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000
FXAK68 PAFC 161246
AFDAFC

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
500 AM AKDT THU MAY 16 2013

.ANALYSIS AND UPPER LEVELS...
A LARGE COLD CORE UPPER LEVEL LOW ENCOMPASSES THE WHOLE OF ALASKA
WITH THE MAIN BELT OF WESTERLIES WILL TO THE SOUTH IN THE
MID-LATITUDES. THE MAIN CYCLONIC CIRCULATION HAS MULTIPLE CENTERS
OVER THE CHUKCHI SEA WHILE A BAGGIER UPPER LEVEL TROUGH HAS BEEN
ABSORBED INTO THE LONG-WAVE. AN AREA OF MOISTURE IS SURGING OVER THE
WESTERN GULF OF ALASKA AHEAD OF A SHORTWAVE TROUGH WITHING THE
SOUTHWESTERLY FLOW. FURTHER UPSTREAM TO THE NORTHWEST...A STRONG
SHORTWAVE IS EMBEDDED IN THE FLOW STRETCHING FROM THE SEWARD
PENINSULA THROUGH EASTERN RUSSIA IS DIGGING TOWARD THE WEST COAST.
MOIST SOUTHWEST FLOW ALOFT HAS SPREAD CLOUD COVER OVER MOST OF THE
SOUTHCENTRAL AS A FRONTAL BOUNDARY APPROACHES THE NORTH GULF COAST.
HIGH PRESSURE OVER THE BERING SEA CONTINUES TO PROMOTE LOW CLOUDS AND
STRATUS FROM THE ALEUTIANS THROUGH THE WEST COAST.

.MODEL DISCUSSION...
GUIDANCE REMAINS IN GOOD SYNOPTIC AGREEMENT WITH THE NATURE OF THE
UPPER TROUGH SHIFTING OVER THE SOUTHWEST MAINLAND THROUGH THE KENAI
PENINSULA. THE TREND ACROSS ALL GUIDANCE HAS BEEN FROM A CONSOLIDATED
NEGATIVE TILT TROUGH TO A STRETCHING/SPLITTING OF THE ENERGY TO A
CLOSED CIRCULATION DEVELOPING IN THE GULF. SYNOPTICALLY THIS MAKES
SENSE GIVEN THE MAIN UPPER LOW CENTER WILL STAY CENTERED NORTH OF THE
BERING STRAIT WHILE THE JET STREAK ON THE EQUATORWARD SIDE OF THE
SYSTEM DIGS TO THE SOUTHEAST. THE EXACT NATURE OF THE TROUGH
PROGRESSION AND VORTICITY REDEVELOPMENT TO THE WEST AS THE JET DIGS
WILL HAVE LARGE IMPLICATIONS ON THE PRECIPITATION FORECAST AND THE
AMOUNT OF LIFT AVAILABLE AS THE TROUGH INTERACTS WITH THE MOISTURE
CURRENTLY MOVING OVER THE SOUTHCENTRAL. THE SECOND FACET TO THE
FORECAST IS THE PROGRESSION OF THE ARCTIC AIR FROM THE NORTHWEST.
CURRENTLY THE NAM IS THE OUTLIER IN TERMS OF QUICKLY MOVING THE
ARCTIC AIR THROUGH THE ALASKA RANGE...THIS IS LIKELY ASSOCIATED WITH
KEEPING THE INITIAL UPPER TROUGH INTACT LONGER BEFORE THE NEW
VORTICITY MAXIMUM/CLOSED LOW DEVELOPS IN THE WESTERN GULF. THE MAIN
THEME TO TAKE FROM THE MODEL GUIDANCE REGARDLESS OF EXACT DETAILS IS
A RESPECTABLE SLUG OF ARCTIC AIR COMBINED WITH A NEGATIVE TILT TROUGH
TO THE WEST OF THE SOUTHCENTRAL BODES VERY WELL FOR SNOWFALL ACROSS
THE AREA.

.SHORT TERM FORECAST...
SOUTHCENTRAL ALASKA...WARM FRONT WILL CONTINUE TO PROGRESS NORTHWARD
IN THE GULF TODAY SPREADING RAIN OVER THE NORTH GULF COAST. HIGHER
ELEVATIONS OF THE CHUGACH COULD SEE A MIX OF RAIN AND SNOW IN THE
MORNING BEFORE CHANGING OVER TO RAIN. RAIN WILL GRADUALLY SPREAD UP
INTO THE SUSITNA VALLEY TODAY WHILE AREAS FROM THE KENAI PENINSULA
THROUGH THE MATANUSKA VALLEY WILL REMAIN MOSTLY DRY AND DOWNSLOPED AS
SOUTHEASTERLY FLOW CROSS-BARRIER REMAINS. LATE TONIGHT INTO FRIDAY
MORNING AS THE SURFACE LOW PRESSURE AND FRONTAL BOUNDARY PUSH INLAND
STEADY RAIN WILL SPREAD OVER THE PREVIOUSLY DOWNSLOPED AREAS. DURING
THE DAY ON FRIDAY THE UNSEASONABLE ARCTIC AIR WILL BEGIN TO FILTER
THROUGH THE ALASKA RANGE. PRECIPITATION WILL LIKELY STAY AS RAIN
THROUGH THE AFTERNOON THEN BEGIN TO MIX WITH AND CHANGE OVER TO SNOW
DURING THE EARLY EVENING HOURS FOR THE COOK INLET REGION WITH A MIX
LINGERING IN THE COPPER RIVER BASIN AND THE NORTHERN GULF COAST.
PRECIPITATION LOOKS TO CONTINUE THROUGH SATURDAY MORNING BEFORE
TAPERING OFF.

SOUTHWEST ALASKA...A TALE OF TWO SYNOPTIC REGIMES OVER THE SOUTHWEST
TODAY. EASTERN BRISTOL BAY AND ALONG THE ALASKA/ALEUTIAN RANGE
REMAINS UNDER THE INFLUENCE OF THE WESTERN GULF LOW PRESSURE BRINGING
THE CHANCE FOR LIGHT RAIN THIS AFTERNOON. FURTHER OUT WEST A COLD
FRONT IS APPROACHING THE KUSKOKWIM DELTA AND WILL BRING VERY LIGHT
SNOW TODAY. THE FRONT WILL MOVE INTO THE BRISTOL BAY AREA TONIGHT AND
BRING SCATTERED SNOW SHOWERS AND A PERIOD OF GUSTY NORTHWESTERLY
FLOW.

BERING SEA AND ALEUTIANS...A COLD FRONT WILL DROP SOUTHWARD OVER THE
BERING TODAY BRINGING A PERIOD OF LIGHT SNOW FIRST TO THE PRIBILOF
ISLANDS THEN TO THE EASTERN ALEUTIANS AND ALASKA PENINSULA TONIGHT.
MOSTLY CLOUDY CONDITIONS WILL PREVAIL ALONG THE BERING SIDE IN
NORTHERLY FLOW.

.LONG TERM FORECAST...
AFTER THE UNSEASONABLY COLD SYSTEM A BIT OF A PATTERN CHANGE IS IN
ORDER. RIDGING LOOKS TO BUILD OVER THE MAINLAND WITH A STRONG UPPER
LOW MOVING INTO THE BERING SEA FROM THE SOUTHWEST. PRECIPITATION WILL
BE PREDOMINATELY CONFINED TO THE WEST WITH MODERATING TEMPERATURES IN
THE EAST.



.AER/ALU...WATCH/WARNING SUMMARY
PUBLIC...NONE
MARINE...HEAVY FREEZING SPRAY...180
         GALE 120 125 130 132 150
FIRE WEATHER...NONE

MTL MAY 13