Small Craft Advisory 508, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
456 AM AKDT THU JUN 18 2015

.ANALYSIS AND UPPER LEVELS...
THE STRONG UPPER LEVEL HIGH REMAINS CENTERED OVER THE NORTHERN
BERING SEA WHICH IS BRINGING NORTHERLY FLOW ALOFT OVER THE MUCH OF
THE MAINLAND. AN UPPER LEVEL DISTURBANCE HAS STARTED TO TRACK OVER
THE KUSKOKWIM DELTA AND LOWER KUSKOKWIM VALLEY THIS MORNING...WHICH
COULD HELP BRING UP SOME GUSTY NORTHERLY WINDS ALONG THE
KUSKOKWIM DELTA TODAY. THE SURFACE HIGH OVER THE BERING HAS
SLIGHTLY WEAKENED AND SHIFTED OVER THE NORTHWEST BERING...AND THE
THERMAL TROUGH OVER SOUTHCENTRAL WILL BEGIN TO BUILD BACK OVER
THE AREA THROUGHOUT THE DAY TODAY. THIS THERMAL TROUGH WILL CONTINUE
TO DEVELOP DURING THE DAYTIME HOURS AND BUILD STRENGTH EACH
DAY...AS IT TRACKS FURTHER NORTH OVER THE SOUTHERN AND CENTRAL
MAINLAND OVER THE NEXT FEW DAYS. FURTHER WEST...A FRONTAL SYSTEM
HAS STARTED TO PUSH INTO THE WESTERN ALEUTIANS FROM THE NORTH
PACIFIC...WITH CURRENT OBSERVATIONS SHOWING WINDS BEGINNING TO
INCREASE AND SHIFT TO THE SOUTHEAST OVER THE WESTERN ALEUTIANS.

&&

.MODEL DISCUSSION...
MODELS REMAIN IN VERY GOOD SYNOPTIC AGREEMENT AS THE RIDGE AND
THERMAL TROUGH ARE DEPICTED WELL INTO THE WEEKEND. THE ATMOSPHERIC
INSTABILITY PARAMETERS ARE BEING COMPARED CLOSELY...AS THE MODELS
ARE IN FAIR AGREEMENT WITH AREAS OF INSTABILITY LINGERING AROUND
SOUTHCENTRAL AND PARTS OF THE SOUTHWEST MAINLAND TODAY AND INTO
THE WEEKEND. MODELS ARE ALSO IN GOOD AGREEMENT WITH FRIDAY AND
SATURDAY BEING MORE CONDUCIVE FOR INCREASED INSTABILITY AND
MOISTURE...LEAVING THE ATMOSPHERE WELL PRIMED FOR AFTERNOON AND
EVENING SHOWERS AND OR THUNDERSTORMS.

.FIRE WEATHER...

CRITICAL FIRE WEATHER CONDITIONS ARE EXPECTED IN THE COPPER RIVER
BASIN AND AGAIN IN THE SUSITNA VALLEY THIS AFTERNOON FOR LOW
RELATIVE HUMIDITIES (15 PERCENT OR LESS). THERE IS ONLY A SLIGHT
RISK OF THUNDERSTORMS OVER THE HIGHER ELEVATIONS TODAY WITH HIGH
PRESSURE OVER SOUTH CENTRAL ALASKA. ON FRIDAY THUNDERSTORMS WILL
BE MORE WIDESPREAD...BUT ARE EXPECTED TO BE MAINLY WET DUE TO THE
HIGHER MOISTURE IN THE AREA AND SLOW MOVEMENT OF ANY STORMS THAT
DEVELOP.

OUT WEST...RH VALUES SHOULD BE SIMILAR OR A BIT HIGHER THAN
YESTERDAY...EXCEPT OVER INLAND BRISTOL BAY WHERE LOCALLY DOWNSLOPE
COMPRESSIONAL WARMING WILL HELP LOWER MINIMUM RH VALUES A BIT.
THIS WILL STILL BE ABOVE CRITICAL FIRE WEATHER THRESHOLDS IN THE
UPPER TEENS TO MID 20S. HOWEVER...INCREASING INSTABILITY FOR THIS
WEEKEND ALONG THE THERMAL TROUGH WILL ALLOW FOR INCREASING
COVERAGE OF SHOWERS AND THUNDERSTORMS INLAND. THESE THUNDERSTORMS
WILL DEVELOP MAINLY ALONG THE MOUNTAINS EACH AFTERNOON AND
EVENING...WITH HEAVIEST CONCENTRATION ALONG THE ALASKA AND
ALEUTIAN RANGES.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (DAYS 1 AND 2)...
THE UPPER LEVEL HIGH OVER THE BERING SEA WILL REMAIN BASICALLY IN
PLACE THROUGH FRIDAY NIGHT. SOUTH CENTRAL ALASKA WILL CONTINUE TO
BE ON THE EASTERN PERIPHERY OF THIS HIGH. ANOTHER FAIRLY HOT DAY
IS ON TAP FOR TODAY. NORTHERLY FLOW ALOFT WILL WEAKEN TODAY AND
TONIGHT. THIS WILL EVOLVE INTO A WEAKNESS IN THE FLOW BY FRIDAY
ALLOWING THE MOISTURE TO INCREASE. AT THE SAME TIME (FRIDAY)...THE
THERMAL TROUGH WILL MAKE A FULL AND DEFINITE SHIFT NORTH OF THE
ALASKA RANGE. LOW LEVEL ONSHORE FLOW WILL DEVELOP AND COOLER
TEMPERATURES ARE EXPECTED ACROSS THE AREA. INSTABILITY WILL
INCREASE ON FRIDAY...AND THIS ALONG WITH THE INCREASE IN MOISTURE
WILL CREATE ISOLATED TO SCATTERED THUNDERSTORMS OVER THE COOK
INLET AREA THROUGH THE SUSITNA AND MATANUSKA VALLEYS.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (TODAY THROUGH SUNDAY)...

THE THERMAL TROUGH OVER THE ALASKA AND ALEUTIAN RANGES WILL SLOWLY
DRIFT WEST TO THE WESTERN SLOPES AND THEN EVENTUALLY TO THE
KUSKOKWIM DELTA BY THE END OF THE WEEKEND. WINDS WILL BE NORTHWEST
TO THE WEST OF THIS FEATURE...AND TO THE SOUTH AND SOUTHWEST ON
THE EAST SIDE. THUS...LOW-LEVEL MOISTURE WILL BE ON THE INCREASE
ACROSS MOST OF THE REGION ALLOWING FOR SOME MODERATION OF MINIMUM
HUMIDITY VALUES...ESPECIALLY AFTER TODAY.

LATER FRIDAY AND ESPECIALLY INTO SATURDAY...INCREASED INSTABILITY
AND AN UPPER LEVEL TROUGH WILL ALLOW FOR THE INCREASED POSSIBILITY
OF SCATTERED SHOWERS AND ISOLATED WET THUNDERSTORMS NEAR AND
ALONG THE THERMAL TROUGH.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (TONIGHT THROUGH FRI)...

STRONG SUBSIDENCE INVERSION OVER THE BERING SEA CONTINUES TO HELP
KEEP LOW STRATUS AND FOG IN PLACE OVER THE BERING THE PAST
SEVERAL DAYS. THIS STABLE AIR MASS DOES NOT SHOW ANY SIGNS OF
DISSIPATING IN THE NEAR FUTURE...WHICH WILL LIKELY KEEP LOW
STRATUS AND FOG INTO THE WEEKEND...WITH SOME OF IT MIXING OUT IN
THE AFTERNOON HOURS EACH DAY. A DISTURBANCE SOUTH OF THE WESTERN
ALEUTIANS CONTINUES TO BRING A CHANCE OF RAIN ALONG WITH FOG TO
THE WESTERN ALEUTIANS. HOWEVER...A LOW IN THE NORTHWESTERN
PACIFIC WILL MOVE THROUGH THE ALEUTIANS BRINGING NEAR GALE-FORCE
WINDS AND RAIN TO THE REGION FRIDAY THROUGH THE WEEKEND.

&&

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

FRIDAY NIGHT THROUGH THE WEEKEND THE UPPER RIDGE OVER THE BERING
FLATTENS OUT ACROSS THE BERING STRAIT TO THE CENTRAL INTERIOR
WHILE A NORTH PACIFIC LOW TRACKS EAST ALONG THE ALEUTIANS. AT THE
SURFACE...A THERMAL LOW ACROSS THE SOUTHERN MAINLAND SHIFTS SLOWLY
NORTH WITH SOUTHERLY FLOW PULLING SURFACE MOISTURE INLAND. THIS
WILL HELP TO MODERATE TEMPERATURES...HOWEVER ABOVE NORMAL RANGES
ARE STILL EXPECTED. A DIURNAL TREND OF AFTERNOON SHOWERS AND
THUNDERSTORMS ACROSS THE SOUTHERN MAINLAND BEGINS FRIDAY NIGHT
WITH THE COMBINATION OF INCREASED LOW LEVEL MOISTURE AND DAY TIME
HEATING MAXIMIZING CONVECTIVE POTENTIAL ALONG MOUNTAIN RANGES.
WEAK FLOW ALOFT WILL KEEP MUCH OF THE ACTIVITY ALONG THE RANGES.

TO THE WEST...THE FRONT THAT MOVES ACROSS THE ALEUTIAN CHAIN AT
THE END OF THE WEEK WILL STALL AND DIMINISH THROUGH SUNDAY. THE
UPPER LEVEL SYSTEM ASSOCIATED WITH THE SURFACE LOW SPINS IN PLACE
SUNDAY NIGHT INTO MONDAY REORGANIZING AND SENDS A WAVE NORTH AT
THE BEGINNING OF THE WEEK. MODELS HAVE NOT NARROWED THE OUTCOMES
FOR THE NEXT WAVE BUT THERE IS POTENTIAL FOR A FRONT TO MOVE NORTH
THROUGH THE GULF. CONTINUED TO USE A CONSERVATIVE APPROACH FOR THE
EXTENDED FORECAST WITH THE ECMWF AS IT PROVIDED THE BEST CONSISTENCY.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...NONE.
FIRE WEATHER...NONE.

&&

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