Area Forecast Discussion
Issued by NWS Anchorage, AK

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000
FXAK68 PAFC 302357
AFDAFC

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
357 PM AKDT SAT APR 30 2016

.ANALYSIS AND UPPER LEVELS...
A VERTICALLY STACKED LOW SWIRLING ABOUT THE SOUTHWESTERN GULF OFF
OF KODIAK ISLAND REMAINS THE DOMINANT STEERING FEATURE FOR ALL OF
SOUTHERN ALASKA OUT TO THE EASTERN ALEUTIANS THIS AFTERNOON. NORTH
AND EAST OF THE LOW...BROAD SOUTH TO SOUTHEASTERLY FLOW IS PUMPING
MOISTURE AND CLOUDINESS INTO THE NORTHERN GULF COAST. AT WHICH
POINT THE MOISTURE IS VERY EFFECTIVELY WRUNG OUT BY THE CHUGACH
AND KENAI MOUNTAINS...LEAVING LITTLE MORE THAN BROAD CLOUDINESS
FOR INLAND LOCALES OF SOUTHCENTRAL. THE PRIMARY WEATHER STORIES
ALONG COOK INLET AND THE VALLEYS REMAIN THE PERIODIC GUSTY
WINDS...ABOVE AVERAGE TEMPERATURES...AND VERY DRY CONDITIONS. MORE
ON THE FORECAST IMPLICATIONS OF THESE WEATHER FACTORS BELOW. WITH
LITTLE OR NO PRECIPITATION EXPECTED FOR THE REST OF THE DAY IN
ANCHORAGE...THE PALTRY 0.02 INCHES OF PRECIPITATION FOR THE MONTH
WILL MAKE THIS THE 3RD DRIEST APRIL ON RECORD. MORE ON THE CLIMATE
STATS FOR ANCHORAGE TOMORROW.

OVER SOUTHWEST ALASKA...AN UNSTABLE ATMOSPHERE REMAINS THE
INGREDIENT MOST LIKELY TO RESULT IN NOTABLE WEATHER THIS EVENING.
THE MOST CLOUD COVER REMAINS NEAR THE ALASKA RANGE...THOUGH EVEN
THIS AREA IS SEEING BREAKS. OTHERWISE ITS A NICE SUNNY DAY OUT
TOWARDS THE COAST...A FACTOR THAT COULD HELP WITH THUNDERSTORM
DEVELOPMENT ONCE AGAIN TODAY. YESTERDAY...STORMS DEVELOPED ALONG A
SEA BREEZE FRONT ALONG THE KUSKOKWIM DELTA IN THE LATE AFTERNOON
AS THE FRONT HELPED PUSH AN AIR MASS THAT HAD SOME MOISTURE WITH
IT UPWARD...WHERE THE UNSTABLE ATMOSPHERE THAT REMAINS ON THIS
MORNINGS BETHEL SOUNDING TOOK OVER.

OVER THE BERING...UPPER LEVEL RIDGING BETWEEN THE UPPER LOW NEAR
KODIAK AND THE NEXT LOW PUSHING OFF THE EAST COAST OF ASIA IS IN
PLACE. THE UPPER LEVEL RIDGING IS INDICATIVE OF MUCH WARMER AIR
ALOFT. THIS CAN BE SEEN COMPARING THIS MORNINGS SHEMYA SOUNDING TO
BETHELS. THUS...IN THIS AREA...INSTABILITY IS ABSENT. A LACK OF
INSTABILITY MEANS A LACK OF VERTICAL MIXING. WITH ABUNDANT LOW
LEVEL MOISTURE FROM THE BERING ITSELF...THE WARM AIR ALOFT HAS
TRAPPED THAT MOISTURE BENEATH THE CAP OF WARM AIR...CAUSING
ABUNDANT LOW LEVEL STRATUS FROM NUNIVAK ISLAND ACROSS MOST OF THE
BERING AND ALEUTIANS.

&&

.MODEL DISCUSSION...
WITH SUCH A STAGNANT WEATHER PATTERN...THE MODELS ALL HAVE BEEN
IN EXCELLENT AGREEMENT TRACKING THE LARGER WEATHER FEATURES ACROSS
SOUTHERN ALASKA...AND SUCH REMAINS THE CASE THIS AFTERNOON.
THUS...THE MAIN FORECAST CHALLENGES REMAIN TRACKING THE SMALLER
SCALE FEATURES THAT HAVE A SIGNIFICANT IMPACT TO THE WEATHER.
THESE FEATURES INCLUDE FORECASTING WHERE ANY AFTERNOON
THUNDERSTORMS WILL DEVELOP AND MOVE...IF THE STRATUS PREVALENT
OVER THE BERING WILL DISSIPATE OR THICKEN AND LOWER INTO FOG...AND
TIMING ANY LOCAL WIND SHIFTS IN THE COOK INLET REGION CAUSED BY
DIFFERENTIAL HEATING OR THE PASSAGE OF MESOSCALE FRONTS. THE
HIGHER RESOLUTION MODELS SUCH AS THE ARW...NAM...AND GEM WERE USED
TO HELP WITH THESE FORECAST CHALLENGES. CONFIDENCE REMAINS VERY
HIGH ON THE BROADER SCALE WEATHER PATTERN...AND AVERAGE FOR THE
SMALLER FEATURES WE ARE TRACKING.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (DAYS 1 AND 2)...
A STRONG LOW NOW IN THE GULF OF ALASKA HAS A FRONT EXTENDING ALONG
THE COAST FROM KODIAK ISLAND TO PRINCE WILLIAM SOUND SOUTHEAST.
THE MAIN CONCERN IS THE GUSTY POST FRONTAL WIND. AFTER THE FRONT
PUSHES INLAND AND WEAKENS TO A TROUGH A RIDGE FORMS IN BEHIND THIS
WHILE ANOTHER TROUGH THEN PUSHES TOWARD THE COAST LATE THIS
EVENING AND SUNDAY MORNING. WHILE WIND IS THE MAIN ISSUE...MODERATE
TO HEAVY RAINFALL IS OCCURRING ALONG THE COASTAL UP SLOPE SIDE OF
THE MOUNTAINS AS WELL AS INTO PORTAGE VALLEY. THE RAIN WILL
DIMINISH IN INTENSITY POST FRONTALLY THOUGH IT WILL CONTINUE
THROUGH SUNDAY IN THE PERSISTENT ONSHORE FLOW PATTERN.

INLAND THE TYPICAL CROSS BARRIER FLOW WILL RESULT IN THE RAIN
SHADOWING ON THE LEE OF THE MOUNTAINS. POST FRONTAL REDUCTION OF
WIND SHOULD ALLOW FOR INCREASED POTENTIAL FOR RAIN ON THE LEE SIDE
OF THE MOUNTAINS LATE THIS EVENING. THE INCREASE IN WIND AS THE
RIDGE BUILDS TONIGHT WILL INCREASE THE RAIN SHADOW AFFECT TONIGHT
INTO SUNDAY MORNING. INTO THE SUSITNA VALLEY AND NORTHERN COPPER
RIVER BASIN ALONG THE ALASKA RANGE EXPECT RAIN TO INCREASE
THROUGH THIS EVENING AND PERSIST THROUGH SUNDAY IN THE UP SLOPE
FLOW REGIME.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (DAYS 1 AND 2)...
MINIMAL CHANGE IS EXPECTED IN THE GENERAL PATTERN OVER SOUTHWEST
ALASKA FOR THE NEXT SEVERAL DAYS. A MODERATELY UNSTABLE AIR MASS
WILL CONTINUE TO SUPPORT ISOLATED TO SCATTERED DIURNAL SHOWERS
WITH PERIODIC SHORTWAVES ROTATING IN FROM THE EAST...ENHANCING AND
MAKING SHOWERS MORE NUMEROUS. ADDITIONALLY ISOLATED THUNDERSTORMS
WILL BE A POSSIBILITY AT TIMES. WITH THE THERMAL TROUGH STRETCHED
ACROSS THE KUSKOKWIM DELTA...STEERING FLOW FROM THE NORTHEAST AND
CLEARER SKIES OVER BOTH THE KUSKOKWIM DELTA AND TO THE NORTHEAST
BRINING GOOD SOLAR HEATING...CONDITIONS LOOK FAVORABLE FOR
ISOLATED THUNDERSTORMS IN THAT LOCATION THIS EVENING.

ON SUNDAY THE THERMAL TROUGH WILL BE FURTHER INLAND WITH THE
INSTABILITY SPREAD OVER LARGER AREA AND THE POTENTIAL FOR A FEW
ISOLATED THUNDERSTORMS OVER A BROADER PORTION OF INLAND SOUTHWEST
ALASKA. HOWEVER THE INSTABILITY SUNDAY DOES LOOKS MARGINAL ENOUGH
THAT A GOOD BIT OF SUNSHINE WILL BE A NECESSARY INGREDIENT TO
TRIGGER ANY THUNDERSTORMS AND CLOUD COVER LIMIT COULD WHERE THEY
DO FORM. ON MONDAY AFTERNOON/EVENING A MORE FOCUSED TROUGH WILL
BRING A STRONGER SWATH OF INSTABILITY TO THE KUSKOKWIM VALLEY AND
NORTHERN BRISTOL BAY AREA AND A BETTER CHANCE FOR ISOLATED OR
POSSIBLY WIDELY SCATTERED THUNDERSTORMS.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (DAYS 1 AND 2)...
THE BROAD VERTICALLY STACKED LOW IN THE SOUTHWESTERN GULF WILL
GRADUALLY WEAKEN AS IT DRIFTS SLOWLY EAST NORTHEAST SUNDAY AND
MONDAY. THE NEXT INCOMING FRONTAL SYSTEM IS CURRENTLY PUSHING INTO
THE WESTERN ALEUTIANS WITH THE PARENT LOW SOUTH OF THE KAMCHATKA
PENINSULA. THE FRONT WILL CONTINUE NORTH AND EAST INTO THE
SOUTHERN BERING AND CENTRAL ALEUTIANS TONIGHT AND SUNDAY. THE LOW
WILL SKIRT JUST SOUTH OF THE WESTERN ALEUTIANS SUNDAY NIGHT THEN
CONTINUE EAST THROUGH THE CENTRAL ALEUTIANS MONDAY AND EASTERN
ALEUTIANS TUESDAY.

&&

.LONG TERM FORECAST (DAYS 3 THROUGH 7...TUE THROUGH SAT)...
THE LONG RANGE PATTERN WILL PRESENT SOME INTERESTING CHALLENGES
AND A POSSIBLE CHANGE IN THE LONG-WAVE REGIME. THIS TIME FRAME
STARTS WITH ONE LOW FILLING AND MOVING INLAND OVER THE GULF OF AK.
MEANWHILE...OVER THE CENTRAL ALEUTIANS...THE NEXT UPPER LEVEL LOW
IS ALREADY TAKING A VERY FAMILIAR TRACK TOWARDS THE GULF. AS THE
FIRST LOW WEAKENS AND MOVES INLAND...THIS SHOULD ALLOW THE
GRADIENT TO RELAX. THIS WILL FINALLY BRING AN END TO THE GUSTY
SOUTHEAST WINDS THAT HAVE BEEN BLOWING THROUGH FAVORED LOCATIONS
SUCH AS THE TURNAGAIN ARM AND THE KNIK RIVER VALLEY. RATHER MOIST
ONSHORE FLOW WILL CONTINUE TO MAKE FOR SOME STEADY RAIN ALONG THE
IMMEDIATE COASTAL AREAS. BUT PRECIPITATION INLAND SHOULD BE
LIMITED TO SHOWERS...MAINLY DRIVEN BY WEAK AFTERNOON INSTABILITY
WITH A COLD POCKET OF RELATIVELY COLD AIR AT 500MB.

BY LATE WED...THE LOW FROM THE CENTRAL ALEUTIANS WILL MAKE ITS WAY
TO THE ALASKA PENINSULA. IT WILL PUSH A WARM OCCLUSION IN FRONT OF
IT. THIS WILL BRING RAIN BACK INTO KODIAK WITH CLOUDS STARTING TO
OVERSPREAD SOUTH CENTRAL. BUT IT IS AT THIS POINT WHEN MODELS
DIVERGE DRAMATICALLY. THE BIGGEST QUESTION IS WHERE THE UPPER
LEVEL LOW SETS UP RESIDENCE. SOME OF THIS DEPENDS ON HOW EFFECTIVE
ENERGY TRACKING THROUGH THE GULF IS AT PUSHING THE NORTH AMERICAN
RIDGE OFF TO THE EAST. THE EC HOLDS THE LOW CENTER THE FURTHEST
NORTH AND WEST...WHILE THE GFS ALLOWS IT TO MIGRATE ALL THE WAY
INTO THE CENTRAL GULF. EARLIER RUNS WERE SHOWING SOME PHASING
OCCURRING WITH THIS LOW AND SOME ENERGY TRACKING ALONG THE JET IN
THE NORTH PACIFIC. THEY WERE SHOWING THE POTENTIAL FOR A TRIPLE-
POINT LOW TO FORM AND TRACK QUICKLY INTO SOUTH CENTRAL. SOLUTIONS
HAVE BACKED OFF THIS IDEA AND NOW MORE SETTLED ON JUST ANOTHER
UPPER LEVEL LOW SITTING OVER THE CENTRAL GULF BY NEXT SAT.

BY LATE NEXT WEEKEND INTO THE WEEK OF 9-13 MAY...THERE SEEMS TO BE
FAIR TO GOOD MODEL AGREEMENT IN SOME FORM OF RIDGE BUILDING OVER
THE EASTERN INTERIOR PORTION OF AK. MODELS HAVE VARIOUS SOLUTIONS
OF HOW THE RIDGE GETS THERE (SOME FROM THE NORTH PAC...SOME FROM
NW CONUS) BUT THEY ALL SHOW SIGNS OF IT. CLIMATOLOGICALLY THIS
MAKES SENSE AS WELL AS WE USUALLY SEE A WARMER STRETCH TOWARD THE
MIDDLE TO END OF MAY.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...GALE WARNING 178.

&&

$$

SYNOPSIS AND MODEL DISCUSSION...JW
SOUTHCENTRAL ALASKA...SA
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JR
LONG TERM...MO



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