Small Craft Advisory 412, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
130 PM AKDT WED MAY 15 2013

.ANALYSIS AND UPPER LEVELS...
A LONGWAVE TROUGH CENTERED OVER THE WEST COAST OF ALASKA CONTINUES TO
ENCOMPASS THE EASTERN BERING SEA AND MOST OF THE MAINLAND. WARM AIR
ADVECTION IS BEGINNING ACROSS SOUTHCENTRAL AS A WEAK LOW SOUTH OF THE
ALEUTIANS IS BEING DRAWN UP INTO THE WESTERN GULF. AN AREA OF
PERSISTENT RAIN AND SNOW SHOWERS IS LOCATED OVER WESTERN PRINCE
WILLIAM SOUND AND THE CHUGACH MOUNTAINS...WITH MORE WIDESPREAD RAIN
APPROACHING KODIAK ISLAND FROM THE SOUTH. OUT WEST...HIGH PRESSURE IN
THE LOWER LEVELS CONTINUES TO MAINTAIN WIDESPREAD FOG AND STRATUS
ACROSS MUCH OF THE BERING SEA REGION. LIGHT NORTHWEST WINDS ARE
BRINGING SOME OF THESE CLOUDS INTO THE KUSKOKWIM DELTA. ELSEWHERE
OVER THE SOUTHWEST MAINLAND...SKIES ARE MOSTLY SUNNY WITH
BELOW-AVERAGE TEMPERATURES.

.MODEL DISCUSSION...
MODELS REMAIN IN GOOD SYNOPTIC AGREEMENT. THE MAIN FORECAST CHALLENGE
ACROSS THE DOMAIN REMAINS HOW THE UNSEASONABLY COLD AIR MOVING INTO
THE REGION ON FRIDAY WILL INTERACT WITH THE MOISTURE LEFTOVER FROM
THE WEAK STORM CURRENTLY MOVING INTO THE GULF. ALL MODELS CONTINUE TO
SHOW SIGNIFICANT COOLING OF THE ATMOSPHERE FRIDAY AFTERNOON AND
EVENING...WITH EVEN SEA-LEVEL LOCATIONS CHANGING OVER TO ALL SNOW.
SNOW AMOUNTS WILL BE PROBLEMATIC AS POTENTIAL FOR ACCUMULATION IS
CLOSELY TIED TO PRECIPITATION RATES. THIS IS WHERE THERE REMAINS A
BIT OF MODEL DISAGREEMENT...AS THE MODELS ARE ALL HINTING AT A WEAK
LOW CENTER FORMING IN THE BASE OF THE UPPER TROUGH BY SATURDAY
MORNING. THE TIMING OF THIS DEVELOPMENT WILL DICTATE HOW STRONG THE
DEEP-LAYER LIFT IS ACROSS THE ANCHORAGE BOWL AND MAT-SU VALLEY. IF
THE LOW DEVELOPS EARLIER THAN EXPECTED...STRONG DYNAMIC LIFT WILL BE
DIMINISHED AND SNOWFALL RATES AROUND THE ANCHORAGE AREA WILL LIKELY
NOT BE ENOUGH TO RESULT IN MORE THAN MINOR SNOW ACCUMULATIONS. THE
MODELS ALL SHOW A SIMILAR TREND...THOUGH THE NAM IS A SLIGHT
OUTLIER WITH A FASTER SPEED OF THE LOW MOVING INTO THE GULF AND ALSO
WITH THE COLD AIR ARRIVAL ON FRIDAY. THUS WILL OPT TO UTILIZE A BLEND
OF THE GFS/ECMWF FOR MOST OF THE FORECAST TODAY.

.SHORT TERM FORECAST...
SOUTHCENTRAL...PERSISTENT SHOWERS OVER THE CHUGACH WILL DIMINISH AS
CROSS-BARRIER WINDS INCREASE THIS EVENING. A CLASSIC DOWNSLOPE
PATTERN WILL DEVELOP ON THURSDAY...KEEPING THE WESTERN KENAI UP TO
ANCHORAGE AND THE MATANUSKA VALLEY MOSTLY DRY. MEANWHILE...LOCATIONS
FURTHER EAST AROUND PRINCE WILLIAM SOUND AND THE NORTH GULF COAST
WILL SEE STEADY RAIN DEVELOP BY MIDNIGHT TONIGHT. BY LATE THURSDAY
NIGHT...DOWNSLOPING WINDS WILL DIMINISH AS THE SURFACE LOW WEAKENS
AND MOVES NORTHWARD. THIS WILL ALLOW RAIN TO SPREAD INTO THE REST OF
THE COOK INLET AREA. THE REAL FUN BEGINS BY FRIDAY AFTERNOON AS THE
COLD AIR ARRIVES FROM WESTERN ALASKA. SNOW SHOULD BEGIN MIXING IN
WITH RAIN BEFORE CHANGING TO ALL SNOW FRIDAY EVENING. THE ANCHORAGE
HILLSIDE WILL CHANGE OVER TO SNOW EARLIEST...AND WILL ALSO HAVE THE
BEST CHANCE TO SEE ACCUMULATION. IT IS IMPORTANT TO NOTE THAT
ACCUMULATING SNOWFALL IN THE ANCHORAGE AREA IS EXTREMELY RARE IN THE
LATTER HALF OF MAY...AND HAS ONLY BEEN RECORDED TWICE BEFORE.
THUS...A HIGHLY UNUSUAL EVENT APPEARS TO BE TAKING AIM AT
SOUTHCENTRAL ALASKA FOR THE START OF THE WEEKEND.

SOUTHWEST MAINLAND...MOSTLY CLEAR SKIES WILL GIVE WAY TO INCREASING
CLOUDS DURING THE DAY THURSDAY AS THE AFOREMENTIONED COLD FRONT
APPROACHES FROM THE NORTH. AVAILABLE ATMOSPHERIC MOISTURE WILL BE
PRETTY MINIMAL AS THE FRONT PASSES...SO ONLY EXPECTING SCATTERED RAIN
AND SNOW SHOWERS AT MOST LOCATIONS. THE EXCEPTION WILL LIKELY BE OVER
THE EASTERN HALF OF THE LOWER KUSKOKWIM VALLEY WHERE AVAILABLE
MOISTURE AND UPSLOPING WINDS WILL BRING THE POTENTIAL FOR SOME SNOW
ACCUMULATION IN THAT AREA ON THURSDAY NIGHT AND FRIDAY.

BERING SEA AND ALEUTIANS...A BROAD AREA OF HIGH PRESSURE WILL REMAIN
IN PLACE AND STRENGTHEN THROUGH THE START OF THE WEEKEND. THIS WILL
MAINTAIN THE WIDESPREAD MARINE FOG AND STRATUS THAT IS EVIDENT ACROSS
THE REGION ON SATELLITE IMAGERY. WINDS GENERALLY OUT OF THE NORTH
WILL ALLOW THIS CLOUDINESS TO CONTINUE TO IMPACT COMMUNITIES MAINLY
ALONG THE BERING SIDE OF THE ALASKA PENINSULA AND ALEUTIAN ISLANDS.

.LONG TERM FORECAST...
COLD AIR WILL MOVE INTO SOUTHERN ALASKA ON FRIDAY AND BY SUNDAY WILL
BE ON THE DECLINE. LONG-RANGE MODELS ARE SUGGESTING A PATTERN SHIFT
FOR THE START OF NEXT WEEK THAT WOULD BE CHARACTERIZED BY MORE ACTIVE
WEATHER OVER THE BERING SEA WITH MODERATING TEMPERATURES (BUT STILL
AT OR BELOW NORMAL) AND SUNNIER WEATHER ACROSS THE MAINLAND FOR NEXT
WEEK.

.AER/ALU...WATCH/WARNING SUMMARY
PUBLIC...NONE
MARINE...GALE 120
FIRE WEATHER...NONE

DIXON MAY 13