Area Forecast Discussion
Issued by NWS Anchorage, AK

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FXAK68 PAFC 230103
AFDAFC

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
403 PM AKST SUN FEB 22 2015

.ANALYSIS AND UPPER LEVELS...

A DEEP TROUGH REMAINS IN PLACE OVER THE BERING SEA AND NORTHERN
PACIFIC...WITH A 100 KT DUE SOUTHERLY JET CLIMBING UP AND OVER A
PERSISTENTLY STRONG RIDGE OVER THE WEST COAST OF NORTH AMERICA.
THIS HAS SET MAINLAND ALASKA UP IN WARM AND MOIST SOUTHERLY FLOW
ORIGINATING FROM UNUSUALLY LOW LATITUDES...AND AS A RESULT THE
MELTDOWN CONTINUES. THE STEADY RAIN FROM YESTERDAY HAS BECOME MORE
SHOWERY IN NATURE AND LARGELY CONFINED TO THE COASTLINE THIS
MORNING. HOWEVER...ANOTHER SIGNIFICANT SHORTWAVE TROUGH RIDING THE
JET NORTHWARD WILL BRING ANOTHER ROUND OF STEADY RAIN TO THE NORTH
GULF COAST AND COPPER RIVER BASIN TONIGHT.

AN ANCHORING UPPER LOW OVER THE NORTHWEST BERING IS ROTATING A
TROUGH THROUGH THE EASTERN BERING TOWARD THE MAINLAND. THIS TROUGH
IS GENERATING ENHANCED AREAS OF SHOWERS ACROSS THE DELTA AND
BRISTOL BAY AREAS. COLDER AIR IN PLACE OVER THE BERING SEA IS
PRODUCING WIDESPREAD SNOW SHOWER ACTIVITY...ESPECIALLY OVER THE
FAR WESTERN ALEUTIANS WHERE LOW-LEVEL LAPSE RATES ARE THE
STEEPEST. THIS COLDER AIR IS STARTING TO WORK BACK INTO THE
SOUTHERN MAINLAND...AND WILL BE THE CATALYST FOR THE DOMINANT
PRECIP TYPE CHANGING BACK OVER TO SNOW BY TOMORROW FOR MUCH OF THE
SOUTHWEST AS WELL AS SOUTHCENTRAL MAINLAND.

&&

.MODEL DISCUSSION...

THE MODELS ARE IN VERY GOOD SYNOPTIC AGREEMENT...AND DEPICT THE
FAIRLY RAPID BREAKDOWN OF THE WESTERN NORTH AMERICAN RIDGE
BEGINNING TONIGHT AND CONTINUING TOMORROW. THIS WILL ALLOW MEAN
FLOW ACROSS MAINLAND ALASKA AND THE GULF TO BECOME SOUTHWESTERLY
AND MORE PROGRESSIVE OVER THE NEXT FEW DAYS. THE LARGEST AREA OF
MODEL DISAGREEMENT REVOLVES AROUND A LOW THAT WILL LIFT OUT OF THE
BASE OF THE LONGWAVE TROUGH AND IMPACT THE SOUTHERN AND EASTERN
GULF TUESDAY AND WEDNESDAY. AT THIS POINT...THE NAM IS A CLEAR
OUTLIER (FASTER) THAN ANY OF THE GLOBAL MODELS...WITH THE
GFS/GEMGLB MUCH SLOWER AND THE ECMWF A MIDDLE GROUND SOLUTION.
WILL INCORPORATE A MIX OF THE GFS/EC FOR FORECAST UPDATES
TODAY...THOUGH IMPACTS TO THE FORECAST AREA WILL BE FAIRLY
MINIMAL...MAINLY IMPACTING PRECIP TIMING ACROSS KODIAK ISLAND.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (DAYS 1 AND 2)...
THE SOUTHERN STREAM OF MOISTURE ACROSS THE GULF AND SOUTHCENTRAL
CONTINUES OVERNIGHT...HOWEVER A SLOW SHIFT IN THE CURRENT WEATHER
REGIME IS UNDERWAY AS A CLOSED LOW OVER KAMCHATKA BEGINS TO BREAK
DOWN THE AMPLIFIED RIDGE OVER THE EASTERN PACIFIC. A COLD FRONT
WILL MOVE FROM THE ALASKA PENINSULA INTO THE WESTERN GULF TONIGHT.
LOW LEVEL WINDS SHIFT OUT OF THE WEST BEHIND THE FRONT ALLOWING A
DRY COOL AIR MASS TO FILTER ACROSS THE SOUTHERN MAINLAND. THE MAIN
CHALLENGE WITH THE FRONT IS THE PRECIPITATION ALONG THE BOUNDARY
AS IT MOVES THROUGH SOUTHCENTRAL. TEMPERATURES ARE GENERALLY WARM
THROUGH THE COLUMN OF AIR ABOVE THE SURFACE AND THEREFORE WILL BE
SLOW TO MODERATE. THIS WOULD KEEP PRECIPITATION MOSTLY RAIN WITH A
CHANGE OVER TO SNOW FOR A SHORT DURATION BEFORE DRYING BEHIND THE
FRONT. THERE IS A POSSIBILITY THAT PRECIPITATION CHANGES OVER TO
SNOW SOONER THAN ANTICIPATED IF THE PARENT LOW ACCELERATES FASTER
TO EAST THAN CURRENTLY PROJECTED.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (DAYS 1 AND 2)...
A MARGINALLY COOLER AIR MASS WILL MOVE INTO SOUTHWEST ALASKA TONIGHT
THROUGH MONDAY AS AN UPPER TROUGH OVER THE CENTRAL TO EASTERN BERING
SEA LIFTS NORTHWARD ACROSS THE REGION. TEMPERATURES WILL STILL BE
ABOVE NORMAL FOR THIS TIME OF YEAR...BUT WILL FALL BELOW FREEZING
CHANGING ANY REMAINING PRECIP TO SNOW. PRECIP HAS BEEN FAIRLY
WIDELY SCATTERED ACROSS THE SOUTHWEST MAINLAND TODAY AND OF A
SHOWERY NATURE. EXPECT PRECIP TO BECOME A LITTLE MORE WIDESPREAD
TONIGHT...BUT WITH MULTIPLE WAVES MOVING THROUGH THE TROUGH THE
LIFT IS NOT FOCUSED OR REALLY STRONG IN ANY ONE PLACE. THUS QPF
AMOUNTS WILL BE PRETTY LIGHT AND PRECIP WILL REMAIN ON THE SHOWERY
SIDE. ANOTHER UPPER TROUGH WILL CLIP THE WEST COAST ON TUESDAY...
LIKELY BRINGING SNOW SHOWERS TO THE KUSKOKWIM DELTA. OTHERWISE...DRY
CONDITIONS WILL DOMINATE IN THE WAKE OF THE CURRENT TROUGH.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (DAYS 1 AND 2)...
AS ONE TROUGH OVER THE CENTRAL TO EASTERN BERING LIFTS NORTHWARD
TONIGHT SHOWERS WILL TAPER OFF FROM THE PRIBILOF ISLANDS TO EASTERN
ALEUTIANS AND SOUTHERN ALASKA PENINSULA. A MORE IMPRESSIVE LOOKING
TROUGH CROSSING THE WESTERN ALEUTIANS WILL LIFT NORTH AND EASTWARD
ACROSS THE CENTRAL BERING AS IT ROTATES AROUND THE EAST SIDE OF A
COLD UPPER LEVEL LOW. MORE UNSTABLE CONDITIONS UNDER THIS TROUGH
WILL PRODUCE LOCALLY HEAVY SHOWERS AND GUSTY WINDS. THE BULK OF
PRECIP WILL REMAIN OVER THE BERING SEA...BUT THE WESTERN TO CENTRAL
ALEUTIANS ON UP TO THE PRIBILOF ISLANDS SHOULD SEE SOME SHOWERS
RIGHT ON THE PERIPHERY OF THE TROUGH. ALTHOUGH SURFACE TEMPERATURES
MAY RISE ABOVE FREEZING ALONG THE ALEUTIANS...THE AIR MASS WILL
REMAIN COLD ENOUGH TO SUPPORT ALL SNOW. THIS WILL CHANGE ON TUESDAY
AS A STORM SYSTEM MOVES UP FROM THE PACIFIC INTO THE WESTERN
BERING SEA...BRINGING STEADY PRECIP AND WARMING TEMPERATURES TO
THE WESTERN ALEUTIANS AND WESTERN BERING.

&&

.LONG TERM FORECAST (DAYS 3 THROUGH 7)...LITTLE IN THE WAY OF
SIGNIFICANT IMPACTS ARE EXPECTED FROM SENSIBLE WEATHER TUESDAY
THROUGH NEXT WEEKEND AS MODELS SUGGEST A TRANSITORY PATTERN OF
FAIRLY WEAK TROUGHS AND MODERATE TO STRONG RIDGES MOVING FROM THE
BERING AND ACROSS THE MAINLAND. MODELS CONTINUE TO TEASE THE
FORMATION OF A MORE HIGHLY AMPLIFIED RIDGE BUILDING OVER THE
EASTERN BERING AND WESTERN ALASKA WITH A STRONG JET STREAK MOVING
OVER TOP OF THIS RIDGE. SUCH AN EVENT WOULD GENERATE IMPACTS IN
THE FORM OF STRONG NORTHERLY OUTFLOW/OFFSHORE WINDS ACROSS
SOUTHCENTRAL. HOWEVER...UNTIL MODELS SHOW CONSISTENCY OVER SEVERAL
PERIODS WILL NOT PUSH THE FORECAST TOO HEAVY IN THAT DIRECTION.

TEMPERATURES WILL MODERATE SOMEWHAT TOWARD LATE FEBRUARY NORMALS
ESPECIALLY AT NIGHT IN THE INTERIOR VALLEYS OF SOUTHCENTRAL
ALASKA...AND WILL REMAIN ABOVE AVERAGE WITH ONSHORE FLOW OVER
SOUTHWEST ALASKA. DAYTIME HIGHS MOST LOCATIONS WILL CONTINUE TO
AVERAGE SEVERAL DEGREES ABOVE AVERAGE ACROSS SOUTHERN ALASKA.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...GALES 185
         HEAVY FREEZING SPRAY 185
FIRE WEATHER...NONE.

&&

$$

SYNOPSIS AND MODEL DISCUSSION...AD
SOUTHCENTRAL ALASKA...KH
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...SEB
LONG TERM...CC


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