Area Forecast Discussion
Issued by NWS Anchorage, AK

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FXAK68 PAFC 301253
AFDAFC

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
453 AM AKDT THU JUL 30 2015

.ANALYSIS AND UPPER LEVELS...

UPPER LEVEL TROUGH AXIS RESIDES FROM THE NORTHWEST TERRITORIES
THROUGH THE YUKON THROUGH THE SOUTHERN GULF TO THE SOUTHEASTERN
BERING. ANOTHER STACKED LOW RESIDES ABOUT 250 MILES SOUTHEAST OF
SHEMYA. AN UPPER LEVEL HIGH RESIDES OVER THE FAR WESTERN
BERING...WITH SUBTLE RIDGING FROM NEAR THE BERING STRAIT EXTENDING
SOUTH AND EAST AND NOSING INTO THE UPPER COOK INLET REGION.

THERE IS VERY LITTLE CHANGE IN THE GEOPOTENTIAL HEIGHT FIELD BETWEEN
THE UPPER LEVEL TROUGH AND EMBEDDED LOWS AND THE UPPER LEVEL
RIDGING...AN INDICATION THAT VERY WEAK FLOW EXISTS ACROSS THE
REGION. THE SURFACE PATTERN IS NOT MUCH DIFFERENT...WITH A WEAK
THERMAL TROUGH OVER THE SOUTHERN INTERIOR OF ALASKA WITH A WEAK
SURFACE RIDGE OVER THE NORTH GULF COAST.

&&

.MODEL DISCUSSION...

OVER THE NEXT DAY OR TWO THE STAGNANT SLOW-CHANGING WEATHER
PATTERN CONTINUES TO AID THE MODELS` EXCELLENT AGREEMENT IN
SHOWING A LACK OF ANY STRONG FEATURES. THEREFORE ONCE AGAIN THE
PRIMARY DRIVERS OF THE WEATHER ARE MUCH SMALLER TRANSIENT FEATURES
THAT ARE GENERALLY NOT WELL RESOLVED BY THE MODELS. BECAUSE OF
THEIR BETTER RESOLUTION AND LESS SMOOTHING...THE LOCAL MODELS WERE
PREFERRED FOR THIS FORECAST PACKAGE. LOCAL CONVERGENCE ZONES IN
OTHERWISE VERY SLOW MOVING FLOW WILL CONTINUE TO PRODUCE SHOWER
ACTIVITY...WITH A FEW THUNDERSTORMS ALONG THE MOUNTAINS IN THE
MOIST ENVIRONMENT. CONFIDENCE REMAINS LOWER THAN NORMAL CONCERNING
THE TIMING OF THESE SHOWERS AND THUNDERSTORMS OVER MOST OF
SOUTHERN ALASKA.

THE MODELS CONTINUE TO AGREE THAT THE TROUGH WILL GRADUALLY BREAK
DOWN OVER MAINLAND ALASKA...LEAVING A CUTOFF LOW OVER THE GULF AND
THE WESTERN BERING RIDGE SPREADING EASTWARD TO REPLACE THE
TROUGH. INITIALLY THE HEIGHT RISES WILL BE DUE TO SUBSIDENCE AND
THE UPPER LOW OVER THE SOUTHWESTERN GULF CUTTING OFF FROM THE COLD
POOL OF AIR ALONG THE ARCTIC COAST...AND NOT SIGNIFICANT WARMING
OF THE UPPER LEVELS UNTIL THE WEEKEND. THIS PATTERN WILL HELP
PROLONG THE INSTABILITY NEEDED FOR THUNDERSTORM DEVELOPMENT NEAR
THE MOUNTAINS AND LOCAL CONVERGENCE ZONES FOR THE REMAINDER OF THE
WORKWEEK.

&&

.FIRE WEATHER...

THE DEEP...MOIST...NEARLY STAGNANT AIR MASS IN PLACE CONTINUES TO
KEEP FIRE WEATHER CONCERNS MINIMAL FOR MOST OF SOUTHERN ALASKA
WITH CONTINUED HIGH RELATIVE HUMIDITY...LIGHT WINDS...AND
EXTENSIVE CLOUD COVER. WET THUNDERSTORMS REMAIN A POSSIBILITY
NEAR THE MOUNTAINS AS THE ATMOSPHERE REMAINS MOIST AND UNSTABLE. A
WARMING AND DRYING TREND REMAINS IN THE FORECAST OVER THE COMING
DAYS WITH THE BUILDING UPPER LEVEL RIDGE INTO SOUTHERN ALASKA.
RELATIVE HUMIDITIES THOUGH SHOULD REMAIN ALMOST ENTIRELY ABOVE 30
PERCENT EVEN WITH OFFSHORE FLOW. WARMING IN THE UPPER LEVELS BY
THE WEEKEND WILL INCREASE STABILITY AND HELP CAP POTENTIAL
CONVECTION.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (DAYS 1 AND 2)...
MUCH OF SOUTHCENTRAL WILL HAVE PERSISTENT CLOUDY AND SHOWERY
WEATHER TODAY AS WEAK SYNOPTIC FORCING WILL PREVENT ANY
SIGNIFICANT CHANGE IN THE WEATHER. THE ATMOSPHERE REMAINS VERY
MOIST HOWEVER...SO ANY SHOWERS THAT DO FORM HAVE THE POTENTIAL TO
PRODUCE BRIEF PERIODS OF MODERATE TO HEAVY RAIN. AFTERNOON/EVENING
THUNDERSTORMS WILL OCCUR ONCE AGAIN MAINLY OVER THE TALKEETNA
MOUNTAINS...ALASKA RANGE AND THE WESTERN KENAI PENINSULA TODAY...HOWEVER
WITH WEAK WINDS ALOFT THERE SHOULD BE LITTLE IF ANY MOVEMENT OF THESE
SHOWERS. WINDS WILL REMAIN LIGHT IN MOST AREAS.

BY FRIDAY...RIDGING ALOFT BUILDING IN FROM THE WEST WILL BEGIN TO
STRENGTHEN AND BRING WARMER AND DRIER CONDITIONS TO MOST AREAS.
THUNDERSTORM ACTIVITY SHOULD BE CONFINED TO THE HIGHER ELEVATIONS
CLOSER TO THE GULF COAST WHERE THE BEST COMBINATION OF LOW-LEVEL
MOISTURE...MID-LEVEL DRYING...AND A MID-LEVEL DISTURBANCE WILL BE
LOCATED. INCREASING OFFSHORE FLOW IN THE LOWER LEVELS WILL LIKELY
MEAN NORTH GULF COAST LOCATIONS SUCH AS SEWARD COULD SEE
TEMPERATURES CLIMB INTO THE 70S FOR HIGHS.

CONVECTION SHOULD BE GREATLY LIMITED FOR SATURDAY TO LOCATIONS
TOWARD THE FAR SOUTHEASTERN COPPER RIVER BASIN AND THE FAR EASTERN
CHUGACH RANGE.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (DAYS 1 AND 2)...
A WEAK THERMAL TROUGH ACROSS THE INTERIOR WITH AN AXIS EXTENDING
INTO THE SOUTHWEST...WILL REMAIN A FOCUS FOR AFTERNOON AND
EVENING SHOWERS AND THUNDERSTORMS TO DEVELOP THROUGH THIS EVENING. CONDITIONS
WILL BECOME MORE STABLE FROM NORTH TO SOUTH FRIDAY...WITH GREATEST
INSTABILITY AND POTENTIAL FOR SHOWERS AND STORMS TO BE LOCATED
TOWARD THE BRISTOL BAY COAST AND NORTHERN ALASKA PENINSULA...FIRST
IN THE AFTERNOON WITH INTERACTION OF LOCALIZED CONVERGENCE
BOUNDARIES (SEA-BREEZE)...AND LATER WELL INTO THE OVERNIGHT WITH
A MID-LEVEL DISTURBANCE APPROACHING FROM THE NORTH.

 &&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (DAYS 1 AND 2)...

RIDGING BUILDS INTO THE CENTRAL BERING THROUGH FRIDAY NIGHT WITH
A WEAKENING UPPER LOW ALONG THE WESTERN AND CENTRAL ALEUTIANS.
THIS IS A RELATIVELY BENIGN SURFACE PATTERN WITH SMALL CRAFT WINDS
CONFINED TO THE WESTERN HALF OF THE ALEUTIANS. CLOUDY CONDITIONS
WILL ENCOMPASS THE BERING WITH WEAK EMBEDDED SHORTWAVES PRODUCING
OCCASIONAL SHOWERS. AREAS OF DENSE FOG IS EXPECTED TO FORM TODAY
INTO FRIDAY AS STABLE CONDITIONS SETTLE ACROSS THE CENTRAL
BERING.


&&

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

AN UPPER LEVEL RIDGE WILL OVERSPREAD THE MAJORITY OF THE STATE
THROUGH THE WEEKEND. WHILE THIS RIDGE BUILDS ACROSS THE STATE...A
PORTION OF THE LONGWAVE TROUGH THAT HAS BEEN CREATING THE SHOWERY
WEATHER ACROSS THE STATE WILL GET CUT OFF AS RIDGING BUILDS ACROSS
THE NORTH PACIFIC. THIS WILL CREATE A CUT OFF LOW SOUTH OF THE
EASTERN ALEUTIANS THAT IS EXPECTED TO DEVELOP ON SATURDAY AS A
SHORTWAVE INTERACTS WITH MULTIPLE FRAGMENTS OF ENERGY ASSOCIATED
WITH THE UPPER LEVEL TROUGH THAT IS GETTING CUT OFF. THE
DETERMINISTIC AND ENSEMBLE MEMBERS HAVE THE DEVELOPMENT OF THIS
LOW PROGGED FAIRLY WELL AS IT DEVELOPS SOUTH OF THE EASTERN
ALEUTIANS SATURDAY. MODELS THEN BEGIN TO STRUGGLE WITH THE
PLACEMENT OF THIS UPPER LEVEL FEATURE AND AN ASSOCIATED SURFACE
LOW THAT DEVELOPS IN RESPONSE TO MID AND UPPER LEVEL CYCLOGENESIS.
THE REASON FOR THE DIFFICULTY IN TIMING AND PLACEMENT IS BECAUSE
STEERING CURRENTS CAN BE QUITE DIFFICULT TO DETERMINE WHEN A LOW
IS NO LONGER LOCATED IN A TROUGH BUT RATHER STUCK BETWEEN TWO
UPPER LEVEL RIDGES. IN SHORT...DEPENDING ON HOW THE LONGWAVE
TROUGH (ROSSBY WAVE) OVER RUSSIA DEVELOPS IN RESPONSE TO A
DEEPENING AREA OF LOW PRESSURE WILL INFLUENCE THE WESTERN RIDGE
OVER THE BERING SEA AND HOW IT WILL FORCE THE AREA OF LOW PRESSURE
IN THE NORTH PACIFIC EAST INTO THE GULF OF ALASKA OR POSSIBLY EVEN
INTO THE ALASKA MAINLAND. WHAT THIS MEANS FOR ALASKA IS THAT A
PERIOD OF WARMER TEMPERATURES...DRIER CONDITIONS AND LESS CLOUDY
SKIES WILL LIKELY BE OBSERVED THROUGH THE WEEKEND AND INTO THE
BEGINNING OF NEXT WEEK. DEPENDING ON THE TRACK OF THE UPPER LEVEL
AND SURFACE LOWS...RAIN CHANCES MAY SNEAK BACK IN TOWARD THE
MIDDLE OF NEXT WEEK BUT CONFIDENCE IS NOT HIGH WITH MORE THAN
SCATTERED SHOWERS AT THIS TIME.

&&




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

&&

$$

SYNOPSIS AND MODEL DISCUSSION...CC
SOUTHCENTRAL ALASKA...CC
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...CC
LONG TERM...MC/CC



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