Heavy Freezing Spray Warning 49, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
448 AM AKDT THU MAR 24 2016

.ANALYSIS AND UPPER LEVELS...

A LOOK AT THE MORNING SATELLITE IMAGERY REVEALS THAT THE LARGE
LOW THAT WAS IN THE GULF OF ALASKA YESTERDAY HAS BEEN WEAKENED
SIGNIFICANTLY BY THE STATIONARY HIGH PRESSURE OVER THE SOUTHERN
MAINLAND AND DEFLECTED TOWARD THE SOUTHERN PANHANDLE. MID AND HIGH
CLOUDS ARE ROTATING CLOCKWISE THROUGH MUCH OF THE SOUTHERN
MAINLAND AROUND THE UPPER HIGH THAT IS CENTERED NEAR THE COPPER
RIVER BASIN...THOUGH EVEN THIS CLOUDINESS WAS NOT ENOUGH TO
PREVENT ANOTHER VERY SPRING-LIKE DAY ACROSS MOST OF THE AREA. WEST
OF THE ALASKA RANGE...THIS MOISTURE IS GETTING ENHANCED BY A WEAK
UPPER LOW SITTING IN THE EASTERN BERING SEA...WITH WIDESPREAD
SHOWER ACTIVITY BREAKING OUT ACROSS THE BRISTOL BAY ZONE AND
MOVING TOWARD THE KUSKOKWIM DELTA. SHALLOW NORTHEASTERLY FLOW
CONTINUES ACROSS THE DELTA...WHICH IS RE-SUPPLYING COLD AIR AT THE
LOWEST LEVELS OF THE ATMOSPHERE AND CREATING A FAVORABLE SCENARIO
FOR SOME FREEZING RAIN. THIS WILL CONTINUE THROUGH MIDDAY BEFORE
WINDS TURN MORE SOUTHERLY AND TEMPERATURES QUICKLY WARM.

ELSEWHERE...SEASONABLY UNSETTLED WEATHER CONTINUES ACROSS THE
ALEUTIANS AS ANOTHER STORM SYSTEM SOUTH OF THE ALEUTIAN ISLANDS
RIDES ALONG THE ZONAL JET STREAM. THIS PLACES MOST OF THE
BERING/ALEUTIAN REGION IN THE MIDST OF A BROAD LONGWAVE
TROUGH...WITH MULTIPLE EMBEDDED SHORTWAVE TROUGHS/LOWS BUT NO
SPECIFICALLY STRONG OR INCLEMENT FEATURES TO NOTE.

&&

.MODEL DISCUSSION...

THE MODELS ARE ONCE AGAIN IN VERY GOOD OVERALL AGREEMENT. THE GFS
HANDLES THE POSITIONING OF THE WEAK SURFACE FEATURES IN THE
EASTERN DOMAIN A BIT DIFFERENTLY THAN THE OTHER MODELS...SO A
NAM/ECMWF BLEND WAS PRIMARILY USED TODAY. HOWEVER...NO MODELS WERE
WHOLESALE TOSSED OUT DURING THE SHORT OR MEDIUM RANGE FORECAST
PERIODS.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (DAYS 1 AND 2)...

HIGH PRESSURE OVER SOUTH CENTRAL ALASKA WILL MOVE EAST TODAY AND
INTO CANADA TONIGHT. THIS WILL OPEN UP MOISTER SOUTHERLY FLOW
ALOFT. AN UPPER LEVEL LOW SOUTH OF THE CENTRAL ALEUTIANS WILL
MOVE TO KODIAK ISLAND FRIDAY AFTERNOON AND INTO THE CENTRAL GULF
BY LATE FRIDAY NIGHT. A FRONT ASSOCIATED WITH THIS LOW WILL MOVE
TO THE GULF COAST ON FRIDAY. SOME OF THE MOISTURE WITH THIS FRONT
WILL WORK ITS WAY INLAND WITH "LIKELY" POPS CANVASSING MUCH OF THE
AREA LATE FRIDAY AND FRIDAY NIGHT. PRECIPITATION TYPE WILL BE A
FORECASTING CHALLENGE. THE USUAL FORECASTING "RULES OF THUMB" FOR
PRECIPITATION TYPE (SUCH AS 1000-850 MB THICKNESSES AND 850 MB
TEMPERATURES) WILL BE NEAR THE RAIN/SNOW THRESHOLDS.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (DAYS 1 AND 2)...

THE UPPER TROUGH LIFTING NORTHWARD FROM BRISTOL BAY WILL PROVIDE
THE PRIMARY FOCUS FOR PRECIPITATION THROUGH THIS EVENING...WITH
THE MAIN FORECAST CONCERN BEING PRECIPITATION TYPE OVER THE
KUSKOKWIM DELTA AS A SHALLOW LAYER OF COLD AIR REMAINS IN PLACE
NEAR THE SURFACE AND PROMOTES FREEZING RAIN THROUGH EARLY
AFTERNOON. THE BEST POTENTIAL FOR ACCUMULATING ICE LOOKS TO BE
ALONG THE COAST WEST OF BETHEL WHERE SURFACE TEMPERATURES WILL
TAKE THE LONGEST TO CLIMB ABOVE FREEZING. PRECIPITATION WILL
TRANSITION TO A MIX OF RAIN AND SNOW AND BECOME MORE SHOWERY IN
NATURE BY LATE TONIGHT AS COLD ADVECTION BEHIND THE DEPARTING
TROUGH ERODES THE WARM LAYER ALOFT AND INCREASES INSTABILITY
ACROSS THE REGION. SHOWERS WILL CONTINUE THROUGH FRIDAY NIGHT BUT
GRADUALLY DIMINISH IN COVERAGE BENEATH A DIFFUSE LONGWAVE TROUGH
WITH SEVERAL EMBEDDED DISTURBANCES.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (DAYS 1 AND 2)...

THE TROUGH LIFTING OVER THE SOUTHWEST MAINLAND WILL ALSO BRING
STEADIER PRECIPITATION TO THE PRIBILOF ISLANDS THROUGH THIS
AFTERNOON BEFORE DEPARTING TO THE NORTH. OTHERWISE...THE SPRAWLING
LONGWAVE WILL MAINTAIN BENIGN SHOWERY CONDITIONS OVER THE EASTERN
BERING THROUGH FRIDAY AFTERNOON BEFORE RIDGING BUILDS IN FROM THE
WEST AND PROMOTES A BRIEF DRYING TREND. MORE ACTIVE WEATHER WILL
RETURN TO THE BERING LATE FRIDAY NIGHT AS A STRONG FRONT MOVES
OVER THE WESTERN AND CENTRAL ALEUTIANS ACCOMPANIED BY GALE FORCE
WINDS AND A MIX OF RAIN AND SNOW.

&&

.LONG TERM FORECAST (DAYS 3 THROUGH 7)...

FOLLOWING A BROAD TROUGH SWINGING EAST THROUGH MAINLAND ALASKA AND
THE GULF ON SATURDAY...THE SUNDAY THROUGH WEDNESDAY TIME PERIOD
CONTINUES TO LOOK VERY FAVORABLE FOR THE ESTABLISHMENT OF A LARGE
BLOCKING PATTERN CONSISTING OF A STRONG HIGH AMPLITUDE UPPER LEVEL
RIDGE CENTERED AROUND 140W AND A LARGE UPPER LOW CENTERED AROUND
170W. THE RESULTING DEEP SOUTHERLY FLOW BETWEEN THESE TWO FEATURES
WILL PUMP ABUNDANT QUANTITIES OF WARM MOIST AIR UP INTO SOUTHWEST
AND SOUTHCENTRAL ALASKA THROUGH MIDWEEK. THE FIRST OF THE
SOUTHERLY FRONTAL WAVES WILL REACH THE ALASKA PENINSULA
SUNDAY...KODIAK ISLAND SUNDAY AFTERNOON AND THEN CONTINUE TO LIFT
NORTH ACROSS SOUTHERN ALASKA SUNDAY NIGHT WITH AN ADDITIONAL WAVE
CATCHING UP MONDAY. THE NEXT FRONTAL WAVE WILL ARRIVE TUESDAY
MORNING WITH AN ADDITIONAL MOISTURE LADEN SYSTEM RIPPLING NORTH ON
WEDNESDAY.

THE NET RESULT WILL BE WIDESPREAD AND HEAVY AT TIMES RAIN...
TEMPERATURES WELL ABOVE SEASONAL NORMALS AND SNOW LEVELS AS HIGH
AS 5000 FT DURING THE WARMEST PORTIONS OF THE SYSTEMS. PRECIPITATION
TOTALS FROM SUNDAY THROUGH WEDNESDAY CURRENTLY LOOK LIKE THEY WILL
RANGE FROM AROUND AN INCH IN THE ANCHORAGE BOWL AND MATANUSKA
VALLEY TO 2 TO 3 INCHES IN THE SUSITNA VALLEY WITH UP TO 5 INCHES
OR MORE POSSIBLE ALONG SOUTH TO SOUTHEAST FACING COASTAL REGIONS.
FOR THE BRISTOL BAY AREA AND LOWER KUSKOKWIM VALLEY PRECIPITATION
TOTALS CURRENTLY LOOK MORE MODERATE WITH 4 DAY TOTALS BETWEEN 1
AND 2 INCHES...HOWEVER SMALL EAST TO WEST CHANGES IN THE POSITION
OF THE STRONG SOUTHERLY JET BRINGING ALL THE MOISTURE NORTH COULD
EASILY SHIFT THE FOCUS OF THE HEAVIEST RAINFALL...EITHER
INCREASING OR DECREASING THESE AMOUNTS SUBSTANTIALLY.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...FREEZING RAIN ADVISORY 155.
MARINE...HEAVY FREEZING SPRAY WARNING 185 412.

&&

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