Small Craft Advisory 796, NWS Anchorage, AK

AREA FORECAST DISCUSSION
SOUTHCENTRAL AND SOUTHWEST ALASKA
125 PM ADT WED SEP 26 2007

.ANALYSIS AND UPPER LEVELS...A LOOK AT SATELLITE IMAGERY THIS
MORNING SHOWS A SLIGHTLY MORE AMPLIFIED PATTERN ACROSS THE ALASKA
REGION. THERE IS A TROUGH CENTERED OVER THE GULF WITH A WEAK
CIRCULATION OVER THE CENTRAL GULF SOUTH OF MIDDLETON ISLAND. A
NEARLY STATIONARY LOW CONTINUES TO DISSIPATE JUST WEST OF NORTON
SOUND. WATER VAPOR IMAGERY SHOWS WEAK SHORT-WAVES ROTATING AROUND
THE SOUTH SIDE OF THE LOW INTO SOUTHWEST ALASKA. FORCING FROM THE
ABOVE FEATURES IS PRODUCING SHOWERS...SOME CONTAINING HEAVY
RAIN...ACROSS MUCH OF THE NORTHERN GULF AND PRINCE WILLIAM SOUND AS
WELL AS SOUTHWEST ALASKA. THERE ARE FEWER SHOWERS AS YOU HEAD INLAND.
OF NOTE ARE THE SHOWERS OVER THE SOUTHERN SUSITNA VALLEY THAT HAVE
BEEN SLOWLY DRIFTING TOWARD ANCHORAGE AND THE NORTHERN KENAI
PENINSULA. THERE IS ALSO A CIRCULATION OVER THE SOUTHERN
SUSITNA...LIKELY A MID LEVEL VORT-MAX. THERE IS ALSO A WEAK
CONVERGENCE ZONE OVER NORTHERN COOK INLET...SO SHOWERS MAY INTENSIFY
AS THEY MOVE INTO ANCHORAGE. A FEW ISOLATED LIGHTNING STRIKES HAVE
ALSO BEEN DETECTED OVER THE GULF. BASED ON HIGH INSTABILITY OF THE
AIR MASS THESE STRIKES ARE LIKELY REAL. 12Z SOUNDINGS ACROSS
SOUTHCENTRAL AND SOUTHWEST ALASKA SHOWED SIGNIFICANT COOLING IN THE
UPPER LEVELS WITH 500MB TEMPS COOLING 5 TO 10 DEGREES THE PAST 24
HOURS DOWN TO ABOUT -30 DEGREES CELSIUS. LIFTED INDICES ARE AROUND
ZERO. WITH THE STATE OF THE ATMOSPHERE IT DOESNT TAKE MUCH SYNOPTIC
LIFT OR SOLAR HEATING TO
SET OFF SOME DECENT SHOWERS.

HEADING WEST IN THE UPPER LEVELS THERE IS A RIDGE CENTERED OVER
THE CENTRAL BERING SEA. THERE IS A STRONG UPPER JET DOWN THE EAST
SIDE OF THE RIDGE (ESTIMATED AT 170 KT) ACROSS THE EASTERN BERING
AND WESTERN AKPENN. THIS IS COMBINATION OF RIDGE BUILDING EASTWARD
(SURFACE AND ALOFT) AND UPPER JET ARE PRODUCING GUSTY WESTERLY WINDS
ACROSS THIS REGION. ALTHOUGH OBSERVATIONS SHOW SMALL CRAFT WINDS
OVER MARINE AREAS...EXPECT THERE ARE SOME GALE FORCE GUSTS THROUGH
GAPS IN THE AKPENN.

FINALLY...THERE IS A CLOSED UPPER LOW WEST OF THE ALEUTIANS WITH A
STRONG SURFACE REFLECTION IN A 975 MB LOW. THE JET IS STRONGER AHEAD
OF THIS LOW...SO DO NOT EXPECT ANYMORE AMPLIFICATION OF THE TROUGH
OR STRENGTHENING OF THE LOW. THE SURFACE LOW HAS OCCLUDED AND THE
GALE FORCE FRONT IS TRACKING EAST ACROSS THE WESTERN ALEUTIANS.

.MODEL DISCUSSION...MODELS CONTINUE TO BE IN GOOD AGREEMENT AND
SHOW EXCELLENT RUN-TO-RUN CONSISTENCY. GFS ENSEMBLE MEMBERS HAVE
VERY LITTLE SPREAD THROUGH ABOUT THE NEXT 4 DAYS...REINFORCING THE
HIGH CONFIDENCE IN THE NUMERICAL MODELS SOLUTION. ONCE AGAIN NO
MAJOR CHANGES WILL BE MADE TO THE FORECAST.

.SHORT TERM FORECAST...

SOUTHCENTRAL AND SOUTHWEST ALASKA...THE UPPER TROUGH OVER THE GULF
WILL SWING EASTWARD TO SOUTHEAST ALASKA THIS EVENING SO SHOWERS WILL
DIMINISH ACROSS SOUTHCENTRAL. THE REMNANTS OF THE WESTERN ALASKA LOW
AS WELL AS WEAK UPPER SHORT-WAVES WILL SLOWLY TRACK ACROSS SOUTHWEST
ALASKA AND WESTERN PORTIONS OF SOUTHCENTRAL TONIGHT. THERE IS ONE
FINAL SHORT-WAVE THAT DOES NOT MOVE THROUGH UNTIL LATE THURSDAY.
SO...THE COMBINATION OF THESE UPPER LEVEL FEATURES WITH A VERY
UNSTABLE AIRMASS IN PLACE WILL CONTINUE THE THREAT OF SHOWERS OVER
THE REGION THROUGH THURSDAY EVENING. THERE WILL ALSO BE SOME SUN
BREAKS WHICH COULD CONTRIBUTE TO INSTABILITY. THE BEST AREA OF UPPER
LEVEL LIFT LOOKS TO BE FROM THE KUSKOKWIM VALLEY TO NORTHERN COOK
INLET AND THE SUSITNA VALLEY. ONLY EXPECT ISOLATED TO SCATTERED
COVERAGE OF SHOWERS...BUT SOME OF THESE SHOWERS WILL LIKELY CONTAIN
HEAVY RAIN. THE HIGH PRESSURE RIDGE OVER THE CENTRAL BERING WILL
SLIDE EASTWARD ACROSS THE AKPENN TONIGHT AND INTO THE GULF AND
SOUTHCENTRAL THURSDAY. COMBINATION OF A FAIRLY TIGHT SURFACE PRESSURE
GRADIENT...PRESSURE RISES...AND STRONG UPPER JET OVERHEAD WILL
PRODUCE GUSTY WINDS ALONG THE AKPENN AND KODIAK ISLAND WATERS
TONIGHT INTO THURSDAY MORNING. BY LATE THURSDAY THE FRONT EXTENDING
FROM THE BERING LOW WILL BE APPROACHING THE SOUTHWEST COAST AND
KODIAK ISLAND...SO RAIN WILL QUICKLY SPREAD ACROSS SOUTHWEST ALASKA
THURSDAY NIGHT AND INTO SOUTHCENTRAL FRIDAY. THERE WILL BE DOWNSLOPE
DRYING ON THE LEE SIDE OF THE ALASKA RANGE AND SOUTHCENTRAL COASTAL
RANGE AS STRONG EASTERLY FLOW DEVELOPS AHEAD OF FRONT. THE BERING
LOW WILL MOVE ACROSS THE AKPENN AND INTO THE GULF LATE FRIDAY SO
FLOW WILL THEN BACK TOWARD THE NORTH. WITH THIS SYSTEM PROGRESSION
LEE SIDE AREAS AND INLAND AREAS WILL SEE VERY LITTLE RAIN...IF ANY.
THE FRONT WILL BECOME PARALLEL TO THE NORTH GULF COAST ON FRIDAY...
SO THERE IS A VERY GOOD CHANCE THAT A BARRIER JET WILL DEVELOP. THE
FORECAST CURRENTLY HAS MAX GALES ADVERTISED...BUT WILL CONSIDER
UPPING THIS TO STORMS. WITH THE UPPER FLOW REMAINING PROGRESSIVE
THIS SYSTEM WILL TRACK EASTWARD ON SATURDAY AND THE SOUTHERN
MAINLAND WILL DRY OUT.

BERING SEA ALEUTIANS...THE LOW PRESSURE SYSTEM WILL QUICKLY TRACK
EASTWARD ACROSS THE BERING SEA AND ALEUTIANS TONIGHT AND THURSDAY
AND MOVE INTO THE GULF FRIDAY. THERE IS WEAK COLD ADVECTION ON THE
BACK SIDE OF THE SYSTEM SO THERE WILL BE SHOWERS OVER THE RELATIVELY
WARM BERING SEA WATERS. NORTHWESTERLY FLOW ON THE BACK SIDE OF THE
LOW WILL ALSO BE QUITE STRONG...WITH MAX GALES OVER THE EASTERN
BERING/EASTERN ALEUTIANS/AKPENN FRIDAY AND FRIDAY NIGHT. WITH COLD
ADVECTION ALOFT THERE MAY BE SOME STORM FORCE GUST THROUGH CHANNELED
AREAS. WITH THE PATTERN CONTINUING TO BECOME MORE AMPLIFIED THE NEXT
SYSTEM WILL BE FORCED FARTHER NORTHWARD AS IT TRACKS INTO THE BERING
SEA FRIDAY NIGHT AND SATURDAY WITH ANOTHER ROUND OF STRONG WINDS AND
RAIN.

.LONG TERM FORECAST...THE SYSTEM OVER THE BERING SEA ON SATURDAY
GRADUALLY ERODES THE RIDGE AHEAD OF IT AND FLATTENS OUT THE PATTERN
AGAIN. TROUGHINESS WILL DOMINATE ALASKA AND THE BERING SEA REGION.
SO EXPECT A SHOWERY REGIME TO SET IN THIS WEEKEND THORUGH MUCH OF
NEXT WEEK. MODELS ALSO INDICATE A SHORT-WAVE TROUGH DROPPING DOWN
FROM THE POLE EARLY TO MID WEEK AND TRANSPORTING COLDER AIR SOUTHWARD
ACROSS ALASKA. SO SNOW LEVELS WILL LIKELY CONTINUE TO SLOWLY FALL
AS WE HEAD THORUGH NEXT WEEK.

.AER/ALU...WATCH/WARNING SUMMARY
PUBLIC...NONE.
MARINE...STORMS 172 175.
         GALES 130 132 138 150 155 160 165 170 171 176 179 180 185.
FIRE WEATHER...NONE.

SEB SEP 07