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000
FXAK68 PAFC 060145
AFDAFC

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
445 PM AKST THU MAR 5 2015

.ANALYSIS AND UPPER LEVELS...
PROGRESSIVE UPPER LEVEL FLOW FROM THE BERING SEA TO MAINLAND ALASKA
WILL BRING ACTIVE WEATHER TO THE REGION OVER THE NEXT COUPLE DAYS.
SATELLITE IMAGERY SHOWS TWO MAIN FEATURES. FIRST IS A LOW AMPLITUDE
SHORT-WAVE TROUGH CROSSING WESTERN ALASKA. VERTICAL MOTION IS
WEAK...BUT LOW LEVELS OUT AHEAD OF IT ARE SATURATED SO WILL SEE
SOME MINOR SNOW ACCUMULATION AS THIS CONTINUES EASTWARD. MID AND
UPPER LEVELS DRY OUT RIGHT BEHIND THE TROUGH UNDER RIDGING ALOFT.
THIS HAS LED TO FOG FORMATION (LOCALLY DENSE) ACROSS THE KUSKOKWIM
DELTA AND PORTIONS OF BRISTOL BAY.

A MUCH MORE SIGNIFICANT STORM SYSTEM (AND THE FOCUS OF THE SHORT-
TERM FORECAST FOR THE NEXT FEW DAYS) HAS MOVED INTO THE WESTERN
BERING SEA. A 150 KT JET STREAM EXTENDS FROM THE NORTHWEST PACIFIC
INTO THE WESTERN BERING SEA. AN UPPER LEVEL LOW AND SHORT-WAVE
TROUGH ARE LIFTING NORTHEASTWARD...BUT ANOTHER SHORT-WAVE JUST
UPSTREAM WILL HELP MAINTAIN A SIGNIFICANT TROUGH AS IT PROGRESS
EASTWARD ACROSS THE BERING SEA. AT THE SURFACE A MODERATE STRENGTH
LOW IS TRACKING NORTHEASTWARD ACROSS THE WESTERN BERING WHILE A
FRONTAL SYSTEM SPREADS EASTWARD OUT AHEAD OF IT. THIS SYSTEM
CONTAINS BOTH STRONG DYNAMICAL LIFT AND DEEP MOISTURE TRANSPORT...
WHICH IS EVIDENT IN THE VERY LARGE AREA OF GREATER THAN 200
PERCENT OF NORMAL PRECIPITABLE WATER VALUES (AS DERIVED FROM
SATELLITE IMAGERY).

&&

.MODEL DISCUSSION...
MODELS ARE IN VERY GOOD AGREEMENT WITH ALL MAJOR FEATURES OVER THE
NEXT FEW DAYS. THUS...FORECAST CONFIDENCE IS GENERALLY HIGH. THERE
ARE HOWEVER A FEW VERY CHALLENGING ASPECTS OF THE FORECAST. FOREMOST
AMONG THESE IS THE STRONG LOW LEVEL SOUTHERLY FLOW ASSOCIATED WITH
THE BERING SEA STORM SYSTEM. AS THESE WINDS MOVE ONSHORE ALONG THE
SOUTHWEST AND SOUTHCENTRAL COASTLINES THEY WILL ADVECT A SHALLOW
LAYER OF WARMER AIR NORTHWARD. TEMPERATURES ALOFT AND PRECIP
INTENSITY WILL FAVOR SNOW IN MANY LOCATIONS DESPITE THE WARMER AIR
AT THE SURFACE...BUT THESE WARMER SURFACE TEMPERATURES COULD REALLY
LIMIT THE ABILITY OF SNOW TO ACCUMULATE. BECAUSE OF THIS WILL FORECAST
NOTABLY LOWER SNOW ACCUMULATION FOR COASTAL BRISTOL BAY...COOK INLET
REGION...AND THE GULF/PRINCE WILLIAM SOUND COASTS COMPARED WITH
INLAND AREAS AND HIGHER ELEVATIONS. THE LOCATION OF MOST UNCERTAINTY
IS VALDEZ WHICH IS CAPABLE OF SIGNIFICANT WET SNOW ACCUMULATION
WITH TEMPERATURES RIGHT AT FREEZING. MOS GUIDANCE INDICATES SNOW
WILL MIX WITH RAIN IN VALDEZ...BUT WILL LEAN TOWARD MORE SNOW THAN
RAIN DUE TO THE EXPECTED HEAVY NATURE OF THE PRECIP. IN
SUMMARY...THERE IS HIGH CONFIDENCE IN THIS EVENT OVERALL WITH
LOWER CONFIDENCE IN DETAILS OF TEMPERATURES/PRECIP-TYPE/SNOW
ACCUMULATION ALONG THE COAST.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (THROUGH SATURDAY
AFTERNOON)...

TWO SNOW EVENTS ARE THE HIGHLIGHTS IN THE SHORT TERM FORECAST
WITH THE FIRST OCCURRING TONIGHT AND THE SECOND QUICKLY FOLLOWING
LATE FRIDAY INTO SATURDAY BRINGING MUCH HIGHER SNOW ACCUMULATIONS
AREA-WIDE.

FOR TONIGHT...THE UPPER LEVEL TROUGH MOVING A WEAK FRONT ACROSS
THE SOUTHWESTERN MAINLAND THIS AFTERNOON PICKS UP EASTWARD
MOMENTUM AROUND MIDNIGHT SWINGING THROUGH THE SUSITNA VALLEY TO
THE COPPER RIVER BY FRIDAY MORNING. SINCE TEMPERATURES CLIMBED
INTO THE UPPER 30S TO LOWER 40S TODAY...PRECIPITATION STARTS INITIALLY
AS RAIN AROUND COOK INLET AND THEN CHANGES TO ALL SNOW AS THE
COLUMN COOLS OVERNIGHT. SNOW AMOUNTS WILL GENERALLY RANGE BETWEEN
1 TO 3 INCHES WITH HIGHER AMOUNTS ACROSS WESTERN FACING MOUNTAIN
SLOPES AND LITTLE TO NO ACCUMULATION ACROSS THE KENAI...KODIAK
ISLAND AND PRINCE WILLIAM SOUND. TEMPERATURES WILL STAY WARM
ENOUGH FOR ALL RAIN ACROSS THE NORTHEASTERN GULF.

THE SECOND MORE ORGANIZED SYSTEM MOVES INTO THE AREA LATE ON
FRIDAY. THE SURFACE LOW ASSOCIATED WITH THIS SYSTEM MOVES INTO
THE WESTERN MAINLAND NEAR THE SEWARD PENINSULA PUSHING A FRONTAL
BOUNDARY TO THE WESTERN ALASKA RANGE LATE FRIDAY. THE AIR MASS
COOLS ENOUGH FOR ALL SNOW TO DEVELOP ALONG THE BOUNDARY FRIDAY
EVENING. A 850 MB LEVEL SOUTHERLY JET BETWEEN 40 TO 50 KT
ACCELERATES UP COOK INLET AND ACROSS THE ANCHORAGE BOWL TO THE
SUSITNA VALLEY AND ANOTHER LOW-LEVEL WIND MAX WILL DEVELOP IN THE
COPPER RIVER VALLEY. AT THE SAME TIME A STRONG UPPER LEVEL WAVE
MOVES OVERHEAD ENHANCING LIFT ALONG THE FRONT. THE COMBINATION OF
A STRONG LOW LEVEL JET AND UPPER LEVEL SUPPORT WILL BRING HEAVY
SNOW AT TIMES ACROSS THE AREA AND THEREFORE A WINTER STORM WARNING
FOR HEAVY SNOW WAS ISSUED FOR THE SUSITNA VALLEY AND SNOW
ADVISORIES WERE ISSUED FOR THE ANCHORAGE BOWL...MATANUSKA AND
COPPER RIVER VALLEYS. THIS EVENT IS EXPECTED TO WIND DOWN SATURDAY
AFTERNOON AS THE MAIN UPPER LEVEL SUPPORT MOVES EAST.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (DAYS 1 AND 2)...
AREAS OF FOG WILL BE A CONCERN AGAIN TONIGHT UNDER THE FLAT RIDGE
SLIDING QUICKLY ACROSS SOUTHWEST ALASKA AHEAD OF THE INCOMING
FRONTAL SYSTEM. WITH RATHER STRONG WESTERLY 500 MB AND SSW 850 MB
JET SUPPORT ALL LOOKS GOOD FOR SOME RATHER SIGNIFICANT QPF AMOUNTS
AS THE FRONTAL SYSTEM PUSHES INLAND FRIDAY AND FRIDAY NIGHT.

BLOWING SNOW WILL BE THE PRIMARY CONCERN FOR THE KUSKOKWIM DELTA
AS STRONG SOUTH THEN SOUTHWEST WINDS SHOULD MAKE IT A WAYS
INLAND...HOWEVER SNOWFALL AMOUNTS WILL BE A BIT MORE MODEST WITH
THE FAST MOVING NATURE OF THE SYSTEM. FOR THE LOWER KUSKOKWIM
VALLEY AND NORTHERN BRISTOL BAY AREAS...SNOW AMOUNTS WILL BE
HIGHER ESPECIALLY IN AREAS UPSLOPE AREAS FAVORED BY THE SSW LOWER
LEVEL WINDS. BLOWING SNOW WILL ALSO BE A CONCERN IN THE NORTHERN
INLAND PORTIONS OF THE BRISTOL BAY ZONE WHERE S TO SW WINDS WILL
FUNNEL AND TEMPERATURES WILL REMAIN COOLER. FROM DILLINGHAM SOUTH
PRECIPITATION MAY BEGIN WITH A COUPLE INCHES OF HEAVY WET SNOW BUT
SHOULD CHANGE OVER TO A MIX OF RAIN AND SNOW FRIDAY EVENING WITH ALL
RAIN FURTHER SOUTH.

A MUCH COOLER AND SHOWERY AIR MASS WILL BEGIN PUSHING INTO THE
AREA ON SATURDAY.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (DAYS 1 AND 2)...
A RATHER STRONG FRONTAL SYSTEM WILL CROSS THE AREA FROM WEST TO
EAST BRINGING WIDESPREAD GALES TO THE ALEUTIANS AND BERING AS
WELL AS AN AREA OF STORMS TO WEST AND CENTRAL BERING OFFSHORE
WATERS. A MUCH COLDER AIR MASS WILL PUSH OVER THE BERING FRIDAY
NIGHT AND SATURDAY WITH SURFACE HIGH PRESSURE BUILDING.

&&

.LONG TERM FORECAST (DAYS 4 THROUGH 7)...
THE UPPER LEVEL WAVE THAT IS EXPECTED TO BRING SIGNIFICANT
SNOWFALL TO MUCH OF THE SOUTHERN MAINLAND ON SATURDAY WILL
CONTINUE TO BRING WIDESPREAD SNOWFALL TO AREAS EAST OF ANCHORAGE
ON SATURDAY NIGHT AND SUNDAY MORNING. SHOWERS WILL STILL REMAIN
IN THE AREA INTO SUNDAY EVENING AS A BROAD UPPER LEVEL TROUGH
SLOWLY MOVES THROUGH THE MAINLAND...BUT THESE WILL NOT BE NEAR AS
WIDESPREAD OR SIGNIFICANT AS THE SNOW EXPECTED ON SATURDAY.

ON MONDAY...THERE REMAINS VERY GOOD AGREEMENT THAT COLD AIR WILL
SPREAD THROUGHOUT THE SOUTHERN MAINLAND WITH CLEARING SKIES THAT
WILL CONTINUE AT LEAST INTO THE MIDDLE OF NEXT WEEK. PRECIPITATION
IN THIS PATTERN WILL BE CONFINED MAINLY TO THE GULF COAST AS A
SERIES OF WEAK LOWS MOVE THROUGH THE GULF...WHICH WILL ALSO HELP
CREATE GUSTY OUTFLOW WINDS THAT WILL PERSIST THROUGH THE MIDDLE OF
NEXT WEEK. OVER THE ALEUTIANS...THERE IS INCREASING CONFIDENCE
THAT A NORTH PACIFIC LOW WILL MOVE INTO THE WESTERN ALEUTIANS ON
SUNDAY EVENING...AND SLOWLY TRACK EASTWARD BEFORE RETREATING BACK
TO THE NORTH PACIFIC ON WEDNESDAY. THIS WILL KEEP THE BULK OF THE
PRECIPITATION AND WIND FROM THIS SYSTEM CONFINED TO THE ALEUTIAN
ISLANDS...WITH SHOWERS FOR THE REMAINDER OF THE BERING SEA AND
AKPENN.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...HEAVY SNOW WARNING 145
         WINTER WEATHER ADVISORY 101 111 141 152 155 161
MARINE...GALE 165 170 173 174 175 176 177 178 179 180 181 185
         HEAVY FREEZING SPRAY 180 185
FIRE WEATHER...NONE.

&&

$$

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