Area Forecast Discussion
Issued by NWS Anchorage, AK

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FXAK68 PAFC 061255 CCA
AFDAFC

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION...CORRECTED
NATIONAL WEATHER SERVICE ANCHORAGE AK
455 AM AKDT WED MAY 6 2015

.ANALYSIS AND UPPER LEVELS...

THE SYNOPTIC PATTERN ACROSS ALASKA AND THE ALASKAN WATERS IS MUCH
MORE COMPLICATED AND DIFFUSE COMPARED TO PREVIOUS DAYS BUT A
QUIETER WEATHER PATTERN IS CURRENTLY IN PLACE. THE UPPER LEVELS
ARE LARGELY DOMINATED BY A LARGE UPPER LOW CENTERED NEAR 48N 171W
WITH RIDGING BEING OBSERVED OVER THE GULF OF ALASKA AS WELL AS THE
FAR WEST BERING SEA. WATER VAPOR IMAGERY THIS MORNING SHOWS A
SHORTWAVE AND ASSOCIATED JET STREAM DESCENDING THE WESTERN
PERIPHERY OF THE UPPER LEVEL LOW. THIS HAS CONTINUED TO ENHANCE
THE AMPLIFICATION OF THE UPPER LEVEL TROUGH WHILE ENHANCING
RIDGING AHEAD OF THE TROUGH IN THE NORTH PACIFIC/GULF OF ALASKA. A
WEAKENING FRONTAL OCCLUSION EXTENDING OVER THE SOUTHERN BERING SEA
AND ACROSS SOUTHWEST ALASKA CONTINUES TO SPARK SOME SHOWERS OVER
SOUTHWEST ALASKA...THE NORTHERN GULF COAST AND GULF OF ALASKA. THE
SFC LOW SOUTH OF THE ALEUTIAN ISLANDS HAS REALLY WEAKENED IN THE
PAST 6 TO 9 HOURS WITH MULTIPLE WEAK LOW CENTERS NOW STARTING TO
SPIN UP VS THE CONCENTRIC LOWS SEEN IN THE PREVIOUS DAYS. THE MOST
POTENT OF THESE LOWS APPEARS NEAR SITKINAK (SOUTHWEST CORNER OF
KODIAK ISLAND) AND HAS AN UPPER LEVEL DISTURBANCE ASSOCIATED WITH
IT.

LOOKING BACK AT THE BROADER PICTURE...THE AMPLIFICATION OF THE
UPPER LEVEL TROUGH IS ALLOWING FOR SUBTROPICAL MOISTURE TO ADVECT
NORTHWARD AHEAD OF THE UPPER LEVEL TROUGH. SATELLITE DERIVED
PERCENT OF NORMAL TOTAL PRECIPITABLE WATER SHOWS AN AIR MASS WITH
OVER TWICE THE NORMAL PRECIPITABLE WATER VALUES MOVING NORTH.
THIS IS ALSO DEPICTED WELL ON INFRARED SATELLITE IMAGERY WITH
COOLING CLOUD TOPS OVER THE NORTH PACIFIC AND INCREASING MOISTURE
ON WATER VAPOR IMAGERY.

&&

.MODEL DISCUSSION...

THE 00Z MODELS INITIALIZED PRETTY WELL FOR THE MOST PART BUT ARE
STRUGGLING MORE THAN IN RECENT NIGHTS DUE TO MORE DIFFUSE FEATURES
AND LESS LARGER SCALE FEATURES TO GRASP ONTO. THE NAM IS THE FIRST
TO BECOME AN OUTLIER...MAINLY OVER THE EASTERN DOMAIN AS IT
APPEARS TO BE A TAD TOO SLOW BRINGING A FRONT THROUGH THE GULF OF
ALASKA AND SLAMMING AGAINST THE NORTHERN GULF COAST THIS AFTERNOON
AND EVENING. THE ECMWF THEN BEGINS TO BECOME A BIT OF AN OUTLIER
HEADING INTO FRIDAY MORNING WITH THE DEVELOPMENT OF A NORTH
PACIFIC LOW THAT QUICKLY TRANSITIONS INTO THE GULF OF ALASKA. THE
ECMWF WANT TO TRY TO CLOSE OFF THE UPPER LEVEL LOW A BIT FURTHER
NORTH THAN THE GFS/NAM/GEM WHICH IN TURN CREATES A SURFACE LOW
FURTHER NORTH THAN DETERMINISTIC AND ENSEMBLE GUIDANCE CURRENTLY
SUGGESTS. BECAUSE OF THE NOTICEABLE ISSUES WITH THE NAM AND
ECMWF...THE GFS WAS GENERALLY USED FOR FORECAST UPDATES WITH A
TOUCH OF THE GEM.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (DAYS 1 AND 2)...
A FAIRLY STRONG UPPER LEVEL SHORT-WAVE WILL AND DEVELOPING WEAK
SURFACE LOW OVER THE NORTH PACIFIC WILL BOTH HEAD INTO THE GULF
TODAY. THIS WILL CAUSE A WEAK OCCLUDED FRONT SITTING NEARLY
STATIONARY OVER THE SOUTHWEST GULF TO RAPIDLY STRENGTHEN AND TRACK
NORTHWARD TOWARD SOUTHCENTRAL. EXPECT INCREASING WINDS AND STEADY
RAIN ACROSS THE GULF KODIAK ISLAND TO PUSH NORTHWARD TO THE KENAI
PENINSULA AND PRINCE WILLIAM SOUND AREAS LATER TODAY THROUGH
TONIGHT. STRONG EASTERLY FLOW OUT AHEAD OF THE FRONT ALONG WITH
GENERALLY STABLE CONDITIONS WILL INITIALLY PRODUCE DOWNSLOPE
DRYING IN THE LEE OF THE KENAI AND CHUGACH MOUNTAINS. AS THE FRONT
PUSHES ONSHORE THURSDAY DOWNSLOPE FLOW WILL WEAKEN AND THE UPPER
LEVEL WAVE WILL MOVE OVERHEAD HELPING TO DESTABILIZE THE MID TO
UPPER LEVELS. THUS...EXPECT EXTENSIVE CLOUD COVER ALONG WITH A
CHANCE OF SHOWERS ACROSS ALL OF INTERIOR SOUTHCENTRAL ON THURSDAY.
ANOTHER PACIFIC STORM WILL BE HEADED FOR THE GULF THURSDAY NIGHT
INTO FRIDAY...SO EXPECT LITTLE OR NO BREAK BETWEEN SYSTEMS.

TAKING A LOOK AT FIRE WEATHER CONDITIONS...A RIDGE WILL BUILD OVER
SOUTHCENTRAL TODAY OUT AHEAD OF THE TROUGH MOVING INTO THE GULF.
THIS WILL PROVIDE ABUNDANT SUNSHINE AND CONTINUED DRY CONDITIONS
OVER INTERIOR PORTIONS OF SOUTHCENTRAL...ESPECIALLY FROM THE MAT-SU
THE COPPER RIVER BASIN. SOME LOCALIZED WINDS WILL DEVELOP AS
PRESSURE GRADIENTS TIGHTEN THIS AFTERNOON AND EVENING...BUT WINDS
WILL REMAIN BELOW RED FLAG CRITERIA IN THE DRIEST LOCATIONS. ON
THURSDAY THE SITUATION WILL BE REVERSED AS INCREASED CLOUD COVER
WILL PROVIDE FOR COOLER TEMPS AND HIGHER RELATIVE
HUMIDITIES...WHILE THE WINDS WILL BE NOTABLY STRONGER AS THE FRONT
PUSHES ONSHORE.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (DAYS 1 AND 2)...
A GENERALLY WET PATTERN IS EXPECTED OVER THE SOUTHWEST MAINLAND
OVER THE NEXT 48 HOURS AS THE SECONDARY LOW SOUTH OF SITKINAK
ISLAND PUSHES NORTHWARD ACROSS THE ALASKA PENINSULA THIS EVENING
AND THEN WILL BECOME STATIONARY OVER BRISTOL BAY THROUGH THURSDAY
MORNING. DUE TO THE SHARPNESS OF THE PARENT UPPER TROUGH CENTERED
ALONG 170W...THERE IS A REASONABLY DEEP MOISTURE SOURCE AVAILABLE
FROM THE CENTRAL PACIFIC WITH THIS LOW. SATELLITE ESTIMATES OF
PRECIPITABLE WATER THIS MORNING INDICATE PW VALUES IN THIS PLUME
OF ABOUT 150 PERCENT OF NORMAL. AS THE LOW BECOMES STATIONARY AND
THE ASSOCIATED FRONT MOVES INLAND OVER THE SOUTHERN ALASKA
COAST...THE MOISTURE PLUME WILL BECOME CUTOFF FROM THE LOW OVER
BRISTOL BAY....THUS ENDING MOST OF THE HEAVIEST RAINFALL BY LATE
THIS EVENING. THE SOUTHEAST TO EASTERLY ORIENTATION OF MOISTURE
PLUME WILL KEEP THE HEAVIEST PRECIPITATION TOTALS MAINLY ON THE
EASTERN SLOPES OF THE ALASKA RANGE THROUGH TONIGHT.

THE LOW WILL CONTINUE TO WEAKEN ON THURSDAY AND ELONGATE INTO A
NORTH- SOUTH ORIENTATED TROUGH CENTERED ALONG THE WESTERN ALASKA
COAST BY LATE THURSDAY NIGHT. THIS WILL KEEP THE REGION IN WEAK
AND UNSTABLE SOUTHERLY FLOW WHICH WILL CONTINUE TO PRODUCE
WIDESPREAD SHOWERS OVER THE REGION. EXPECT THE HEAVIEST RAINFALL
TOTALS ON THURSDAY MAINLY ALONG THE KILBUCK AND AHKLUN MOUNTAINS
AS THE LOW LEVEL FLOW WILL FAVOR OROGRAPHIC LIFT IN THOSE AREAS.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (DAYS 1 AND 2)...
IN THE NEAR TERM...THE REGION WILL CONTINUE TO BE UNDER THE
INFLUENCE OF THE LARGE AND WEAKENING VERTICALLY STACKED LOW SOUTH
OF THE ALEUTIAN CHAIN. AT THE SURFACE...THE SECONDARY LOW
CURRENTLY SOUTH OF SITKINAK WILL MOVE OVER BRISTOL BAY TODAY AND
WILL BEGIN TO DISSIPATE THURSDAY. A SURFACE RIDGE WILL SLOWLY
MOVE EASTWARD ACROSS THE REGION THROUGH LATE THURSDAY NIGHT AS A
WEAK LOW MOVES EASTWARD TO A POSITION ABOUT 450 NM SOUTH OF ATTU.
THIS PATTERN WILL GENERALLY KEEP UNSETTLED WEATHER ALONG THE
EASTERN BERING SEA AND ALASKA PENINSULA WITH WIDESPREAD RAIN
SHOWERS...MAINLY ALONG THE BERING SIDE OF THE ALASKA PENINSULA.
AREAS OF FOG...DENSE AT TIMES...ARE ALSO EXPECTED OVER THE EASTERN
HALF OF THE BERING SEA AND THE PRIBILOF ISLANDS THROUGH THE
PERIOD. GENERALLY TRANQUIL CONDITIONS WILL DOMINATE THE WESTERN
HALF OF THE REGION UNTIL LATE THURSDAY NIGHT AS THE FRONT
ASSOCIATED WITH THE LOW SOUTH OF ATTU BEGINS TO IMPACT THE WESTERN
ALEUTIANS.

&&

.LONG TERM FORECAST (FRIDAY THROUGH MONDAY NIGHT)...
A FRONT ASSOCIATED WITH A DEVELOPING NORTH PACIFIC LOW WILL
QUICKLY MOVE INTO THE GULF FRIDAY...REINFORCING WET AND WINDY
CONDITIONS ALONG THE GULF COAST. THE JET WILL SLIDE TO THE EAST
AND WEAKEN THROUGHOUT THE DAY ON FRIDAY WITH THIS SYSTEM...ALLOWING
FOR IT TO BECOME A SEMI-PERMANENT FEATURE UNDER A CUTOFF LOW THAT
WILL CONTINUE THROUGH THIS WEEKEND. THIS PATTERN WILL BRING
CONTINUED RAIN TO MUCH OF THE GULF COAST INTO THIS WEEKEND...WITH
RAIN BEING MORE SHOWERY IN NATURE FOR INLAND SOUTHCENTRAL. THIS
SYSTEM WILL THEN BEGIN TO DISSIPATE ON SUNDAY...ALLOWING FOR
CLEARING SKIES FOR LATE THIS WEEKEND AND EARLY NEXT WEEK.

SHOWERY CONDITIONS OVER THE SOUTHWEST MAINLAND ON FRIDAY WILL
BEGIN TO CLEAR ON SATURDAY...AS WEAK OFFSHORE FLOW DEVELOPS
BEHIND THE GULF LOW AND HIGHER PRESSURE BEGINS TO BUILD IN FROM
THE EASTERN BERING. THIS WILL BRING A PERIOD OF BENIGN AND CALM
WEATHER TO THE SOUTHWEST MAINLAND FOR SUNDAY AND MONDAY AS THE
BERING LOW TRACKS FURTHER NORTH...AND THE ABOVE MENTIONED WEAK
RIDGING BUILDS IN. OVER THE BERING...THERE IS DECENT AGREEMENT
THAT A NORTH PACIFIC LOW WILL BEGIN TO TRACK INTO THE CENTRAL
ALEUTIANS SATURDAY MORNING...THEN CROSS OVER INTO THE BERING
OVERNIGHT SATURDAY. THERE IS STILL SOME MODEL DISCREPANCY
REGARDING THE STRENGTH AND LOCATION OF THIS SYSTEM...BUT THERE IS
GOOD AGREEMENT THAT THIS WILL BE A FAIRLY WEAK ONE.
OTHERWISE...CONFIDENCE REMAINS BELOW AVERAGE IN THE LONG TERM
FORECAST OVER THE BERING THIS WEEKEND INTO EARLY NEXT WEEK.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...GALES 119 120 130 131 138.
FIRE WEATHER...NONE.

&&

$$

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


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