Small Craft Advisory 683, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
415 PM AKDT SUN AUG 30 2015

.ANALYSIS AND UPPER LEVELS...

UPPER LEVELS...A LONGWAVE TROUGH IS NOW ESTABLISHED OVER MAINLAND
ALASKA...WITH AN UPPER LEVEL LOW CENTERED OVER THE NORTHEASTERN
GULF. THE JET STREAM EXTENDS FROM THE WEST COAST OF ALASKA ACROSS
THE ALASKA PENINSULA...THEN TURNS EAST ACROSS THE SOUTHERN GULF.
RIDGING IS IN PLACE OVER THE CENTRAL BERING AND ALEUTIANS...WHILE
A CUTOFF UPPER LEVEL LOW IS CENTERED BETWEEN ATTU AND THE
KAMCHATKA PENINSULA.

SURFACE ANALYSIS...A SURFACE LOW CENTERED OVER THE NORTHEASTERN
GULF IS CREATING STRONG OFFSHORE WINDS ACROSS MUCH OF SOUTHCENTRAL
THIS AFTERNOON. A SHORTWAVE DIVING SOUTH ACROSS THE WESTERN
INTERIOR COMBINED WITH TERRAIN ENHANCEMENT IS CAUSING LIFT FOR
RAIN AND SNOW SHOWERS ALONG THE WEST SLOPES OF THE ALASKA RANGE.
COLD ADVECTION ALONG THE KUSKOKWIM DELTA COAST IS KEEPING SKIES
RELATIVELY CLEAR. OVER THE BERING...A STRONG DEFORMATION ZONE IS
IN PLACE SEPARATING A VERY MOIST...WARM...UNSTABLE AIR MASS SOUTH
AND WEST OF THE BOUNDARY FROM THE COLD...DRY...MORE STABLE AIR
MASS OVER THE NORTHERN BERING. VISIBLE SATELLITE IMAGERY SHOWS
EXTENSIVE CLOUD COVER FROM THE DEFORMATION ZONE SOUTH WITH SHOWERY
CONDITIONS OVER MUCH OF THE ALEUTIANS. A STATIONARY FRONT REMAINS
OVER THE WESTERN ALEUTIANS...CAUSING LOCALLY DENSE FOG.

&&

.MODEL DISCUSSION...

THE MODELS ARE ALL IN VERY GOOD AGREEMENT FOR THE NEXT FEW DAYS. THE
SHORTWAVE WILL MOVE SOUTHEAST ACROSS SOUTHCENTRAL TONIGHT. THE
SURFACE LOW OVER THE NORTHEASTERN GULF WILL BECOME A BROAD AREA OF
LOW PRESSURE OVER THE NEXT DAY OR TWO. HIGH PRESSURE OVER THE
BERING WILL MOVE INTO SOUTHWEST ALASKA BY TUESDAY AND TIGHTEN THE
PRESSURE GRADIENT ACROSS THE SOUTHERN MAINLAND. A NORTH PACIFIC
LOW WILL MOVE THROUGH THE CENTRAL ALEUTIANS MONDAY NIGHT AND INTO
THE EASTERN BERING ON TUESDAY. A FRONT WILL REMAIN STATIONARY OVER
THE WESTERN ALEUTIANS FOR THE NEXT SEVERAL DAYS. THE NAM WAS THE
PREFERRED MODEL TODAY OWING TO ITS HIGHER RESOLUTION AND BETTER
HANDLING OF THE ONGOING LOCALIZED GAP WIND FLOWS.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (DAYS 1 AND 2)...
AN UPPER LEVEL LOW MOVING INTO SOUTHCENTRAL FROM THE WESTERN
INTERIOR WILL BRING INCREASING CLOUD COVER AND A CHANCE OF
SHOWERS ALONG THE TALKEETNA AND CHUGACH MOUNTAINS THIS EVENING.
THERE WILL BE ENOUGH COLD AIR ASSOCIATED WITH THIS LOW TO BRING
SNOW LEVELS AS LOW AS 2000 FT...SO THERE IS A GOOD CHANCE THAT
THESE SHOWERS WILL BRING ANOTHER FRESH DUSTING OF SNOW TO THE
HIGHER ELEVATIONS TONIGHT. THERE IS ALSO A SLIGHT CHANCE THAT SNOW
MAY REACH THE ANCHORAGE HILLSIDE...HOWEVER ACCUMULATIONS SHOULD BE
LIGHT. WITH THIS INCREASED CLOUD COVER...MANY OF THE USUAL COLD
SPOTS OF THE MATANUSKA VALLEY AND ANCHORAGE BOWL MAY BE SLIGHTLY
WARMER THAN THE LAST FEW NIGHTS THOUGH ISOLATED AREAS MAY DIP
BELOW FREEZING TONIGHT. AT THIS POINT HOWEVER IT DOES NOT APPEAR
AS IF IT WILL BE WIDESPREAD ENOUGH TO WARRANT A FREEZE WARNING.

AS THE UPPER LOW PROGRESSES INTO THE NORTHERN GULF TOMORROW
MORNING...A SURFACE LOW WILL FORM ONCE AGAIN IN THE VICINITY OF
PRINCE WILLIAM SOUND...BRINGING ANOTHER ROUND OF OUTFLOW WINDS
THROUGHOUT THE NORTH GULF COAST AND GUSTY NORTHERLY WINDS TO
INTERIOR AREAS. THIS SYSTEM SHOULD BRING SIMILAR WINDS TO MOST
AREAS THE SYSTEM ON FRIDAY NIGHT BROUGHT...WITH THE STRONGEST
WINDS EXPECTED THROUGH THE BARREN ISLANDS...AS WELL AS USUAL
COASTAL GAP WIND AREAS OF SEWARD...WHITTIER...AND THOMPSON PASS IN
THE MORNING HOURS. THESE WINDS SHOULD BEGIN TO DIMINISH IN THE
AFTERNOON AS THE SURFACE LOW MOVES OUT OF THE AREA...ALLOWING FOR
THE PRESSURE GRADIENT TO SLACKEN. AFTERWARDS...WEAK RIDGING WILL
MOVE INTO THE AREA...ALLOWING FOR GENERALLY QUIET CONDITIONS WITH
SEASONABLE TEMPERATURES INTO TUESDAY.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (DAYS 1 AND 2)...
THE LOW OVER MAINLAND ALASKA COMBINED WITH RIDGING OVER THE BERING
IS PRODUCING A TIGHT PRESSURE GRADIENT OVER THE SOUTHWEST CAUSING
STRONG NORTHWEST WINDS. THESE WINDS WILL WEAKEN THROUGH MONDAY AS
THE LOW PUSHES OFF INTO THE GULF. THE NORTHWEST FLOW IS ALSO
EXPECTED TO BRING SHOWERS TO THE SOUTHWEST AND THE LOW IS PULLING
DOWN ENOUGH COLD AIR FROM THE NORTH FOR THE RAIN TO MIX WITH SNOW
IN THE LOWER KUSKOKWIM VALLEY AND ALASKA RANGE. THE SHOWERS WILL
THEN DIMINISH AS THE RIDGING MOVES OVER THE SOUTHWEST LATER ON
MONDAY.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (DAYS 1 AND 2)...
THE WEAKENING FRONT OVER THE CENTRAL BERING IS STALLING IN ITS
EASTWARD PROGRESSION AS IT IS BEING HELD UP BY THE RIDGING OVER
THE BERING. FOR NOW THE RIDGING IS KEEPING THE PRIBILOF ISLANDS
OUT OF THE RAIN BUT AS THE RIDGING PUSHES OFF TO THE EAST ON
MONDAY REMNANTS OF THE FRONT WILL BRING IN SOME RAIN. THIS WILL BE
FOLLOWED BY A NEW FRONT PUSHING IN FROM THE SOUTH. WEST OF THE
WESTERN ALEUTIANS ANOTHER LOW IS SLOWLY TRACKING NORTH AND WILL
BRING RAIN TO THE WESTERN ALEUTIANS OVERNIGHT. AS THE LOW PUSHES
OFF TO THE NORTH THE RAIN WILL BECOME LIGHTER AND MORE SHOWERY.

&&

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

MODELS REMAIN IN GOOD AGREEMENT THROUGH FRIDAY WITH HIGH PRESSURE
OVER THE EASTERN BERING SEA SHIFTING EAST ACROSS THE ALASKA
MAINLAND ON WEDNESDAY AND ANOTHER SERIES OF UPPER LEVEL LOWS
ACROSS THE WESTERN BERING SEA AND NORTH PACIFIC. THERE IS SOME
DISCREPANCY ON HOW AN UPPER LOW SITTING EAST OF THE KAMCHATKA
PENINSULA INTERACTS...IF AT ALL...WITH A SECOND UPPER LOW PUSHING
OFF THE SOUTHERN TIP OF THE KAMCHATKA PENINSULA ON WEDNESDAY. THIS
CREATES SOME PLACEMENT ISSUES AS THE NAM TRIES TO KEEP SOME
INTERACTION BETWEEN THE TWO SYSTEMS WHILE GLOBAL MODELS HINT AT VERY
LITTLE INTERACTION. THE NET RESULT OF THIS INTERACTION WILL BE
SEEN AS THE UPPER LOW FALLS APART AND BECOMES MORE OF AN OPEN WAVE
HEADING INTO SATURDAY. SIGNIFICANT DEVIATIONS IN THE MODELS
DEVELOP IN THE SHORTWAVE PLACEMENT OF THIS WAVE AS IT ENTERS THE
WESTERN GULF OF ALASKA AND THEREFORE PRECIPITATION ALONG THE GULF
COAST. IN ADDITION...THE UPPER LEVEL TROUGH LOOKS TO SHARPLY
AMPLIFY LATE SATURDAY WITH THE INJECTION OF A POTENT POSITIVE
VORTICITY ANOMALY. THE GFS WANTS TO UNDERCUT THE BUILDING RIDGE
AND LEAD TO A WET ZONAL FLOW PATTERN ACROSS ALASKA WHILE THE ECMWF
USES THE COLD AIR DROPPING OUT OF SIBERIA TO AMPLIFY THE TROUGH
AND THEREFORE THE RIDGE BUILDING BACK ACROSS THE ALASKA MAINLAND.
THE ECMWF SOLUTION APPEARS TO BE THE MORE PROBABLE ONE BASED OFF
OF PATTERN RECOGNITION...ESPECIALLY WITH ABUNDANT COLD AIR
DROPPING OUT OF SIBERIA/THE ARCTIC.

THE BERING SEA WILL LIKELY REMAIN UNDER A LARGE UPPER LEVEL TROUGH
LEADING TO YET ANOTHER PERIOD OF SHOWERS AS COLD AIR ALOFT KEEPS
MID-LEVEL LAPSE RATES FAVORABLE FOR CONVECTIVE SHOWERS. SOUTHWEST
AND SOUTHCENTRAL ALASKA SHOULD REMAIN MOSTLY DRY WITH A FEW
SHOWERS THROUGH MUCH OF THE EXTENDED...ESPECIALLY IF THE ECMWF
SOLUTION VERIFIES WITH THE RE-AMPLIFICATION OF THE RIDGE OVER THE
MAINLAND. THE ONE HICCUP COMES ON SATURDAY AS THE UPPER LEVEL LOW
MENTIONED EARLIER BEGINS PUSHING INTO THE WESTERN GULF OF ALASKA
AND MAY CLIP THE ALASKA PANHANDLE. THIS WOULD BRING SOME RAIN
MAINLY TO THE BRISTOL BAY AREA AND COASTAL LOCATIONS. THE PERIOD
OF NORMAL TO SLIGHTLY COOLER THAN AVERAGE TEMPERATURES LOOKS TO
PERSIST THROUGH THE MAJORITY OF THE EXTENDED PERIOD.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...GALE 121 127 130-139.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...DS
SOUTHCENTRAL ALASKA...DEK
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...DK
LONG TERM...MC