Small Craft Advisory 807, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
1245 PM AKDT MON OCT 14 2013

.ANALYSIS AND UPPER LEVELS...A HIGH AMPLITUDE PROGRESSIVE UPPER
LEVEL PATTERN CONTINUES TO SUPPORT FAST MOVING STORM SYSTEMS WITH
RAPIDLY CHANGING WEATHER. A LOOK AT WATER VAPOR IMAGERY THIS MORNING
SHOWS A DEEP VERTICALLY STACKED LOW PASSING JUST NORTH OF SHEMYA
WITH A DIGGING TROUGH EXTENDING SOUTHWARD ALL THE WAY TO AROUND
35N LATITUDE. THE TROUGH IS TAPPING INTO TROPICAL MOISTURE WHICH
IS QUICKLY STREAMING NORTHWARD TOWARD THE CENTRAL ALEUTIANS. COOLING
TEMPERATURES IN INFRARED SATELLITE IMAGERY ARE AN INDICATION OF
STRONG SYNOPTIC SCALE LIFT FROM THE DIGGING TROUGH AIDED BY A
STRONG SUBTROPICAL JET. THIS WILL HELP TRANSLATE THAT TROPICAL
MOISTURE ALOFT INTO HEAVY RAIN AT THE SURFACE. LOW LEVEL COLD
ADVECTION IS WRAPPING AROUND THE ALEUTIAN LOW...WITH A COLD FRONT
EXTENDING FROM THE EASTERN ALEUTIANS SOUTHWARD INTO THE PACIFIC.
MODELS DON'T CURRENTLY INDICATE DEVELOPMENT OF A SURFACE LOW ALONG
THIS FRONT...BUT WITH SUCH STRONG DYNAMICS IN PLACE THIS IS
SOMETHING THAT IS POSSIBLE AND SHOULD BE MONITORED.

MEANWHILE...AN OCCLUDED FRONT EXTENDS FROM THE LOW CENTER NEAR
SHEMYA NORTH AND EAST ACROSS THE BERING SEA AND THEN BACK
SOUTHEASTWARD ALONG THE SOUTHWEST ALASKA COAST. THE FRONT IS
WEAKENING AS IT STRETCHES OUT FAR FROM THE PARENT LOW CENTER. AN
UPPER LEVEL SHORT-WAVE OVER THE EASTERN BERING SEA IS HELPING
SUPPORT RAIN ALONG THE FRONT. ANOTHER SHORT-WAVE IS CROSSING
SOUTHCENTRAL ALASKA WITH RAIN GRADUALLY DIMINISHING FROM SOUTHWEST
TO NORTHEAST THIS MORNING.

.MODEL DISCUSSION...MODELS ARE IN GOOD AGREEMENT WITH EVOLUTION
OF THE SYNOPTIC SCALE PATTERN WHICH PROVIDES FAIRLY HIGH CONFIDENCE
IN IDENTIFYING THE ZONE WHERE THE TROPICAL FIRE HOSE WILL BE POINTED
AND WHERE THE STRONGEST WINDS WILL BE OBSERVED. THERE ARE SOME MINOR
DIFFERENCES IN SURFACE LOW TRACKS AND TIMING...PARTICULARLY WITH A
LOW TRACKING UP THE WEST SIDE OF THE UPPER RIDGE WEDNESDAY NIGHT
AND THURSDAY. THIS AFFECTS THE EXACT TIMING OF HEAVIEST RAIN AS WELL
AS STRENGTH OF WINDS ACCOMPANYING THIS SYSTEM. THUS...FORECAST
CONFIDENCE DROPS A BIT FOR SOUTHWEST AND SOUTHCENTRAL ALASKA DURING
THIS TIME PERIOD.

.SHORT TERM FORECAST...THE PRIMARY WEATHER ASSOCIATED WITH THIS
HIGH AMPLITUDE PATTERN IS WARM WET AND WINDY. IN MANY OF THESE VERY
HIGH AMPLITUDE CASES THE TROUGH AND RIDGE REMAIN LARGELY IN THE SAME
PLACE FOR A WEEK OR MORE SO THAT THE SAME GENERAL AREA GETS SLAMMED
ONE STORM AFTER ANOTHER. IN THIS CASE THE UPPER LEVEL FLOW IS MORE
DYNAMIC...SO THERE WILL BE CHANGES IN TROUGH/RIDGE PLACEMENT AND
ORIENTATION AND THE AREA OF HEAVIEST RAIN AND STRONGEST WIND WILL
CHANGE WITH EACH SYSTEM MOVING THROUGH. FOLLOWING ARE SOME SPECIFICS
WHICH HIGHLIGHT THE HIGHEST IMPACT WEATHER WHICH WILL BE EXPERIENCED
OVER THE NEXT FEW DAYS.

THE OCCLUDED FRONT WILL DISSIPATE AS IT MOVES INLAND OVER SOUTHWEST
ALASKA TONIGHT WITH LITTLE TO NONE OF THE FRONTAL RAIN SURVIVING
INTO SOUTHCENTRAL ALASKA. THERE WILL BE SOME TIGHTENING OF SURFACE
PRESSURE GRADIENTS OVER SOUTHCENTRAL ALASKA AS THE REMNANTS OF THE
FRONT AND THE FOLLOWING UPPER SHORT-WAVE MOVE THROUGH TONIGHT. NOT
ONLY WILL THIS AFFECT THE TYPICAL EAST-WEST GAPS SUCH AS TURNAGAIN
ARM AND THE KNIK RIVER VALLEY...BUT GRADIENTS ARE FAVORABLE FOR
GUSTY WINDS TO BE MORE WIDESPREAD ACROSS SOUTHCENTRAL.

THE ALEUTIAN LOW AND ASSOCIATED TROUGH WILL PROGRESS EASTWARD
TONIGHT WITH TROPICAL MOISTURE RAPIDLY ADVECTING NORTHEASTWARD TO
SOUTHWEST ALASKA OVERNIGHT AND INTO SOUTHCENTRAL BY TUESDAY MORNING.
MOST MODELS INDICATE A SURFACE WAVE TRACKING UP INTO BRISTOL BAY
LATE TONIGHT AND THEN ACROSS SOUTHWEST ALASKA TUESDAY...BUT WILL
KEEP AN EYE OUT FOR THE POSSIBILITY OF FORMATION OF A SURFACE LOW.
IN EITHER CASE PRESSURE GRADIENTS EAST OF THIS FEATURE WILL TIGHTEN
UP QUITE A BIT...BRINGING STRONG WINDS TO PORTIONS OF SOUTHWEST AND
SOUTHCENTRAL ALASKA. ASSUMING THE MODELS ARE CORRECT AND THIS REMAINS
A FRONTAL WAVE IT LOOKS LIKE TURNAGAIN ARM AND HIGHER ELEVATIONS
WILL REMAIN BELOW HIGH WIND WARNING LEVELS. THE LACK OF A STRONG
FRONTAL PASSAGE OR SIGNIFICANT INVERSION ABOVE MOUNTAIN TOP SUPPORT
THIS IDEA. HOWEVER...CONDITIONS DO ONCE AGAIN LOOK FAVORABLE FOR
STRONGER WINDS TO MOVE INTO SOME OF THE LOWER ELEVATIONS...WITH WARM
SURFACE TEMPERATURES PROVIDING ENOUGH INSTABILITY FOR SOME VERTICAL
MIXING OF WINDS AND THE PRESENCE OF FAVORABLE PRESSURE GRADIENTS
ALLOWING WINDS TO SPREAD NORTHWARD (OR INLAND). IT IS WORTH
HIGHLIGHTING THAT ALTHOUGH WINDS WILL BE FAIRLY WIDESPREAD THE
MAGNITUDE OF THIS SYSTEM IS LESS THAN THE PAST TWO OR THREE AND
DAMAGING WINDS ARE NOT EXPECTED.

WITH SUCH FAST STEERING FLOW THE HEAVY RAIN WILL NOT BE PROLONGED
FOR ANY ONE LOCATION. EXPECT RAIN TO DIMINISH OVER MUCH OF THE
SOUTHERN MAINLAND TUESDAY AFTERNOON THROUGH EVENING. MEANWHILE...AS
THE UPPER TROUGH IS PROGRESSING EASTWARD THE DOWNSTREAM RIDGE WILL
BUILD FROM THE EASTERN GULF INTO SOUTHCENTRAL ALASKA. THIS WILL SHIFT
THE FIRE HOSE OF TROPICAL MOISTURE A BIT SO THAT IT IS POINTED TOWARD
THE ALASKA PENINSULA AND ON UP TO SOUTHWEST ALASKA. THIS LEADS INTO
THE TIME PERIOD OF MORE UNCERTAINTY AS A SURFACE LOW TRACKS UP THE
WEST SIDE OF THE RIDGE WEDNESDAY NIGHT AND THURSDAY TIME FRAME. THERE
IS POTENTIAL FOR A STRONGER WIND EVENT AS THIS LOW PASSES BY. THE
TRACK WILL ALSO DETERMINE WHETHER THE FIRE HOSE SHIFTS ONCE AGAIN.
WILL HAVE TO WAIT UNTIL MODELS HAVE A BETTER HANDLE ON THIS TO NAIL
DOWN THE DETAILS.

OUT WEST...THE STORM TRACK WILL BE QUITE DIFFERENT AS SYSTEMS ENTER
FROM THE WEST INTO THE BACK SIDE OF THE TROUGH. LOOKS LIKE ONE
MODERATE STRENGTH SYSTEM WILL MOVE OUT OF ASIA TONIGHT AND QUICKLY
TRACK ACROSS THE ALEUTIANS TUESDAY NIGHT AND WEDNESDAY. THE SYSTEM
AFTER THAT...A FORMER TYPHOON...WILL LIKELY BE QUITE A BIT STRONGER.
THIS WILL BECOME THE FOCUS FOR THE END OF THE WEEK AS THE VERY ACTIVE
WEATHER PATTERN CONTINUES.

.LONG TERM FORECAST...THE FORMER TYPHOON MOVING INTO THE
BERING/ALEUTIANS AT THE END OF THIS WEEK WILL BE THE PRIMARY STORM
FOR THE DAY 4 THROUGH 7 TIME PERIOD...LIKELY BRINGING ANOTHER ROUND
OF RAIN AND WIND TO THE ALASKA REGION. THERE ARE SOME INDICATIONS
THAT THE PERSISTENT SHORT-WAVES ROUNDING THE TROUGH AND PUSHING UP
AGAINST THE RIDGE WILL GRADUALLY WEAKEN THE RIDGE AND ALLOW COOLER
AIR TO MOVE BACK INTO MAINLAND ALASKA. HOWEVER...WITH THERE TENDS
TO BE A MODEL BIAS TOWARD BREAKING DOWN THESE HIGH AMPLITUDE RIDGES
TOO QUICKLY...SO WILL WAIT TO SEE IF THERE IS ANY RUN-TO-RUN
CONSISTENCY WITH THIS SOLUTION.

.AER/ALU...WATCH/WARNING SUMMARY
PUBLIC...NONE.
MARINE...STORM WARNING...178.
         GALE WARNING...130 131 132 138
                        150 155 173 174 175 176 177 185.
FIRE WEATHER...NONE.

SEB OCT 13