Ice Storm in Alaska, March 30 to 31, 2016

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
454 PM AKDT WED MAR 30 2016

.ANALYSIS AND UPPER LEVELS...

THE UPPER-LEVEL BLOCKING RIDGE REMAINS IN PLACE OVER THE EASTERN
PACIFIC WITH THE RIDGE AXIS EXTENDING NORTHWESTWARD FROM THE
EASTERN GULF THROUGH THE SEWARD PENINSULA. UNDERCUTTING THE RIDGE
TO THE WEST IS A VERTICALLY STACKED NORTH PACIFIC LOW CENTERED
SOUTH OF THE EASTERN ALEUTIANS. THESE TWO FEATURES ARE CREATING A
HIGHLY AMPLIFIED WEATHER PATTERN FOR SOUTHERN ALASKA WITH STRONG
SOUTHERLY FLOW UPSTREAM OF THE RIDGE AXIS. THIS DEEP LAYER
SOUTHERLY FLOW IS CONTINUING TO ADVECT SUBTROPICAL MOISTURE TO THE
SOUTHERN MAINLAND AROUND THE TOP OF THE RIDGE. AS A DIFFUSE
SURFACE WARM FRONT PUSHES NORTHWARD THROUGH THE INTERIOR BRISTOL
BAY REGION AND UP THE WESTERN GULF COAST...SURFACE TEMPERATURES
CONTINUE TO WARM BEHIND THE FRONT. THIS IS ALLOWING FOR A PRIMARY
PRECIPITATION TYPE OF RAIN EAST OF THE KUSKOKWIM MOUNTAINS AND
OVER THE WESTERN GULF COAST INTO THE SUSITNA VALLEY. MEANWHILE
FURTHER WEST NORTHEASTERLY SURFACE FLOW HOLDS ON THROUGH THE
KUSKOKWIM DELTA...KEEPING ENOUGH COLD AIR SOCKED IN AT THE SURFACE
TO KEEP THE PRECIPITATION THIS AFTERNOON AS SNOW NORTHWEST OF THE
KUSKOKWIM MOUNTAINS. THE AFOREMENTIONED SURFACE WARM FRONT EXTENDS
SOUTHWEST THROUGH THE EASTERN BERING SEA AND CENTRAL ALEUTIANS
BRINGING NORTHERLY GALE FORCE WINDS AND RAIN AND SNOW SHOWERS TO
THE ALEUTIANS AND PRIBILOF ISLANDS.

&&

.MODEL DISCUSSION...

MODELS REMAIN IN GOOD AGREEMENT WITH REGARDS TO SYNOPTIC FEATURES.
RECENT OBS AND MODEL RUNS HAVE INDICATED A SLIGHTLY FASTER
NORTHWESTWARD ADVANCEMENT OF THE WARM FRONT THROUGH THE BERING SEA
THIS AFTERNOON INTO TONIGHT...SO HAVE MADE MINOR CHANGES TO SPEED
UP THE PRECIPITATION AND WINDS AS THE FRONT TRACKS WESTWARD
THROUGH THE ALEUTIANS. MINOR DIFFERENCES ARISE IN THE DEVELOPMENT
AND TRACK OF A TRIPLE POINT LOW TRACKING NORTHWARD OVER THE ALASKA
PENINSULA FRI MORNING HOWEVER THE IMPACTS TO THE FORECAST ARE
MINIMAL. A BLEND OF THE NAM AND GFS WAS USED FOR THE FORECAST IN
THE WEST. IN THE EAST...A NAM SOLUTION WAS PREFERRED AS THE NAM
APPEARS TO HAVE THE BEST HANDLE ON THE PRESSURE GRADIENTS LEADING
TO THE GAP WINDS THROUGH SOUTHCENTRAL...WHICH IS THE PRIMARY
FORECAST CHALLENGE OF THE DAY IN THE EAST DOMAIN.

&&

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

WITH THE FRONTAL SYSTEM CURRENTLY STRETCHED ACROSS THE WEST SIDE
OF COOK INLET AND INTO THE SUSITNA VALLEY...EAST TO SOUTHEASTERLY
GAP WINDS WILL CONTINUE STRONGLY THROUGH THIS EVENING BEFORE
GRADUALLY DIMINISHING OVERNIGHT. THE FRONT WILL CROSS THE
MOUNTAINS AND MOVE INTO SOUTHWEST ALASKA TONIGHT WITH SKIES
CLEARING OUT AS HIGH PRESSURE BUILDS IN ALONG THE NORTHERN GULF
COAST. WHEN AND WHERE FOG WILL FORM OVERNIGHT IS ONE OF THE MAIN
FORECAST CHALLENGES TODAY. THE RECENT RAINFALL...CLEARING SKIES
AND BUILDING UPPER LEVEL RIDGE ALL FAVOR THE FORMATION OF FOG
WHILE THE SOUTHEASTERLY WINDS WILL HELP PREVENT IT OVER ALL BUT
THE NORTHEASTERN GULF COAST WHERE THE SOUTHEAST FLOW BANKS MARINE
AIR UP AGAINST THE MOUNTAINS. THE BIG QUESTION HOWEVER REMAINS IF
THE SOUTHEASTERLY WINDS WILL REMAIN WIDESPREAD ENOUGH THROUGH THE
EARLY MORNING HOURS TO KEEP THINGS ADEQUATELY STIRRED UP AND FOG
PATCHY AND LIMITED UPSLOPE AND WIND SHELTERED AREAS.

THURSDAY CONTINUES TO LOOK LIKE A WARM...DRY AND SUNNY DAY FOR
ALMOST ALL OF SOUTH CENTRAL ALASKA. THE EXCEPTION WILL BE KODIAK
ISLAND WHERE CLOUDS FROM THE NEXT APPROACHING FRONTAL SYSTEM
BEGIN SPREADING NORTH. THE FRONT WILL CROSS KODIAK ISLAND THURSDAY
NIGHT AND CONTINUE NORTH ACROSS THE KENAI PENINSULA EARLY FRIDAY
MORNING AND INTO MAINLAND SOUTHCENTRAL ALASKA DURING THE DAY
FRIDAY.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (DAYS 1 AND 2)...
WARM AND WET CONDITIONS WILL CONTINUE TO PUSH THROUGH THE
SOUTHWEST MAINLAND THIS EVENING AS A WARM FRONT TRAVERSES THE
REGION THROUGH THURSDAY AFTERNOON. RAIN OVER THE BRISTOL BAY AREA
WILL PERSIST THROUGH THURSDAY AFTERNOON...WHEREAS THE SNOW OVER
THE KUSKOKWIM VALLEY AND KUSKOKWIM DELTA WILL GRADUALLY START TO
MIX WITH RAIN LATER TONIGHT. AREAS THAT WILL STAY PREDOMINANTLY
SNOW THROUGH LATE THURSDAY MORNING WILL BE MOSTLY WEST OF
SLEETMUTE AND AREAS FROM BETHEL NORTH AND WEST...WHICH WILL ALSO
SEE THE HIGHEST ACCUMULATION AMOUNTS. ISOLATED POCKETS OF FREEZING
RAIN ARE ALSO EXPECTED THIS EVENING WEST OF SLEETMUTE...AS WARM
AIR CONTINUES TO PUSH NORTHWARD TONIGHT...BRIEFLY OVERRUNNING A
LINGERING COLDER AIRMASS AT THE SURFACE. GUSTY NORTHEASTERLY WINDS
ALONG THE KUSKOKWIM DELTA COAST WILL PERSIST THROUGH LATE
TONIGHT...BEFORE SHIFTING TO THE SOUTHEAST BY THURSDAY MORNING.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (DAYS 1 AND 2)...
NORTH TO NORTHEAST FLOW IS EXPECTED TO CONTINUE OVER MOST OF THE
BERING SEA THROUGH THE SHORT TERM AS THE LARGE RIDGE REMAINS
FIRMLY IN PLACE OVER THE NORTHEASTERN PACIFIC. A WARM FRONT MOVING
THROUGH THE EASTERN AND CENTRAL BERING/ALEUTIANS THIS EVENING
WILL BRING SNOW TO THE PRIBILOF ISLANDS THIS EVENING...AND SPREAD
RAIN WESTWARD ALONG THE EASTERN AND CENTRAL ALEUTIANS THROUGH
TOMORROW MORNING. WITH GALE FORCE GUSTS ALREADY BEING OBSERVED
OVER THE PRIBILOF ISLANDS AND EASTERN ALEUTIANS...AREAS OF
BLOWING SNOW ARE EXPECTED TO DEVELOP OVER THE PRIBILOF ISLANDS
LATER THIS EVENING. WINDS ARE EXPECTED TO DECREASE OVER THE
REGION BY FRIDAY...AS THE GRADIENT WEAKENS OVER MUCH OF THE
REGION.

&&

.LONG TERM FORECAST (DAYS 3 THROUGH 7)...
THE WEEKEND WILL BRING A SLIGHT COOLING TREND TO THE SOUTHERN
MAINLAND AS THE UPPER RIDGE OVERHEAD IS DISPLACED TO THE NORTH AND
EAST...HOWEVER TEMPERATURES WILL STILL REMAIN ABOVE SEASONAL
NORMAL VALUES AND SLOWLY REBOUND THROUGH THE FOLLOWING WEEK AS
BROAD SOUTHERLY FLOW CONTINUES ACROSS THE GULF AND THE RIDGE AXIS
RESIDES JUST NORTH OF THE AREA OVER THE CENTRAL INTERIOR. THE
ONSHORE FLOW WILL ALSO MAINTAIN HIGHER CHANCES FOR PRECIPITATION
ALONG THE GULF COAST...WITH MORE SPORADIC SHOWERY CONDITIONS
INLAND OWING TO DOWNSLOPE FLOW IN THE LEE OF THE COASTAL MOUNTAIN
RANGES. FARTHER WEST...A LONGWAVE TROUGH WILL ENCOMPASS THE BERING
REGION AND KEEP A CHANCE OF PRECIPITATION OVER THE BERING AND
PORTIONS OF THE SOUTHWEST MAINLAND FOR MUCH OF THE NEXT WEEK.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...WSW 152 155.
MARINE...GALES 131 132 173 174 175 176 177 179 180 181 185 352
411 412 413 414.

&&

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