Small Craft Advisory 333, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
522 AM AKDT THU APR 16 2015

.ANALYSIS AND UPPER LEVELS...

UPPER LEVEL TROUGHING CONTINUES TO BE THE DOMINANT MODE AS THE
REGION REMAINS ENCOMPASSED BY A BROAD UPPER LEVEL JET STREAM. THE
MAIN FEATURES OF INTEREST WITHIN THIS BROAD TROUGH INCLUDE A
WEAKENING SHORTWAVE TROUGH MOVING INLAND OVER SOUTHCENTRAL THIS
MORNING...AND ANOTHER TRAILING WEAK NEGATIVE TILT TROUGH
STRETCHING FROM NUNIVAK ISLAND TO KODIAK ISLAND. A MORE DEFINED
POTENTIAL VORTICITY ANOMALY CAN BE SEEN ON WATER VAPOR SATELLITE
IMAGERY ALONG THE NORTH PACIFIC JET STREAM TRACKING SOUTH OF THE
CENTRAL ALEUTIANS. FINALLY...A SHORTWAVE TROUGH SOUTH OF KAMCHATKA
CAN BE SEEN RAPIDLY MOVING EAST AS IT ENCOUNTERS THE BROADER UPPER
TROUGH OVER THE BERING SEA. EVENTUALLY...A WARM FRONT ASSOCIATED
WITH THE LOW SOUTH OF THE ALEUTIAN CHAIN WILL DEVELOP INTO A
STRONGER SURFACE LOW WHICH WILL EVENTUALLY TRACK INTO THE EASTERN
BERING SEA OVER THE WEEKEND...BRINGING ACTIVE WEATHER TO SOUTHERN
ALASKA AND THE BERING SEA/ALEUTIANS THROUGH THE WEEKEND.

&&

.MODEL DISCUSSION...

THE MODELS ARE IN GOOD SHORT TERM AGREEMENT...AND VERY GOOD
OVERALL AGREEMENT WITH THE GENERAL PATTERN THROUGH THE WEEKEND. IN
THE SHORTER TERM...THE MODELS DIVERGE WITH PRECIPITATION PATTERNS
ACROSS SOUTHCENTRAL LATE TODAY INTO TONIGHT AS A SECONDARY TROUGH
AXIS MOVES THROUGH. THE PATTERN IS CONDUCIVE TO MORE LOCAL SCALE
PRECIPITATION PATTERNS...AND THAT WILL LEND SOME UNCERTAINTY FROM
THE KENAI PENINSULA NORTHWARD TO ANCHORAGE. AS A RESULT...A
BLENDED AND MORE CONSERVATIVE APPROACH WILL BE USED FOR THIS
MORNING. LATER ON...THE NUMERICAL GUIDANCE BEGINS TO DIVERGE WITH
MORE NOTABLE FEATURES AS EARLY AS LATE FRIDAY AS A STRONG LOW
MOVES FROM THE NORTH PACIFIC INTO THE EASTERN BERING SEA. THE
GENERAL TRACK...SPEED...AND INTENSITY OF THE LOW RESULT IN GREATER
UNCERTAINTY WITH RESPECT TO IMPACTS AND PRECIPITATION
PATTERNS...ESPECIALLY LATE FRIDAY AND SATURDAY. THE LARGEST
UNCERTAINTY IS ACROSS THE BERING SEA AND EASTERN ALEUTIANS WITH
THE PLACEMENT OF POTENTIAL HEAVY SNOW ACROSS THE EASTERN BERING
AND SOUTHWEST COAST. THE EXTREMES IN MODEL SOLUTIONS ARE THE GFS
AND ECMWF...WITH THE FORMER MUCH FARTHER EAST AND FASTER WITH THE
LOW TRACK. AS A RESULT...A BLENDED APPROACH FAVORING THE FASTER
AND FARTHER EAST GFS WAS USED...BUT THE OTHER EXTREME...THE
ECMWF...WAS ALSO CONSIDERED BUT IN SMALLER PROPORTIONS.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (DAYS 1 AND 2)...

RAIN AND SNOW SHOWERS WILL REMAIN A FORECAST CHALLENGE THROUGH
TONIGHT AS A WEAK NORTH GULF LOW MOVES INLAND TODAY. THE MOST
WIDESPREAD SHOWERS WILL BE OVER COASTAL LOCATIONS...BUT THERE WILL
BE INCREASING CHANCES FOR INLAND SHOWERS FROM THE KENAI PENINSULA
NORTHWARD AS A SECONDARY UPPER LOW TRACKS IN LATE THIS AFTERNOON
THROUGH TONIGHT. THE MOST FAVORABLE LOCATIONS FOR POTENTIAL
ACCUMULATING SNOW WILL BE ACROSS THE HIGHER TERRAIN OF THE CHUGACH
AND KENAI MOUNTAINS...AND NORTHWARD INTO THE SUSITNA VALLEY.
COLDER AIR BEHIND THE TROUGH WILL ALSO RESULT IN LOWERING SNOW
LEVELS ACROSS COASTAL LOCATIONS AND THE POSSIBILITY FOR SNOW DOWN
TO SEA LEVEL BY THURSDAY NIGHT.

ON FRIDAY...THE UPPER DISTURBANCE WILL TRACK NORTHWARD OUT OF THE
AREA...WITH PRECIPITATION GENERALLY REMAINING OVER THE HIGHER
ELEVATION MOUNTAINS...AND ACROSS COASTAL LOCATIONS. A MUCH MORE
STOUT FRONTAL ZONE WILL BEGIN MOVING INTO THE GULF OF ALASKA
FRIDAY AFTERNOON AND EVENING...BRINGING STEADY RAIN TO KODIAK
ISLAND...AND EVENTUALLY THE NORTHERN GULF COAST.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (DAYS 1 AND 2)...
THROUGH TONIGHT...NORTHERLY FLOW WILL CONTINUE OVER THE SOUTHWEST
MAINLAND AS THE REGION REMAINS UNDER THE INFLUENCE OF THE WEAK
RIDGE OVER THE BERING SEA AND THE BROAD LOW OVER SOUTH CENTRAL
ALASKA. A MIX OF RAIN AND SNOW TODAY WILL BE CONFINED TO THE
INTERIOR SECTIONS OF THE ALEUTIAN RANGE AND THE LOWER KUSKOKWIM
VALLEY. AS THE RIDGE PUSHES EASTWARD...SKIES WILL REMAIN PARTLY
CLOUDY OVER THE COASTAL AREAS. TEMPERATURES TONIGHT WILL REMAIN
BELOW NORMAL FOR MOST OF THE REGION WITH SINGLE DIGIT LOWS
EXPECTED AGAIN ALONG THE UPPER KUSKOKWIM COAST.

AS THE WARM FRONT ASSOCIATED WITH THE LOW CURRENTLY SOUTH OF ADAK
APPROACHES THE REGION FRIDAY MORNING...PRECIPITATION CHANCES WILL
INCREASE ALONG THE ALASKA PENINSULA AS THE FRONT SWINGS NORTHWARD
DURING THE DAY. WINDS WILL ALSO BEGIN TO INCREASE ALONG THE
ALEUTIAN RANGE WITH GUSTS TO 45 MPH BEGINNING FRIDAY AFTERNOON.
TEMPERATURES WILL BEGIN TO WARM TOMORROW AS WARM AIR ADVECTION
STARTS TO PUSH INTO THE REGION WITH THE FRONT.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (DAYS 1 AND 2)...
THE EASTERN BERING SEA AND ALEUTIANS WILL REMAIN UNDER THE
INFLUENCE OF THE RIDGE TODAY WITH GENERALLY LIGHT WINDS OVER MOST
OF THE REGION THIS MORNING. AS THE LOW SOUTH OF THE ALEUTIAN CHAIN
CONTINUES ITS EASTWARD MOVEMENT AND DEEPENS...GALE FORCE WINDS
ARE EXPECTED IN THE COASTAL WATERS IN THE CENTRAL ALEUTIANS SOUTH
OF THE ISLANDS. AS THE FRONT MOVES NORTHWARD THROUGH THE EASTERN
BERING...NORTHERLY WINDS WILL INCREASE OVER THE EASTERN HALF OF
THE BERING SEA AND PRIBILOF ISLANDS. WINDS ARE CURRENTLY EXPECTED
TO INCREASE TO UPPER GALE FORCE BUT AREAS OF STORM FORCE WINDS MAY
BE POSSIBLE OVER THE EASTERN BERING SEA LATE TOMORROW NIGHT.

AS THE NORTHERLY FLOW WILL BRING COLDER AIR INTO THE
REGION...AREAS OF FREEZING SPRAY CAN BE EXPECTED WELL INTO THE
SOUTHERN BERING SEA WITH HEAVY FREEZING SPRAY EXPECTED IN PORTIONS
OF THE NORTHERN BERING.

&&

.LONG TERM FORECAST (DAYS 3 THROUGH 7)...
THE DEEP LOW SOUTH OF THE ALEUTIANS WILL CROSS THE AKPEN SATURDAY
AND MOVE ALONG THE WEST COAST OF THE STATE. THIS WILL BRING IN
CONTINUED WARM AND MOIST SOUTHERLY FLOW TO SOUTHCENTRAL AK INTO
MONDAY WHEN THE SURFACE LOW DISSIPATES. THIS WILL RESULT IN MAINLY
RAIN SHOWERS. ON THE WESTERN SIDE OF THE LOW OVER THE BERING COLD
NORTHERLY FLOW WILL BE ASSOCIATED WITH SNOW SHOWERS. MODEL
GUIDANCE IS IN DECENT AGREEMENT WITH THIS OVERALL SYNOPTIC
PATTERN. HOWEVER...THERE ARE DIFFERENCES WITH HOW ELONGATED THE
LOW BECOMES ALONG THE WEST COAST WHICH WILL IMPACT THE STRENGTH OF
THE WINDS CIRCULATING AROUND THE LOW...ESPECIALLY ON THE SOUTHERN
SIDE NEAR BRISTOL BAY. A MORE CONSOLIDATED LOW CENTER IS THE
PREFERRED CHOICE WITH THIS SYSTEM.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...
 STORMS 130 131
 GALES 119 120 132 136-139 150 155 160 165 170-176 179 180
 HEAVY FREEZING SPRAY 180
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...JA
SOUTHCENTRAL ALASKA...JA
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...RF
LONG TERM...DK