Small Craft Advisory 572, NWS Anchorage, AK

AREA FORECAST DISCUSSION
SOUTHCENTRAL AND SOUTHWEST ALASKA
125 PM ADT SAT JUL 7 2007

.ANALYSIS AND UPPER LEVELS...LITTLE CHANGE FROM YESTERDAY IN THE
UPPER LEVELS WITH A REX BLOCK EXTENDING ACROSS THE BERING SEA
AND NORTHWEST ALASKA TO THE BEAUFORT SEA. THERE ARE TWO CLOSED
LOWS SOUTH AND EAST OF THIS RIDGE...ONE STATIONARY OVER THE SOUTHERN
GULF OF ALASKA...AND ANOTHER OVER THE NORTH PACIFIC TRACKING SLOWLY
EASTWARD NEAR 44N 164W. A NEARLY ZONAL UPPER JET EXTENDS ACROSS THE
NORTH PACIFIC BETWEEN 35N AND 45N TO AROUND THE POSITION OF THE UPPER
LOW AND THEN BUCKLES SLIGHTLY AT THE POSITION OF A DEVELOPING SURFACE
LOW BEFORE HEADING NORTHEAST TOWARD BRITISH COLUMBIA. THE LOW IS IN
GOOD POSITION TO CONTINUE DEEPENING AND UPPER LOW WILL MOVE OVERHEAD
WITHIN THE NEXT 6 TO 12 HOURS.

NORTHEAST FLOW ALONG THE EAST SIDE OF THE REX BLOCK CONTINUES TO
BRING SHORT-WAVES FROM THE YUKON SOUTHWESTWARD ACROSS SOUTHCENTRAL
AND SOUTHWEST ALASKA. ONE WAVE COMBINED WITH AN ALREADY UNSTABLE
AIRMASS PRODUCED THUNDERSTORMS ALL NIGHT ACROSS THE INTERIOR OF
ALASKA. AS THESE STORMS APPROACHED AND CROSSED THE ALASKA RANGE
THEY RAN INTO A MORE STABLE AIR MASS...SO NO THUNDERSTORMS HAVE
BEEN OBSERVED SOUTH OF THE RANGE. RADAR IMAGERY ALSO SHOWS CHARACTER
OF PRECIP MOVING ACROSS THE RANGE HAS BECOME MORE STRATIFORM.
SURFACE OBS SHOW AREAS OF RAIN AND FOG ALONG THE SOUTH SIDE OF THE
RANGE. RAIN WILL CONTINUE TO SPREAD SOUTHWARD ACROSS THE SUSITNA
VALLEY AND COPPER RIVER BASIN THIS AFTERNOON.

THERE IS FAIRLY EXTENSIVE MAINLY MID TO HIGH CLOUD COVER OVER
SOUTHWEST ALASKA THIS MORNING AND ISOLATED SHOWERS. VISIBLE
SATELLITE IMAGERY SHOWS CUMULUS BUILDING IN CLEAR AREAS AND WHERE
CLOUDS ARE THIN. THERE ARE ALSO ISOLATED THUNDERSTORMS JUST NORTH
OF THE KUSKOKWIM VALLEY TRACKING SOUTHWESTWARD. SO EVEN WITH A
STABLE SOUNDING OUT OF MCGRATH THIS MORNING...THERE APPEARS TO BE
ENOUGH SURFACE HEATING GOING FOR ISOLATED THUNDERSTORMS TO FORM
LATER THIS AFTERNOON. WEAK UPPER WAVES WILL ALSO PROVIDE A LITTLE
LIFT TO HELP OUT.

MOST OF THE EFFECTS OF THE UPPER LOW OVER THE GULF ARE BEING FELT
OVER WATER. THE EXCEPTION IS KODIAK ISLAND WHERE BANDS OF CLOUDS
AND RAIN SHOWERS CONTINUE TO ROTATE ONSHORE.

BACK OVER THE BERING SEA UNDER THE RIDGE IS AND ABUNDANCE OF STRATUS
AND FOG. BASED ON THE SPARSE NETWORK OF OBSERVATIONS OUT THERE FOG
DOES NOT APPEAR TO BE WIDESPREAD AT THIS TIME...MOST OBS ARE SHOWING
STRATUS.

.MODEL DISCUSSION...MODELS CONTINUE TO DISPLAY SIGNIFICANT
DIFFERENCES IN THE EVOLUTION OF DEVELOPING SYSTEM OVER THE NORTH
PACIFIC. THE GFS ENSEMBLE STILL SHOWS A LARGE SPREAD OF SOLUTIONS.
HOWEVER THE OPERATIONAL RUN CONTINUES TO MATCH UP WELL WITH THE
ENSEMBLE MEAN AND SHOWS GOOD RUN-TO-RUN CONSISTENCY. SINCE THIS
SOLUTION IS WHAT THE PAST FEW FORECASTS HAVE BEEN BASED ON AND DONT
HAVE ENOUGH INFORMATION TO CHANGE TRACKS YET...WILL STICK WITH THE
GFS. HOWEVER...WILL MONITOR THE EVOLUTION OF THIS SYSTEM CLOSELY THE
NEXT 12 HOURS. THERE ARE ASPECTS OF THE NAM/UKMET SOLUTIONS THAT MAKE
A LOT OF SENSE. ALL MODELS SHOW THE UPPER LOW MOVING OVERHEAD THE
SURFACE LOW WITHIN THE NEXT 6 TO 12 HOURS AND SLOWING PROGRESSION.
THE NAM/UKMET ARE DIFFER FROM THE GFS IN SHOWING A WEAK TRIPLE POINT
LOW FORMING AS THE LOW CENTER ELONGATES AND THE FRONT IS PUSHED OFF
TO THE NORTHEAST...AND THIS LOW BECOMES DOMINANT. THIS IN TURN
RESULTS IN A FASTER AND MORE EASTERLY SURFACE LOW TRACK. THE NAM
IS FASTEST AND FARTHEST EAST WHILE THE UKMET IS IN BETWEEN THE NAM
AND GFS. THIS EVOLUTION MAKES SENSE...BUT THE UPPER LEVELS COMPLICATE
THINGS. AS THE SYSTEM GETS CLOSER TO THE GULF AND RIDGE BUILDS AHEAD
OF DEVELOPING SYSTEM THE SOUTHWESTERN GULF LOW WILL BE PULLED
SOUTHWEST AND AROUND THE BACK SIDE OF THE NEW SYSTEM. THE NAM/UKMET
SOLUTIONS MAKE THIS WESTERN LOW THE DOMINANT UPPER CLOSED FEATURE
WHILE THE GFS SOLUTION MAKES THE NEW SYSTEM CIRCULATION DOMINANT.
THIS OF COURSE IS THE KEY TO WHICH DIRECTION THE LOW WILL BE FORCED.
SO...WILL MONITOR THESE ASPECTS TO DETERMINE WHICH SOLUTION IS
CORRECT IN UPCOMING FORECASTS.

.SHORT TERM FORECAST...

SOUTHCENTRAL ALASKA...THE WESTERN HALF OF THE COPPER BASIN...THE
SUSITNA VALLEY...AND THE MAT VALLEY LOOK LIKE A SURE BET TO GET A
WETTING RAIN TONIGHT AS UPPER WAVES TRACK SOUTHWESTWARD ACROSS THE
AREAS. EXTENT OF PRECIP ALONG THE EDGE OF THESE AREAS...INCLUDING
ANCHORAGE...IS TRICKIER. WITH BEST LIFT OFF TO THE NORTH AND
WEST...THINK ANY RAIN THAT FALLS FROM THE SOUTHERN COPPER BASIN TO
PRINCE WILLIAM SOUND TO THE KENAI PENINSULA WILL BE VERY LIGHT. FOR
ANCHORAGE PATTERN WILL LIKELY BE SIMILAR TO RECENT CASES WITH HIGHER
AMOUNTS AS YOU HEAD NORTH AND WEST. THERE ARE MORE UPPER WAVES TO
MOVE THROUGH SUNDAY SO EXTENSIVE CLOUD COVER WILL REMAIN. AFTER A
BREAK SUNDAY MORNING BEHIND THE FIRST WAVE THERE WILL BE A SHOT AT
SHOWERS OR LIGHT RAIN THROUGH SUNDAY EVENING. THE UPPER LEVEL FLOW
WILL THEN BEGIN TO SHIFT SUNDAY NIGHT TO EAST TO SOUTHEAST IN
RESPONSE TO THE PACIFIC SYSTEM TRACKING TOWARD KODIAK. THIS WILL
RESULT IN ANOTHER SHORT BREAK SUNDAY NIGHT INTO MONDAY. HOWEVER...AN
UPPER SHORT-WAVE AND SURFACE FRONT WILL APPROACH SOUTHCENTRAL FROM
THE SOUTH LATE MONDAY...SPREADING RAIN TO THE COAST AND SHOWERS
INLAND MONDAY NIGHT INTO TUESDAY.

SOUTHWEST ALASKA...THE KEY TO THE EXTENT OF CONVECTION OVER SOUTHWEST
ALASKA THE NEXT FEW DAYS IS CLOUD COVER. WEAK UPPER WAVES CONTINUE
TO MOVE FROM NORTHEAST TO SOUTHWEST ACROSS THE REGION...MAINTAINING
QUITE A BIT OF CLOUDINESS OVERHEAD AND LIMITING SURFACE HEATING.
MODELS INDICATE WARMER LOW LEVEL AIR ADVECTING INTO THE SOUTHWEST
ALASKA SUNDAY RESULTING IN MORE INSTABILITY. HOWEVER...THE STRONGER
WAVES MOVING ACROSS SOUTHCENTRAL TONIGHT WILL TRACK ALONG THE ALASKA
AND ALEUTIAN RANGES SUNDAY...BRINGING RAIN ALONG THE MOUNTAINS AND
POSSIBLY EVEN WEST OF THE MOUNTAINS. THIS COULD SPREAD A THICK LAYER
OF CLOUDS WESTWARD OVER MUCH OF THE REGION AND LIMIT SURFACE HEATING
AND CONVECTION. THEREFORE...THINK THE BEST CHANCE OF THUNDERSTORMS
ON SUNDAY WILL BE ALONG THE KUSKOKWIM MOUNTAINS. IF THERE ARE SOME
BREAKS IN THIS AREA THE SHORT-WAVES COULD PROVIDE SOME ADDITIONAL
LIFT. MONDAY LOOKS A LITTLE BETTER FOR BREAKS IN THE CLOUDS SO
EXPECT AT LEAST ISOLATED THUNDERSTORMS FROM THE KUSKOKWIM VALLEY
TO THE DELTA. ON TUESDAY SOUTHWEST ALASKA WILL BE BETWEEN THE SYSTEM
APPROACHING THE GULF AND A TROUGH DROPPING DOWN FROM THE NORTH...SO
ONCE AGAIN IF THERE ARE ANY BREAKS IN THE CLOUDS IT COULD BE AN
ACTIVE THUNDERSTORM DAY.

BERING SEA...THE COMBINATION OF SHORT-WAVE DROPPING DOWN INTO THE
NORTHERN BERING AND SYSTEM MOVING TO AKPENN WILL HELP SHOVE RIDGE
WESTWARD ON TUESDAY. THIS WILL BRING A CHANCE OF PRECIP TO THE
EASTERN BERING AND SOUTHERN AKPENN AS WELL AS A LITTLE WIND.
OTHERWISE...CONTINUED FOG AND STRATUS UNDER THE RIDGE.

.LONG TERM FORECAST...MODELS INDICATE BERING RIDGE FINALLY BREAKING
DOWN DURING THE WEEK AS SYSTEMS PUSH ON IT FROM ALL SIDES. THE UPPER
LOW SOUTH OF THE AKPENN TUESDAY LOOKS LIKE IT WILL DRIFT NORTH OVER
SOUTHWEST ALASKA AND SIT IN PLACE. SO THERE IS NO LETUP IN SITE FOR
THE CLOUDY AND WET WEATHER ACROSS THE SOUTHERN MAINLAND AS WE LOOK
TO THE END OF NEXT WEEK. THE EASTERN BERING...EASTERN
ALEUTIANS...SOUTHERN AKPENN MAY SEE SOME PRECIP BY THE END OF THE
WEEK. FURTHER WEST WILL REMAIN DRY...BUT SYSTEMS WILL BE APPROACHING
BY NEXT WEEKEND AND THIS COULD POTENTIALLY BE THE BEGINNING OF A
LARGER PATTERN CHANGE.

.AER/ALU...WATCH/WARNING SUMMARY
PUBLIC...NONE.
MARINE...NONE.
FIRE WEATHER...NONE.

SEB JUL 07