Area Forecast Discussion
Issued by NWS Anchorage, AK

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FXAK68 PAFC 170039
AFDAFC

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
439 PM AKDT THU APR 16 2015

.ANALYSIS AND UPPER LEVELS...

A DEEP LONGWAVE TROUGH REMAINS OVER THE NORTH PACIFIC WITH STRONG
RIDGING OVER EASTERN RUSSIA AND A WEAKER BUT STRENGTHENING RIDGE
OVER BRITISH COLUMBIA. THE FIRST FEATURE OF NOTE IS A WEAKENING
LOW THAT IS MOVING INLAND FROM THE GULF ACROSS THE KENAI PENINSULA
TODAY. WEAKENING CROSS-MOUNTAIN WINDS HAVE ALLOWED PRECIP TO BREAK
OUT AROUND COOK INLET...FALLING MOSTLY IN THE FORM OF RAIN BELOW
1500 FEET. COLD AIR WRAPPING IN BEHIND THIS SYSTEM IS PRODUCING
ROBUST SHOWER ACTIVITY ACROSS THE GULF. THIS TOO IS FALLING MOSTLY
AS RAIN FROM WHITTIER TO VALDEZ AND CORDOVA...BUT WILL SLOWLY
CHANGE TO SNOW AS THE ATMOSPHERE CONTINUES TO SLOWLY COOL THIS
EVENING.

OUT WEST...A FAIRLY GARDEN-VARIETY GALE-FORCE LOW SOUTH OF DUTCH
HARBOR IS MOVING STEADILY TOWARD THE GULF...WITH NORTHERLY WINDS
AND SNOW SHOWERS ACROSS MUCH OF THE BERING. THIS LOW IS FIGHTING
FOR SURVIVAL UNDERNEATH WEAKLY CONFLUENT UPPER LEVEL FLOW THAT IS
LIMITING ITS STRENGTH AT THE MOMENT. A SIGNIFICANT JET STREAK
MOVING DOWN THE FRONT OF THE RUSSIAN RIDGE IS ABOUT TO DROP INTO
THE BASE OF THE LONGWAVE TROUGH AND WILL BE THE CATALYST FOR A
SERIES OF COMPLEX INTERACTIONS IN THE WEATHER PATTERN OVER THE
NEXT FEW DAYS.

&&

.MODEL DISCUSSION...

DESPITE THE COMPLEX INTERACTIONS OVER THE NEXT FEW DAYS...THE
MODELS...AND ESPECIALLY THE GLOBAL MODELS (GFS/ECMWF/GEMGLB)...ARE
IN SURPRISINGLY GOOD AGREEMENT IN THE FIRST FEW DAYS OF THE
FORECAST. ALL MODELS SHOW THE LOW CURRENTLY SOUTH OF THE ALEUTIANS
SENDING THE ASSOCIATED FRONT UP INTO THE NORTHERN GULF BY FRIDAY
EVENING. THE LOW ITSELF WILL THEN GET A REINVIGORATING BOOST FROM
THE SIGNIFICANT JET STREAK ROUNDING THE BASE OF THE TROUGH AT
ABOUT THE SAME TIME. THIS WILL GENERATE ANOTHER FRONT THAT WILL
MOVE THROUGH SOUTHCENTRAL ALASKA LATER THIS WEEKEND. THE MAIN
DISCREPANCY AMONG THE AVAILABLE MODELS LIES IN THE SPEED/TIMING
WITH WHICH THE NAM TRANSFERS MOST OF THE ENERGY FROM THE NORTH
PACIFIC LOW INTO A NEW LOW SOUTH OF THE BERING STRAIT. THE NAM IS
TRYING TO SIMULATE THIS TRANSITION ABOUT 12 HOURS SOONER THAN THE
OTHER MODELS...AND WOULD HAVE SIGNIFICANT IMPACT ON SNOW/BLOWING
SNOW POTENTIAL FOR PORTIONS OF THE DELTA COAST AND PRIBILOF
ISLANDS. BECAUSE THE NAM IS NOT A GLOBAL MODEL IT STRUGGLES WITH
FEATURES MOVING QUICKLY IN FROM OUTSIDE OF ITS DOMAIN...MOST
COMMONLY DURING HIGHLY AMPLIFIED/MERIDIONAL FLOW. THUS...THE NAM
EVOLUTION OVER THE NEXT FEW DAYS WAS REJECTED IN FAVOR OF THE MORE
CONSISTENT GLOBAL MODELS.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (DAYS 1 AND 2)...
SHOWERS WILL CONTINUE ACROSS MUCH OF THE AREA THROUGH TONIGHT AS
AN UPPER LEVEL TROUGH LIFTS NORTHWARD FROM THE GULF...WITH COLD
AIR FILTERING IN BEHIND THE TROUGH CHANGING PRECIPITATION TO
MOSTLY SNOW BY LATE TONIGHT. THE BEST CHANCE FOR ACCUMULATING SNOW
WILL REMAIN OVER HIGHER TERRAIN NEAR THE COAST SUCH AS TURNAGAIN
OR THOMPSON PASSES AND ALSO NORTHERN AND WESTERN PARTS OF THE
SUSITNA VALLEY. A POWERFUL SYSTEM WILL THEN FOLLOW CLOSELY BEHIND
THE DEPARTING TROUGH ON FRIDAY...WITH A STORM FORCE FRONT REACHING
THE COAST BY FRIDAY NIGHT AND BRINGING STRONG GAP WINDS TO
FAVORABLE AREAS SUCH AS TURNAGAIN ARM AND THE MATANUSKA VALLEY
THROUGH MUCH OF THE WEEKEND. THIS SYSTEM WILL ALSO BRING ABUNDANT
PRECIPITATION TO AREAS ALONG THE COAST...WITH A RAIN/SNOW MIX
GRADUALLY CHANGING OVER TO MOSTLY RAIN BY SATURDAY AS WARMER AIR
MOVES INTO THE AREA IN SOUTHERLY FLOW. STRONG CROSS BARRIER FLOW
OVER THE MOUNTAINS WILL KEEP INLAND AREAS MOSTLY DRY FRIDAY NIGHT
AND SATURDAY.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (TONIGHT THROUGH SATURDAY)...
PARTLY TO MOSTLY CLOUDY SKIES AND LIGHT WINDS WILL REMAIN FOR MOST
OF THE SOUTHWEST MAINLAND UNTIL TOMORROW AFTERNOON WHEN THE
FRONTAL SYSTEM CURRENTLY JUST SOUTH OF THE CENTRAL ALEUTIANS WILL
BEGIN TO IMPACT THE AKPEN. THIS WILL BRING RAIN OR A WET SNOW TO
BRISTOL BAY AS WELL AS STRONG EASTERLY WINDS THROUGH FRIDAY
EVENING. THIS RAIN WILL SWITCH TO SNOW AS IT REACHES THE KUSKOKWIM
DELTA LATE TOMORROW NIGHT...WHICH WILL ALSO COINCIDE WITH
INCREASING NORTHERLY WINDS ALONG THE COAST. THIS WILL BRING AREAS
OF BLOWING SNOW TO COASTAL COMMUNITIES THROUGH SATURDAY
MORNING...BUT WITH LITTLE IF ANY TRANSPORTABLE SNOW CURRENTLY IN
THE AREA AND JUST A FEW INCHES OF ACCUMULATION EXPECTED...AN
ADVISORY IS NOT EXPECTED AT THIS TIME. BY SATURDAY
AFTERNOON...PRECIP THROUGHOUT THE AREA SHOULD SWITCH OVER TO RAIN
WITH CONTINUED SOUTHEASTERLY FLOW.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (TONIGHT THROUGH
SATURDAY)...
THE FOCUS IN THE SHORT TERM FORECAST WILL REMAIN CENTERED AROUND
THE NORTH PACIFIC LOW CURRENTLY SOUTHWEST OF DUTCH HARBOR. WINDS
FROM THE ALEUTIAN ISLANDS INTO THE AKPEN WILL INCREASE THIS
EVENING...WITH RAIN AND SNOW BUILDING INTO THE AREA FROM THE SOUTH
BY TOMORROW MORNING ALONG THE FRONT. THIS LOW WILL BRING
PERSISTENT RAIN AND SNOW WITH NORTHERLY WINDS TO DUTCH HARBOR
THROUGH THIS WEEKEND. FURTHER NORTH IN THE PRIBILOFS...THERE
REMAINS SIGNIFICANT DISCREPANCIES IN THE MODELS HAS TO HOW FAR
WEST A DEFORMATION BAND SETTING UP OVER THE EASTERN BERING...WITH
THE NAM BEING AN OUTLIER BY BRINGING SIGNIFICANT SNOWFALL AND
STRONG NORTHERLY WINDS TO THE ISLAND. FOR NOW THE FORECAST KEEPS
MUCH OF THE SNOWFALL JUST OFF TO THE EAST...BUT LATER MODELS WILL
NEED TO BE CLOSELY MONITORED AS THIS SYSTEM HAS THE POTENTIAL TO
BRING BLIZZARD CONDITIONS TO THE AREA.

&&

.LONG TERM FORECAST (SUNDAY THROUGH THURSDAY)...

THE AREA OF LOW PRESSURE SOUTH OF THE ALEUTIAN ISLANDS WILL CROSS
THE ALASKA PENINSULA EARLY SUNDAY MORNING. THIS WILL BRING A
PERIOD OF NORTHERLY GALE FORCE WINDS TO THE EASTERN BERING SEA
INCLUDING THE PRIBILOF ISLANDS WHILE SOUTHWEST ALASKA WILL SEE A
PERIOD OF GUSTY SOUTHEASTERLY WINDS. PRECIPITATION WILL OVERSPREAD
MUCH OF SOUTHWEST AND SOUTHCENTRAL ALASKA AS THE LOW CROSSES THE
ALASKA PENINSULA...WITH MAINLY RAIN OBSERVED ACROSS INLAND AREAS
AND A MIX OF RAIN AND SNOW ACROSS THE COASTAL AREAS AS A BRIEF
PERIOD OF SLIGHTLY WARMER TEMPERATURES WILL BE OBSERVED.
SOUTHCENTRAL ALASKA WILL SEE MAINLY RAIN ACROSS MOST LOCATIONS
WHILE HIGHER ELEVATIONS WILL BE MORE PRONE TO SEEING PERIODS OF
SNOWFALL. STRONG WINDS WILL PUSH THROUGH PORTAGE VALLEY AND
TURNAGAIN ARM WITH THE ORIENTATION OF THE FRONT AS IT PUSHES NORTH
TOWARD NUNIVAK ISLAND. ONCE THE LOW REACHES NUNIVAK ISLAND (ABOUT
60N)...THE NORMAL DOWN INLET GRADIENT ACROSS COOK INLET WILL
REVERSE AND BECOME UP INLET WHICH SHOULD BRING SOME GUSTY WINDS
INTO WEST ANCHORAGE ON SUNDAY.

THE OVERALL SYNOPTIC PATTERN DOES NOT APPEAR TO SHIFT MUCH IN THE
EXTENDED PERIOD WITH THE ARCTIC JET STREAM REMAINING IN THE NORTH
PACIFIC. THIS PATTERN IS FAVORABLE FOR LOW PRESSURE SYSTEMS TO
PUSH OFF EITHER JAPAN OR RUSSIA...PASS SOUTH OF THE ALEUTIAN
ISLANDS AND EVENTUALLY TOWARD THE GULF OF ALASKA. MODELS ARE
PRESENTLY HINTING AT THIS SCENARIO BEGINNING TO DEVELOP AGAIN BY
MIDWEEK. OVERALL...THE TYPE OF WEATHER PATTERN OBSERVED OVER THE
PAST 7 TO 10 DAYS WILL LIKELY PERSIST AT LEAST ANOTHER 7 TO 10
DAYS.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...STORM WARNING 119 120 130 131.
 GALE WARNING 125 129 132 136 137 138 139 141 150 155 160 165 170
  171 172 173 174 175 176 179 180 181 185.
 HEAVY FREEZING SPRAY 179 180.
FIRE WEATHER...NONE.

&&

$$

SYNOPSIS AND MODEL DISCUSSION...AD
SOUTHCENTRAL ALASKA...CB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...DEK
LONG TERM...MC



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