Small Craft Advisory 390, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
507 AM AKDT THU MAY 7 2015

.ANALYSIS AND UPPER LEVELS...

THE UPPER LEVEL PATTERN IS QUITE CHAOTIC THIS MORNING WITH MULTIPLE
WEAK SHORTWAVES PROPAGATING ALONG THE MAIN UPPER LEVEL LOW ANCHORED
AT THE BASE OF THE UPPER LEVEL TROUGH OVER SOUTHWEST ALASKA AND
THE NORTH PACIFIC. IN ADDITION...MULTIPLE FRAGMENTS OF ENERGY ARE
ALSO OBSERVED ACROSS THE KAMCHATKA PENINSULA...THE NORTH PACIFIC
AND THE BEAUFORT/CHUKCHI SEAS. THE OVERALL IMPACTS OF THE NUMEROUS
SHORTWAVES AND CHAOTIC UPPER LEVEL PATTERN HAS LED TO MULTIPLE
WEAK SURFACE LOWS EXTENDING FROM THE NORTH PACIFIC SOUTH OF THE
ALASKA PENINSULA TO THE BERING STRAIT. A JET STREAK OF BETWEEN 90
TO 100 KT AT 500 MB HAS BEGUN TO ROUND THE BASE OF THE TROUGH AS
SEEN ON WATER VAPOR IMAGERY AND HAS BEGUN TO CHANGE THE ORIENTATION
OF THE UPPER LEVEL TROUGH. CURRENTLY THE TROUGH HAS A POSITIVE
ORIENTATION WHICH IS NOT VARY FAVORABLE FOR THE DEVELOPMENT OF A
CONCENTRIC SFC BASED LOW BUT THE ORIENTATION WILL TAKE ON A MORE
NEGATIVE TILT THIS AFTERNOON WITH GOOD DIFFLUENCE IN THE LEFT EXIT
REGION OF THE JET. THIS IN TURN WILL DEVELOP A CONCENTRIC SFC BASED
LOW THAT WILL BE TALKED ABOUT LATER IN THE DISCUSSION.

IN THE MEANTIME...A GALE FORCE FRONT (AS SEEN ON SCATTEROMETER
IMAGERY OVERNIGHT) CONTINUES TO IMPACT THE WESTERN GULF OF ALASKA
AND SHOULD WEAKEN QUITE SIGNIFICANTLY EARLY THIS MORNING. RAIN
WILL CONTINUE ACROSS THE NORTHERN AND WESTERN GULF COAST AS THE
FRONT STALLS IN THE NORTHERN GULF AND WEAKENS. THE WESTERN KENAI
PENINSULA...ANCHORAGE AND THE MATANUSKA VALLEY HAVE BEEN DRY FOR
THE MOST PART DUE TO A STABLE LAYER AT MOUNTAIN TOP WITH SOUTHEASTERLY
WINDS AT MOUNTAIN TOP AS WELL. WHILE RADAR ECHOES HAVE BEEN OBSERVED
OVERNIGHT...NO PRECIPITATION HAS BEEN PICKED UP BY OBSERVATION
EQUIPMENT. THE LAST TWO FEATURES OF NOTE THIS MORNING ARE AN
OCCLUDED SFC BASED LOW CENTERED NEAR 43N 170E AND A WEAKENING
UPPER LEVEL LOW OVER SIBERIA. THE SFC LOW HAS CONTINUED TO SHIFT
EASTWARD WITH THE UPPER LEVEL STEERING CURRENTS. THE WEAKENING
UPPER LOW OVER SIBERIA WILL BECOME AN OPEN WAVE LATE THIS MORNING
AND APPEARS ON TRACK TO PHASE WITH THE UPPER LEVEL LOW ASSOCIATED
WITH THE AFOREMENTIONED SFC LOW AND REDEVELOP THE LOW ON FRIDAY.

&&

.MODEL DISCUSSION...

MODELS INITIALIZED FAIRLY WELL FOR THE MOST PART GIVEN THE MULTIPLE
FRAGMENTS OF ENERGY AND LACK OF LARGE SYNOPTIC FEATURES TO GRASP
ONTO. THE NAM APPEARS TO BE THE SIGNIFICANT OUTLIER THIS MORNING
DUE TO THE WAY IT HANDLES THE JET STREAK ROUNDING THE BASE OF THE
TROUGH. THE NAM DEPICTS THE JET STREAK ENERGY TRANSLATING FURTHER
EASTWARD OVER THE GULF OF ALASKA WHILE THE ECMWF/GEM/GFS ALL SHOW
A SIMILAR SOLUTIONS WITH THE ENERGY REMAINING SLIGHTLY FURTHER
WEST. THIS COMPLICATES THE POSITION OF THE SURFACE AND UPPER LEVEL
LOW WITH THE NAM MUCH FURTHER NORTH AND QUICKER THAN THE OTHER
GUIDANCE. THE ECMWF TRIES TO DEVELOP A TRIPLE POINT LOW FRIDAY
EVENING BUT WITH THE NEGATIVE TILT OF THE TROUGH AND THE AFFLUENT
AMOUNT OF DYNAMIC ENERGY ALOFT...THE LIKELIHOOD OF A TRIPLE POINT
LOW IS MINIMAL. OUT WEST...THE NAM IS ALSO AN OUTLIER AS IT
APPEARS TO BE NEARLY 6 HOURS FASTER THAN THE OTHER GUIDANCE COME
FRIDAY MORNING. THE NAM/ECMWF WAS DISCARDED FOR THE MORNING
PACKAGED WITH A BLEND OF THE GFS AND GEM USED FOR FORECAST
UPDATES.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (DAYS 1 AND 2)...
A GALE FORCE OCCLUDED FRONT OVER THE NORTHERN GULF WILL MOVE ONSHORE
THIS AFTERNOON WHILE AN UPPER LEVEL SHORT-WAVE MOVES OVERHEAD. THE
BULK OF RAIN WILL REMAIN ALONG THE COAST AS STRONG DOWNSLOPE FLOW
KEEPS THE LEE OF THE KENAI AND CHUGACH MOUNTAINS MOSTLY DRY. CROSS
BARRIER FLOW WILL WEAKEN IN THE AFTERNOON WHILE SURFACE BASED
INSTABILITY INCREASES. THUS WOULD EXPECT ISOLATED TO SCATTERED
SHOWERS INLAND. THE BIGGEST IMPACT HOWEVER WILL BE DEVELOPMENT OF
STRONG GUSTY SOUTHEASTERLY WINDS AS THE FRONT MOVES ONSHORE...FROM
TURNAGAIN ARM/ANCHORAGE TO THE KNIK RIVER VALLEY/PALMER AND ALONG
THE COPPER RIVER INTO THE GLENNALLEN AREA. DESPITE THE OVERALL
INCREASE IN ATMOSPHERIC MOISTURE THE SURFACE WILL REMAIN QUITE
DRY TODAY...PARTICULARLY IN THE COPPER RIVER BASIN AND PARTS OF
THE MATANUSKA VALLEY. WITH SLIGHTLY COOLER TEMPERATURES EXPECTED
TODAY UNDER FAIRLY EXTENSIVE CLOUD COVER...RHS ARE EXPECTED TO
REMAIN JUST ABOVE RED FLAG THRESHOLDS.

A DEVELOPING STORM SYSTEM OVER THE NORTHEAST PACIFIC WILL QUICKLY
MOVE INTO THE GULF WITH VERY LITTLE OR NO BREAK IN RAIN FOR MUCH
OF THE GULF COAST. WITH A STRONG UPPER LEVEL JET AND LONG FETCH
OF TROPICAL MOISTURE EXPECT WIDESPREAD MODERATE TO HEAVY RAIN FOR
THE PRINCE WILLIAM SOUND REGION. THERE IS UNCERTAINTY IN THE EXACT
TRACK OF THE STORM...BUT BASED ON THE STRENGTH OF UPPER LEVEL
FORCING AND ABUNDANT MOISTURE CHANCES ARE PRETTY HIGH THAT SOME
RAIN WILL MAKE IT OVER THE COASTAL RANGE AND AFFECT THE REST OF
SOUTHCENTRAL FRIDAY THROUGH FRIDAY NIGHT. DETERMINING HOW MUCH
RAIN WILL FALL AS WELL AS THE DURATION OF RAIN FOR INLAND AREAS
IS NEARLY IMPOSSIBLE UNTIL THERE IS A BETTER HANDLE ON THE STORM
TRACK. AS IS SO OFTEN THE CASE THE LOW LEVEL FLOW WILL DETERMINE
WHETHER DOWNSLOPING WILL PLAY A ROLE. BOTTOM LINE...IT IS LOOKING
PRETTY WET HEADING INTO THE WEEKEND.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (DAYS 1 AND 2)...
A WEAK SURFACE LOW ALONG WITH A UPPER LEVEL SHORTWAVE TROUGH WILL
MOVE NORTHWARD OVER THE ALASKA PENINSULA THIS MORNING AND INTO
BRISTOL BAY WHERE IT WILL DISSIPATE BY LATE THIS EVENING. THIS
SYSTEM IS PRODUCING WIDESPREAD AREAS OF RAIN THIS MORNING FROM THE
ALASKA PENINSULA TO THE KUSKOKWIM DELTA. NUMEROUS SHOWERS AND
AREAS OF RAIN ARE EXPECTED TO CONTINUE THROUGH THE OVERNIGHT HOURS
TONIGHT AS THE ATMOSPHERE WILL CONTINUE TO STAY VERY MOIST WITH
SOUTHERLY FLOW ALOFT AND SUFFICIENT FORCING REMAINING FROM THE
DISSIPATING SHORTWAVE TROUGH.

AS THE NEW VERTICALLY STACKED LOW MOVES TO NEAR KODIAK ISLAND BY
FRIDAY...THE ASSOCIATED FRONT WILL PUSH THROUGH THE SOUTHCENTRAL
COAST AND OVER THE ALEUTIAN AND ALASKA RANGES DURING THE DAY ON
FRIDAY. THIS WILL PRODUCE WIDESPREAD AREAS OF RAIN MAINLY OVER
THE BRISTOL BAY REGION AND THE LOWER KUSKOKWIM VALLEY. AS THE FLOW
BECOMES CYCLONIC OVER THE REGION ON FRIDAY AFTERNOON...WINDS WILL
SHIFT TO THE NORTH TO NORTHEAST AND BRING DRIER AIR TO THE
KUSKOKWIM DELTA. THIS SHOULD END MOST CHANCES OF PRECIPITATION BY
FRIDAY AFTERNOON ALONG THE KUSKOKWIM COAST WITH THE EXCEPTION OF
THE WESTERN SIDE OF THE KILBUCK MOUNTAINS WHICH MAY SEE SOME RAIN
SHOWERS FRIDAY AFTERNOON DUE TO UPSLOPE FLOW.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (DAYS 1 AND 2)...
CONDITIONS ARE GENERALLY QUIET OVER MUCH OF THE BERING SEA AND
ALEUTIANS AS A RIDGE OF HIGH PRESSURE PROPAGATES EASTWARD OVER THE
CENTRAL BERING SEA THROUGH FRIDAY. AS THE RIDGE TRANSITS
EASTWARD THROUGH FRIDAY...AREAS OF FOG OVER THE EASTERN BERING
SEA AND PRIBILOF ISLANDS ARE EXPECTED THROUGH TOMORROW. RAIN
SHOWERS ARE EXPECTED OVER THE EASTERN ALEUTIANS AND THE ALASKA
PENINSULA THROUGH TONIGHT AS THE WEAK LOW MOVES TODAY INTO BRISTOL
BAY AND DISSIPATES TONIGHT. PRECIPITATION WILL CONTINUE INTO FRIDAY
AS THE LOW SOUTH OF KODIAK BECOMES THE DOMINATE FEATURE. THIS LOW
WILL BRING WITH IT SUFFICIENT WRAP-AROUND MOISTURE AND
INSTABILITY TO KEEP RAIN CHANCES UP ALONG THE ALASKA PENINSULA.

OUT WEST...A LOW CURRENTLY OVER THE NORTH CENTRAL PACIFIC WILL
MOVE TO NEAR ADAK BY LATE FRIDAY AND DEEPEN. THE ASSOCIATED FRONT
WILL BEGIN TO IMPACT THE CENTRAL AND WESTERN ALEUTIANS BY EARLY
FRIDAY MORNING. WITH THE WARM SOUTHERLY PUSH ASSOCIATED WITH THIS
SYSTEM...PRECIPITATION SHOULD STAY AS ALL RAIN. WINDS WILL
INCREASE ALONG THE FRONT WITH MINIMAL GALES EXPECTED OVER THE
CENTRAL ALEUTIANS FRIDAY NIGHT.

&&

.LONG TERM FORECAST (DAYS 3 THROUGH 7)...
THIS WEEKEND THROUGH EARLY NEXT WEEK REMAINS A CHALLENGING
FORECAST AS TWO STORM SYSTEMS MOVE INTO THE FORECAST AREA...ONE
IN THE GULF AND ANOTHER IN THE BERING. CONFIDENCE IS LOWER THAN
AVERAGE IN THE EXTENDED TIME FRAME AS MODEL SOLUTIONS LACK
AGREEMENT BEYOND FRIDAY NIGHT. THE GENERAL OUTLOOK ACROSS THE
SOUTHERN MAINLAND AND GULF IS FOCUSED AROUND A STACKED LOW OVER
THE GULF THAT BECOMES STATIONARY FRIDAY NIGHT THROUGH SATURDAY. A
BLOCKING RIDGE ACROSS THE ALCAN BORDER WORKS AGAINST THE LOW
PREVENTING AN EASTWARD PROGRESSION AND NORTHWARD MOVEMENT IS
BLOCKED BY A STRONGER LOW DIGGING TOWARD THE BERING STRAIT. THE
MOST LIKELY OUTCOME TO THIS PATTERN IS THE LOW IN THE GULF WEAKENS
AND EVENTUALLY BECOMES AN OPEN WAVE AS THE STRONGER SYSTEM NEAR
THE BERING STRAIT KICKS IT EAST.

EXPECT GUSTY WINDS IN THE VICINITY OF THE LOW CENTERS WITH
PERIODS OF RAIN ACROSS AREA WATERS THIS WEEKEND. ALSO GUSTY WINDS
ARE EXPECTED THROUGH CHANNELED TERRAIN ALONG PRINCE WILLIAM SOUND
AND THE EASTERN KENAI WHICH TAPER DOWN AS THE LOW WEAKENS.
RIDGING BETWEEN THE LOW PRESSURE SYSTEMS KEEP THE THE SOUTHERN
MAINLAND MOSTLY DRY...WITH ISOLATED TO SCATTERED SHOWERS STILL
POSSIBLE INLAND AS MOISTURE INCREASES AND SHORT WAVES MOVE FROM
THE TWO UPPER LEVEL LOWS.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...GALES 119 125 130 131 176.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...MC
SOUTHCENTRAL ALASKA...SEB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...RF
LONG TERM...TP