Small Craft Advisory 742, NWS Anchorage, AK

AREA FORECAST DISCUSSION
SOUTHCENTRAL AND SOUTHWEST ALASKA
200 PM ADT TUE SEP 11 2007

.ANALYSIS AND UPPER LEVELS...A VERY LARGE-SCALE SYSTEM IS AFFECTING
THE WEATHER ACROSS THE STATE TODAY. SATELLITE IMAGERY SHOWS A MATURE
VERTICALLY STACKED LOW WITH OCCLUSION WRAPPED AROUND SEVERAL TIMES
SITTING NEARLY STATIONARY OVER THE CENTRAL BERING SEA NEAR 58N 176W.
THE LOW IS BEGINNING TO SLOWLY WEAKEN BUT THERE IS STILL A VERY TIGHT
SURFACE PRESSURE GRADIENT ACROSS MUCH OF THE BERING SEA AND
ALEUTIANS...WITH GALE FORCE WINDS COVERING MUCH OF THIS REGION.
SHORT-WAVES ROTATING AROUND LOW CENTER ARE PRODUCING BANDS OF
RAIN/SHOWERS OVER THE BERING AND PRIBILOFS...WITH JUST A FEW SHOWERS
AS YOUR HEAD SOUTH TO THE ALEUTIANS. THE OCCLUDED FRONT EXTENDS
NORTHWARD ACROSS FAR EASTERN RUSSIA THEN CURVES AROUND TO THE WESTERN
ALASKAN COAST AND SOUTHWARD ACROSS THE AKPEN (NEAR SAND POINT) AND
ON INTO THE PACIFIC. A WATER VAPOR IMAGERY LOOP SHOWS A STRONG
SHORT-WAVE RIDING NORTHWARD ALONG THE FRONT. THERE ARE ALSO HINTS
(IN THE FORM OF SMALL NOTCHES ALONG THE WEST EDGE OF THE FRONTAL
CLOUD BAND) OF VERY WEAK SURFACE WAVES ALONG THE FRONT SOUTH OF THE
AKPEN. THERE IS LIKELY ONE DIRECTLY UNDERNEATH THE UPPER
SHORT-WAVE...BUT THERE ARE TOO MANY HIGH CLOUDS BLOCKING THE VIEW
NEEDED TO VERIFY THIS. A COUPLE OTHER WEAKER UPPER WAVES JUST AHEAD
OF THE FRONT HAVE BEEN TRACKING NORTH-NORTHWEST TOWARD SOUTHWEST
ALASKA THIS MORNING. EARLY THIS MORNING THERE WAS NOT MANY RETURNS
TO SPEAK OF ON THE BETHEL OR KING SALMON RADARS. BY MID MORNING
ECHOES FILLED THE RADAR AND HAVE HELD STEADY EVER SINCE. GUSTY WINDS
CONTINUE AHEAD OF THE FRONT ACROSS BRISTOL BAY AND THE KODIAK ISLAND
REGION. LOW LEVEL WIND SHEAR WAS NOTED ON THE BETHEL AND KING SALMON
SOUNDINGS THIS MORNING WITH 40 TO 50 KTS WITHIN THE FIRST COUPLE
THOUSAND FEET. THIS HAS LIKELY SPREAD TO MCGRATH BY NOW AND AS THE
FRONT PROGRESSES EASTWARD WILL AFFECT SITE SUCH AS HOMER AND
ANCHORAGE.

THE UPPER JET ON THE SOUTHWEST SIDE OF THE BERING LOW REALLY HELPED
AMPLIFY THE PATTERN LAST NIGHT AND THIS MORNING. THUS...THE UPPER
RIDGE BUILT OVER THE EASTERN HALF OF ALASKA AND KEPT MOST OF THE
FRONTAL CLOUDS OFF TO THE WEST. THIS RESULTED IN THE DEVELOPMENT OF
AREAS OF STRATUS AND FOG...ESPECIALLY FOR THE PRINCE WILLIAM SOUND
AND COPPER RIVER BASIN REGIONS. ANY FOG THAT FORMED WEST OF THESE
REGIONS DISSIPATED EARLIER. BUT THESE OTHER AREAS HAVE BEEN SLOW
TO IMPROVE. THE LATEST VISIBLE SATELLITE IMAGERY SHOWS SIGNIFICANT
IMPROVEMENT BETWEEN NOON AND 1 PM. SO EXPECT SOME SUN THIS AFTERNOON
BEFORE THE CLOUDS THICKEN UP. THE AMPLIFIED PATTERN HAS ALSO
STRENGTHENED THE UPPER JET BETWEEN TROUGH AND RIDGE WITH 100KT
PLUS NOW HEADED STRAIGHT NORTH OVERHEAD OF THE SURFACE FRONT ACROSS
THE AKPEN AND WESTERN ALASKA. THIS JET IS PUMPING LOTS OF TROPICAL
MOISTURE UP INTO THE STATE.

.MODEL DISCUSSION...THE MODELS ARE IN GOOD AGREEMENT IN HANDLING OF
CURRENT SYSTEM...INCLUDING MOVEMENT OF FRONT AND UPPER TROUGH ACROSS
THE SOUTHERN MAINLAND THE NEXT COUPLE DAYS. YESTERDAY THE GFS
DEPICTION OF A STRONG SHORT-WAVE TRACKING FROM NEAR THE LOW CENTER TO
SOUTHCENTRAL LATE WEDNESDAY WAS IN QUESTION (THIS IS IN ADDITION TO
THE WAVE TALKED ABOUT ABOVE TRACKING NORTHWARD ALONG THE FRONT). THE
GFS STILL SHOWS THIS FEATURE AND NAM NOW HINTS AT IT. THIS WILL HELP
INTENSIFY PRECIP OVER SOUTHCENTRAL LATE WEDNESDAY.

THERE IS STILL GREAT UNCERTAINTY IN NEXT MAJOR PACIFIC SYSTEM
TRACKING TO THE GULF FRIDAY. THERE IS VERY POOR RUN-TO-RUN
CONSISTENCY AND A VERY LARGE SPREAD IN GFS ENSEMBLES. THE LATEST
TREND IS FASTER AND SIGNIFICANTLY FARTHER EAST...WHICH WOULD LEAD
TO A VERY DIFFERENT FORECAST THAN CURRENTLY ADVERTISED. THEREFORE...
WILL CONTINUE TO STICK WITH CURRENT FORECAST AND WAIT UNTIL MODELS
HAVE A BETTER HANDLE ON THIS SYSTEM.

.SHORT TERM FORECAST...

SOUTHCENTRAL/SOUTHWEST...RAIN HAS MOVED OVER KODIAK ISLAND BUT WILL
BE SLOW TO SPREAD TO THE REST OF SOUTHCENTRAL TONIGHT DUE TO HIGH
AMPLITUDE RIDGE. MULTIPLE SHORT-WAVES TALKED ABOUT ABOVE WILL TRACK
NORTH OVERHEAD OF THE SURFACE FRONT ACROSS KODIAK ISLAND AND
SOUTHWEST ALASKA THIS AFTERNOON THIS AFTERNOON AND
EVENING...RESULTING IN CONTINUED WET CONDITIONS ACROSS THIS REGION.
STRONG EASTERLY WINDS HAVE BEEN LIMITING PRECIP ALONG THE LEE SIDE
OF MOUNTAINS IN BRISTOL BAY. BUT AS THE FRONT MOVES EASTWARD TONIGHT
WINDS WILL DIMINISH AND RAIN WILL ALONG THE ALEUTIAN RANGE AND UP
INTO THE KUSKOKWIM VALLEY. THE COMBINATION OF UPPER SHORT-WAVES AND
INTENSIFYING JET ON EAST SIDE OF TROUGH WILL CAUSE UPPER TROUGH TO
BECOME NEGATIVELY TILTED BY LATE TONIGHT. THIS WILL HELP NUDGE UPPER
RIDGE EASTWARD...ALLOW SURFACE FRONT TO MAKE EASTWARD PROGRESSION
INTO THE WESTERN GULF...AND SPREAD RAIN UP INTO THE WESTERN HALF OF
SOUTHCENTRAL OVERNIGHT. THE UPPER TROUGH WILL CONTINUE TO TILT
WEDNESDAY AND THEN LIFT NORTHWARD ACROSS THE STATE LATE WEDNESDAY
AFTERNOON THROUGH WEDNESDAY NIGHT PRODUCING WIDESPREAD RAIN. AT THE
SURFACE...PRESSURE GRADIENT WILL TIGHTEN ACROSS SOUTHCENTRAL BY LATE
TONIGHT AS FRONT MOVES INTO WESTERN GULF. ALSO...MODELS INDICATE
FRONTAL WAVE MOVING UP TO JUST SOUTH OF THE KENAI PENINSULA LATE
TONIGHT. THIS WILL LIKELY INTENSIFY THE GRADIENT TO THE NORTH. A
HIGH WIND WATCH WAS ISSUED LAST NIGHT FOR TURNAGAIN ARM...BUT DECIDED
TO CANCEL THAT BECAUSE LOOKS LIKE EVEN IF ADD A BIT TO MODEL FORECAST
GRADIENT LOOKS LIKE WOULD STILL REMAIN BELOW WARNING LEVELS. SURFACE
WAVE IS PROGGED TO TRACK NORTH OVER THE KENAI PENINSULA WHICH MEANS
GRADIENT WILL QUICKLY FALL. IF IT WERE TO TRACK WEST OF ANCHORAGE
THERE WOULD BE A MUCH BETTER CHANCE OF REACHING WARNING LEVELS. THE
STRONG WINDS WILL LIKELY LIMIT LEE SIDE PRECIP TONIGHT INTO
WEDNESDAY...BUT DYNAMICS ALOFT ARE IMPRESSIVE SO THINK THERE WILL
BE AT LEAST A PERIOD OF LIGHT RAIN IN ALL LOCATIONS. FLOW WILL THEN
SHIFT TO SOUTHWEST LATE WEDNESDAY AS UPPER TROUGH MOVES THROUGH.
BEHIND THE UPPER TROUGH SOUTHWEST FLOW ALOFT WILL PERSIST THURSDAY
INTO FRIDAY...RESULTING IN SHOWERS...ESPECIALLY OVER SOUTHWEST
ALASKA WHICH WILL BE CLOSER TO THE UPPER LOW CENTER OVER THE BERING.

BERING SEA/ALEUTIANS...AS THE UPPER TROUGH BECOMES TILTED AND LIFTS
NORTHWARD THE BERING LOW WILL TRACK NORTH TOWARD THE BERING STRAIT.
RAIN/SHOWERS AND GALE FORCE WINDS WILL SPREAD EASTWARD TONIGHT TO THE
EASTERN BERING AND THEN WINDS AND PRECIP WILL GRADUALLY DIMINISH AS
WE HEAD THROUGH THE DAY WEDNESDAY. BY THURSDAY WINDS WILL GENERALLY
BE LIGHT ACROSS THE REGION WITH JUST A FEW SHOWERS LEFT FROM THE
SYSTEM. A SHORT-WAVE UPPER RIDGE BUILDS IN FRIDAY...SO IT IS LOOKING
LIKE DRYING TREND WILL CONTINUE.

.LONG TERM FORECAST...LOW CONFIDENCE IN EXTENDED FORECAST FOR THE
WEEKEND THROUGH NEXT WEEK...ESPECIALLY FOR SOUTHCENTRAL ALASKA. IT
DOES LOOK LIKE MAJOR LOW WILL HEAD SOMEWHERE IN THE GULF RESULTING
IN A GOOD CHANCE OF RAIN AND WINDS ACROSS THE GULF/SOUTHCENTRAL FOR
THE WEEKEND WITH DRYING TREND AFTER THAT. WITH NEAR ZONAL FLOW ALOFT
OUT WEST WILL LOOK FOR ANOTHER MAJOR SYSTEM TO MOVE INTO THE
BERING/ALEUTIANS BY THE BEGINNING OF NEXT WEEK.

.AER/ALU...WATCH/WARNING SUMMARY
PUBLIC...NONE.
MARINE...GALES 130 160 170 171 172 175 179 180.
FIRE WEATHER...NONE.

SEB SEP 07