Small Craft Advisory 408, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
528 AM AKDT WED MAY 13 2015

.ANALYSIS AND UPPER LEVELS...
THERE IS AN AMPLIFIED UPPER LEVEL RIDGE LOCATED AT 40N 150W IN
THE NORTH PACIFIC EXTENDING NORTHWARD THROUGH THE WESTERN GULF
INTO THE SOUTHCENTRAL REGION. THIS COINCIDES WITH THE STRONG
SOUTHERLY MERIDIONAL JET STREAK OVER THE EASTERN ALEUTIANS..WHICH
IS BUILDING THE AMPLIFIED RIDGE DOWNSTREAM. THE ASCAT SCATTEROMETER
WINDS ARE SHOWING LIGHT WINDS THROUGH THE WESTERN GULF AND THE
INTERIOR OF SOUTHCENTRAL ALASKA. UPSTREAM THERE IS A CLOSED LOW
AT 500 MB JUST SOUTH OF ADAK WITH A LONGWAVE TROUGH OVER THE
NORTHWESTERN PACIFIC REGION. THE SURFACE REFLECTS A GALE FORCE LOW
IN THE WESTERN BERING SEA WITH A WEATHER FRONT MOVING THROUGH THE
SOUTHWEST ALASKA COASTLINE AND EXTENDING THROUGH THE EASTERN
ALEUTIANS THIS MORNING. ASCAT SCATTEROMETER WINDS IMAGERY IS
SHOWING THE STRONGEST WINDS ON THE BACKSIDE OF THIS LOW PRESSURE
SYSTEM. SATELLITE IMAGERY IS ALSO SHOWING A LARGE SWATH OF
MOISTURE OVER THE EASTERN BERING/ALEUTIANS SPREADING INTO SOUTHWEST
ALASKA. RADAR IS SHOWING A BAND OF RAIN THROUGH THE ALASKA
PENINSULA AND INTO THE INTERIOR OF THE KUSKOKWIM DELTA THIS
MORNING.

&&

.MODEL DISCUSSION...
THE MODELS ARE DOING BETTER WITH THE NORTH PACIFIC LOW OVER THE
WESTERN BERING SEA AS IT TRACKS TO THE NORTHERN BERING BY
THURSDAY. THE MODELS ARE STRUGGLING WITH THE WEAK LOW THAT FORMS
JUST SOUTH OF DUTCH HARBOR BY LATE WEDNESDAY NIGHT AS IT TRACKS
EASTWARD SOUTH OF THE CHAIN THROUGH REST OF THE WEEK. THE ECMWF
WAS THE PREFERRED MODEL OF CHOICE FOR THE DIFFERENT SYNOPTIC
FEATURES. FORECAST CONFIDENCE IS AVERAGE GOING INTO THE WEEKEND.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (DAYS 1 AND 2)...
THE UPPER LEVEL RIDGE OVER SOUTHCENTRAL ALASKA WILL DRIFT EASTWARD
TODAY AS A FRONT MOVES INTO THE WESTERN GULF OF ALASKA. THIS FRONT
WILL BRING RAIN TO KODIAK ISLAND FOR THE NEXT FEW DAYS AS IT
STALLS AGAINST THE RIDGE IN THE NORTHERN AND EASTERN GULF. THIS
WILL ALSO INCREASE THE PRESSURE GRADIENT OVER THE AREA BRINGING IN
GUSTY SOUTHEAST WINDS THROUGH CHANNELED TERRAIN. AT THIS TIME IT
LOOKS LIKE ANCHORAGE NORTHWARD WILL NOT SEE ANY PRECIPITATION FROM
THIS SYSTEM. THE EASTERN KENAI PENINSULA SHOULD GET INTO THE RAIN
WHILE THE WESTERN SIDE WILL SE LIMITED RAINFALL DUE TO THE
DOWNSLOPE.

&&

.FIRE WEATHER...
THE COPPER RIVER BASIN WILL SEE CRITICAL FIRE WEATHER CONDITIONS
ON THURSDAY AS THEY REMAIN UNDER THE UPPER LEVEL RIDGE AND
THEREFORE WILL REMAIN DRY IN CONJUNCTION WITH THE GUSTY WINDS. IT
IS EXPECTED THAT THERE WILL BE ENOUGH MOISTURE IN THE MAT-SU AREAS
ON THURSDAY TO KEEP RH VALUES ABOVE CRITICAL LEVELS WHEN THE WINDS
GET GUSTY THERE. THIS IS NOT A GUARANTEE AND ATTENTION SHOULD BE
PAID FOR THAT AREA AS WELL. CRITICAL FIRE WEATHER CONDITIONS MAY
PERSIST INTO FRIDAY IN THE COPPER RIVER BASIN BUT IT IS A LITTLE
EARLY TO MAKE THAT CALL YET.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (DAYS 1 AND 2)...
WIDESPREAD RAIN WILL MOVE OVER THE SOUTHWEST MAINLAND TODAY AS
THE BERING SEA FRONT PUSHES INLAND. PRECIPITATION WILL BRIEFLY
BECOME MORE SHOWERY IN NATURE BY THIS EVENING INTO TONIGHT ACROSS
THE KUSKOKWIM DELTA IN THE WAKE OF THE FRONT. BUT ABUNDANT
RAINFALL WILL CONTINUE TO IMPACT THE REGION THROUGH THE LATTER
PART OF THE WEEK AS A SERIES OF WEAK DISTURBANCES MOVE OVERHEAD IN
DEEP SOUTHERLY FLOW. A 90-100 KT JET STREAK NOSING INTO THE AREA
FROM THE NORTHERN PACIFIC WILL PROVIDE ADDITIONAL UPPER LEVEL
SUPPORT. THE HEAVIEST PRECIPITATION WILL BE FOCUSED FROM THE
BRISTOL BAY REGION INTO THE KUSKOKWIM VALLEY WHERE UPWARDS OF 1
INCH OF RAIN WILL BE POSSIBLE BY FRIDAY.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (DAYS 1 AND 2)...
RAIN AND STRONG WINDS WILL CONTINUE ON A DOWNWARD TREND THROUGH
THIS MORNING ACROSS THE EASTERN BERING AS THE GALE FORCE FRONT
PUSHES INLAND OVER SOUTHWEST ALASKA. THIS WILL LEAVE THE MUCH OF
THE BERING IN A SHOWERY REGIME THROUGH TONIGHT AS SPRAWLING LOW
PRESSURE CONTINUES TO DOMINATE THE REGION. A NEW LOW CENTER WILL
THEN BEGIN TO FORM LATE TONIGHT SOUTH OF DUTCH HARBOR AND TRACK
EASTWARD THROUGH THURSDAY NIGHT...ONCE AGAIN ENHANCING
PRECIPITATION CHANCES FROM THE EASTERN ALEUTIANS TO THE ALASKA
PENINSULA.

&&

LONG TERM FORECAST (FROM TUESDAY AFTERNOON DISCUSSION) (DAYS 3
THROUGH 7...FRIDAY THROUGH TUESDAY)... THE GENERAL PATTERN THROUGH
MUCH OF THE LONG TERM FORECAST REMAINS UNCHANGED...WHICH LOOKS TO
CONTINUE THROUGH EARLY NEXT WEEK. MODELS CONTINUE TO INDICATE A
PERSISTENT BLOCKING NORTH AMERICAN RIDGE STANDING FAST OVER THE
GULF AND SOUTHCENTRAL ALASKA THROUGH EARLY NEXT WEEK...WITH A DEEP
TROUGH OVER THE BERING AND NORTH PACIFIC PRODUCING MULTIPLE
DISTURBANCES THAT RIDE UP ALONG THE RIDGE FROM THE NORTH PACIFIC.
WITH THIS PATTERN WE ARE EXPECTING RELATIVELY WARM AND MOIST
SOUTHERLY FLOW OVER MUCH OF THE MAINLAND...ALONG WITH ROUND AFTER
ROUND OF RAIN AND GUSTY WINDS AS EACH SYSTEM TRACKS THROUGH THE
EASTERN ALEUTIANS AND INTO THE SOUTHWEST MAINLAND AND WESTERN
GULF. HOWEVER...THE TIMING AND STRENGTH OF THESE SYSTEMS IS NOT
YET SOLIDIFIED BETWEEN THE CURRENT MODEL SOLUTIONS. THIS LEADS TO
LOWER FORECAST CONFIDENCE IN THE TIMING/STRENGTH OF THE WINDS AND
PRECIPITATION FOR EACH OF THESE SYSTEMS...AS WELL AS THE STRENGTH
OF THE RIDGE AND ASSOCIATED GAP WINDS AROUND SOUTHCENTRAL ALASKA.
WE WILL CONTINUE TO MONITOR CONDITIONS...ESPECIALLY OVER THE
COPPER RIVER BASIN...TO SEE IF ANY DRY LOCATIONS WILL EXPERIENCE
STRONG ENOUGH WINDS TO TRIGGER A FIRE WEATHER WATCH.

THE PREFERRED SOLUTION FOR THIS FORECAST PACKAGE WAS THE
ECMWF...WHICH HAS BEEN CONSISTENTLY SHOWING A SLIGHTLY MORE
RESILIENT/STRONGER RIDGE BUILDING OVER THE GULF AND SOUTHCENTRAL.
THE THOUGHT PROCESS BEHIND THIS WAS WITH MORE DIURNAL HEATING
RESULTING IN WARMER LOW LEVEL TEMPERATURES...AND WARMER AIR BEING
ADVECTED UP FROM THE NORTH PACIFIC...THAT THE RIDGE WILL LIKELY
STRENGTHEN. THE EC ALSO SHOWS THE MAIN CORRIDOR OF MOISTURE FOR
EACH OF THESE SYSTEMS SETTING UP OVER THE SOUTHWEST MAINLAND AND
WESTERN PORTION OF THE GULF. THIS SWATH OF MOISTURE WILL
EVENTUALLY GET PUSHED FURTHER NORTH AND EASTWARD BY TRAILING
SYSTEMS WORKING THEIR WAY UP THE DEFIANT RIDGE. THIS PLACES THE
MOISTURE OVER THE WESTERN GULF AND WESTERN PORTION OF SOUTHCENTRAL
ALASKA...UNTIL THE NEXT SYSTEM MOVES IN FROM THE NORTH PACIFIC AND
RESETS THE POSITION OF THE MOISTURE BACK OVER THE SOUTHWEST
MAINLAND. THIS REMAINS DEPENDENT THE STRENGTH AND RESILIENCE OF
THE NORTH AMERICAN RIDGE...AS WELL AS THE TIMING AND STRENGTH OF
THE SYSTEMS ROTATING AROUND THE TROUGH TO THE WEST.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...GALE WARNING 178.
FIRE WEATHER...FIRE WEATHER WATCH 141.

&&

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