Area Forecast Discussion
Issued by NWS Anchorage, AK

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000
FXAK68 PAFC 300032
AFDAFC

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

.ANALYSIS AND UPPER LEVELS...

THE UPPER LEVEL PATTERN OVER SOUTHERN ALASKA REMAINS IN A STAGNANT
STATE. A TROUGH WITH TWO UPPER LEVEL LOWS IS CENTERED OVER
SOUTHERN ALASKA. ONE OF THE LOWS IS CENTERED OVER THE KUSKOKWIM
DELTA...AND THE OTHER IS OVER YAKUTAT. SMALLER EMBEDDED
DISTURBANCES ARE ROTATING AROUND THE LOWS PRODUCING LOCALIZED
ENHANCED LIFT. THE 00Z SOUNDINGS FROM AROUND SOUTHERN ALASKA
CONTINUE TO SHOW A DEEP MOISTURE-RICH AIR MASS IN PLACE. THUS THE
ENHANCED LIFT CAUSED BY THE SMALL DISTURBANCES IS TAPPING THE
ABUNDANT MOISTURE TO PRODUCE SHOWER ACTIVITY. TERRAIN ADDING TO
THAT LIFT IS CAUSING SOME THUNDERSTORM DEVELOPMENT AGAIN TODAY
ACROSS THE ALASKA RANGE...TALKEETNAS...AND THE KENAI MOUNTAINS.

OVER THE WESTERN BERING...A RIDGE IS IN PLACE WITH A VERTICALLY
STACKED LOW CUT OFF SOUTH OF ATTU ISLAND...KEEPING THE WEATHER
UNSETTLED ALONG THE WESTERN ALEUTIANS. THE JET STREAM IS FLOWING
ACROSS THE CENTRAL AND EASTERN BERING...BETWEEN THE WESTERN RIDGE
AND THE UPPER TROUGH.

&&

.MODEL DISCUSSION...

OVER THE NEXT DAY OR TWO THE MODELS REMAIN IN EXCELLENT AGREEMENT
IN SHOWING A LACK OF ANY STRONG FEATURES. 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. AS THE UPPER LOW OVER THE SOUTHWESTERN
GULF CUTS OFF FROM THE COLD POOL OF AIR ALONG THE ARCTIC
COAST...IT WILL ALLOW THE WESTERN BERING RIDGE TO SPREAD EASTWARD.
INITIALLY THE HEIGHT RISES WILL BE DUE TO SUBSIDENCE. SIGNIFICANT
WARMING OF THE UPPER LEVELS WILL NOT BEGIN 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 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...THOUGH RELATIVE
HUMIDITIES SHOULD REMAIN ABOVE CRITICAL LEVELS. WARMING
TEMPERATURES 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 THROUGH THURSDAY 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...ESPECIALLY OVER
THE WESTERN KENAI PENINSULA THIS EVENING WHERE A WEAK SHORTWAVE
IS MOVING OVERHEAD. AFTERNOON/EVENING THUNDERSTORMS WILL OCCUR
ONCE AGAIN MAINLY OVER THE TALKEETNA MOUNTAINS...ALASKA RANGE AND THE
WESTERN KENAI PENINSULA THROUGH THURSDAY...HOWEVER WITH WEAK
WINDS ALOFT THERE SHOULD BE LITTLE IF ANY MOVEMENT...AND THESE
SHOWERS WILL REMAIN CONFINED TO THE HIGHER ELEVATIONS. ONE
EXCEPTION TO THIS WILL BE OVER THE WESTERN KENAI PENINSULA...WHERE
EASTERLY FLOW WILL ALLOW THUNDERSTORMS FORMING OVER THE CHUGACH
MOUNTAINS TO DRIFT OVER THE LOWER ELEVATIONS THROUGH THURSDAY.
WINDS WILL REMAIN LIGHT IN MOST AREAS...WITH THE EXCEPTION OF GAP
WIND PRONE AREAS SUCH AS TURNAGAIN ARM...WHICH WILL SEE CONTINUED
SOUTHEASTERLY WINDS THROUGH THIS EVENING. 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 OF THE NORTHERN
SUSITNA VALLEY AND COPPER RIVER BASIN ON FRIDAY...AND SHOULD BE
LESS WIDESPREAD THAN THE NEXT TWO DAYS AS INSTABILITY WEAKENS.

&&

.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 SHOWER AND THUNDERSTORM DEVELOPMENT THROUGH THE END
OF THE WEEK. CONDITIONS WILL BE SLIGHTLY MORE STABLE ACROSS THE
KUSKOKWIM DELTA AS IT COMES UNDER THE INFLUENCE OF A COOLER
MARITIME ENVIRONMENT. PATCHY FOG WILL MAINLY AFFECT THE COASTAL
AREAS OF THE KUSKOKWIM...HOWEVER AREAS INLAND MAY EXPERIENCE BRIEF
PERIODS AS TEMPERATURES COOL TO NEAR THE DEW POINT DURING THE
LATE NIGHT HOURS.


&&

.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 ON THURSDAY
AS STABLE CONDITIONS SETTLE ACROSS THE CENTRAL BERING.

&&

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

AN UPPER LEVEL RIDGE WILL BEGIN TO BUILD ACROSS THE INTERIOR OF
ALASKA FROM THE BERING SEA FRIDAY AFTERNOON AND 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 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...HOW THE LONGWAVE TROUGH (ROSBY 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...RMC
SOUTHCENTRAL ALASKA...DEK
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KH
LONG TERM...MC



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