Small Craft Advisory 812, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
137 PM AKDT THU OCT 17 2013

.ANALYSIS AND UPPER LEVELS...A VERY HIGH AMPLITUDE PATTERN REMAINS
IN PLACE ACROSS ALASKA AND THE NORTH PACIFIC. A RIDGE CENTERED
OVER THE FAR EAST PACIFIC EXTENDS NORTHWARD ACROSS SOUTHEAST
ALASKA AND ALL THE WAY UP TO THE BEAUFORT SEA. A PARALLEL TROUGH
EXTENDS FROM THE EASTERN BERING SEA SOUTHWARD ALONG 160W DOWN INTO
THE TROPICS. TROPICAL MOISTURE CONTINUES TO FLOW NORTHWARD IN THE
ZONE BETWEEN THE RIDGE AND TROUGH. A DEFORMATION ZONE HAS BEEN
NEARLY STATIONARY OVER SOUTHWEST ALASKA THE PAST COUPLE DAYS
RESULTING IN PERSISTENT STEADY RAIN (HEAVY AT TIMES). THIS MORNING
THE REMNANTS OF A VERTICALLY STACKED LOW FROM THE TROPICS ARE
TRACKING NORTHWARD TOWARD KODIAK ISLAND...WHICH WILL BRING IN
ANOTHER BATCH OF RAIN TO THE SOUTHERN MAINLAND...BUT ULTIMATELY
BREAK THE LONG FETCH OF TROPICAL MOISTURE.

MEANWHILE THE FORMER TYPHOON WIPHA IS APPROACHING THE WESTERN
ALEUTIANS AS A DEEP EXTRA-TROPICAL LOW...WITH CENTRAL PRESSURE
ESTIMATED SOMEWHERE AROUND 960 MB. AN OCCLUDED FRONT HAS STARTED
TO MARCH EASTWARD ACROSS THE WESTERN BERING AND ALEUTIANS BRINGING
WIDESPREAD RAIN AND STRONG WINDS. SCATTEROMETER WINDS FROM OVERNIGHT
SHOWED A LARGE AREA OF STORM FORCE WINDS SURROUNDING THE LOW
CENTER...BUT BASED ON THE STRENGTH OF THE LOW LEVEL JET (MODELS
SHOWING 75KTS AT 925MB) SUSPECT THERE IS AN AREA OF HURRICANE FORCE
WINDS ON THE SOUTH SIDE OF THE LOW.

.MODEL DISCUSSION...MODELS HAVE FINALLY COME INTO AGREEMENT WITH
TRACK AND TIMING OF BOTH THE UPPER LEVEL SHORT-WAVE HEADED TOWARD
THE SOUTHERN MAINLAND AND THE ASSOCIATED SURFACE LOW/COLD FRONT
SOUTH OF THE ALASKA PENINSULA. THUS...FORECAST CONFIDENCE IS
HIGHER... ALTHOUGH THERE ARE STILL SOME CHALLENGES TO FORECASTING
THE WINDS FOR ANCHORAGE OVERNIGHT INTO FRIDAY. MODELS ARE RELATIVELY
SIMILAR WITH TRACK OF THE DEEP LOW OUT WEST...BUT THERE ARE
DIFFERENCES IN STRUCTURE AND STRENGTH OF THE OCCLUDED FRONT AND
TRIPLE POINT AS IT MOVES TO MAINLAND ALASKA FRIDAY AND SATURDAY.
THE STRENGTH OF THE TRIPLE POINT WILL DETERMINE HOW MUCH DOWN INLET
FLOW DEVELOPS ACROSS COOK INLET. THIS IS KEY TO WHETHER WINDS IN
ANCHORAGE REMAIN GUSTY OUT OF THE SOUTHEAST BETWEEN SYSTEMS OR SHIFT
TO THE NORTHEAST AND LIGHTEN UP UNTIL THE NEXT FRONT GOES THROUGH
FRIDAY NIGHT OR SATURDAY.

.SHORT TERM FORECAST...

MAINLAND ALASKA...THE CHALLENGE OF THE DAY IS THE WIND FORECAST
FOR THE ANCHORAGE AREA TONIGHT INTO FRIDAY MORNING. WINDS HAVE
DIMINISHED ACROSS MUCH OF THE LOWER ELEVATIONS OF ANCHORAGE THIS
MORNING AS WEAK DOWN INLET FLOW DEVELOPED (OUT AHEAD OF THE SURFACE
LOW SOUTH OF THE ALASKA PENINSULA). THIS DOWN INLET GRADIENT WILL
STRENGTHEN AS THE LOW TRACKS ACROSS BRISTOL BAY THIS AFTERNOON AND
EVENING. HOWEVER...AS HAS BEEN THE CASE WITH RECENT STORMS
INSTABILITY IN THE NEAR SURFACE LAYER IS RESULTING IN A LOT OF
TURBULENT MIXING. THE BIG QUESTION IS HOW MUCH WIND MIXES DOWN TO
LOWER ELEVATIONS AS THE EAST-WEST PRESSURE GRADIENT ACROSS TURNAGAIN
ARM INCREASES TONIGHT. IT SEEMS THE MOST LIKELY AREAS TO SEE SOME
OF THE EASTERLIES WOULD BE RIGHT ALONG THE MOUNTAINS (EAST
ANCHORAGE/LOWER HILLSIDE/EAGLE RIVER). THE REST OF THE ANCHORAGE
BOWL SHOULD REMAIN ON THE LIGHT SIDE UNTIL THE COLD FRONT MOVES
THROUGH LATE TONIGHT. THE OTHER ASPECT THAT WORKS AGAINST WIDESPREAD
STRONG WINDS ACROSS ALL OF ANCHORAGE IS THAT THE EAST-WEST PRESSURE
GRADIENTS PEAK PRIOR TO FRONTAL PASSAGE. THUS...AS THE FRONT MOVES
THROUGH AND NORTH-SOUTH PRESSURE GRADIENTS BECOME FAVORABLE FOR
WIND TO PUSH INTO ALL OF ANCHORAGE THE TURNAGAIN ARM JET WILL
ALREADY BE TRENDING DOWNWARD. BASED ON THIS...WILL LOWER THE
FORECAST WINDS FOR THE ANCHORAGE BOWL AND MAINTAIN THE STRONG WIND
HEADLINE FOR THE LOWER HILLSIDE AND EAST ANCHORAGE.

AS HAS ALSO BEEN THE CASE IN THE PAST COUPLE STORMS...STRONG LOW
LEVEL SOUTH TO SOUTHEAST FLOW WILL LIMIT RAINFALL AMOUNTS IN
DOWNSLOPE AREAS...BUT MUCH OF SOUTHWEST ALASKA AND UPSLOPE AREAS OF
SOUTHCENTRAL WILL SEE ANOTHER DOUSING. THERE WILL ACTUALLY BE A
BRIEF BREAK IN RAIN ACROSS MUCH OF THE SOUTHERN MAINLAND AS THE
UPPER SHORT-WAVE EXITS TO THE NORTH ON FRIDAY. HOWEVER...THE OCCLUDED
FRONT FROM THE LOW OUT WEST WILL BE RIGHT ON ITS HEELS WITH RAIN
MOVING BACK IN FRIDAY AFTERNOON ACROSS SOUTHWEST ALASKA AND FRIDAY
NIGHT FOR SOUTHCENTRAL. IT LOOKS LIKE YET ANOTHER WET AND WINDY
WEEKEND AS THIS SYSTEM TRACKS TO THE EASTERN BERING SEA. AS MENTIONED
THERE IS STILL SOME UNCERTAINTY IN THE STRENGTH OF THE FRONT...BUT
AT THE MOMENT IT LOOKS SIMILAR IN STRENGTH TO WHAT HAS BEEN OBSERVED
WITH THE LAST COUPLE STORMS (SUB-WARNING LEVEL WINDS). A COUPLE
NOTABLE DIFFERENCES WITH THIS NEXT SYSTEM ARE THE LACK OF A DIRECT
TAP TO TROPICAL MOISTURE AND THE FACT THAT THE WARM SECTOR WILL
REMAIN SOUTH OF THE MAINLAND. AS A RESULT...DON'T EXPECT QUITE AS
MUCH RAINFALL AND TEMPERATURES WILL BEGIN TO COOL DOWN.

BERING SEA/ALEUTIANS...THE GREATEST IMPACT OF THE DEEP LOW HEADED
FOR THE ALEUTIANS WILL BE STRONG WINDS AND HIGH SEAS ALONG THE
WESTERN ALEUTIANS...WITH SEAS 40 TO 50 FEET AND HURRICANE FORCE
WIND GUSTS SOUTH OF THE CHAIN. ALL AREAS WILL EXPERIENCE MORE
MODERATE WINDS AND HEAVY RAIN...WHICH IS NOTHING OUT OF THE
ORDINARY FOR THIS AREA OF THE WORLD. ARCTIC AIR WILL MOVE IN BEHIND
THE SYSTEM GRADUALLY ADVECTING ACROSS THE BERING SEA TO MAINLAND
ALASKA. THIS WILL PRODUCE INCREASED INSTABILITY WHICH IS SUPPORTIVE
OF GUSTY WINDS AND SHOWERS.

.LONG TERM FORECAST...THE CURRENT ALEUTIAN LOW WILL WILL AID IN
SHIFTING THE LONG-WAVE PATTERN EASTWARD...WITH THE UPPER RIDGE
MOVING EASTWARD INTO WESTERN CANADA AND A BROAD BUT STILL HIGH
AMPLITUDE TROUGH SETTING UP FROM THE BERING SEA TO MAINLAND ALASKA.
THE LACK OF A DIRECT TROPICAL CONNECTION MEANS TEMPERATURES WILL
MODERATE ACROSS THE REGION. STILL GENERALLY EXPECT WARMER THAN
NORMAL TEMPERATURES FOR THIS TIME OF YEAR...BUT MUCH CLOSER TO
NORMAL THAN WE HAVE BEEN. THE OTHER MAJOR CHANGE OS A SLOWDOWN
IN THE STORM TRACK WHICH SHOULD ALLOW FOR MORE SIGNIFICANT BREAKS
WITH DRY WEATHER BETWEEN STORM SYSTEMS NEXT WEEK...ESPECIALLY FOR
MAINLAND ALASKA. AS FAR AS THE FORECAST OF SPECIFIC LOWS IN THE
EXTENDED PERIOD...MODEL SPREAD IS VERY LARGE SO DON'T HAVE
CONFIDENCE IN ANY PARTICULAR SOLUTION.

.AER/ALU...WATCH/WARNING SUMMARY
PUBLIC...NONE.
MARINE...HURRICANE FORCE WIND WARNING...177 178.
         STORM WARNING...173 174 175 176.
         GALE WARNING...119 125 130 132 138
                        150 155 160 165 170 171 172 179 180 185.
FIRE WEATHER...NONE.

SEB OCT 13