Small Craft Advisory 716, NWS Anchorage, AK

AREA FORECAST DISCUSSION
SOUTHCENTRAL AND SOUTHWEST ALASKA
140 PM AKDT WED SEP 9 2009

.ANALYSIS AND UPPER LEVELS...UPPER LOW IS TRANSITIONING FROM THE
EASTERN BERING SEA ACROSS THE ALASKA PENINSULA INTO THE PACIFIC
THIS MORNING. A SECOND WEAKER AND MORE ELONGATED UPPER LOW IS
DROPPING DOWN THE BACK SIDE OF THE UPPER TROUGH ACROSS THE EASTERN
BERING. WEAK WAVES EMBEDDED WITHIN THE CYCLONIC CIRCULATION AROUND
THE MAIN UPPER CENTER CONTINUE TO MOVE ACROSS THE SOUTHERN
MAINLAND...PRODUCING AREAS OF RAIN OR SHOWERS. LOW LEVEL RIDGE IS
BUILDING BEHIND ONE A WAVE MOVING ACROSS SOUTHCENTRAL THIS MORNING.
THIS IS BEGINNING TO KICK UP WINDS FROM PASSAGE CANAL TO PORTAGE
VALLEY AND TURNAGAIN ARM. A TIGHT PRESSURE GRADIENT ON THE BACKSIDE
OF THE TROUGH IS COMBINING WITH COLD ADVECTION TO PRODUCE GALE TO
STORM FORCE WINDS FROM THE EASTERN BERING SOUTHWARD ACROSS THE
EASTERN ALEUTIANS AND SOUTHERN TIP OF THE ALASKA PENINSULA. WINDS
WERE FURTHER ENHANCED THIS MORNING BY THE PASSAGE OF A SHORT-WAVE
ON THE WEST SIDE OF THE MAIN UPPER CENTER THROUGH THE EASTERN
ALEUTIANS. ONE STATION IN DUTCH HARBOR RECORDED A PEAK WIND GUST
OF 64 KT THIS MORNING...BUT IT WAS BRIEF AND WINDS HAVE DIMINISHED
SLIGHTLY. A STORM FORCE LOW DOWN ALONG THE BASE OF THE
TROUGH...AROUND 45N 157W...IS TRACKING NORTHEASTWARD TOWARD THE
GULF AND WILL BE A SIGNIFICANT WEATHER MAKER FOR THE NEXT COUPLE
DAYS. A RIDGE CURRENTLY DOMINATES THE CENTRAL TO WESTERN BERING...BUT
FORMER TROPICAL STORM DUJUAN IS ABOUT TO BE ABSORBED INTO THE
UPPER CIRCULATION AROUND UPPER LOW CENTERED OVER THE SEA OF OKHOTSK.
THIS WILL DRIVE THE STORM NORTHEASTWARD TOWARD THE WESTERN
ALEUTIANS.

.MODEL DISCUSSION...THERE IS A HUGE DIFFERENCE BETWEEN CENTRAL
PRESSURE OF FORMER TROPICAL STORM...986 MB ON GFS THIS AFTERNOON
AND 969 MB ON THE ECMWF. HOWEVER...THE POSITIONS MATCH AND CENTRAL
PRESSURE COMES INTO AGREEMENT BY THE TIME THE LOW REACHES THE
WESTERN ALEUTIANS. SOLUTIONS THEN DIVERGE AGAIN AS THE LOW MOVES
INTO THE BERING. BOTH MODELS ARE FAIRLY CONSISTENT WITH THEIR
PARTICULAR SOLUTIONS. SINCE CURRENT FORECAST WAS BASED OFF GFS
WILL STICK WITH THAT FOR NOW. MODELS APPEAR TO HAVE A BETTER
HANDLE ON THE SYSTEM HEADED FOR THE GULF...BUT TRACK DOES DIVERGE
A BIT ONCE THE LOW MOVES INTO THE GULF...WITH THE ECMWF BEING THE
FASTEST SOLUTION. THIS WILL HAVE A SIGNIFICANT IMPACT ON THE WIND
FORECAST OVER THE NORTHERN GULF. GFS TRACK HAS GOOD SUPPORT FROM
ITS ENSEMBLES AND CURRENT FORECAST IS NOT TOO FAR FROM THIS...SO
WILL CONTINUE TO BASE FORECAST OFF OF THE GFS.

.SHORT TERM FORECAST...

SOUTHCENTRAL ALASKA...SHOWERS WILL CONTINUE TONIGHT...PRIMARILY
ALONG THE COAST...AS UPPER WAVES CONTINUE TO ROTATE ACROSS THE
REGION. THIS SYSTEM LOOKS A LITTLE STRONGER THAN CURRENTLY
ADVERTISED...SO HAVE BUMPED UP WINDS OVER THE MARINE AREAS. AS
THE LOW MOVES INTO THE GULF THE OCCLUDED FRONT WILL PUSH NORTH
AND WEST ACROSS SOUTHCENTRAL THURSDAY AND THURSDAY NIGHT...WITH
THE BULK OF RAIN FALLING ALONG THE COAST AS DOWNSLOPING WILL KEEP
INLAND AREAS MOSTLY DRY. A SURFACE RIDGE WILL BUILD ALONG THE NORTH
GULF COAST BEHIND THIS FRONT...PRODUCING RAPID PRESSURE RISES...WHICH
WILL RESULT IN STRONG EASTERLIES FROM PRINCE WILLIAM SOUND TO
PASSAGE CANAL TO TURNAGAIN ARM. THIS RIDING ALONG THE COAST WILL
ALSO SET UP A BARRIER JET THURSDAY NIGHT AND FRIDAY...ASSUMING THE
GFS SOLUTION OF KEEPING THE LOW OVER THE CENTRAL GULF IS CORRECT.
WINDS WILL GRADUALLY DIMINISH FRIDAY NIGHT AND SATURDAY AS THE
LOW WEAKENS. RAIN WILL CONTINUE IN THE PRINCE WILLIAM SOUND REGION
WITH JUST A FEW SHOWERS ELSEWHERE.

SOUTHWEST ALASKA...SHOWERS WILL DIMINISH OVER SOUTHWEST ALASKA
TONIGHT AS THE UPPER LOW CURRENTLY ALONG THE ALASKA PENINSULA
MOVES INTO THE GULF (OVERHEAD OF THE APPROACHING SURFACE LOW).
THE OCCLUDED FRONT FROM THE GULF LOW WILL MOVE TO THE ALASKA
RANGE WHERE IT WILL FALL APART FRIDAY MORNING. THIS WILL
PRODUCE A FEW SHOWERS ALONG THE ALASKA AND ALEUTIAN RANGES.
OTHERWISE...EXPECT MOSTLY DRY CONDITIONS ACROSS THE REGION
THURSDAY AND FRIDAY. THERE IS SOME UNCERTAINTY IN THE TRACK
OF THE NEXT STORM ACROSS THE BERING SEA...BUT AT THIS POINT
IT LOOKS LIKE MOISTURE FROM THIS WILL REACH THE SOUTHWEST
COAST BY SATURDAY.

BERING SEA/ALEUTIANS...STRONG NORTHERLIES WILL CONTINUE ACROSS
THE EASTERN BERING/EASTERN ALEUTIANS/SOUTHERN ALASKA PENINSULA
THROUGH THURSDAY AND THEN DIMINISH AS THE RIDGE SLIDES IN FROM
THE WEST. THE FORMER TROPICAL SYSTEM AND ASSOCIATED STORM FORCE
FRONT WILL MOVE ACROSS THE ALEUTIANS THURSDAY AND THEN RAPIDLY
WEAKEN AS THE SOUTHERN PORTION OF THE FRONT IS PUSHED EASTWARD
AND THE NORTHERN PORTION NORTHWARD. THE UPPER CENTER BACK OVER
THE SEA OF OKHOTSK WILL THEN MOVE TO THE BERING SEA AND PROGRESS
TOWARD THE EASTERN BERING BY SATURDAY. STRONG NORTHERLIES WILL
LIKELY DEVELOP ON THE BACK SIDE OF THIS SYSTEM AS A RIDGE BUILDS
BEHIND IT (AHEAD OF ANOTHER SYSTEM APPROACHING FROM THE WEST
PACIFIC).

.LONG TERM FORECAST...THERE IS GOOD RUN-TO-RUN CONSISTENCY ON
SYSTEM OVER EASTERN BERING ON SATURDAY DROPPING SOUTHEASTWARD
INTO THE NORTHEAST PACIFIC SUNDAY AND REMAINING THERE THROUGH
AT LEAST MID WEEK. UPPER WAVES EMBEDDED WITHIN THE UPPER TROUGH
MAY ROTATE BACK TOWARD THE SOUTHERN MAINLAND...BUT ITS TOO EARLY
AT THIS POINT TO SAY WHETHER ANY OF THOSE WILL BE OF SIGNIFICANCE.
MEANWHILE...ANOTHER SYSTEM WILL MOVE TO THE ALEUTIANS...BUT THERE
IS A LARGE SPREAD IN MODEL SOLUTIONS AND POOR CONTINUITY...SO
CONFIDENCE IN TRACKING AND STRENGTH OF THIS NEXT SYSTEM IS VERY
LOW.

.AER/ALU...WATCH/WARNING SUMMARY
PUBLIC...NONE.
MARINE...STORM WARNING...120 130 132 175 176.
         GALE WARNING...125 129 138 155 165 170 171.
FIRE WEATHER...NONE.

SEB SEP 09