Small Craft Advisory 546, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
135 PM AKDT FRI AUG 5 2011

.ANALYSIS AND UPPER LEVELS...A LARGE ANOMALOUSLY DEEP UPPER LEVEL
LOW IS CENTERED OVER THE BERING STRAIT/FAR NORTHERN BERING SEA.
THERE ARE ACTUALLY TWO LOW CENTERS DUMB-BELLING AROUND WITH ONE
OVER THE SEWARD PENINSULA MOVING NORTHWARD AND ANOTHER DROPPING
SOUTHEASTWARD OUT OF SIBERIA INTO THE NORTHERN BERING. BOTTOM LINE
IS THIS PATTERN CONTINUES TO BRING COOL AND WET CONDITIONS TO MUCH
OF MAINLAND ALASKA.

A SHARP UPPER LEVEL SHORT-WAVE AND SURFACE COLD FRONT CONTINUE TO
PROGRESS EASTWARD ACROSS SOUTHCENTRAL ALASKA THIS MORNING...WITH
A BAND OF MODERATE TO HEAVY RAIN RIGHT ALONG THE FRONTAL BOUNDARY.
LOW LEVEL COLD ADVECTION BEHIND THE FRONT HAS CREATED ENOUGH
INSTABILITY IN THE BOUNDARY LAYER (AS OBSERVED IN AREA SOUNDINGS
THIS MORNING) TO PRODUCE MOSTLY CLEAR SKIES FROM THE WESTERN GULF
WESTWARD TO PORTIONS OF SOUTHWEST ALASKA AND THE EASTERN BERING SEA.
FREEZING LEVELS HAVE FALLEN SIGNIFICANTLY BEHIND THE FRONT...WITH
THE LOWEST OBSERVED LEVEL AROUND 2500 FEET IN BETHEL AT 12Z THIS
MORNING. THIS UNSEASONABLY COLD AIR WILL IMPACT HIGH TEMPERATURES...
PRIMARILY OVER SOUTHWEST ALASKA...WITH FORECAST HIGHS FOR TODAY
AVERAGING 6 TO 10 DEGREES BELOW NORMAL.

IF YOU FOLLOW THE COLD FRONT SOUTHWARD TO THE NORTH-CENTRAL PACIFIC
IT CURVES BACK NORTHWESTWARD TO THE CENTRAL ALEUTIANS AND BECOMES
A WARM FRONT. THIS WARM FRONT IS DRIFTING NORTHEASTWARD BRINGING
WITH IT MUCH WARMER LOW LEVEL AIR. THE RESULT OF THIS IS EVIDENT
IN VISIBLE SATELLITE IMAGERY WHICH SHOWS STRATUS SLOWLY SPREADING
BACK NORTH AND EASTWARD INTO THE CENTRAL TO EASTERN BERING AND
TOWARD THE ALASKA PENINSULA.

THE LOW DROPPING OUT OB SIBERIA IS ALREADY SPREADING CLOUDS AND
RAIN TOWARD THE WEST COAST OF ALASKA...SO EXPECT CONDITIONS TO
QUICKLY HEAD DOWNHILL THIS AFTERNOON AND EVENING. THIS SYSTEM
WILL ALSO BRING WITH IT A REINFORCING SHOT OF COLD AIR.

.MODEL DISCUSSION...GFS/ECMWF/UKMET HAVE ALL TRENDED TOWARD A
MORE NORTHERLY TRACK OF THE UPPER LOW AS IT CROSSES SOUTHWEST
ALASKA SATURDAY AND HEAD INTO INTERIOR ALASKA SUNDAY. THIS WILL
HAVE MINOR AFFECTS ON LOCATION AND TIMING OF PRECIPITATION ACROSS
THE SOUTHERN MAINLAND. DIFFERENCES ARE ALSO NOTED AT THE SURFACE
WITH DEVELOPMENT OF A SURFACE LOW ALONG THE FRONT SOUTH OF SEWARD
SUNDAY MORNING. ALTHOUGH PREFER NAM IN OTHER PLACES DUE TO ITS
SUPERIOR RESOLUTION AND HANDLING OF TERRAIN PLAN TO TREND TOWARD
THE GFS/ECMWF/UKMET SOLUTION WITH THIS LOW. MINOR DIFFERENCES
ALSO EXIST WITH TRACK OF NEXT SURFACE LOW DROPPING OUT OF SIBERIA
ON SUNDAY. PLAN TO USE AN AVERAGE POSITION FOR NOW.

.SHORT TERM FORECAST...

SOUTHCENTRAL ALASKA...THE SOUTH END OF THE UPPER SHORT-WAVE WILL
SPEED RAPIDLY EASTWARD ACROSS THE GULF TONIGHT CAUSING THE TROUGH
TO BECOME NEGATIVELY TILTED. THIS MEANS THE SURFACE COLD FRONT AND
TROUGH WILL HANG UP OVER THE COPPER RIVER BASIN THROUGH TONIGHT.
THE SOUTHERN AND WESTERN SIDES OF THE MOUNTAINS IN PARTICULAR WILL
CONTINUE TO SEE MODERATE TO HEAVY RAIN AT TIMES THROUGH AT LEAST
THE EVENING HOURS BEFORE THINGS START TO DIMINISH AFTER MIDNIGHT.
THE REST OF SOUTHCENTRAL WILL CONTINUE TO SEE IMPROVING CONDITIONS
THIS AFTERNOON AND EVENING...ESPECIALLY FROM THE MAT-SU TO ANCHORAGE
AND THE KENAI PENINSULA WHERE MOSTLY SUNNY SKIES WILL MAKE A BRIEF
APPEARANCE.

CONDITIONS WILL HEAD DOWNHILL DURING THE DAY SATURDAY OVER THESE
SAME AREAS AS NEXT SHORT-WAVE TROUGH AND SURFACE FRONTAL SYSTEM
MOVE IN. AS WITH THE PAST COUPLE EVENTS THE DYNAMICS WILL BE STRONG
AND MOISTURE ABUNDANT. SO ONCE AGAIN EXPECT WIDESPREAD MODERATE TO
HEAVY RAINFALL. WITH A DEEP LAYER OF SOUTH-SOUTHWEST FLOW AND A
VORT-MAX PROGGED TO TRACK RIGHT UP THE INLET AND THEN OVER THE
TALKEETNA MOUNTAINS...EXPECT THE HEAVIEST RAINFALL ALONG THE WEST
SIDE OF THE CHUGACH AND TALKEETNA MOUNTAINS. THIS SHOULD ALLOW
STREAMS IN THE WESTERN SUSITNA VALLEY (WHERE A FLOOD ADVISORY IS
CURRENTLY IN EFFECT) TO CONTINUE TO GRADUALLY FALL. WILL NEED TO
KEEP AN EYE ON RIVERS AND STREAMS FURTHER EAST IN THE MATANUSKA
VALLEY FOR POTENTIAL SWELLING OF WATER LEVELS TO BANK FULL. ALL OF
THIS RAIN WILL CONTINUE EASTWARD TO PRINCE WILLIAM SOUND AND THE
COPPER RIVER BASIN SATURDAY NIGHT. SIMILAR TO TODAY...CONDITIONS
WILL IMPROVE FROM WEST TO EAST ACROSS SOUTHCENTRAL SUNDAY AND SUNDAY
AS EVERYTHING EXITS TO THE NORTH AN EAST. CYCLONIC UPPER LEVEL FLOW
WILL CONTINUE HOWEVER AND WEAK EMBEDDED WAVES WILL PRODUCE AT LEAST
A FEW CLOUDS AND SHOWERS.

THIS WHOLE SCENARIO LOOKS LIKE IT WILL PLAY OUT ONCE AGAIN ON MONDAY
AS ANOTHER MORE SIGNIFICANT SHORT-WAVE TROUGH ROTATES AROUND THE
UPPER LOW INTO SOUTHCENTRAL. AT THIS POINT IN TIME IT LOOKS LIKE
IT WILL BE WEAKER SO RAINFALL AMOUNTS WILL LIKELY BE LOWER.

SOUTHWEST ALASKA...AFTER A BRIEF BREAK IN RAIN AND CLOUDS IN SOME
LOCATIONS...WIDESPREAD RAIN WILL RETURN TO SOUTHWEST ALASKA LATE
TONIGHT THROUGH TOMORROW. THE UPPER LEVEL SHORT-WAVE WILL MOVE AT
A DECENT CLIP SO DON'T EXPECT STEADY RAIN TO LAST MORE THAN 12 HOURS.
ANOTHER SHORT BREAK CAN BE EXPECTED LATE SATURDAY INTO SUNDAY MORNING
BEFORE THE NEXT IN WHAT IS TURNING OUT TO BE A LONG LINE OF SYSTEMS
MOVES IN SUNDAY AFTERNOON THROUGH MONDAY. WITH A RE-SUPPLY OF COLD
AIR ALOFT WITH EACH OF THESE SYSTEMS SURFACE TEMPERATURES (ESPECIALLY
AFTERNOON HIGHS) WILL REMAIN WELL BELOW NORMAL FOR THIS TIME OF YEAR.

BERING SEA/ALEUTIANS...ACTIVE WEATHER WILL REMAIN FOCUSED FROM THE
NORTHERN BERING TO SOUTHWEST ALASKA WITH SYSTEMS JUST CLIPPING THE
EASTERN ALEUTIANS AND SOUTHERN ALASKA PENINSULA. WEAK HIGH PRESSURE
WILL DOMINATE THE WESTERN ALEUTIANS SO STRATUS AND FOG WILL PERSIST.

.LONG TERM FORECAST...CONFIDENCE IS INCREASING THAT A PATTERN CHANGE
WILL OCCUR IN THE MIDDLE TO END OF NEXT WEEK. ECMWF/GFS BOTH DEPICT
ENERGY FROM THE UPPER LOW SPLITTING AND MOVING IN OPPOSITE DIRECTIONS
HELPING TO ELONGATE INTO AN EAST-WEST TROUGH AT THE HIGHER LATITUDES.
THIS IN TURN WILL ALLOW A RIDGE TO BUILD OVER THE BERING SEA MID WEEK
AND HEAD EASTWARD TOWARD THE MAINLAND AS WE HEAD THROUGH THE WEEK.
ENSEMBLES SUPPORT THIS GENERAL SCENARIO. IF THE PATTERN DOES EVOLVE
THIS WAY EXPECT THE SOUTHERN MAINLAND TO FINALLY SEE A FEW DAYS IN
A ROW OF DRIER AND WARMER WEATHER. THE WESTERN ALEUTIANS AND WESTERN
BERING SEA WOULD ALSO BECOME MORE ACTIVE ON THE WEST SIDE OF THE
RIDGE LATE NEXT WEEK.

.AER/ALU...WATCH/WARNING SUMMARY
PUBLIC...FLOOD ADVISORY 145.
MARINE...NONE.
FIRE WEATHER...NONE.

SEB AUG 11