY THURSDAY AFTERNOON AND THE OCCLUDED FRONT FROM THE LOW
SOUTH OF THE ALEUTIANS WILL MOVE INTO THE GULF...QUICKLY SPREADING
MORE PRECIPITATION NORTHWARD. THE BULK OF THE PRECIPITATION WITH
THIS FRONT WILL FALL ALONG THE COAST DUE TO DOWNSLOPING ON THE LEE
OF THE COASTAL RANGE...BUT SHORT-WAVES ALOFT COMBINED WITH ABUNDANT
MOISTURE AND SOME OVERRUNNING WILL BRING A CHANCE OF PRECIPITATION
TO THE REST OF SOUTHCENTRAL. CONTINUED WARM AIR ADVECTION WILL
CHANGE ALL COASTAL AREAS TO RAIN BY FRIDAY WITH THE POTENTIAL FOR
SOME INLAND AREAS WARMING ENOUGH TO AT LEAST MIX WITH RAIN. THE
STORM IS NOW EXPECTED TO TRACK TO PRINCE WILLIAM SOUND FRIDAY NIGHT
WHILE AN UPPER TROUGH SWINGS NORTHEASTWARD TOWARD THE REGION AND
STRONG COLD ADVECTION ENSUES ON THE BACK SIDE OF THE SYSTEM. IF ALL
OF THIS COMES TOGETHER THERE COULD BE A SNOWSTORM IN STORE FOR MUCH
OF SOUTHCENTRAL(INCLUDING ANCHORAGE AND VICINITY) OVER THE WEEKEND.

SOUTHWEST ALASKA...THE LOW TRACKING ACROSS THE NORTHERN BERING MAY
CLIP THE KUSKOKWIM DELTA COAST WITH A LITTLE WIND AND SNOW TONIGHT.
OTHERWISE...NORTHERLY WINDS WILL INCREASE ACROSS THE REGION TONIGHT
THROUGH THURSDAY AS THE PACIFIC LOW TRACKS SOUTH OF THE ALASKA
PENINSULA. PRECIPITATION AHEAD OF THE OCCLUDED FRONT WILL BACK INTO
EASTERN PORTIONS OF SOUTHWEST ALASKA LATE THURSDAY THROUGH FRIDAY
MORNING. TEMPERATURES WILL REMAIN COLD ENOUGH FOR SNOW AND
ACCUMULATION WILL GENERALLY REMAIN LIGHT AS THE BULK OF THE LIFT
WITH THE SYSTEM STAYS TO THE EAST. AS THE LOW TRACKS TO PRINCE
WILLIAM SOUND FRIDAY AND FRIDAY NIGHT COLD AIR WILL ADVECT ACROSS
SOUTHWEST ALASKA...PRODUCING GUSTY NORTHWEST FLOW AND SNOW IN
UPSLOPE AREAS OF THE ALASKA AND ALEUTIAN RANGES. THE UPPER TROUGH
MAY HELP ENHANCE SNOWFALL OVER PORTIONS OF THE KUSKOKWIM DELTA AS
IT SWINGS NORTHEASTWARD FRIDAY NIGHT AND SATURDAY. OTHERWISE...EXPECT
A DRYING TREND BEHIND THIS TROUGH HEADING INTO THE WEEKEND.

BERING SEA/ALEUTIANS...THE ALEUTIANS AND ALASKA PENINSULA WILL
REMAIN NORTH OF THE PACIFIC LOW AND NORTH OF THE OCCLUDED FRONT
WITH LITTLE OR NO WARM ADVECTION. AS INDICATED BY THE COLD BAY
12Z SOUNDING THE SURFACE TEMPERATURE WAS NEAR FREEZING BUT THERE
IS A VERY STEEP LAPSE RATE WITH TEMPERATURES ABOVE THE SURFACE
WELL BELOW ZERO. THE VERTICAL PROFILE ALONG THE EASTERN TO CENTRAL
ALEUTIANS IS LIKELY A BIT WARMER...BUT STILL COLD ENOUGH TO PRODUCE
SNOW. THEREFORE...EXPECT PRECIPITATION TYPE TO BE SNOW ACROSS THE
REGION. MOISTURE WILL CONTINUE TO NUDGE NORTHWARD THIS AFTERNOON AND
TONIGHT AND WINDS WILL INCREASE TO 40 TO 60 MPH. THEREFORE...EXPECT
BLOWING SNOW TO REDUCE VISIBILITIES. IT DOES APPEAR THAT
PRECIPITATION WILL REMAIN GENERALLY LIGHT WITH THE STEADY AND
HEAVIER PRECIPITATION ALONG THE OCCLUDED FRONT REMAINING JUST
TO THE SOUTH OF THE ALEUTIANS AND ALASKA PENINSULA. THUS...DO NOT
EXPECT VISIBILITIES TO GET LOW ENOUGH TO WARRANT A BLIZZARD WARNING.
AS THE LOW TRACKS EASTWARD ON FRIDAY THE FLOW WILL SHIFT TO THE
NORTH AND COLD ADVECTION WILL BEGIN ACROSS ACROSS MUCH OF THE
BERING SEA...PRODUCING WIDESPREAD SNOW SHOWERS...ESPECIALLY ALONG
THE ALEUTIAN CHAIN AND ALASKA PENINSULA. EXPECT THE USUAL STRONGER
WIND GUSTS THROUGH CHANNELED AREAS. WINDS AND SNOW SHOWERS WILL
GRADUALLY DIMINISH FRIDAY AND FRIDAY NIGHT FROM WEST TO EAST.
MEANWHILE...THE NEXT STORM SYSTEM OR SYSTEMS WILL ENTER FROM THE
WEST FRIDAY AND SATURDAY...BUT THERE IS GREAT UNCERTAINTY IN THE
TIMING AND PLACEMENT OF THESE.

.LONG TERM FORECAST...THERE IS GREAT UNCERTAINTY HEADING INTO THE
EXTENDED PERIOD...SO HAVE LOW CONFIDENCE IN THE FORECAST. MEDIUM
RANGE MODELS ARE CONSISTENT IN INDICATING A MORE AMPLIFIED PATTERN
WITH RIDGING OVER WESTERN NORTH AMERICA AND A TROUGH OVER THE
BERING/NORTH PACIFIC. THIS WOULD LEAD TO A LESS ACTIVE PATTERN
THAN OF LATE.

.AER/ALU...WATCH/WARNING SUMMARY
PUBLIC...NONE.
MARINE...STORM WARNING...165 170 172.
         GALE WARNING...120 130 140 150 160 165 171 175 176 180.
         HEAVY FREEZING SPRAY WARNING...130 160 180 185.
FIRE WEATHER...NONE.

SEB NOV 08