Small Craft Advisory 809, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
116 PM AKDT TUE OCT 15 2013

.ANALYSIS AND UPPER LEVELS...A LARGE VERTICALLY STACKED LOW IS
CENTERED OVER THE NORTH-CENTRAL BERING SEA WITH A HIGH AMPLITUDE
TROUGH EXTENDING SOUTHWARD WELL INTO THE PACIFIC. THE LOW IS
WEAKENING AND LIFTING NORTHWARD WHILE THE TROUGH DIGS AS
SHORT-WAVES DROP DOWN THE BACK SIDE OF THE TROUGH. AMPLIFICATION
OF THE TROUGH IS FURTHER SUPPORTED BY A STRONG SUBTROPICAL JET
(175 TO 200 KT) AND LOW LEVEL COLD AIR ADVECTION.

MEANWHILE THE FOCUS FOR WEATHER OVER MAINLAND ALASKA THIS MORNING
IS A SHORT-WAVE MOVING UP THE EAST SIDE OF THIS TROUGH AND OVER
TOP OF A STRONG HIGH AMPLITUDE RIDGE WHICH EXTENDS FROM THE
NORTHEAST PACIFIC UP TO MAINLAND ALASKA. MUCH OF THE MOISTURE
WITH THIS ORIGINATED IN THE TROPICS AND THIS IS CONTRIBUTING TO
MODERATE TO HEAVY RAIN ACROSS PORTIONS OF SOUTHCENTRAL AND SOUTHWEST
ALASKA. AT THE SURFACE A TRIPLE POINT IS TRACKING NORTHWARD ALONG
THE SOUTHWEST COAST THIS MORNING WITH A WARM FRONT EXTENDING
EASTWARD TO SOUTHCENTRAL AND A COLD FRONT TRAILING SOUTHWARD ACROSS
THE EASTERN BERING AND ALASKA PENINSULA. THE BIG IMPACT OF THIS
FRONTAL SYSTEM IS STRONG SOUTH TO SOUTHEAST LOW LEVEL WINDS. THIS
IN TURN IS PRODUCING DOWNSLOPING IN SOME AREAS (RESULTING IN MUCH
LIGHTER RAINFALL) AND UPSLOPE IN OTHER AREAS (ENHANCING RAINFALL).

.MODEL DISCUSSION...MODELS CONTINUE TO DEPICT SIMILAR SOLUTIONS
WITH LARGE SCALE FEATURES. THE MOST NOTABLE DIFFERENCES IN THE
SHORT-TERM ARE WITH THE INTERACTION OF THE DIGGING TROUGH OVER
THE NORTH PACIFIC AND A CLOSED SPINNING WAY DOWN AROUND 40N 153W.
THE EVOLUTION OF THIS HAS CHANGED A BIT SINCE MODEL RUNS YESTERDAY
MORNING. MOST SOLUTIONS NOW INDICATE FORMATION OF A LOW ALONG THE
SOUTH END OF THE COLD FRONT AND WEAKENING OF THE ORIGINAL LOW IN
THE TROPICS. THIS NEW LOW IS THEN THE ONE TO TRACK NORTHWARD TO
SOUTHWEST ALASKA. DIFFERENCES IN SPEED AND TRACK OF THIS LOW
PERSIST AND AFFECT THE STRENGTH OF WINDS ACROSS THE GULF OF ALASKA
AND SOUTHERN MAINLAND AS WELL AS THE DISTRIBUTION OF PRECIP
WEDNESDAY AND THURSDAY...SO FORECAST CONFIDENCE IN THE DETAILS IS
BELOW AVERAGE. THERE IS MUCH HIGHER CONFIDENCE IN THE GENERAL
IMPACTS. THE DIGGING TROUGH WILL TAP INTO TROPICAL MOISTURE (WHICH
WILL ONCE AGAIN STREAM NORTHWARD TO MAINLAND ALASKA) AND THERE WILL
BE FAIRLY WIDESPREAD STRONG WINDS AS THE LOW PRESSURE SYSTEM MOVES
THROUGH THE REGION.

.SHORT TERM FORECAST...RAIN WILL DIMINISH ACROSS MUCH OF SOUTHWEST
AND SOUTHCENTRAL ALASKA THIS AFTERNOON AND EVENING AS THE UPPER
SHORT-WAVE OVER THE EASTERN BERING ALONG WITH THE SURFACE TRIPLE
POINT AND ASSOCIATED FRONTAL SYSTEM QUICKLY EXIT TO THE NORTH.
HOWEVER...A NARROW BAND OF RAIN WILL PERSIST...SUPPORTED BY A
PIPELINE OF TROPICAL MOISTURE BETWEEN THE UPPER TROUGH AND RIDGE.
THE UPPER RIDGE WILL BUILD NORTH AND WESTWARD IN THE WAKE OF THE
SHORT-WAVE WHICH WILL PUSH THIS BAND OF RAIN WESTWARD ACROSS
SOUTHWEST ALASKA TONIGHT THROUGH WEDNESDAY.

MEANWHILE...THE NEXT SYSTEM MOVING UP FROM THE TROPICS (AS DISCUSSED
ABOVE) WILL BRING ANOTHER BATCH OF RAIN TO THE REGION BY WEDNESDAY
AFTERNOON. THERE IS UNCERTAINTY IN THE TRACK OF THIS SYSTEM WHICH
MEANS SOME UNCERTAINTY IN THE PRECIP AND WIND FORECASTS. BASED ON
THE SPREAD OF SOLUTIONS IT LOOKS LIKE MOST OF THE RAIN WILL
INITIALLY HEAD TO KODIAK ISLAND...THE ALASKA PENINSULA...AND INTO
SOUTHWEST ALASKA WHILE SOUTHCENTRAL ALASKA STAYS MOSTLY DRY. AS THE
SYSTEM PROGRESSES NORTHWARD WEDNESDAY NIGHT AND THURSDAY RAIN WILL
THEN SPREAD INTO SOUTHCENTRAL.

SINCE THIS SYSTEM WILL BE STRONGER THAN THE TRIPLE POINT AND FRONTAL
SYSTEM TODAY...EXPECT WINDS WILL ALSO BE STRONGER. THUS...THERE IS
POTENTIAL FOR REACHING HIGH WIND LEVELS ALONG TURNAGAIN ARM AND
HIGHER ELEVATIONS SOMETIME LATE THURSDAY. WILL HOLD OFF ON A HIGH
WIND WATCH UNTIL HAVE MORE CONFIDENCE IN LOW INTENSITY AND TRACK.
MORE SIGNIFICANTLY THOUGH...EXPECT YET ANOTHER STRONG WIND EVENT
FOR MANY OF THE LOWLANDS (INCLUDING ANCHORAGE) AS A COLD FRONT
SWEEPS THROUGH BEHIND THIS LOW.

STRONG SOUTHEAST LOW LEVEL FLOW WILL BE VERY CONDUCIVE TO PRODUCING
DOWNSLOPE DRYING ON THE WEST SIDE OF MOUNTAIN RANGES. THUS...EXPECT
LITTLE RAIN IN THE LEE OF THE KENAI AND CHUGACH MOUNTAINS. ON THE
OTHER HAND...EXPECT HEAVY RAIN IN UPSLOPE AREAS. A FLOOD ADVISORY
IS CURRENTLY IN EFFECT FOR MINOR FLOODING ALONG KENAI LAKE AND THE
UPPER KENAI RIVER THROUGH WEDNESDAY AFTERNOON. WATER LEVELS ARE
FALLING NOW...BUT THESE RAINS ON THURSDAY HAVE POTENTIAL TO CAUSE
WATER LEVELS TO RISE BACK ABOVE FLOOD STAGE.

OUT WEST...A SYSTEM NEAR THE KAMCHATKA PENINSULA WILL RAPIDLY CROSS
THE ALEUTIANS TONIGHT AND DROP DOWN INTO THE NORTH PACIFIC BY LATE
WEDNESDAY AS IT MOVES DOWN THE BACK SIDE OF THE UPPER TROUGH. THE
REMNANTS OF POWERFUL TYPHOON WIPHA (CURRENTLY AFFECTING JAPAN) WILL
THEN MAKE ITS WAY TO THE ALEUTIANS THURSDAY AS A VERY DEEP
EXTRA-TROPICAL LOW. THIS WILL BE A VERY LARGE STORM SYSTEM SO
EXPECT IT TO IMPACT THE ENTIRE FORECAST AREA FROM THE BERING SEA
TO SOUTHERN MAINLAND OF ALASKA AS IT PROGRESSES EASTWARD LATE THIS
WEEK. AS HAS BEEN THE STORY LATELY...THIS MEANS ANOTHER ROUND OF
RAIN AND WIND.

.LONG TERM FORECAST...THE FORMER TYPHOON WILL BE THE MAIN WEATHER
MAKER FOR LATE THIS WEEK AND INTO THE WEEKEND. THERE ARE MODEL
DIFFERENCES AS FAR AS WHERE THIS LOW SETTLES IN DURING THAT TIME
FRAME. HOWEVER...THE GENERAL PATTERN OF A VERY LARGE CYCLONIC
CIRCULATION CENTERED OVER THE EASTERN BERING/WESTERN ALASKA WITH
NUMEROUS SHORT-WAVES TRANSITING AROUND IT IS THE SAME. THUS...
EXPECT CONTINUES PERIODS OF RAIN ACROSS THE FORECAST AREA. IN
PARTICULAR...WITH A LOW IN THAT GENERAL POSITION SOUTHWEST ALASKA
WOULD BE IN THE FAVORABLE SPOT FOR FREQUENT RAIN AND HEAVY RAIN.
COOLER AIR WILL WRAP ALL THE WAY AROUND THIS SYSTEM AND INTO
MAINLAND ALASKA...SO EXPECT A GRADUAL COOLING OF SURFACE
TEMPERATURES...CLOSER TO NORMAL THAN THE CURRENTLY BALMY
TEMPERATURES BEING OBSERVED ACROSS MAINLAND ALASKA.

MEDIUM RANGE MODELS ARE ALSO INDICATING EXPLOSIVE CYCLOGENESIS
OVER THE NORTH PACIFIC DURING THE EARLY TO MIDDLE PART OF NEXT
WEEK...BUT SINCE IT RELIES ON THE PRESENCE OF A SHORT-WAVE
ROUNDING THE BERING TROUGH HAVE LITTLE CONFIDENCE IN SUCH A
SOLUTION AT THIS POINT IN TIME.

.AER/ALU...WATCH/WARNING SUMMARY
PUBLIC...FLOOD ADVISORY 121 125 (KENAI LAKE AND KENAI RIVER).
MARINE...GALE WARNING...130 131 132 150 155 175 176 177 178 185.
FIRE WEATHER...NONE.

SEB OCT 13