Small Craft Advisory 659, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
418 PM AKDT FRI AUG 21 2015

.ANALYSIS AND UPPER LEVELS...
THE MOST OBVIOUS AND IMPRESSIVE FEATURE ACROSS THE FORECAST AREA
TODAY IS AN IMPRESSIVE LOW PRESSURE CENTER THAT IS IN THE PROCESS
OF RAPIDLY STRENGTHENING IN THE GULF OF ALASKA. THE DEVELOPING
FRONT IS LIKELY ALREADY PRODUCING GALE-FORCE WINDS IN THE NORTH
WEST AND SOUTH QUADRANTS. IN A LOT OF WAYS THIS DEVELOPING STORM
HAS A LOT GOING FOR IT...WITH FAVORABLE JET STREAK DYNAMICS ALONG
WITH AN INCREASINGLY NEGATIVELY TILTED UPPER TROUGH AND A DEEP
LAYER OF WARM AND UNSTABLE AIR AVAILABLE IN THE GULF. THIS IS
RESULTING IN A SURFACE LOW THAT HAS DEEPENED ABOUT 10 MB IN THE
LAST 12 HOURS...AND WILL LIKELY STRENGTHEN ANOTHER 10 BEFORE
BOTTOMING OUT IN THE HIGH 980S LATE TONIGHT.

ADDITIONALLY...BECAUSE THE UPPER TROUGH IS RAPIDLY CLOSING OFF
INTO AN UPPER LOW AT THE SAME TIME THE SURFACE LOW IS DEEPENING
RAPIDLY...A DEEP LAYER OF ASCENT IN THE AFOREMENTIONED WARM AIR
IS GENERATING AND MAINTAINING AN UNUSUAL OUTBREAK OF STRONG
CONVECTIVE SHOWERS AND THUNDERSTORMS OVER THE NORTH AND NORTHWEST
COASTAL WATERS OF THE GULF THAT SHOULD CONTINUE THROUGH THE
AFTERNOON.

INLAND...THICK MID AND HIGH LEVEL CLOUDS ARE INHIBITING SIGNIFICANT
DIURNAL HEATING IN MOST AREAS...BUT NOT ENOUGH TO STOP CONVECTIVE
SHOWERS ALREADY FORMING ALONG THE TALKEETNA MOUNTAINS AND WESTERN
COPPER RIVER BASIN. SOME OF THE SHOWERS FROM THE NORTHWEST GULF
ARE ALSO MAKING IT ACROSS THE CHUGACH AND KENAI MOUNTAINS TOWARD
THE POPULATION CENTERS ON THE WESTERN KENAI PENINSULA AND ALSO
ANCHORAGE.

WEST OF THE ALASKA RANGE...WINDS ARE GENERALLY OUT OF THE
NORTHWEST AS DEEP LAYER HIGH PRESSURE IN THE BERING SEA SLOWLY
EXPANDS EASTWARD. SOME SHOWERS ARE FORMING ALONG THE WEST SIDE OF
THE ALASKA RANGE...BUT IN GENERAL THE AIRMASS ACROSS THE SOUTHWEST
MAINLAND IS GENERALLY UNDER A HEAVY MARITIME INFLUENCE AFTER DAYS
OF ONSHORE FLOW.

WAY OUT WEST...THE BERING SEA/ALEUTIAN REGION IS EXPERIENCING
LIGHT WINDS AND WIDESPREAD MARINE FOG/STRATUS WITH WARM AIR
ADVECTION AND SUBSIDENCE (FROM THE RIDGE OF HIGH PRESSURE)
REINFORCING LOW-LEVEL TEMPERATURE INVERSIONS. MEANWHILE...THE
ALASKA PENINSULA IS SEEING GUSTY NORTHWEST WINDS AS MASS MOVES
TOWARD THE RAPIDLY STRENGTHENING LOW PRESSURE CENTER IN THE GULF.

&&

.MODEL DISCUSSION...
THE MODELS ARE IN GOOD SYNOPTIC AGREEMENT...WITH ONLY MINOR
DIFFERENCES IN THE EXACT TRACK AND PRESSURE OF THE LOW IN THE
GULF. THE GEM REGIONAL IS VERIFYING THE BEST AS OF 18Z/10 AM THIS
MORNING...AND WAS UTILIZED IN THE BEGINNING OF THE SHORT TERM
FORECAST TODAY. THE MOST APPRECIABLE MODEL DIFFERENCE AT THIS TIME
INVOLVES A FAST-MOVING SHORTWAVE TROUGH EXPECTED TO MOVE DOWN THE
FRONT/EAST SIDE OF THE BROAD BERING SEA RIDGE AND INTO THE GULF ON
SUNDAY. NAMELY...THE GLOBAL MODELS ARE IN AGREEMENT THAT THIS
FEATURE WILL DIG OUT A LITTLE MORE OF THE UPPER TROUGH IN THE GULF
OF ALASKA AND EVEN PULL/DEFORM THE SURFACE LOW POSITION IN THE
GULF A BIT TO THE SOUTH. THE NAM COMPLETELY MISSES THIS
FEATURE...AND AS A RESULT THE 12Z NAM WAS DISCARDED DURING THIS
TIME PERIOD.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (DAYS 1 AND 2)...
SATELLITE IMAGERY DEPICTS A RAPIDLY DEEPENING LOW IN THE GULF OF
ALASKA THIS EVENING...WITH A SECONDARY REGION OF WEAKER ASCENT
ACROSS ALL OF SOUTHERN ALASKA. EVEN WITH EXTENSIVE CLOUD COVER
OVERHEAD...ISOLATED TO SCATTERED MOUNTAIN SHOWERS...ALONG WITH
ISOLATED THUNDERSTORMS ALONG THE KENAI PENINSULA...HAVE MANAGED TO
FORM IN ASSOCIATION WITH DIURNAL HEATING THIS AFTERNOON AND EARLY
EVENING. MORE EXTENSIVE SHOWERS WITH EMBEDDED THUNDERSTORMS HAVE
FORMED OVER THE NORTHERN GULF AND INTO PORTIONS OF SOUTHERN PRINCE
WILLIAM SOUND. AS THE LOW DEEPENS IN THE GULF OVERNIGHT...INLAND
SHOWERS WILL RAPIDLY WEAKEN AS THE FLOW BECOMES MORE OFFSHORE...
RESULTING IN INCREASING NORTHERLY GAP WINDS. THIS WILL LEAD TO A
GENERAL DRYING TREND ACROSS INTERIOR ALASKA ON SATURDAY WITH THE
EXCEPTION OF THE COPPER BASIN...CHUGACH MOUNTAINS...AND TALKEETNA
MOUNTAINS AS MOISTURE FROM THE GULF LOW SURGES INLAND. THE LOW
WILL WEAKEN AND DRIFT EAST SUNDAY...BUT BREEZY OFFSHORE WINDS WILL
LINGER...ALONG WITH SHOWER ACTIVITY MAINLY FROM THE WESTERN COPPER
BASIN EASTWARD AS SHOWERS AND CLOUDS REMAIN IN THE WAKE OF THE
DEPARTING LOW.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (DAYS 1 AND 2)...
AN AREA OF INSTABILITY ALONG THE ALEUTIAN RANGE WILL KEEP A
SLIGHT CHANCE OF THUNDERSTORMS THROUGH EARLY EVENING. OTHERWISE
HIGH PRESSURE OVER THE BERING AND LOW PRESSURE OVER THE GULF WILL
KEEP SHOWERY NORTHWEST FLOW OVER THE AREA TONIGHT...AND ALONG THE
KUSKOKWIM RIVER THROUGH THE WEEKEND. SOME STRONG WIND GUSTS ARE
EXPECTED THROUGH CHANNELED TERRAIN OF THE AKPEN.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (DAYS 1 AND 2)...
NORTHWEST FLOW OVER THE EASTERN BERING WILL KEEP SOME SHOWERS
ALONG THE BERING COAST TONIGHT. AS HIGH PRESSURE BUILDS ACROSS THE
BERING...SHOWERS WILL END OVER MUCH OF THE CHAIN. THE NEXT FRONT
WILL ENTER THE WESTERN ALEUTIANS WITH SMALL CRAFT WINDS AND RAIN
SAT...INCREASING TO GALES ON SUN.

&&

.LONG TERM FORECAST (DAYS 3 THROUGH 7)...
AN UPPER LEVEL LOW OVER THE GULF OF ALASKA WILL EXIT TO THE SOUTHEAST
BY MONDAY...MAKING ROOM FOR A NORTH PACIFIC RIDGE WHICH WILL BUILD
OVER MUCH OF THE SOUTHERN MAINLAND. BY THE END OF THE WEEKEND...THE
PREVIOUSLY ESTABLISHED GUSTY NORTHWEST FLOW WILL BEGIN TO DIMINISH
AS THE SURFACE LOW OVER THE GULF TRACKS FURTHER EASTWARD. THAT
NORTHWEST FLOW WILL CONTINUE TO USHER IN SOME DRIER AIR ACROSS
MUCH OF THE MAINLAND...WITH THE EXCEPTION OF A FEW WEAK WAVES
COMING ACROSS THE ALASKA RANGE EARLY IN THE WEEK THAT CARRY THE
POTENTIAL TO TRIGGER SOME AFTERNOON/EVENING SHOWERS ALONG HIGHER
TERRAIN.

THE MODELS THROUGH TUESDAY ARE IN FAIR OVERALL AGREEMENT...BUT
BEGIN TO DIVERGE BY MIDWEEK. MAINLY THEY CONTINUE TO DIFFER WITH
REGARDS TO THE NORTHERN ENERGY STREAM PUSHING OFF EASTERN SIBERIA
AND AN UPPER LOW APPROACHING THE WESTERN ALEUTIANS TUESDAY
MORNING. THE ECMWF...BEING THE MORE FAVORABLE SOLUTION...PHASES
THESE TWO SYSTEMS INTO A BROAD TROUGH DIGGING THROUGH THE BERING
AND INTO THE SOUTHWEST MAINLAND BY THURSDAY...AND PUSHING THE
NORTH PACIFIC RIDGE FARTHER SOUTH. THE 12Z RUN OF THE GFS KEEPS
THE RIDGE FARTHER NORTH WITH THE NORTHERN ENERGY STREAM TRACKING
THROUGH THE SOUTHWEST MAINLAND BY FRIDAY...THEN REBUILDING
STRONGER RIDGING OVER THE BERING INTO THE WEEKEND. BY NEXT
WEEKEND...THE MODEL SPREAD GROWS EVEN LARGER WITH THE GFS KEEPING
RIDGING OVER THE BERING AND THE EC DIGGING ANOTHER TROUGH THROUGH
THE BERING FROM THE NORTH. EVEN THOUGH MODELS HAVE COME INTO
BETTER AGREEMENT THROUGH EARLY NEXT WEEK...OVERALL FORECAST
CONFIDENCE REMAINS LOW WITH THE OVERALL PATTERN GIVEN THE
AFOREMENTIONED MODEL DIFFERENCES FROM TUESDAY THROUGH NEXT
WEEKEND. MODELS HAVE STARTED TO AGREE ON THE REMNANTS OF ATSANI
TRACKING FARTHER WEST OF THE WESTERN ALEUTIANS THAN PREVIOUSLY
DEPICTED...KEEPING THE SYSTEM WEST OF THE DATE LINE AND SOUTH OF
50N THROUGH THE WEEKEND.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...GALE WARNING 119 120 130 131 132 150.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...AD
SOUTHCENTRAL ALASKA...JA
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...DS
LONG TERM...TP