Area Forecast Discussion
Issued by NWS Anchorage, AK

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000
FXAK68 PAFC 012331
AFDAFC

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
331 PM AKDT SUN MAY 1 2016

.ANALYSIS AND UPPER LEVELS...
THE VERTICALLY STACKED LOW CENTERED OFF THE COAST OF KODIAK
ISLAND HAS BARELY MOVED SINCE YESTERDAY. IT CONTINUES TO ACT AS
THE STEERING SYSTEM FOR SEVERAL OTHER MUCH SMALLER FEATURES MOVING
ACROSS MAINLAND SOUTHERN ALASKA. MOISTURE FLOW UP THE EAST SIDE OF
THE ALASKA RANGE CONTINUES TO PRODUCE STEADY RAINFALL OVER THE
UPSLOPE AREAS OF THE WESTERN SUSITNA VALLEY AND WESTERN COOK
INLET. A MORE UNSTABLE AIR MASS MOVING INTO THE COAST CONTINUES TO
CAUSE SCATTERED SHOWERS ALONG THE NORTH GULF COAST AS THE CONSTANT
SOUTHEASTERLY FLOW OFF THE GULF MOVES A CONTINUOUS SUPPLY OF
MOISTURE ASHORE. THICKER CLOUD COVER OVER MUCH OF SOUTHCENTRAL HAS
HELPED TEMPER THE RISING TEMPERATURES A BIT...DESPITE THE DRY
CONDITIONS PREVAILING TO THE LEE OF THE CHUGACH AND KENAI
MOUNTAINS.

SOUTHWESTERN ALASKAS WEATHER ALSO IS BEING CONTROLLED BY THE LOW
OVER THE GULF...THOUGH LESS MOISTURE ALOFT HAS ONCE AGAIN ALLOWED
ABUNDANT MORNING SUNSHINE TO DESTABILIZE THE ATMOSPHERE TO THE
LEE OF THE ALASKA RANGE. THIS HAS RESULTED IN A BLOSSOMING CUMULUS
FIELD THIS AFTERNOON OVER THE AREA FROM NORTH OF PILOT POINT
THROUGH SPARREVOHN. THE WOOD RIVER MOUNTAINS ARE HELPING LIFT THE
AIR MASS ON THE WINDWARD SIDE OF THOSE MOUNTAINS...RESULTING IN
MORE NUMEROUS SHOWERS AND ISOLATED THUNDERSTORMS APPEARING ON KING
SALMON RADAR FROM DILLINGHAM NORTHWARD. A FEW LIGHTNING STRIKES
HAVE ALSO OCCURRED NEAR ANIAK. TOWARDS THE KUSKOKWIM DELTA NEARLY
FULL SUNSHINE CONTINUES FOR ALL AREAS AWAY FROM THE IMMEDIATE
COAST...WITH SOME CUMULUS STARTING TO DEVELOP EAST OF BETHEL.

FOR THE BERING SEA AND ALEUTIANS...UPPER LEVEL RIDGING PERSISTS
OVER THE CENTRAL AND EASTERN BERING AND ALEUTIANS THIS AFTERNOON.
LIGHT NORTHWESTERLY WINDS IN THE LOW LEVELS ARE DRAWING MOISTURE
FROM THE SEA SURFACE. THIS MOISTURE THEN IS HITTING THE WARM AIR
ABOVE IT...CAPPING ITS ASCENT. THE RESULT IS WIDESPREAD STRATUS
OVER NEARLY THE ENTIRE BERING...WITH THE EXCEPTION OF THE SOUTH
SIDE OF NUNIVAK ISLAND...AND SOUTHEAST OF THE PRIBILOFS...WHERE
THE STRATUS IS MORE BROKEN. THE NEXT VERTICALLY STACKED LOW IN THE
SERIES IS MOVING TOWARDS THE WESTERN ALEUTIANS...WITH THE
ASSOCIATED OCCLUDED FRONT HAVING MOVED THROUGH THE CHAIN EARLIER
TODAY. THIS LOW WILL BECOME THE DOMINANT WEATHER MAKER FOR MUCH OF
THE SOUTHERN BERING OVER THE COMING DAYS.

&&

.MODEL DISCUSSION...
WITH VERY FEW CHANGES IN THE WEATHER ANYWHERE IN SOUTHERN
ALASKA...THE MODELS REMAIN IN VERY GOOD AGREEMENT REGARDING THE
EVOLUTION OF THE BROAD WEATHER PATTERN IN THE SHORT-TERM. THE
HIGHER RESOLUTION MODELS WERE FAVORED FOR THEIR BETTER HANDLING OF
THE LOCAL WEATHER AND MESOSCALE FEATURES CRISS-CROSSING THE AREA.
MODEL DIFFERENCES EMERGE FOR THE MID AND LONG TERM FORECASTS WITH
THE GFS SHOWING THE LOW CURRENTLY MOVING TOWARDS THE WESTERN
ALEUTIANS RECURVING ALONG THE BRISTOL BAY COAST...WHILE THE NAM
KEEPS MOST OF THE ENERGY SOUTH AND INTO THE GULF. THE GFS WAS
PREFERRED FOR THIS SCENARIO...THOUGH WITH SOME FLIP-FLOPPING IN
THE MODELS HAVING OCCURRED THE PAST COUPLE DAYS...CONFIDENCE BY
THIS POINT WANES CONSIDERABLY. FOR THE SHORTER-TERM THE FORECAST
CONFIDENCE IS HIGH.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (DAYS 1 AND 2)...
THE MAIN CONCERN THROUGH MONDAY CONTINUES TO BE THE STRONGER GAP
AND CROSS BARRIER WIND ACROSS SOUTH CENTRAL ALASKA. AN OCCLUDED
FRONT IS PUSHING IN ALONG THE NORTH GULF COAST TODAY AND THAT WILL
BE FOLLOWED BY A RIDGE THAT IS BUILDING IN ALONG THE NORTH GULF
COAST. THE BUILDING RIDGE WILL SERVE TO ACCELERATE WINDS THROUGH
PORTAGE VALLEY AND ALONG TURNAGAIN ARM AS WELL AS ACROSS THE
ANCHORAGE HILLSIDE. RAIN ASSOCIATED WITH THE FRONT WILL MAINLY
STAY ON THE COASTAL SIDE OF THE MOUNTAINS AS THE DOWN SLOPE DRYING
ON THE LEE OF THE MOUNTAINS IS KEEPING ANY SIGNIFICANT RAIN FROM
FALLING. THE UPSLOPE FLOW INTO THE ALASKA RANGE SHOULD RESULT IN
INCREASED CHANCES FOR SHOWERS ALONG THE RANGE.

ANOTHER AREA OF CONCERN IS THE INCREASING POTENTIAL FOR
THUNDERSTORM ACTIVITY IN THE COPPER RIVER BASIN BY TOMORROW
AFTERNOON. THE VERTICAL TEMPERATURE SPREAD IS INCREASING ALONG
WITH THE MODELS INDICATING INCREASINGLY MORE UNSTABLE CONDITIONS
PARTICULARLY IN THE NORTHERN AND EASTERN COPPER RIVER BASIN. THERE
IS PLENTY OF MOISTURE TO ALLOW FOR THE DEVELOPMENT OF
THUNDERSTORMS AND WE WILL BE ADDING SOME ISOLATED INTO THE
FORECAST.

THE PATTERN WILL PERSIST THROUGH MONDAY WITH SHOWERS ACROSS THE
GULF AND INCREASING WINDS DEVELOPING AS ANOTHER TROUGH MOVES UP
FROM THE SOUTHERN GULF.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (DAYS 1 THROUGH 3)...
WITH THE GRADUALLY WEAKENING VERTICALLY STACKED LOW IN THE
WESTERN GULF KEEPING SOUTHWEST ALASKA IN WEAK EASTERLY FLOW ALOFT
DIURNALLY ENHANCED INSTABILITY SHOWERS AND THE POTENTIAL FOR
ISOLATED THUNDERSTORMS WILL BE THE MAIN FORECAST CHALLENGE. THIS
AFTERNOON AND EVENING THE THERMAL TROUGH WILL EXTEND FROM INTERIOR
ALASKA ACROSS THE LOWER KUSKOKWIM VALLEY AND INTO THE NORTHERN
BRISTOL BAY AREA. THE FEATURE WILL BE FAIRLY DIFFUSE WITH MARGINAL
INSTABILITY SPREAD OVER MUCH OF SOUTHWEST ALASKA FROM AROUND
BETHEL WEST...BRINGING A SLIGHT CHANCE FOR A WEAK THUNDERSTORM OR
TWO THIS AFTERNOON OR EVENING.

ON MONDAY THE THERMAL TROUGH BECOMES BETTER DEFINED WITH STRONGER
INSTABILITY CONCENTRATED ALONG ITS AXIS. WHILE THIS WILL FOCUS THE
THUNDERSTORM THREAT OVER A SMALLER AREA GEOGRAPHICALLY (RUNNING
FROM THE LOWER YUKON RIVER VALLEY...ACROSS THE SOUTHERN HALF OF
THE KUSKOKWIM MOUNTAINS...THE HOLITNA RIVER VALLEY...THE EASTERN
KILBUCK AND AHKLUN MOUNTAINS...THE TAYLOR MOUNTAINS...THE NUSHAGAK
RIVER VALLEY AND THE WOOD RIVER LAKES AREA)...THE PROBABILITY OF
SEEING ISOLATED TO WIDELY SCATTERED THUNDERSTORMS IS HIGHER WITH A
BETTER CHANCE OF A GREATER NUMBER OF LIGHTNING STRIKES OUT OF
THEM.

ON TUESDAY THE THERMAL TROUGH RETREATS BACK NORTH WITH MORE
MINIMAL INSTABILITY LIMITING CONVECTION TO ISOLATED WITH A FEW
AREAS OF SCATTERED SHOWERS IN THE AFTERNOON AND EVENING. ON
WEDNESDAY A SURFACE LOW CROSSING BRISTOL BAY AND ASSOCIATED TROUGH
ROTATING ACROSS SOUTHWEST ALASKA WILL BRING MORE NUMEROUS AND
WIDESPREAD SHOWERS TO THE AREA.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (DAYS 1 THROUGH 3)...
THE SURFACE LOW TO THE SOUTHWEST OF THE WESTERN ALEUTIANS WILL
TRACK EAST NORTHEAST TO INTERSECT THE ALEUTIAN CHAIN AROUND KISKA
MONDAY AFTERNOON WHILE THE ASSOCIATED SURFACE FRONT STRETCHES FROM
WEST TO EAST ACROSS THE BERING AROUND 56N. LOW WILL CONTINUE EAST
ALONG THE ALEUTIAN CHAIN MONDAY NIGHT AND TUESDAY THEN UP THE
ALASKA PENINSULA ALONG THE BERING SIDE TUESDAY NIGHT. THE ORIGINAL
SURFACE LOW WILL THEN CURL NORTH ACROSS BRISTOL BAY AND INTO
SOUTHWEST ALASKA WEDNESDAY WITH THE ASSOCIATED FRONT AND A
DEVELOPING TRIPLE POINT LOW SWINGING INTO THE SOUTHWESTERN GULF.

&&

.LONG TERM FORECAST (DAYS 3 THROUGH 7...WED THROUGH SUN)...
THE EXTENDED PORTION HAS HINGED THE PAST FEW DAYS ON HOW STRONG
THE RIDGING FROM THE NORTH PACIFIC WOULD HOLD AND HOW ENERGY
WOULD EVOLVE SURROUNDING THE UPPER LEVEL LOW NEAR THE ALASKA
PENINSULA WED-THU. MODELS DO SHOW SOME DISPARITY ON HANDLING WHAT
HAPPENS WITH THE REMNANTS OF THE UPPER LEVEL LOW OVER BRISTOL BAY.
THE GFS HOLDS IT TOGETHER BETTER THAN THE OTHER AND THIS SEEMS
QUITE PLAUSIBLE GIVEN THE PATTERN. THUS...WE KEPT POPS RATHER HIGH
THROUGH THE SOUTHWEST INTO THU. BEYOND THAT...FORECAST CONFIDENCE
CONTINUES TO INCREASE...ALBEIT SLOWLY...AS MODELS CONVERGE ON A
SOLUTION. THE RIDGE DOES BUILD RATHER QUICKLY ALONG THE ALCAN
BORDER WED-THU. THIS SETS THE STAGE FOR THE AKPEN UPPER LEVEL LOW
TO WORK IN TANDEM WITH SOME ENERGY FROM THE NORTH PACIFIC RIDING
UP THE WESTERN PERIPHERY OF THAT RIDGE. THIS WILL RESULT IN SOME
PHASING WITH A TRIPLE POINT LOW TAKING SHAPE OVER THE 45N 170W AND
WORKING ITS WAY TOWARD KODIAK BY EARLY FRI. BUT WHILE THE SYNOPTIC
PATTERN LOOKS SIMILAR FROM MODEL TO MODEL...THERE ARE STILL SOME
SIGNIFICANT DIFFERENCES. BY 06Z FRI...THE ECMWF MODEL HAS THE
SURFACE LOW JUST SOUTH OF KODIAK AT 995MB WHILE THE GFS PAINTS
IT AT 979MB. THE MODELS THEN AGREE THAT THE LOW WILL TRACK TOWARD
THE NE GULF THROUGH SAT. IT WILL HAVE AN HEALTHY AMOUNT OF
MOISTURE COMING WITH IT. THE QUESTION IS...EXACTLY WHERE DOES THE
LOW TRACK GO AND HOW MUCH CROSS BARRIER FLOW WILL BE PRESENT OVER
SOUTH CENTRAL AK. USING THE PAST FEW WEEKS AS A BASELINE...THE
INCLINATION IS TO THINK THE COAST WILL ONCE AGAIN RECEIVE A FEW
INCHES OF RAIN WHILE AREAS IN THE LEE OF THE MOUNTAINS MIGHT ONLY
SEE A FEW PASSING SHOWERS.

SUN INTO NEXT WEEK...WHILE THERE ARE STILL SOME SIGNS OF
RIDGING...FORECAST CONFIDENCE HAS NOT INCREASED. THE BIGGEST
CHANGE FROM YESTERDAY IS A FAIRLY DEEP LOW THAT QUICKLY MOVES INTO
PLACE OVER THE BERING SEA. IF THIS WERE TO VERIFY...AK COULD
EASILY RETURN TO THE SAME PATTERN WE HAVE BEEN IN THE PAST FEW
WEEKS.

&&


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

&&

$$

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



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