Small Craft Advisory 296, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
546 AM AKDT SAT APR 4 2015

.ANALYSIS AND UPPER LEVELS...

A STRONG NORTHERLY UPPER LEVEL JET STREAK CONTINUES TO REMAIN
NEARLY STATIONARY BETWEEN A NARROWING UPPER LEVEL RIDGE TO THE
WEST OVER WESTERN ALASKA AND A LARGE UPPER LOW SOUTH OF THE
SOUTHEASTERN GULF OF ALASKA. A PAIR OF AMPLIFIED UPPER LEVEL
DISTURBANCES PERSIST IN SOUTHWEST UPPER LEVEL FLOW OVER THE
BERING SEA...WITH ONE DISTURBANCE WITH ATTACHED SURFACE LOW OVER
THE NORTHERN BERING AND ATTENDANT SURFACE FRONT EXTENDING SOUTH
AND EAST ACROSS SOUTHWEST ALASKA. A SECOND SPEEDY UPPER WAVE WITH
SURFACE LOW ABOUT 275 NM SOUTHEAST OF THE SOUTHERN TIP OF THE
KAMCHATKA PENINSULA AND ABOUT 550 NM WEST-SOUTHWEST OF ATTU HAS
ITS SIGHTS SET ON THE BERING SEA ALONG A TRACK MUCH LIKE ITS
PREDECESSOR. BETWEEN THESE TWO LOWS RESIDES TRANSITORY SHORTWAVE
RIDGING.

THE ENTRANCE REGION OF THE STRONG NORTHERLY JET STREAK OVER
SOUTHCENTRAL ALASKA AND THE NORTHERN GULF IS HELPING TO PROMOTE
WEAK LOW PRESSURE TROUGHING OVER THE NORTHERN GULF...WITH SUBSIDENCE
ON THE LEFT ENTRANCE REGION SUPPORTING OFFSHORE GAP FLOWS THROUGH
THE TYPICALLY FAVORED AREAS.

&&

.MODEL DISCUSSION...
MODELS ARE ALL IN GOOD AGREEMENT DEALING WITH THE PATTERN CHANGE
THAT WILL BE OCCURRING OVER THE STATE THIS WEEKEND...AS MULTIPLE
SEASONABLY STRONG LOWS MOVE ACROSS THE BERING.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (TODAY THRU SUN NIGHT)...

AS THE UPPER LEVEL TROUGH CURRENTLY OVER SOUTHCENTRAL MOVES EAST
INTO THE YUKON BY THIS EVENING...INCREASING INSTABILITY WILL
INITIATE WIDESPREAD SHOWER ACTIVITY OVER THE WRANGELL
MOUNTAINS...CHUGACH RANGE...AND ALASKA RANGE...BUT THESE SHOULD
LARGELY NOT IMPACT THE ROAD SYSTEM TOO MUCH...IF AT ALL. THUS WE
HAVE SIGNIFICANTLY CUT BACK ON SNOW ACCUMULATIONS OVER THE EASTERN
COPPER RIVER BASIN.

GAP WINDS WILL ALSO SUBSIDE THROUGH THE DAY AS THE UPPER LEVEL
SUPPORT FOR THE WEAK SURFACE TROUGH OVER THE NORTHERN GULF ALSO
MOVES AWAY FROM THE AREA.

THE NEXT SYSTEM TO WATCH WILL BE A WEAKENING SYSTEM CURRENTLY
MOVING INLAND OVER SOUTHWEST ALASKA AND MOVING TOWARD
SOUTHCENTRAL LATER TODAY. AS THE UPPER LEVEL WAVE APPROACHES LATE
THIS AFTERNOON AND TONIGHT WE SHOULD SEE LIGHT SNOW (POSSIBLY
MIXING WITH RAIN TOWARD SEA LEVEL) DEVELOP ACROSS THE REGION FROM
WEST TO EAST. RIGHT NOW IT IS DIFFICULT TO SAY FOR SURE WHAT KIND
OF ACCUMULATIONS ARE POSSIBLE...GIVEN THE HIGH AMOUNT OF SOLAR
RADIATION AND RESULTANT HEATING OF THE LOWER LAYERS OF THE
ATMOSPHERE...BUT HIGHER ELEVATIONS SUCH AS THE HILLSIDE AND TOWARD
HATCHER PASS STAND THE BEST CHANCE AT SEEING ACCUMULATING SNOW. ISOLATED
TO SCATTERED SNOW AND RAIN SHOWERS MAINLY ALONG THE MOUNTAINS
UNDER TRANSITORY RIDGING PERSISTS THROUGH SUNDAY BEFORE THE NEXT
SYSTEM MOVES INTO THE AREA FROM THE WEST.

THIS NEXT STRONGER WAVE ASSOCIATED WITH THE LOW CURRENTLY
SOUTHWEST OF ATTU WILL LIKELY PUSH OVERRUNNING PRECIPITATION UP
THE COOK INLET REGION LATE SUNDAY...SETTING UP A PERIOD OF
UNSETTLED WEATHER FOR THE EARLY TO MIDDLE PART OF THE WEEK.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (DAYS 1 AND 2)...
THE MAIN WEATHER FEATURE FOR THE SOUTHWEST MAINLAND TODAY IS THE
997 MB LOW NEAR SAINT MATTHEW ISLAND AND ITS ASSOCIATED FRONT
RUNNING SOUTHEASTWARD ALONG THE WESTERN COAST TO BRISTOL BAY.
THIS FRONT WILL BECOME STATIONARY TODAY AND BEGIN TO DISSIPATE
OVERNIGHT. THE MAIN IMPACTS FROM THIS FRONT WILL BE SNOW OVER MUCH
OF THE KUSKOKWIM DELTA WITH LOCAL AMOUNTS OF UP TWO INCHES THIS
MORNING. LIGHTER SNOW AMOUNTS ARE EXPECTED OVER THE BRISTOL BAY
REGION TODAY AS THE MORNING SOUNDING FROM KING SALMON SHOWS A
LARGE DRY LAYER NEAR THE SURFACE SO MUCH OF THE RADAR RETURNS
CURRENTLY OVER THE REGION ARE IN THE FORM OF VIRGA.

A BRIEF DRYING PERIOD IS EXPECTED OVERNIGHT AS THE FIRST FRONT
DISSIPATES AND A SECOND FRONT ASSOCIATED WITH A RAPIDLY DEEPENING
LOW CURRENTLY NEAR ATTU MOVES AT A BRISK PACE THROUGH THE BERING
TO A POSITION NEAR SAINT MATTHEW ISLAND LATE SUNDAY NIGHT.
PRECIPITATION WITH THIS SYSTEM WILL START OFF AS SNOW ALONG THE
WEST COAST BEFORE WARM AIR FROM THE NORTHERN PACIFIC CHANGES THE
PRECIPITATION TO A RAIN SNOW MIX. WINDS OVER THE REGION SHOULD
REMAIN GENERALLY LESS THAN 25 TO 30 MPH AS THE CORE OF THE STRONG
WINDS WILL REMAIN OVER THE BERING SEA.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (DAYS 1 AND 2)...
THE 997 MB LOW NEAR ST MATTHEW ISLAND AND THE ASSOCIATED FRONT IS
SLOWLY DISSIPATING THIS MORNING WITH THE STRONGEST WINDS LIMITED
TO THE BERING SEA AND THE PRIBILOF ISLANDS. THIS PATTERN WILL
CHANGE QUICKLY TODAY AS ANOTHER RAPIDLY DEVELOPING LOW CURRENTLY
NEAR ATTU WILL DEEPEN FROM 992 MB TO 982 MB WEST OF ST PAUL
TONIGHT THEN DEEPEN FURTHER TO THE LOWER 970S NEAR SAINT MATTHEW
ISLAND. THE MAIN IMPACT FROM THIS SYSTEM WILL BE STRONG GALE FORCE
WINDS OVER MUCH OF THE CENTRAL BERING SEA AND ALEUTIAN CHAIN WEST
OF COLD BAY WITH STORM FORCE WINDS EXPECTED OVER THE SOUTHERN
BERING AND CENTRAL ALEUTIANS. WITH THE LARGE FETCH ASSOCIATED WITH
THIS SYSTEM AND THE LONG DURATION OF THE GALE FORCE WINDS...SEAS
IN EXCESS OF 20 FT CAN BE EXPECTED IN PORTIONS OF THE AREA THROUGH
SUNDAY.

&&

.LONG TERM FORECAST (DAYS 3 THROUGH 7)...

AS THE UPPER LEVELS SETTLE INTO A PROGRESSIVE PATTERN...TWO
SYSTEMS WILL MOVE STEADILY ACROSS OUR FORECAST AREA MONDAY THROUGH
FRIDAY. THE FIRST WILL BE A BERING LOW AND ASSOCIATED FRONT THAT
MOVES FROM NEAR SAINT MATTHEW ISLAND TOWARD THE KUSKOKWIM DELTA INTO
TUE...WITH THE FRONT SLOWLY PUSHING ACROSS SOUTHWEST INTO
SOUTHCENTRAL. THE FRONT LINGERS ALONG THE EASTERN GULF COAST INTO
THURSDAY. THE SECOND SYSTEM WILL BE A FRONT THAT SWEEPS ACROSS
THE ALEUTIANS MON NIGHT THROUGH WED...WESTERN AK WED AND THU...AND
WHICH THEN SPAWNS A TRIPLE POINT LOW ON THU THAT MOVES NEAR THE
KENAI ON FRI. THERE IS INCREASING CONFIDENCE WITH THE FIRST SYSTEM
AS THE EC AND GFS ARE IN GOOD AGREEMENT. THE FORECAST UTILIZES
SOME EC THROUGH WED...AND THEN DEFERS TO WPC THROUGH FRI AS THE
SPEED OF SYSTEMS MOVING THROUGH NEAR ZONAL FLOW IS BEST HANDLED BY
AN ENSEMBLE APPROACH.

THE FIRST SYSTEM WILL BRING RAIN AND SNOW TO THE ALEUTIANS AND SW
AK INTO TUE AND RAIN ALONG THE GULF COAST WITH A MIX INLAND EARLY
NEXT WEEK. THE SECOND SYSTEM IS FORECAST TO BE COLDER...WITH A
BETTER CHANCE OF SNOW MOVING INTO SW AK WED INTO THU. FOR
SOUTHCENTRAL...RAIN IS EXPECTED ALONG THE GULF COAST. THESE
SCENARIOS HAVE A TENDENCY TO WARM WITH SUBSEQUENT MODEL
RUNS...BUT FOR NOW...SOME SNOW FOR INTERIOR AREAS OF SOUTHCENTRAL
REMAINS A POSSIBILITY LATE NEXT WEEK.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...STORMS 173 175 176 177.
         GALES 165 170 172 174 178 179 181.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...CC
SOUTHCENTRAL ALASKA...CC
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...RLF
LONG TERM...DS/KH