Small Craft Advisory 783, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
130 PM AKDT SUN OCT 6 2013

.ANALYSIS AND UPPER LEVELS...A LARGE UPPER LEVEL LOW IS ANCHORED
OVER THE NORTHERN BERING SEA WITH A ZONAL SUB-TROPICAL JET TO THE
SOUTH STREAMING ACROSS THE SOUTHERN BERING SEA. A SERIES OF FAIRLY
WEAK SHORT-WAVES ARE LIFTING NORTHEASTWARD UP THE EAST SIDE OF THE
LOW PRODUCING AREAS OF RAIN (OR SHOWERS) ACROSS SOUTHWEST AND
SOUTHCENTRAL ALASKA. MEANWHILE...THE FOCUS OF THE SHORT-TERM
WEATHER IS A STRONG SHORT-WAVE DIGGING SOUTHEASTWARD ACROSS THE
NORTHWEST PACIFIC. THIS WILL CREATE FAVORABLE CONDITIONS FOR
DEVELOPMENT OF A MAJOR AUTUMN STORM WHICH WILL IMPACT MAINLAND
ALASKA EARLY THIS WEEK.

.MODEL DISCUSSION...THERE CONTINUES TO MODEL DIFFERENCES IN THE
EXACT TRACK AND STRENGTH OF THE DEVELOPING SURFACE LOW MONDAY
THROUGH TUESDAY. HOWEVER...THE SPREAD IN SOLUTIONS IS NARROWING
AS WE GET CLOSER IN TIME AND THERE HAS BEEN A DEFINITE TREND
TOWARD A DEEPER/STRONGER SOLUTION. THUS...WILL CONTINUE THE TREND
OF STRENGTHENING FORECAST WINDS. AS FAR AS TRACK...THE ECMWF
PROVIDES A NICE MIDDLE GROUND AMONG THE VARIOUS MODELS.

.SHORT TERM FORECAST...RAIN WILL TAPER OFF OVER MOST OF THE SOUTHERN
MAINLAND TONIGHT AS THE PRIMARY UPPER LEVEL WAVES EXIT TO THE NORTH.
MEANWHILE...THE DIGGING NORTHWEST PACIFIC TROUGH WILL CAUSE
AMPLIFICATION OF THE DOWNSTREAM RIDGE AND STRENGTHENING OF THE
SUBTROPICAL JET...LEADING TO CYCLOGENESIS OVER THE NORTH-CENTRAL
PACIFIC. THIS DEVELOPING SYSTEM WILL TAP INTO TROPICAL MOISTURE
WHICH WILL BE RAPIDLY TRANSPORTED NORTHWARD TOWARD ALASKA. WHILE
THERE ARE ENOUGH INGREDIENTS IN PLACE TO DEVELOP A MODERATE STRENGTH
LOW...THE REAL KEY HERE IS A SECONDARY SHORT-WAVE DROPPING DOWN OUT
OF RUSSIA WHICH WILL PHASE WITH THE FIRST TROUGH EARLY MONDAY MORNING
LEADING TO MORE SIGNIFICANT INTENSIFICATION OF THE SURFACE LOW MONDAY
THROUGH MONDAY NIGHT.

ALTHOUGH THERE IS UNCERTAINTY IN THE EXACT TRACK AND STRENGTH OF
THIS LOW...THERE IS HIGH CONFIDENCE THAT THIS WILL BE A MAJOR EARLY
SEASON STORM IMPACTING ALL OF THE SOUTHERN MAINLAND OF ALASKA. THE
MORE GENERAL IMPACTS WILL BE WIDESPREAD HEAVY RAIN AND STRONG WINDS.
THE STORM WILL MOVE AT A GOOD CLIP WHICH MEANS THE DURATION OF HEAVY
RAIN AND STRONG WINDS IN ANY ONE SPOT WILL TEND TO BE ON THE SHORT
SIDE. BASED ON THIS...THE THREAT OF FLOODING ALONG RIVERS AND
STREAMS LOOKS TO BE MINIMAL. FOCUSING IN ON MORE SPECIFIC THREATS
WHICH ARE LIKELY WITH THIS SYSTEM...

THE GENERAL LOW TRACK IS VERY FAVORABLE FOR HIGH WINDS ALONG WEST
TURNAGAIN ARM AND HIGHER ELEVATIONS AND FORECAST CONFIDENCE IS HIGH
FOR THESE AREAS. THE MORE DIFFICULT FORECAST IS DETERMINING HOW MUCH
WIND WILL GET INTO LOWER ELEVATIONS OF ANCHORAGE. ONE ASPECT OF THIS
STORM WHICH LOOKS FAVORABLE FOR TURNAGAIN ARM WINDS BENDING INTO
ANCHORAGE IS THE SURFACE LOW CROSSING THE 60 DEGREES NORTH LATITUDE
LINE AROUND OR PRIOR TO PASSAGE OF THE OCCLUDED FRONT THROUGH
ANCHORAGE. A SLIGHT CHANGE IN THIS TIMING COULD RESULT IN MUCH WEAKER
WINDS. THE OTHER MAIN ASPECT TO CONSIDER FOR WINDSTORMS IS STABILITY.
IN THIS CASE...THE FRONT OCCLUDES WELL BEFORE IT REACHES
ANCHORAGE...SO LOW LEVEL WARMING IS MINOR AND ANY INVERSIONS THAT
FORM WILL BE WEAK. A MORE UNSTABLE ENVIRONMENT LEADS TO MORE VERTICAL
MIXING OF WINDS...BUT ALSO WORKS AGAINST THE VERY STRONG ERRATIC
WINDS COMMON WITH MOUNTAIN WAVES. WHEN YOU CONSIDER ALL OF THIS
WOULD EXPECT THAT THE STRONGER WINDS WOULD MIX DOWN LOWER THAN A
TYPICAL HIGH WIND EVENT...PERHAPS TO THE LOWER HILLSIDE....BUT WOULD
NOT EXPECT WIDESPREAD HIGH WINDS DOWN IN THE LOWER ELEVATIONS. ONE
OTHER MAJOR FACTOR THAT MUST BE TAKEN INTO ACCOUNT IS THAT THIS IS
AN EARLY SEASON STORM IN WHICH THE GROUND IS NOT YET FROZEN AND
LEAVES ARE STILL ON MANY OF THE TREES...SO LOWER WINDS SPEEDS CAN
CAUSE TREES AND POWER LINES TO FALL THAN IN THE MIDDLE OF WINTER.
BOTTOM LINE...IT LOOKS LIKE ALL OF ANCHORAGE WILL EXPERIENCE SOME
STRONG WINDS MONDAY NIGHT THROUGH TUESDAY MORNING...BUT WILL NEED
TO WAIT UNTIL CLOSER TO THE EVENT TO NAIL DOWN EXACTLY WHAT SPEEDS
CAN BE EXPECTED. FOR NOW A HIGH WIND WATCH IS IN EFFECT FOR ALL OF
THE ANCHORAGE AREA.

SHIFTING WESTWARD ANOTHER POTENTIAL IMPACT IS COASTAL FLOODING ALONG
THE KUSKOKWIM BAY. THE LOW IS EXPECTED TO MAKE LANDFALL SOMEWHERE
ALONG THE KUSKOKWIM DELTA MONDAY NIGHT WITH STRONG WESTERLIES AND
HIGH SEAS MOVING IN BEHIND THE LOW DURING THE DAY TUESDAY. STORM
SURGE GUIDANCE INDICATES MINOR FLOODING IS POSSIBLE WITH HIGH TIDE
TUESDAY AFTERNOON...BUT THIS GUIDANCE IS BASED ON THE GFS SOLUTION
WHICH IS STRONGER THAN MOST OF THE OTHER MODELS. THE EXPECTED WINDS
AND SEAS ALONG THE SOUTHWEST COAST IS ALSO HEAVILY DEPENDENT ON THE
LOW TRACK. THEREFORE...WILL NOT ISSUE ANYTHING YET AND WILL
RE-EVALUATE THE THREAT WHEN CONFIDENCE IN THE LOW TRACK INCREASES.

FINALLY...WITH THE LOW TRACKING INLAND AND PROGRESSIVE UPPER LEVELS
LEADING TO HIGH PRESSURE QUICKLY BUILDING IN BEHIND IT...EXPECT
STRONG PRESSURE RISES ACROSS SOUTHWEST ALASKA TUESDAY AFTERNOON
THROUGH TUESDAY NIGHT. THESE PRESSURE RISES CONTRIBUTE TO STRONGER
WINDS EXTENDING FARTHER INLAND THAN NORMAL...ESPECIALLY ACROSS THE
KUSKOKWIM DELTA (INCLUDING BETHEL) AND BRISTOL BAY AREAS.

COINCIDENT WITH THESE STRONGER WINDS ON THE BACK SIDE...SOLDER AIR
WILL ALSO MOVE IN...POSSIBLY CHANGING ANY REMAINING PRECIPITATION
TO SNOW. ANOTHER UPPER LOW/TROUGH WILL SWING OUT OF ASIA INTO THE
BERING SEA ON TUESDAY...BUT WITH ESTABLISHMENT OF A LESS AMPLIFIED
PATTERN THIS SYSTEM WILL LIKELY BE MUCH WEAKER.

.LONG TERM FORECAST...UPPER LEVEL FLOW LOOKS LIKE IT WILL REMAIN
PROGRESSIVE SUPPORTED BY A STRONG SUBTROPICAL JET CENTERED OVER THE
NORTH PACIFIC TO SOUTHERN BERING SEA. BASED ON THIS EXPECT ACTIVE
WEATHER TO CONTINUE ACROSS THE REGION WITH TEMPERATURES REMAINING
WARM ENOUGH FOR PREDOMINANTLY RAIN FOR IN THE LOWLANDS.

.AER/ALU...WATCH/WARNING SUMMARY
PUBLIC...HIGH WIND WATCH 101.
MARINE...GALE WARNING...130 131 132 136 137 138
                        155 160 165 170 172 180.
FIRE WEATHER...NONE.

SEB OCT 13