Small Craft Advisory 909, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
1230 PM AKST WED DEC 14 2011

.ANALYSIS AND UPPER LEVELS...SATELLITE IMAGERY SHOWS TWO POWERFUL
SYSTEMS APPROACHING THE ALEUTIANS AND ALASKA PENINSULA. THE FIRST
LOW WHICH DEVELOPED OVER THE NORTH PACIFIC YESTERDAY IS NOW ABOUT
250 MILES SOUTH-SOUTHWEST OF DUTCH HARBOR WITH AN ESTIMATED CENTRAL
PRESSURE AROUND 955 MB. AS UPPER LEVEL TROUGH CONTINUED TO DIG
SOUTHEASTWARD OVERNIGHT THE TROUGH BECAME NEGATIVELY TILTED AND
FORCED THIS LOW TO TRACK NORTHWESTWARD. THIS MOTION IS SLOWING
AND EXPECT THE LOW TO STALL SOMETIME SOON. MEANWHILE A SECOND LOW
DEVELOPED AT THE BASE OF THE TROUGH AND AT THE HEAD OF A STRONG
UPPER JET. THIS LOW IS MARGINALLY WEAKER AND IS TRACKING NORTHWARD.
AS THE JET CONTINUES EASTWARD THIS LOW WILL ALSO BEGIN TO CURVE BACK
TOWARD THE NORTHWEST AND WILL INTERACT WITH THE FIRST LOW...LIKELY
CONSOLIDATING INTO A SINGLE LOW. THE FORECAST CHALLENGE OF THE DAY
IS FIGURING OUT WHEN AND WHERE THESE LOWS WILL MERGE. THIS WILL
HAVE SIGNIFICANT IMPACTS ON THE WIND FORECAST FROM COLD BAY TO
DUTCH HARBOR. FOR NOW...HIGH WIND WARNINGS ARE IN PLACE FOR THE
SOUTHERN ALASKA PENINSULA AND EASTERN ALEUTIANS AND WINDS ARE
RAPIDLY INCREASING THIS MORNING.

LOOKING DOWNSTREAM FROM THIS SYSTEM A WEAK TRIPLE POINT LOW WHICH
FORMED YESTERDAY SOUTH OF KODIAK ISLAND IS NOW DISSIPATING EAST OF
KODIAK ISLAND AS UPPER LEVEL SHORT-WAVE IS MOVING QUICKLY ACROSS
THE GULF. THIS TROUGH IS PRODUCING AREAS OF LIGHT SNOW FROM PRINCE
WILLIAM SOUND TO THE COPPER BASIN. A BAND OF SNOW ALSO DEVELOPED
NEAR ANCHORAGE THIS MORNING AND HAS BEEN TRACKING NORTH AND WEST
INTO THE SOUTHERN SUSITNA VALLEY. IT APPEARS THAT LOW LEVEL
ISENTROPIC LIFT IS COMBINING WITH A SHORT-WAVE TO THE NORTH (WHICH
IS JUST BARELY CLIPPING THIS AREA) TO PRODUCE THIS BAND OF SNOW.
THE AREA OF LIFT WILL HEAD UP THE SUSITNA VALLEY SO EXPECT SNOW
TO DIMINISH IN THE SOUTH THE NEXT COUPLE HOURS AND SPREAD TO
NORTHERN PORTIONS OF THE VALLEY.

.MODEL DISCUSSION...THE ECMWF IS VERIFYING BEST WITH THE TWO LOWS
OUT WEST AND IS THE PREFERRED MODEL WITH THE EVOLUTION OF THESE
SYSTEMS. THE NAM IS NOT CARRYING THE SECOND LOW...BUT ONCE LOWS
MERGE INTO ONE THE TRACK IS SIMILAR TO THE ECMWF. THEREFORE...WILL
MAKE USE OF BOTH THE ECMWF AND NAM TO MAKE FORECAST ADJUSTMENTS.

.SHORT TERM FORECAST...

SOUTHCENTRAL ALASKA...THE FRONTAL SYSTEM ASSOCIATED WITH THE LOWS
OUT WEST WILL MOVE INTO THE GULF TONIGHT AND THEN RAPIDLY WEAKEN
AS IT MOVES ONSHORE OF THE NORTH GULF COAST THURSDAY AFTERNOON.
WITH THE PARENT LOW REMAINING BACK OVER THE EASTERN BERING AND
THE UPPER TROUGH CONTINUING TO BUILD EASTWARD THE UPPER LEVEL
ENERGY WILL BECOME MUCH WEAKER AS IT REACHES SOUTHCENTRAL ALASKA.
THEREFORE...EXPECT MOST OF THE PRECIP TO BE FOCUSED ALONG THE FRONT
AND TO BE FAIRLY SHORT-LIVED. THE PRINCE WILLIAM SOUND REGION WILL
SEE THE HEAVIEST PRECIP RIGHT AS THE FRONT MOVES THROUGH. AS IS
TYPICAL EASTERN PRINCE WILLIAM SOUND WILL SEE A PERIOD OF HEAVY
PRECIPITATION ONCE WINDS SHIFT TO SOUTHERLY BEHIND THE FRONT
CREATING UPSLOPE FLOW. THE UPPER TROUGH WILL EXIT THURSDAY NIGHT
INTO FRIDAY MORNING...BUT ANOTHER WEAK UPPER WAVE WILL MOVE INTO
THE GULF FRIDAY BRINGING A GOOD SHOT OF PRECIP TO  MAINLY COASTAL
AREAS LATE FRIDAY AND FRIDAY NIGHT. THERE IS QUITE A BIT OF
UNCERTAINTY IN THE DETAILS OF THIS BUT THE BOTTOM LINE IS IT
LOOKS LIKE IT WILL BE WEAK AND DISORGANIZED SO DO NOT EXPECT ANY
SIGNIFICANT IMPACTS.

SOUTHWEST ALASKA...STRONG FRONT WILL MAKE ITS WAY INTO BRISTOL BAY
LATE TONIGHT AND THEN RAPIDLY WEAKEN AS IT MOVES NORTHWARD DURING
THE DAY THURSDAY. THE BRISTOL BAY REGION WILL SEE THE HIGHEST IMPACT
WITH STRONG EASTERLY WINDS. THESE WINDS WILL COMBINE BRIEFLY WITH
SNOW RIGHT ALONG THE FRONT TO PRODUCE LOW VISIBILITIES IN BLOWING
SNOW. VISIBILITIES MAY VERY BRIEFLY REACH WHITE-OUT CONDITIONS BUT
DO NOT EXPECT THESE CONDITIONS TO LAST LONG ENOUGH TO WARRANT A
BLIZZARD WARNING. WITH EASTERLY FLOW ANY PRECIP THAT FALLS ALONG
THE ALASKA PENINSULA AND ALEUTIAN RANGE WILL BE ON THE LIGHT SIDE.
THE MORE INTENSE SNOW WILL FALL ACROSS AREAS NORTH AND WEST OF KING
SALMON. BY THE TIME THE FRONT REACHES THE KUSKOKWIM VALLEY AND
KUSKOKWIM DELTA EXPECT SNOW TO BE VERY LIGHT SO BLOWING SNOW SHOULD
NOT BE A HUGE PROBLEM...ALTHOUGH THERE WILL BE SOME VISIBILITY
REDUCTION ALONG THE KUSKOKWIM DELTA COAST. AFTER THIS FRONT MOVE
THE THREAT OF SHOWERS WILL PERSIST THROUGH FRIDAY AND SATURDAY AS
WEAK UPPER LEVEL FEATURES MOVE ACROSS THE REGION...BUT EXPECT MOST
AREA WILL REMAIN ON THE DRY SIDE.

BERING SEA/ALEUTIANS...THE STRONGEST WINDS WILL OCCUR AHEAD OF THE
FRONT THIS ALONG THE SOUTHERN ALASKA PENINSULA AND EASTERN ALEUTIANS.
HOWEVER...AS THE LOW CROSSES THE EASTERN ALEUTIANS TONIGHT AND DRIFTS
INTO THE EASTERN BERING FRIDAY STRONG SOUTHERLIES WILL PERSIST ALONG
THE PENINSULA. WINDS WILL DROP OFF BEHIND THE FRONT BUT WILL THEN
INCREASE BY MORNING. THE CURRENT HIGH WIND WARNING FOR THE ALASKA
PENINSULA GOES THROUGH 5 AM THURSDAY BUT WILL HAVE TO CONSIDER
EXTENDING ADJUSTING THIS. WILL WAIT UNTIL MORE MODEL RUNS COME IN
TO SEE IF THERE IS BETTER AGREEMENT IN THE TRACK OF THIS LOW BEFORE
MAKING ANY CHANGES TO THE EXISTING WARNING. THE LOW WILL RAPIDLY
WEAKEN THURSDAY NIGHT AND FRIDAY SO WINDS WILL COME DOWN QUICKLY
DURING THIS PERIOD. MEANWHILE ANOTHER PACIFIC SYSTEM WILL MOVE TO
THE WESTERN TO CENTRAL ALEUTIANS THURSDAY NIGHT AND FRIDAY. ALTHOUGH
THE CENTRAL PRESSURE IS EXPECTED TO DROP INTO THE 970S THE PRESSURE
ACROSS THE REGION WILL ALREADY BE QUITE LOW ACROSS THE REGION SO
WINDS WILL NOT BE AS MUCH AS THEY WOULD BE OTHERWISE.

.LONG TERM FORECAST...A TROUGH CENTERED OVER THE BERING WITH STRONG
UPPER JET ACROSS THE NORTH PACIFIC AND NORTHEASTWARD TOWARD MAINLAND
ALASKA WILL MAINTAIN AN ACTIVE PATTERN THROUGH THE EXTENDED PERIOD.
FORECAST CONFIDENCE IN TIMING AND LOCATION OF INDIVIDUAL SYSTEMS IS
LOW.

.AER/ALU...WATCH/WARNING SUMMARY
PUBLIC...HIGH WIND WARNING 181 185.
MARINE...HURRICANE FORCE WIND WARNING...155 165 170.
         STORM WARNING...150 160 171 172 179.
         GALE WARNING...120 125 130 132 138 175 180 185.
         HEAVY FREEZING SPRAY WARNING...180 185.
FIRE WEATHER...NONE.

SEB DEC 11