Small Craft Advisory 742, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
222 PM AKDT THU SEP 25 2014

.ANALYSIS AND UPPER LEVELS...THE OFFSHORE WIND EVENT ALONG THE
NORTH GULF COAST IS GRADUALLY COMING TO AN END THIS MORNING AS
WEAK HIGH PRESSURE BUILDS OVERHEAD. THE FOCUS TURNS TO A
DEVELOPING SURFACE LOW SOUTH OF THE EASTERN ALEUTIANS. AN UPPER
LOW AND SHORT-WAVE TROUGH OVER THE SOUTHERN BERING SEA IS DIGGING
SOUTHEASTWARD HELPING TO STRENGTHEN THE SURFACE LOW AND PULL IT
NORTHWARD TOWARD DUTCH HARBOR. OUTSIDE OF THIS STORM SYSTEM AND A
DISSIPATING LOW OVER THE SOUTHERN BERING QUIET CONDITIONS DOMINATE
THE WEATHER ACROSS THE REGION.

&&

.MODEL DISCUSSION...MODELS HAVE REALLY STRUGGLED THE PAST FEW DAYS
WITH HANDLING OF THE SYSTEM SOUTH OF THE ALEUTIANS. NOW THAT IT
HAS FORMED THE MODELS HAVE COME INTO FAIRLY GOOD AGREEMENT THROUGH
ABOUT FRIDAY. WITH STRONG UPPER SHORT-WAVE AND SUBTROPICAL JET
JUST OUT AHEAD OF IT DO PREFER THE STRONGER OF THE SOLUTIONS.
NOTABLE MODEL DIFFERENCES ARISE OVER THE GULF BEGINNING FRIDAY
NIGHT WITH HANDLING OF UPPER LEVEL WAVE MOVING OUT AHEAD OF WHAT
IS AT THAT POINT A LARGE WELL ESTABLISHED UPPER LEVEL CLOSED LOW.
SIMILARLY THERE ARE DIFFERENCES IN STRENGTH AND POSITION OF AN
INVERTED TROUGH WHICH FORMS OVER THE GULF IN RESPONSE TO THE UPPER
WAVE. THE BIGGEST CHALLENGE IS DETERMINING HOW FAR NORTH AND WEST
THESE FEATURES MOVE...WHICH WILL AFFECT WINDS OVER THE NORTHERN
AND WESTERN GULF AND PRECIP CHANCES ON THE LEE SIDE OF THE
CHUGACH AND KENAI MOUNTAINS. BASED ON A STRONGER PARENT SYSTEM
WILL TREND TOWARD A STRONGER AND MORE AGGRESSIVE SURFACE TROUGH AS
PER THE NAM AND ECMWF SOLUTIONS. AS THE ORIGINAL LOW CONTINUES TO
TRACK EASTWARD JUST SOUTH OF THE GULF ON SUNDAY MODEL SPREAD
BECOMES MUCH LARGER. WILL USE AN AVERAGE OF THE NAM/GFS TRACK
WHICH PROVIDES BEST CONTINUITY WITH PREVIOUS GRIDS. FORECAST
CONFIDENCE STARTS OUT HIGH IN THE FIRST 24 HOURS OF THE FORECAST
DROPPING TO BELOW AVERAGE BY THE END OF DAY 3.

&&

.SHORT TERM FORECAST...

SOUTHCENTRAL ALASKA...LOW LEVEL TEMPERATURES WILL BEGIN TO MODERATE
EVER SO SLIGHTLY TONIGHT ACROSS SOUTHCENTRAL. HOWEVER...WITH CALM
TO LIGHT WINDS ACROSS NEARLY THE ENTIRE REGION AND CLEAR SKIES IT
WILL BE ANOTHER NIGHT OF EFFECTIVE RADIATIONAL COOLING. EXPECT
SIMILAR OR COLDER TEMPERATURES TO WHAT HAVE BEEN OBSERVED THE LAST
COUPLE NIGHTS. FRIDAY NIGHT WILL START OUT CLEAR BUT CLOUDS WILL
MOVE IN AHEAD OF THE APPROACHING STORM SYSTEM OVERNIGHT INTO
SATURDAY MORNING WHICH WILL STALL THE DROP IN TEMPERATURES. THE
TOP OF THOMPSON PASS (AT NEARLY 2700 FEET) MAY BRIEFLY SEE A
PERIOD OF SNOW OR MIXED RAIN AND SNOW RIGHT AS PRECIP ARRIVES
EARLY SATURDAY MORNING...BUT WARMER AIR WILL MOVE IN QUICKLY FROM
THE GULF AND RAISE SNOW LEVELS UP TO 4000 FEET OR HIGHER. THE BULK
OF PRECIP WILL REMAIN ALONG THE COAST...BUT DEPENDING ON HOW FAR
NORTH AND WEST TROUGH PUSHES MAY SEE A BRIEF PERIOD OF LIGHT RAIN
IN ANCHORAGE AND THE MAT VALLEY SOMETIME SATURDAY AFTERNOON OR
EVENING. ANTECEDENT CONDITIONS WILL WORK AGAINST MUCH FALLING WITH
DRY AIR MASS IN PLACE AND LOW LEVEL OFFSHORE FLOW HELPING TO
MAINTAIN THAT DRY AIR. THE SYSTEM WILL EXIT TO THE EAST ON
SUNDAY... ALTHOUGH THERE IS UNCERTAINTY IN THE TRACK AND
ULTIMATELY HOW QUICKLY SOUTHCENTRAL WILL CLEAR OUT.

SOUTHWEST ALASKA...THE SOUTHWEST MAINLAND WILL REMAIN JUST NORTH
OF THE ALEUTIAN LOW. THE ALASKA PENINSULA SOUTH OF KING SALMON MAY
SEE A PERIOD OF LIGHT RAIN. OTHERWISE...DRY CONDITIONS WITH
OFFSHORE FLOW WILL DOMINATE. A TROUGH DIGGING OUT OF THE RUSSIAN
ARCTIC MAY AFFECT INTERIOR SOUTHWEST WITH SOME CLOUDS AND A FEW
SHOWERS SUNDAY...BUT WITH LIMITED MOISTURE AND THE GREATEST
VERTICAL MOTION FARTHER NORTH AND EAST...MOST AREAS WILL REMAIN
DRY.

BERING SEA/ALEUTIANS...LATEST SATELLITE TRENDS INDICATE SOME VERY
STRONG UPWARD MOTION JUST NORTH OF THE EASTERN ALEUTIANS OUT
AHEAD OF DIGGING SHORT-WAVE. EXPECT SIMILAR CONDITIONS TO DEVELOP
RIGHT OVER THE EASTERN ALEUTIANS (INCLUDING DUTCH HARBOR AREA)
TONIGHT AS THIS UPPER WAVE INTERACTS WITH THE SURFACE LOW.
THUS...EXPECT PERIODS OF HEAVY RAIN. MEANWHILE...WITH THE
DEEPENING LOW HEADED TO A POSITION IMMEDIATELY SOUTH OF THE CHAIN
AND WITH INDICATIONS OF FORMATION OF A BENT-BACK OCCLUSION (AS
THE UPPER LOW CURRENTLY OVER THE SOUTHERN BERING MOVES OVER TOP OF
THE SURFACE LOW AND MARGINALLY COLDER AIR MOVES DOWN THE BACK SIDE
OF THE LOW)...PLAN TO UP FORECAST WINDS A BIT ALONG THE EASTERN
ALEUTIANS TONIGHT INTO FRIDAY. SOME OF THE HEAVY RAIN AND STRONG
WINDS WILL ALSO AFFECT THE SOUTHERN ALASKA PENINSULA AS THE LOW
TRACKS EAST-SOUTHEAST ON FRIDAY. AS THIS FIRST LOW EXITS A MATURE
LOW WILL MOVE IN FROM THE NORTHWEST PACIFIC AND TRACK SOUTH OF THE
ALEUTIAN CHAIN THIS WEEKEND. THE CHAIN WILL BE RIGHT ON THE
NORTHERN PERIPHERY OF THIS SYSTEM SO EXPECT LITTLE IMPACT.

&&

.LONG TERM FORECAST...
MODELS HAVE CONTINUED TO TREND MORE AGGRESSIVE IN DIGGING AN
UPPER LEVEL TROUGH OVER THE EASTERN MAINLAND AND SOUTHCENTRAL AK
EARLY NEXT WEEK...WITH THE 12Z RUNS OF THE GFS/EC/GEM ALL
DEVELOPING A CLOSED LOW OVER THE NORTHEASTERN GULF ON MONDAY NIGHT
INTO TUESDAY. THIS WILL RESULT IN COLDER TEMPERATURES AS WELL AS
ANOTHER ROUND OF GUSTY OUTFLOW WINDS ALONG THE COAST IN NORTHWEST
FLOW. MOST OF THE AREA SHOULD REMAIN DRY GIVEN THE LIMITED
MOISTURE AVAILABLE...BUT A FEW SHOWERS COULD ACCOMPANY THE PASSAGE
OF THE TROUGH...PARTICULARLY FARTHER NORTH ALONG THE ALASKA RANGE
AND INTO THE COPPER RIVER BASIN WHERE A FEW SNOW SHOWERS COULD MIX
IN AS COLDER AIR FILTERS DOWN FROM THE NORTH. DRY AND SEASONABLY
COOL CONDITIONS WILL THEN PERSIST THROUGH THE REST OF THE WEEK AS
A RIDGE BUILDS IN BEHIND THE DEPARTING TROUGH.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...STORM WARNING...172.
         GALE WARNING...138 155 165 170 171.
FIRE WEATHER...NONE.

&&

SEB/CB SEP 14