Small Craft Advisory 345, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
507 AM AKDT MON APR 20 2015

.ANALYSIS AND UPPER LEVELS...
AN UPPER LEVEL CLOSED STACKED LOW IS POSITIONED NEAR NUNIVAK
ISLAND WITH THE SURFACE LOW BEING REFLECTED JUST WEST OF THE
AREA. THIS LOW HAS SEVERAL UPSTREAM SHORTWAVE TROUGHS AROUND THIS
SYNOPTIC FEATURE. AN ASSOCIATED NEGATIVE TILTED TROUGH EXTENDS
THROUGH THE SOUTHWEST ALASKA REGION INTO KODIAK ISLAND AND THE
SOUTHWESTERN GULF. THE EASTERN GULF HAS A 100 KNOT SOUTHERLY JET
STREAM POSITIONED THROUGH THE NORTHERN GULF EXTENDING THROUGH THE
EASTERN COPPER RIVER BASIN. THE RADAR IMAGERY IS REFLECTING
NUMEROUS RAIN SHOWERS IN THE NORTHERN GULF...PRINCE WILLIAM
SOUND...EASTERN KENAI PENINSULA AND THROUGH COOK INLET INTO THE
LOWER SUSITNA VALLEY. THE SURFACE TEMPERATURES FOR THE NORTHERN
COASTAL AREAS ARE ALL IN THE UPPER 30S TO LOWER 40S. ASCAT
SCATTEROMETER WIND IMAGERY IS SHOWING THE GULF WITH 20 KNOT
SOUTHERLY FLOW AND NORTHWESTERLY FLOW OVER THE CHIRIKOF ISLANDS.

OVER THE BERING SEA AND THE ALEUTIANS REGION...THERE IS A
NORTHERLY 90 KNOT JET STREAM POSITION OVER THE WESTERN BERING.
THIS JET STREAM TRANSITIONS TO A ZONAL FLOW WITH A 110 KNOT JET
STREAM SOUTH OF THE CENTRAL TO EASTERN ALEUTIANS. THE COLD AIR ON
THE BACKSIDE OF THE UPPER LEVEL LOW IS PRODUCING STRONG GUSTY WEST
TO NORTHWESTERLY WINDS. THIS FEATURE IS ACCOMPANIED BY NUMEROUS
SNOW SHOWERS IN THE BERING SEA AND ALONG THE ALEUTIAN CHAIN.

&&

.MODEL DISCUSSION...
ALL MODELS REMAIN IN GOOD AGREEMENT WITH THE SHORTWAVE TROUGH
MOVING INTO THE WESTERN GULF AND A WEAK RIDGE OVER THE ALASKA
PENINSULA EXTENDING INTO THE ALEUTIAN RANGE THIS MORNING. ON THE
BERING SIDE THE MODELS ARE IN DECENT AGREEMENT WITH THE SURFACE
LOW OVER SAINT MATTHEW ISLAND AND AN ASSOCIATED TROUGH EXTENDING
ALONG THE SOUTHWEST ALASKA COASTLINE THIS MORNING. BY TUESDAY
MORNING AN AMPLIFIED RIDGE BUILDS INTO THE GULF AND SOUTHCENTRAL.
THE FORECAST CONFIDENCE IS AVERAGE WITH SOME UNCERTAINTY WITH THE
TROUGH PUSHING THROUGH SOUTHCENTRAL AND THE AMOUNT OF SNOWFALL THE
ANCHORAGE BOWL WILL RECEIVE TODAY. THE NAM WAS THE PREFERRED
MODEL OF CHOICE FOR SOUTHCENTRAL. ACROSS SOUTHWEST ALASKA...THE
GEM-REGIONAL WAS PREFERRED DUE TO BETTER HANDLING OF THE COLD AIR
ON THE BACKSIDE OF THE LOW IN THE NORTHERN BERING SEA.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (DAYS 1 AND 2)...
THE ATMOSPHERE OVER SOUTHCENTRAL REMAINED VERY UNSTABLE OVERNIGHT
AS THERE WERE NUMEROUS RAIN AND SNOW SHOWERS OVER THE AREA AND
EVEN SOME LIGHTNING STRIKES DETECTED IN THE PRINCE WILLIAM SOUND
REGION AS WELL AS IN THE NORTHWEST SUSITNA VALLEY NEAR THE HIGHER
PEAKS OF THE ALASKA RANGE. A COLD FRONT PUSHING INTO THE REGION
FROM THE SOUTHWEST WILL TURN PRECIPITATION TO ALL SNOW IN THE COOK
INLET AND SUSITNA VALLEY REGIONS TODAY. TEMPERATURES WILL BE AT
THEIR HIGHEST DURING THE EARLY MORNING OVER MUCH OF THE AREA FROM
THE KENAI PENINSULA NORTHWARD DUE TO THIS FRONT. THE ATMOSPHERE
BEHIND THIS FRONT WILL BE COLD ENOUGH FOR SNOW SO EVEN IF IT WARMS
INTO THE 40S THE TEMPERATURE WILL DROP RAPIDLY WHEN A SNOW SHOWER
MOVES OVERHEAD. THOSE TWO REASONS ARE WHY MOST AREAS WILL SEE
HIGHS WELL ABOVE FREEZING YET STILL HAVE SNOW AS THE
PRECIPITATION TYPE.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (DAYS 1 THROUGH 3)...
DISTURBANCES WRAPPING AROUND THE MAIN CLOSED LOW CENTER OFF OF
NUNIVAK ISLAND WILL SUPPORT ANOTHER ROUND OF SNOW SHOWERS OVER
MOST OF SOUTHWEST TODAY AND TONIGHT. THERE IS ENOUGH INSTABILITY
IN THE ATMOSPHERE TO SUPPORT ISOLATED THUNDERSTORMS ACROSS
PORTIONS OF THE BRISTOL BAY AND THE LOWER KUSKOKWIM VALLEY. SNOW
SHOWERS...REGION WIDE...MAY BECOME INTENSE AT TIMES WITH HEAVY
SNOW SQUALLS AND BLOWING SNOW...AND WILL PERSIST INTO MONDAY
NIGHT. FOR THIS REASON...A WINTER WEATHER ADVISORY WILL BE ISSUED
FOR THE WESTERN CAPES...THE AREA WHERE THE STRONGEST WINDS AND
POTENTIAL HEAVIEST SNOWFALL WILL BE. ON TUESDAY...THE ATMOSPHERE
WILL SLOWLY STABILIZE...WITH SNOW SHOWERS BECOMING LESS WIDESPREAD
AND INTENSE...ALTHOUGH ISOLATED INTENSE SHOWERS WILL STILL BE
POSSIBLE DURING THE AFTERNOON...ESPECIALLY ACROSS THE BRISTOL BAY
AND KUSKOKWIM DELTA REGIONS.

SHOWERS WILL BEGIN TO DIMINISH TUESDAY EVENING ALTHOUGH SNOW
THREATS WILL REMAIN ACROSS THE BRISTOL BAY AND LOWER KUSKOKWIM
DELTA AS AN ELONGATED FRONTAL ZONE DEVELOPS INTO WEDNESDAY. THERE
IS MUCH UNCERTAINTY WITH THE EXACT PLACEMENT OF THIS FRONT...BUT
IT IS EXPECTED TO BE A RATHER NARROW ZONE OF POTENTIAL
ACCUMULATING SNOW.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (DAYS 1 THROUGH 3)...
DISTURBANCES WRAPPING AROUND A PARENT LOW NEAR NUNIVAK ISLAND
WILL KEEP THE CENTRAL AND EASTERN BERING SEA/ALEUTIANS/PRIBILOF
ISLANDS UNDER THE THREAT OF SNOW SHOWERS AND SQUALLS INTO MONDAY
NIGHT BEFORE SHOWERS SLOWLY DIMINISH. A FAST MOVING WARM FRONT
WILL TRACK FROM THE WESTERN ALEUTIANS TUESDAY AFTERNOON AND REACH
THE EASTERN ALEUTIANS BY WEDNESDAY AFTERNOON...BRINGING LOW END
GALE FORCE WINDS AND RAIN AND SNOW TO THE REGION.

&&

.LONG TERM FORECAST (WEDNESDAY THROUGH SUNDAY)...

THE LONG TERM FORECAST WILL BE HIGHLY DEPENDENT UPON SOME HIGH
AMPLITUDE FEATURES EXTENDING FROM KAMCHATKA INTO THE GULF OF
ALASKA. ON WEDNESDAY...TWO AREA OF LOW PRESSURES...ONE OVER THE
GULF OF ALASKA AND THE OTHER OVER THE WESTERN BERING SEA...WILL
BOTH BRING GALE FORCE WINDS TO THOSE LOCATIONS. THE LOW OVER THE
SOUTHERN GULF OF ALASKA WILL BE QUICK TO DISSIPATE AS THE LOW OVER
THE WESTERN BERING SEA BARRELS EASTWARD AND ABSORBS THE REMNANTS
OF THE GULF LOW ON THURSDAY. MODEL AGREEMENT CONTINUES TO INCREASE
WITH THESE TWO LOWS ALTHOUGH MODELS HAVE SPED THE SYSTEMS UP BY
ABOUT 12 HOURS DUE TO A SLIGHTLY LESS AMPLIFIED PATTERN. A JET
STREAK ON FRIDAY WILL HELP TO AMPLIFY THE LONGWAVE TROUGHING
DEVELOPING AS THE INITIAL LOW OVER KAMCHATKA ELONGATES AND GETS
TRAPPED ON THE EAST SIDE BY A BUILDING PACIFIC NORTH AMERICAN
RIDGE. PREVIOUS MODEL RUNS HINTED AT THIS RIDGE NOSING ACROSS
ALASKA BUT NOW IT LOOKS LIKE IT WILL STAY EAST OF ALASKA DUE TO
THE TRACK OF THE UPPER LEVEL LOW. A SECOND KAMCHATKA LOW WILL PUSH
OFF THE PENINSULA ON SATURDAY AND TAKE A SIMILAR PATH TO THE ONE
OBSERVED MIDWEEK. OVERALL...THE SYNOPTIC PATTERN DOES NOT LOOK TO
CHANGE MUCH IN THE COMING 7 TO 10 DAYS AS THE JET STREAM CONTINUES
TO STAY OVER THE NORTH PACIFIC.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...WINTER WEATHER ADVISORY 161.
MARINE...GALE 165 178.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...PD
SOUTHCENTRAL ALASKA...EZ
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JRA
LONG TERM...MC