Small Craft Advisory 793, NWS Anchorage, AK

AREA FORECAST DISCUSSION
SOUTHCENTRAL AND SOUTHWEST ALASKA
145 PM ADT TUE SEP 25 2007

.ANALYSIS AND UPPER LEVELS...SYSTEMS CONTINUE TO SPEED ACROSS THE
REGION IN A VERY PROGRESSIVE PATTERN...RESULTING IN RAPIDLY CHANGING
CONDITIONS AT THE SURFACE. THE MAIN CENTER OF CIRCULATION IS A MATURE
VERTICALLY STACKED LOW SPINNING JUST SOUTH OF SAINT LAWRENCE ISLAND.
SEVERAL SHORT-WAVES ARE EMBEDDED IN THE LARGE CYCLONIC CIRCULATION
AROUND THIS LOW. A LOOK AT SATELLITE IMAGERY SHOWS A NEGATIVELY
TILTED SHORT-WAVE TROUGH FROM THE SUSITNA VALLEY TO AROUND
YAKUTAT...TRACKING EASTWARD TOWARD THE YUKON. AT THE SURFACE THERE
IS A WEAK CIRCULATION SOUTHWEST OF YAKUTAT AS WELL AS INLAND OVER
THE YUKON. RAIN CONTINUES OVER THE COPPER BASIN WITH SHOWERS TO THE
WEST FROM THE SUSITNA VALLEY SOUTHEASTWARD TO EASTERN PRINCE WILLIAM
SOUND. SOME OF THESE SHOWERS CONTAIN MODERATE TO HEAVY RAIN WITH
ATMOSPHERE DESTABILIZING BEHIND THE TROUGH AS COLDER AIR ALOFT MOVES
IN. PRECIPITATION IS ALSO IN THE FORM OF SNOW AT HIGHER ELEVATIONS
(GENERALLY ABOVE 2500 FEET) INCLUDING NABESNA AND PAXSON. THERE IS
AN AREA OF CLEARING OVER THE WESTERN GULF TO COOK INLET. HEADING
WEST TO THE NEXT FEATURE IN LINE...THERE IS A SHORT-WAVE TROUGH
OVER THE EASTERN BERING. IT IS BEGINNING TO BECOME NEGATIVELY TILTED
AND TO DIG SOUTHEASTWARD DUE TO A BUILDING RIDGE TO THE WEST. WITH
THIS CHANGE IN FLOW SHOWERS AHEAD OF THE SHORT-WAVE ARE NOW TURNING
MORE TO THE NORTH AND INTO SOUTHWEST ALASKA. RADAR IMAGERY SHOWS
WIDESPREAD SHOWERS HAVE MOVED ONSHORE OVER THE KUSKOKWIM DELTA AND
WESTERN BRISTOL BAY...TRACKING INLAND. EXPECT INCREASING SHOWERS OVER
THE REST OF SOUTHWEST ALASKA AS WE HEAD THROUGH THE AFTERNOON. THIS
IS ALSO CAUSING A SLIGHT AMPLIFICATION TO SURFACE RIDGE ALONG AKPEN
AND WESTERN GULF WATERS...RESULTING IN DECENT PRESSURE RISES.
COMBINATION OF PRESSURE RISES AND INSTABILITY IS PRODUCING GUSTY
WINDS FROM SOUTHWEST ALASKA TO THE WESTERN GULF AND COOK INLET. THE
ISALLOBARIC MAX WILL MOVE EAST ACROSS THE GULF THIS AFTERNOON AND
EVENING. THERE ALSO APPEARS TO BE A WEAK MID TO UPPER LEVEL
CIRCULATION EMBEDDED IN THE SHORT-WAVE...BUT DO NOT EXPECT ANY
DEVELOPMENT. BACK OVER THE CENTRAL TO WESTERN BERING THERE ARE QUITE
A FEW BREAKS WITH ONLY ISOLATED TO SCATTERED SHOWERS. LOOKING EVEN
FARTHER WEST TO THE REGION EAST OF KOREA AND SOUTH OF KAMCHATKA THERE
IS ANOTHER HIGHER AMPLITUDE TROUGH WITH A MODERATE SURFACE LOW
UNDERNEATH AND ANOTHER SURFACE LOW DEVELOPING JUST AHEAD OF THE
TROUGH. A DECENT JET MAX ON THE BACK SIDE OF THIS SHORT-WAVE OF ABOUT
130KT IS HELPING TO AMPLIFY THE TROUGH AND CAUSE IT TO BECOME
NEGATIVELY TILTED. THIS WILL FAVOR THE DEVELOPMENT OF THE SURFACE LOW
AHEAD OF THE TROUGH. THIS IS ALSO THE CAUSE OF AMPLIFICATION OF
SHORT-WAVE RIDGE TO THE EAST WHICH THEN TRANSLATES ON DOWN THE LINE
TO THE EASTERN BERING SHORT-WAVE AS MENTIONED ABOVE. THIS WILL BE THE
NEXT SYSTEM TO MOVE TO THE WESTERN BERING AND ALEUTIANS.

.MODEL DISCUSSION...THE MODELS CONTINUE TO BE IN GOOD AGREEMENT WITH
EXCELLENT RUN-TO-RUN CONSISTENCY. THIS IS SURPRISING CONSIDERING
THE MODELS DO NOT USUALLY HANDLE THIS PROGRESSIVE PATTERN VERY WELL.
ANYWAY...BECAUSE OF THIS CONFIDENCE LEVEL IS HIGH AND ONLY MINOR
ADJUSTMENTS WILL BE MADE TO THE CURRENT FORECASTS.

.SHORT TERM FORECAST...

SOUTHCENTRAL...THE SHORT-WAVE OVER THE EASTERN BERING WILL CONTINUE
TO AMPLIFY AS IT MOVES INTO SOUTHWEST ALASKA AND THE WESTERN GULF
THIS EVENING. MEANWHILE SOUTHCENTRAL WILL BRIEFLY DRY OUT THIS
AFTERNOON AND EVENING AND BECOME PARTLY TO MOSTLY SUNNY MOST AREAS
UNDER THE INFLUENCE OF A SHORT-WAVE RIDGE ALOFT. COMBINATION OF
CLEARING SKIES AND REMNANT LOW LEVEL MOISTURE WILL LIKELY PRODUCE
AREAS OF STRATUS AND FOG...ESPECIALLY OVER THE COPPER BASIN WHERE
THE NEXT BATCH OF SHOWERS WILL NOT ARRIVE UNTIL WEDNESDAY MORNING.
AS THE TROUGH APPROACHES SOUTHCENTRAL LOW LEVEL FLOW WILL SHIFT TO
SOUTHEAST AND THERE WILL BE ENOUGH OF A SURFACE GRADIENT TO PRODUCE
TURNAGAIN ARM WINDS OF AROUND 20 TO 30 KTS. DO NOT THINK LOW LEVEL
FLOW WILL BE STRONG ENOUGH TO HAVE A CONSIDERABLE AFFECT ON LEE SIDE
PRECIP. IN ADDITION ATMOSPHERE WILL REMAIN UNSTABLE SO IT WONT TAKE
A WHOLE LOT OF FORCING TO PRODUCE PRECIP. THEREFORE...EXPECT SHOWERS
TO MOVE INTO MUCH OF SOUTHCENTRAL OVERNIGHT. THE SHORT-WAVE WILL
QUICKLY TRACK TO SOUTHEAST ALASKA THURSDAY MORNING...SO ONCE AGAIN
EXPECT IMPROVING CONDITIONS ACROSS SOUTHCENTRAL AS THE DAY WEARS ON.
FREEZING LEVELS WILL CONTINUE TO COME DOWN A BIT TONIGHT WITH A
LITTLE LOW-LEVEL COOLING. SO SNOW MAY MAKE IT DOWN TO 200 FEET OR SO.
FAIR CONDITIONS WILL HOLD OVER SOUTHCENTRAL WEDNESDAY NIGHT INTO
THURSDAY. A FRONT EXTENDING FROM A DEVELOPING LOW MOVING TO THE
BERING SEA WILL THEN TRACK TO KODIAK ISLAND LATE THURSDAY AND TO THE
REST OF SOUTHCENTRAL FRIDAY. WITH LOW PROGGED TO TRACK TO BRISTOL BAY
THERE WILL BE A STRONG EAST-WEST PRESSURE GRADIENT...SO EXPECT STRONG
TURNAGAIN ARM WINDS AND DOWNSLOPE DRYING. WILL BEGIN TO INDICATE
THESE WINDS IN THE NEW FORECAST. THERE IS ALSO THE POTENTIAL FOR A
BARRIER JET TO SET UP ALONG THE NORTH GULF COAST AS THE FRONT BECOMES
PARALLEL TO THE COASTLINE ON FRIDAY. FOR NOW WILL STICK WITH GALES
AND WAIT UNTIL WE GET A LITTLE CLOSER IN TIME BEFORE DECIDING WHETHER
TO UP WINDS TO STORMS.

SOUTHWEST ALASKA...WIDESPREAD SHOWERS ASSOCIATED WITH EASTERN BERING
SHORT-WAVE WILL MOVE ACROSS THE REGION TONIGHT. WEAKER WAVES EMBEDDED
IN THE FLOW AROUND NORTHERN BERING LOW WILL CONTINUE TO PRODUCE
SHOWERS ON WEDNESDAY...BUT EXPECT LESS WIDESPREAD AREAL
COVERAGE...ESPECIALLY INLAND. THERE WILL BE SOME DRYING WEDNESDAY
NIGHT INTO THURSDAY...BUT THE FRONT WILL BE UPON THE COAST BY
THURSDAY AFTERNOON. LIKE OVER SOUTHCENTRAL THERE WILL BE SOME
DOWNSLOPING WINDS ALONG THE ALEUTIAN AND ALASKA RANGES. HOWEVER AS
THE LOW TRACKS TOWARD BRISTOL BAY THURSDAY NIGHT FLOW WILL QUICKLY
SHIFT TO SOUTH...SO EXPECT WIDESPREAD RAIN ACROSS SOUTHWEST ALASKA.

BERING SEA/ALEUTIANS/AKPEN...SHOWERS WILL GRADUALLY DECREASE TONIGHT
ACROSS THE EASTERN BERING AND AKPEN AS SHORT-WAVE MOVES EASTWARD.
SURFACE RIDGE BUILDING OVER CENTRAL BERING AND ALEUTIANS AHEAD OF
NEXT SYSTEM WILL PRODUCE A TIGHT NORTH-SOUTH PRESSURE GRADIENT OVER
THE EASTERN BERING/AKPEN. THIS WILL CONTINUE TO PRODUCE GUSTY
WESTERLIES THROUGH WEDNESDAY MORNING. THE RAPIDLY DEVELOPING LOW
OVER THE NORTHWEST PACIFIC WILL TRACK TO WEST OF THE WESTERN
ALEUTIANS WEDNESDAY WITH A STRONG GALE FORCE FRONT MOVING EAST
ACROSS THE ALEUTIANS. THE LOW WILL GET WRAPPED UP ON THURSDAY WITH
A VERY TIGHT GRADIENT ALONG THE OCCLUSION SOUTH OF THE LOW CENTER.
WILL UP FORECAST WINDS TO STORM FORCE LATE WEDNESDAY NIGHT AND
THURSDAY ACROSS THE CENTRAL AND WESTERN ALEUTIANS. EXPECT SHOWERS
BEHIND THE LOW FRIDAY WITH THE NEXT SYSTEM ON THE DOORSTEP TO THE
WEST.

.LONG TERM FORECAST...VERY PROGRESSIVE HEMISPHERIC FLOW CONTINUES.
THEREFORE...EXPECT SYSTEMS TO CONTINUE TO TRACK FROM WEST TO EAST
ACROSS THE REGION. IT DOES APPEAR THAT A LITTLE BIT COOLER AIR WILL
GRADUALLY PULLED DOWN FROM THE NORTH AS EACH SYSTEM COMES
THROUGH...SO FREEZING LEVELS MAY DROP A BIT FURTHER.
HOWEVER...WITHOUT SOME AMPLIFICATION OF THE PATTERN DONT SEE ANY
STRONG COLD ADVECTION TO BRING SNOW TO AREAS NEAR SEA LEVEL.

.AER/ALU...WATCH/WARNING SUMMARY
PUBLIC...NONE.
MARINE...GALES 170 171 172 175 176.
FIRE WEATHER...NONE.

SEB SEP 07