Area Forecast Discussion
Issued by NWS Anchorage, AK

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FXAK68 PAFC 050037
AFDAFC

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
437 PM AKDT TUE AUG 4 2015

.ANALYSIS AND UPPER LEVELS...
THE PRIMARY FEATURE AFFECTING THE AREA THIS AFTERNOON IS A NEARLY
STATIONARY VERTICALLY STACKED LOW LOCATED SOUTH OF KODIAK ISLAND.
THE LOW IS CAUSING UNSETTLED WEATHER ACROSS KODIAK AND THE ALASKA
PENINSULA. A SURFACE HIGH BENEATH AN UPPER LEVEL RIDGE IS CENTERED
OVER THE CENTRAL AND WESTERN BERING ALLOWING FOR MARINE STRATUS TO
PERSIST OVER NEARLY ALL OF THE BERING WEST OF 165W. WITH SURFACE
RIDGING IN PLACE OVER MUCH OF SOUTHCENTRAL AS WELL...PRINCE WILLIAM
SOUND AND THE NORTH GULF COAST WILL ONLY SEE A BRIEF RESPITE FROM
THE MARINE STRATUS. ELSEWHERE ACROSS SOUTHCENTRAL...THE SKIES ARE
NEARLY CLOUDLESS.

&&

.MODEL DISCUSSION...
FOR THE FIRST 48 HOURS OF THE FORECAST...THE MODELS ALL GENERALLY
AGREE WITH THE EVOLUTION OF THE PATTERN WITH MINOR DIFFERENCES IN
THE STRENGTH OF THE UPPER LEVEL LOW SOUTH OF KODIAK. THAT LOW WILL
SLOWLY DRIFT EASTWARD INTO THE GULF WITH THE BERING RIDGE BUILDING
INTO THE EASTERN BERING AND A NEW UPPER LEVEL LOW MOVING INTO THE
WESTERN ALEUTIANS...EFFECTIVELY SETTING UP AN OMEGA BLOCK.
MEANWHILE...A TROUGH IN THE POLAR JET WILL DIG SOUTHWARD ACROSS
THE EASTERN HALF OF THE STATE AS AN EASTERLY WAVE PUSHES ACROSS
THE COPPER RIVER BASIN. THERE REMAIN LARGE DIFFERENCES AS TO THE
SPEED AND STRENGTH OF THE TROUGH ALONG WITH HOW IT INTERACTS WITH
THE EASTERLY WAVE ON THURSDAY. THE NAM IS THE STRONGEST AND THE
FURTHEST SOUTH AND WEST WITH THE TROUGH...WHILE THE OTHER MODELS
ARE ALL LESS ROBUST. FOR THE AFTERNOON PACKAGE...THE NAM WAS
DISCOUNTED IN FAVOR OF THE GLOBAL MODELS. THE STRENGTH OF THE
TROUGH WILL HAVE THE LARGEST IMPACT ON THE COVERAGE OF
THUNDERSTORMS ALONG THE ALASKA RANGE AND THE WRANGELLS.

&&

.FIRE WEATHER...
FULL SUNSHINE FOR MUCH OF SOUTHERN ALASKA TODAY CONTINUES THANKS
TO DRY AIR AT THE LOWER LEVELS AS EVIDENCED BY THE 00Z ANCHORAGE
SOUNDING. AS HAS BEEN TYPICALLY THE CASE THIS SUMMER...THE LOWEST
RH LEVELS ARE IN THE COPPER RIVER BASIN...THOUGH RH VALUES IN THE
30-40 PERCENT RANGE ARE ALSO PRESENT IN THE KUSKOKWIM VALLEY AND
INTERIOR BRISTOL BAY. WEAK SURFACE FEATURES CONTINUE TO KEEP THE
WINDS LIGHT IN THESE AREAS. THE THREAT FOR THUNDERSTORMS WILL
INCREASE WEDNESDAY AND THURSDAY TOWARDS THE ALASKA RANGE AND
TALKEETNAS AS AN UPPER TROUGH DIGS SOUTHWARD AND PROVIDES
ADDITIONAL FORCING.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (DAYS 1 AND 2)...
HIGH CLOUDINESS WILL CONTINUE TO SPREAD SLOWLY NORTHWARD ALONG THE
KENAI PENINSULA AND NORTHERN GULF...THOUGH THE PARENT LOW SOUTH OF
KODIAK HAS PROBABLY REACHED ITS MAX NORTHWARD EXTENT AND HAS BEGUN
TO FALL APART QUICKLY AND DRIFT EASTWARD. THE FORECAST CHALLENGE
OF THE DAY HAS BEEN THE POTENTIAL FOR THUNDERSTORMS...MAINLY ALONG
THE TALKEETNA MOUNTAINS AND ALASKA RANGE. THE HIGH PRESSURE ALOFT
HAS HUNG QUITE TOUGH OVERNIGHT...WHICH HAS KEPT THE LID ON
CONVECTIVE POTENTIAL THIS AFTERNOON.

THIS SHOULD CHANGE TOMORROW AS THE CAPPING INVERSION OBSERVED
AROUND 700 MB ON THE 00Z ANCHORAGE SOUNDING SHOULD CONTINUE TO
WEAKEN. IN ADDITION...THE TROUGH APPROACHING FROM THE ARCTIC WILL
BEGIN TO PROVIDE SOME LIFT LATE IN THE DAY TO THE FAR NORTHERN
PORTIONS OF THE FORECAST AREA. THUS WILL CONTINUE TO MENTION
THUNDERSTORM POTENTIAL IN THE FAR NORTH TOMORROW. THE GLOBAL
MODELS HAVE THUS FAR PROVED THE MOST CONSISTENT AND RELIABLE FOR
THE EVOLUTION OF THE NORTHERN TROUGH FOR THE NEXT FEW DAYS...WHICH
KEEPS MOST OF THE ENERGY NORTH OF UPPER COOK INLET BEFORE DROPPING
IT THROUGH THE COPPER RIVER BASIN ON THURSDAY. THIS SHOULD PRODUCE
MORE WIDESPREAD SHOWER ACTIVITY DURING THE DAY THURSDAY IN THIS
AREA. THE CURRENT EXPECTED SCENARIO IS LIKELY TO KEEP ANCHORAGE
AND THE OTHER MAIN POPULATION CENTERS IN THE AREA MOSTLY OR
COMPLETELY DRY...A CHANGE FROM PREVIOUS DAYS.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (DAYS 1 AND 2)...
HIGH PRESSURE RIDGING ALOFT CONTINUES TO DOMINATE MOST OF THE
WEATHER ACROSS SOUTHWEST...WITH CONTINUED WARM TEMPERATURES AND
LARGELY DRY CONDITIONS. A LOW NEAR KODIAK ISLAND CONTINUES TO
SPIN AND SPREAD MOISTURE OVER THE ALASKA PENINSULA...WITH SOME OF
THIS MOISTURE AND CONVERGENCE EXPECTED TO HELP SPARK SHOWERS OVER
PORTIONS OF THE LOWER KUSKOKWIM VALLEY AND BRISTOL BAY REGIONS. ON
THURSDAY...A TROUGH WILL MOVE IN FROM THE NORTH...HELPING TO SPARK
OFF SHOWERS...ALONG WITH ISOLATED THUNDERSTORMS ALONG AND NEAR THE
ALASKA RANGE. A SLIGHT COOLING WILL FOLLOW BEHIND THIS TROUGH...BUT
TEMPERATURES WILL STILL REMAIN ON THE WARMER SIDE THROUGH THURSDAY.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (DAYS 1 AND 2)...
HIGH PRESSURE RIDGING WILL DOMINATE MOST OF THE BERING THROUGH
THURSDAY. A STRENGTHENING FRONTAL ZONE WILL EVENTUALLY FORM INTO
A NEAR GALE FORCE LOW OVER THE WESTERN ALEUTIANS AND BERING SEA
THURSDAY NIGHT...SPREADING RAIN AND GUSTY EASTERLY WINDS OVER THE
CENTRAL AND WESTERN ALEUTIAN CHAIN. ELSEWHERE...NORTHERLY FLOW
BEHIND A DEPARTING LOW WILL MAINTAIN PATCHY FOG AND STRATUS OVER
THE EASTERN ALEUTIAN CHAIN AND THE PRIBILOF ISLANDS...AND
LINGERING SHOWERS EAST OF UNALASKA.

&&

.LONG TERM FORECAST (DAYS 3 THROUGH 7)...
THE MAIN CHALLENGE IN THE LONG TERM FORECAST IS A STORM SYSTEM
THAT IS CURRENTLY MOVING TO THE WESTERN ALEUTIANS. CONDITIONS WILL
INITIALLY BE DRIER ACROSS MOST OF THE SOUTHERN MAINLAND THURSDAY
NIGHT THROUGH SATURDAY MORNING...EXCEPT FOR ISOLATED TO SCATTERED
SHOWERS THAT LINGER OVER THE COPPER RIVER AND GULF FROM AND
EXITING SHORTWAVE. TO THE WEST...THE STORM SYSTEM CENTERS NEAR
SHEMYA PUSHING A FRONT INTO THE EASTERN BERING AND ALASKA
PENINSULA ON FRIDAY. THERE IS LITTLE PROGRESSION WITH THIS SYSTEM
UNTIL SATURDAY AND THEN IT PICKS UP EASTWARD MOMENTUM.

THE FRONT IS EXPECTED TO PUSH INTO THE SOUTHWEST THIS WEEKEND
SPREADING RAIN AND GUSTY WINDS ALONG ITS BOUNDARY AS IT MOVES
THROUGH. MODELS ARE STRUGGLING WITH THE TIMING AND TRACK OF THIS
SYSTEM AS THEY ARE HANDLING DYNAMICS FROM A SECOND SYSTEM THAT
MOVES INTO THE NORTH COAST DURING THE SAME TIME FRAME. ONLY MINOR
ADJUSTMENTS WERE MADE TODAY USING THE ECMWF TO MAINTAIN CONSISTENCY.
IT IS LIKELY THAT RAIN OVERSPREADS SOUTHCENTRAL ON SUNDAY WITH A
WETTER PATTERN FOR THE SOUTHERN MAINLAND THROUGH TUESDAY AS AN
UPPER TROUGH CENTERS OVER THE STATE. A RIDGE SHIFTS BACK OVER THE
WESTERN BERING ON SUNDAY SHIFTING EAST THROUGH THE BEGINNING OF
NEXT WEEK.

&&


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

&&

$$

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


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