Area Forecast Discussion
Issued by NWS Anchorage, AK

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FXAK68 PAFC 291312
AFDAFC

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
512 AM AKDT WED JUL 29 2015

.ANALYSIS AND UPPER LEVELS...

A STRONG UPPER-LEVEL JET STREAK RUNNING NEARLY NORTH-SOUTH ACROSS
THE BERING SEA AND CENTRAL ALEUTIANS REPRESENTS THE DIVIDING LINE
BETWEEN MID-LEVEL RIDGING OVER THE WESTERN BERING AND THE BROAD
MID-LEVEL LOW OVER THE ALASKA MAINLAND...THOUGH THERE IS LITTLE TO
NO REFLECTION OF THIS JET STREAK IN THE LOWER LEVELS OF THE
ATMOSPHERE.

SHOWER ACTIVITY HAS INCREASED OVERNIGHT OVER UPPER COOK INLET AND
THE COPPER RIVER BASIN IN ASSOCIATION WITH A WEAK LOW-LEVEL JET
FEATURE THAT IS CURRENTLY DEVELOPING AS INDICATED BY LATEST MODEL
GUIDANCE. IN ASSOCIATION WITH THIS ADDED LATENT HEATING...AN AREA
OF INCREASING HEIGHTS WILL DEVELOP OVER SOUTHCENTRAL ALASKA
THROUGH THURSDAY...AND THEN WILL RETROGRADE WEST AND MERGE WITH A
BUILDING HIGH OVER THE BERING BY SATURDAY. THIS WILL MEAN A
PROGRESSIVE REDUCTION IN AREAL EXTENT OF SHOWER AND THUNDERSTORM
ACTIVITY ACROSS SOUTHCENTRAL AND SOUTHWEST ALASKA THURSDAY THROUGH
SATURDAY.

&&

.MODEL DISCUSSION...

WITH THE SOUTH-SOUTHWEST LOW LEVEL JET STREAKS REMAINING IN PLACE
OVER UPPER COOK INLET...SUSITNA VALLEY...AND THE COPPER RIVER
BASIN...COMBINED WITH AN ALREADY MOIST ENVIRONMENT...EXPECT
SHOWER ACTIVITY TO BE MORE NUMEROUS IN THESE AREAS THROUGH
TONIGHT...ESPECIALLY GIVEN THE STRENGTH OF THE THERMAL TROUGH JUST
TO THE NORTH AND WEST.

OTHERWISE...MODELS REMAIN IN EXCELLENT AGREEMENT REGARDING THE
EVOLUTION OF THE LARGE SCALE PATTERN SHOWING A GRADUAL BREAK-DOWN
OF THE UPPER TROUGH OVER MAINLAND ALASKA BEING REPLACED BY A
GRADUALLY STRENGTHENING RIDGE OVER INTERIOR ALASKA AND AN UPPER
LEVEL LOW CENTERED OVER THE SOUTHERN AND EASTERN GULF. BEGINNING
THURSDAY AND LASTING THROUGH SATURDAY...SHOWER ACTIVITY SHOULD
DIMINISH IN INTENSITY AND COVERAGE AND BECOME CONFINED CLOSER TO
THE COAST AS MID-LEVEL HEIGHTS AND TEMPERATURES RISE OVER INTERIOR SOUTHWEST
AND SOUTHCENTRAL ALASKA. CONFIDENCE IS LOWEST IN THE EXTENT TO
WHICH DIURNAL CONVECTIVE ACTIVITY IS SUPPRESSED ADDITIONALLY EACH
DAY.

&&

.FIRE WEATHER...

FIRE WEATHER CONCERNS ARE MINIMAL AGAIN TODAY WITH A MOIST AND
CONVECTIVELY UNSTABLE AIR MASS IN PLACE ACROSS MUCH OF SOUTHERN
ALASKA...KEEPING RH VALUES HIGH WITH EXTENSIVE CLOUD COVER AND
LIGHT WINDS. A GRADUAL WARMING AND DRYING TREND IS EXPECTED OVER
ALL OF SOUTHERN ALASKA IN THE COMING DAYS THOUGH WARMING
TEMPERATURES ALOFT WILL DECREASE INSTABILITY FOR THUNDERSTORMS AS
WELL.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (WEDNESDAY AND THURSDAY)...

PERSISTENCE IS THE NAME OF THE GAME TODAY AS CLOUDINESS AND
WIDESPREAD SHOWER ACTIVITY WILL CONTINUE TODAY ACROSS MOST OF
SOUTHCENTRAL ALASKA. WINDS REMAIN GENERALLY LIGHT ACROSS THE
ENTIRE AREA...AS THE REGION REMAINS TRAPPED UNDER WEAK LARGE-
SCALE FEATURES. SOME LOW-LEVEL CONVERGENCE OVER WESTERN TURNAGAIN
ARM HAS PROVIDED THE TRIGGER FOR SOME MODERATE RAIN SHOWERS ACROSS
THE ANCHORAGE BOWL THIS MORNING...WHICH SHOULD DISSIPATE BY MID
MORNING. HOWEVER...BY THIS AFTERNOON GARDEN-VARIETY CONVECTIVE
ACTIVITY RESUMES AGAIN AS THE OVERALL AIRMASS REMAINS FAIRLY
UNSTABLE AND VERY MOIST. THE BEST CHANCE FOR THUNDERSTORMS FOR
EACH OF THE NEXT FEW DAYS WILL CONTINUE TO BE FOUND OVER THE
NORTHERN TIER OF ZONES (COPPER RIVER BASIN AND SUSITNA
VALLEY)...THOUGH NUMEROUS CONVERGENCE BOUNDARIES OVER THE WESTERN
KENAI MAY HELP INITIATE A FEW THUNDERSTORMS THERE AS WELL. ANY
SHOWERS OR THUNDERSTORMS THAT DO FORM WILL BE CAPABLE OF VERY HEAVY
RAINFALL OVER THE NEXT DAY OR TWO AS THE AIRMASS REMAINS UNUSUALLY
MOIST.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (DAYS 1 AND 2...WED AND
THU)...THE WEAK UPPER LEVEL SYSTEM THAT TRIGGERED CONVECTION
TUESDAY AFTERNOON WILL SET UP AS A CLOSED LOW RIGHT NEAR BRISTOL
BAY TODAY. THIS WILL LEAD TO ANOTHER DAY OF AFTERNOON AND EVENING
THUNDERSTORMS...BUT THEY SHOULD BE SLIGHTLY LESS WIDESPREAD THAN
YESTERDAY AS THE BEST FORCING WILL BE OFFSHORE.

AS WE GET INTO THURSDAY...WE START TO SEE HEIGHTS RISE ACROSS MUCH
OF THE AREA AS A RIDGE BUILDS IN FROM THE BERING. THIS WILL HELP
TEMPERATURES TO WARM AND THE ASSOCIATED THERMAL TROUGH TO BECOME
MORE ESTABLISHED. BUT AT THE SAME TIME THE THERMAL TROUGH WILL
HAVE TO FIGHT AGAINST INCREASING STABILITY UNDER THE RIDGE. SO
WHILE SHOWERS WILL STILL LIKELY BE ABUNDANT...THUNDERSTORMS
SHOULD BE EVEN MORE LIMITED AND MAINLY HOLDING TO THE HIGHEST
TERRAIN.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (DAYS 1 AND 2...WED AND
THU)...THREE MAIN FEATURES WILL DRIVE SENSIBLE WEATHER ACROSS THE
BERING THE NEXT COUPLE OF DAYS. FROM WEST TO EAST...A FAIRLY MOIST
LOW NEAR THE WESTERN ALEUTIANS. NEXT IS A SURFACE HIGH BUILDING
FROM WEST TO EAST ACROSS THE CENTRAL BERING. FINALLY...AN UPPER
LEVEL JET DIVING FROM NORTH TO SOUTH THROUGH THE EASTERN BERING.
THE LOW WILL SKIRT THE SOUTHERN EDGE OF THE WESTERN ALEUTIANS WITH
SOME RAIN AND SMALL CRAFT EASTERLY FLOW AS IT PEAKS TODAY. IT WILL
THEN WEAKEN AND DRIFT SOUTHEAST THURSDAY. THE RIDGE WILL TILT OVER
INTO THE WESTERN HALF OF THE MAINLAND THROUGH THURSDAY. THIS WILL
KEEP AREAS OF DRIZZLE AND FOG...DENSE AT TIMES...FROM THE CENTRAL
ALEUTIANS THROUGH THE PRIBILOF ISLANDS. FINALLY...THE JET DIVING
DOWN WILL HELP TO PUSH ANOTHER WEAK LOW FROM BRISTOL BAY INTO THE
NORTH PACIFIC. THIS WILL KEEP SOME RAIN CHANCES FROM THE ALASKA
PENINSULA BACK THROUGH COLD BAY. THE HEAVIEST PRECIPITATION
SHOULD BE ON THE BERING SIDE.

&&

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

DETERMINISTIC AND ENSEMBLE GUIDANCE IS IN MUCH BETTER AGREEMENT
TODAY IN REGARDS TO A BIT OF A PATTERN CHANGE TAKING PLACE LATE
THIS WEEK AND INTO THE WEEKEND. THE LONGWAVE TROUGH THAT HAS BEEN
CENTERED ACROSS ALASKA WILL AMPLIFY SOUTHWARD A LITTLE BIT AND
MOVE INTO THE NORTH PACIFIC ON FRIDAY AND MULTIPLE UPPER LEVEL LOW
CENTERS BEGIN TO CLOSE OFF THE UPPER LEVEL PATTERN. AT THE SAME
TIME...HIGH PRESSURE WILL BE BUILDING ACROSS THE BERING SEA AND
WILL BEGIN TO NUDGE INTO THE INTERIOR OF ALASKA BEFORE
OVERSPREADING THE STATE EARLY OVER THE WEEKEND. THE UPPER LEVEL
PATTERN BECOMES A LITTLE COMPLICATED AND HARD TO FIGURE OUT AS THE
TWO AFOREMENTIONED UPPER LEVEL LOWS MAY OR MAY NOT INTERACT WITH
ONE ANOTHER. WHILE THERE HAS BEEN A BIT OF CHANGE FROM THE 00Z AND
12Z RUNS OF THE ECMWF/GFS/GEFS...IT APPEARS THE LOWS WILL
ULTIMATELY PHASE INTO ONE CONCENTRIC LOW OVER THE NORTH PACIFIC ON
SUNDAY BEFORE EITHER PUSHING INTO THE NORTHERN GULF OF ALASKA OR
FURTHER WEST TOWARD THE ALASKA PANHANDLE. THIS UPPER LOW AND ITS
SURFACE FEATURE WILL PLAY A HUGE ROLE IN DETERMINING RAINFALL AS
WELL AS WINDS (OUTFLOW OR GAP) THROUGH CHANNELED TERRAIN OF
SOUTHCENTRAL ALASKA. DETAILS WILL BECOME CLEARER IN COMING
DAYS...AS THE PHASING OF TWO LOWS TENDS TO BE POORLY HANDLED BY
MODELS ESPECIALLY 5-6 DAY OUT.

&&


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

&&

$$

SYNOPSIS AND MODEL DISCUSSION...CC
SOUTHCENTRAL ALASKA...AD
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...MSO
LONG TERM...MMC


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