Small Craft Advisory 718, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
200 PM AKDT WED SEP 18 2013

.ANALYSIS AND UPPER LEVELS...
A LOOK AT THE MORNING WEATHER MAP CONTINUES TO SHOW A DEEP UPPER
TROUGH ENCOMPASSING MOST OF MAINLAND ALASKA. THE UPPER TROUGH THAT
BROUGHT RAIN TO PRINCE WILLIAM SOUND AND THE COPPER RIVER BASIN HAS
NOW WEAKENED AND DRIFTED INTO THE INTERIOR...WHILE A COLD UPPER
TROUGH EXTENDED DOWN FROM AN UPPER LOW OVER NORTHWEST ALASKA IS
SWINGING THROUGH SOUTHCENTRAL. LIFT ASSOCIATED WITH THIS TROUGH
RESULTED IN A PERIOD OF STEADY RAIN FORMING OVER EAST ANCHORAGE AND
THE MAT-SU VALLEY THIS MORNING...AND IS NOW BEING REPLACED
INSTABILITY SHOWERS FORMING OVER COOK INLET AND MOVING TO THE
NORTHEAST. SNOW CONTINUES TO FALL ABOVE 2000-2500 FT IN THE NORTHERN
SUSITNA VALLEY AND COPPER RIVER BASIN...WITH AN INCH OR TWO OF SLUSHY
ACCUMULATION OBSERVED AT A FEW LOCATIONS. A BROAD RIDGE OF HIGH
PRESSURE TOPPED BY A RELATIVELY ZONAL 140 KT JET REMAINS OVER THE
NORTH PACIFIC AND SOUTHERN BERING SEA. A DISSIPATING FRONT MOVED OVER
THE TOP OF THE RIDGE THIS MORNING...BRINGING LITTLE MORE THAN
SPRINKLES TO THE PRIBILOF ISLANDS THIS MORNING. MEANWHILE...GUSTY
WINDS ASSOCIATED WITH THE REMNANTS OF A FORMER TROPICAL SYSTEM
WEAKENING OVER THE KAMCHATKA PENINSULA OF RUSSIA ARE DIMINISHING.

.MODEL DISCUSSION...
SATELLITE IMAGERY SHOWS A COMPLEX LOW DEVELOPING DEEP IN THE BASE OF
THE LONGWAVE TROUGH OVER THE NORTHEAST PACIFIC. THERE ARE MULTIPLE
UPPER LEVEL WAVES COMBINING AT THIS MOMENT...AND THE MODELS ALL AGREE
THAT EVENTUALLY THIS SYSTEM WILL GET MORE ORGANIZED AND STRENGTHEN AS
IT HEADS TOWARD THE SOUTHERN PANHANDLE OVER THE NEXT 24-36 HOURS. THE
MODELS HAVE ALL SHOWED A FASTER/FURTHER NORTH TRACK WITH THE LOW ONCE
IT DEVELOPS...WHICH IS NOT SURPRISING CONSIDERING THE STRONG 120 KT
SOUTH-TO-NORTH JET THAT WILL BE CLIMBING OUT OF THIS TROUGH. THE NAM
IS A NOTABLE OUTLIER COMPARED TO THE OTHER MODELS...LAGGING BEHIND
THE CONSENSUS...AND WAS REMOVED FROM CONSIDERATION FOR THE AFTERNOON
FORECAST. MEANWHILE...MODELS ALSO CONTINUE TO STRUGGLE WITH THE
TIMING AND INTENSITY OF THE LOW THAT WILL DEVELOP OVER THE SOUTHERN
BERING SEA ON FRIDAY MORNING. THERE IS BETTER AGREEMENT THAN
YESTERDAY...ESPECIALLY AT THE UPPER LEVELS...BUT THE AMOUNT OF COLD
AIR ENTRAINED BY THE SYSTEM IS NOT IN GOOD AGREEMENT AND IS HAVING AN
IMPACT ON HOW STRONG THE SURFACE LOW (AND RESULTANT WIND FIELD) WILL
BECOME. THE ECMWF IS PULLING IN THE MOST COLD AND AS A RESULT HAS THE
STRONGEST SURFACE LOW. MEANWHILE THE NAM IS ON THE OTHER END OF THE
SPECTRUM. UNTIL SOME BETTER MODEL AGREEMENT DEVELOPS...WILL STICK
WITH THE GFS SOLUTION WHICH IS CLOSE TO THE INHERITED GRIDS.

.SHORT TERM FORECAST...
SOUTHCENTRAL...WIDESPREAD PRECIPITATION ASSOCIATED WITH THE UPPER
SHORTWAVE WILL CONTINUE TO END FROM WEST TO EAST TONIGHT...WITH THE
COPPER RIVER BASIN AND VALDEZ/CORDOVA THAT LAST PLACES TO SEE DRYING
CONDITIONS. CONVECTIVE SHOWERS OVER UPPER COOK INLET WILL ALSO
DISSIPATE THIS EVENING AS WEAK SUBSIDENCE ALOFT GRADUALLY SPREADS
INTO THE REGION FROM THE WEST. CLOUDS WILL BE A BIT SLOWER TO
CLEAR...AND LINGER ON WEST-FACING MOUNTAIN RANGES THE LONGEST...BUT
WILL EVENTUALLY YIELD TO INCREASING SUNSHINE ON THURSDAY. MANY PLACES
WILL SEE THE COLDEST TEMPERATURES OF THE SEASON SO FAR ON THURSDAY
NIGHT...WITH LOCATIONS AS FAR SOUTH AS THE KENAI PENINSULA DROPPING
BELOW FREEZING BY FRIDAY MORNING.

SOUTHWEST...DRY NORTHWEST FLOW WILL PREVAIL OVER MOST OF THE AREA
THROUGH THE END OF THE WEEK. THE BERING SEA STORM WILL PASS TO THE
SOUTH OF THE AREA...WITH CLOUDINESS AND SLIGHT RAIN CHANCES CONFINED
LARGELY TO THE COAST. OVERNIGHT TEMPERATURES WILL CONTINUE NEAR OR
SLIGHTLY BELOW NORMAL...WITH TEMPERATURES DROPPING EASILY INTO THE
20S OVER INTERIOR AREAS.

BERING SEA AND ALEUTIANS...MORE ACTIVE WEATHER RETURNS TO THE REGION
THURSDAY NIGHT AS A STRONG LOW DEVELOPS WITH GOOD JET SUPPORT AND
SOME COLD AIR FILTERING DOWN THROUGH THE BERING STRAIT. MOST ALEUTIAN
LOCATIONS WILL SEE RAIN AND GUSTY WINDS AS THIS STORM GOES PAST. WITH
THE EXPECTED STORM TRACK AS WELL AS THE VERY FAST FORWARD SPEED OF
THE STORM ITSELF...EXPECT THE EASTERN ALEUTIANS WILL SEE STRONGER
WINDS THAN OTHER AREAS. AT THIS POINT THIS WILL LIKELY HAPPEN FRIDAY
AFTERNOON AND EVENING WITH STRONG WINDS BLOWING FROM THE WEST.

.LONG TERM FORECAST...
THE LONG RANGE MODELS ARE BEGINNING TO SUGGEST THAT THE DEEP LONGWAVE
TROUGH WILL RETROGRADE WESTWARD TO THE BERING SEA BY THE MIDDLE OF
NEXT WEEK. THIS SUGGEST SOUTHERN ALASKA COULD TRANSITION RETURN TO A
SLIGHTLY WARMER AND WETTER PATTERN FOR MOST AREAS...WHILE THE ACTIVE
STORM TRACK AND UNSETTLED WEATHER WOULD SHIFT BACK TO THE BERING SEA
REGION.

.AER/ALU...WATCH/WARNING SUMMARY
PUBLIC...NONE
MARINE...GALE 130
FIRE WEATHER...NONE

AMD SEP 13