Blizzard in Alaska, March 25 to 26, 2013

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
400 PM AKDT SUN MAR 24 2013

.ANALYSIS AND UPPER LEVELS...
AN UPPER-LVL LOW NEAR THE SW TIP THE KAMCHATKA PENINSULA IS DRIFTING
NORTH. AN ELONGATED FRONTAL ZONE NEARS THE WESTERN ALEUTIAN
CHAIN...WITH A SURFACE WAVE ALONG IT 100 MILES SOUTHWEST OF KISKA
ROTATING EAST ALONG WITH A SHIELD OF SNOW AND RAIN. THIS WAVE'S
SUPPORT ALOFT IS CUTTING OFF FROM THE PARENT KAMCHATKA LOW AND
WEAKENING THE UPPER-RIDGE DOWNSTREAM OF IT IN THE CENTRAL BERING SEA.
THE RIDGE IS FURTHER BEING WEAKENED BY A VIGOROUS POLAR VORTEX
SWINGING DOWN FROM WESTERN AK TOWARDS THE SOUTHERN MAINLAND.
OVERALL...CONDITIONS ARE RATHER BENIGN OVER THE WESTERN AOR...WITH
DRY WEATHER FROM THE SW MAINLAND BACK THROUGH THE CENTRAL TO EASTERN
BERING SEA. THE ARCTIC AIRMASS OVER CENTRAL INTERIOR AK CONTINUES TO
KEEP TEMPERATURES BELOW AVERAGE BACK THROUGH THE KUSKOKWIM
DELTA/VALLEY.

FURTHER EAST...A LOW CENTERED AROUND 100 MILES EAST OF KODIAK CITY IS
ADVANCING NORTH TOWARDS THE SOUTHERN MAINLAND. PRECIPITATION IS
WANING OVER KODIAK ISLAND BEHIND THIS FEATURE BUT WINDS ARE
INCREASING AS COLD ADVECTION STARTS TO FILTER IN. ONSHORE FLOW HAS
CONTINUED TO PRODUCE SNOW ACROSS PRINCE WILLIAM SOUND...THOUGH WARM
ADVECTION HAS POKED FAR ENOUGH NORTH TO CHANGE PRECIPITATION TO ALL
RAIN ALONG THE IMMEDIATE COAST NEAR CORDOVA. ISENTROPIC LIFT
OVERNIGHT LAST NIGHT INTO EARLY MORNING HAS GIVEN WAY TO AN AREA OF
DEFORMATION ALOFT IN BETWEEN THE ADVANCING GULF LOW AND THE
AFOREMENTIONED POLAR VORTEX ROTATING SOUTH...ALLOWING FOR A BROAD
AREA OF STEADY SNOW ACROSS UPPER COOK INLET.

.MODEL DISCUSSION...
IT SHOULD BE NOTED THAT THE MODELS ARE DOING A RATHER POOR JOB
CAPTURING SURFACE TEMPERATURES OVER THE SOUTHERN MAINLAND...IN SOME
CASES ON THE ORDER OF SEVERAL DEGREES TOO WARM. OUTSIDE OF THAT...
MODELS HAVE ADEQUATE AGREEMENT AND CAPTURE THE MAIN FEATURES ACROSS
THE AOR SATISFACTORILY. MODELS STILL SHOW THE WAVE SW OF KISKA MOVING
TO NEAR ADAK AT 1000 MB MON AFTERNOON THEN TRACKING IT TO AROUND 300
MILES SOUTHEAST OF UNALASKA ON TUE. POSITION DIFFERENCES ARE A TOUCH
LARGER THAN PREVIOUS MODEL RUNS WITH THIS FEATURE. A STRONGER PACIFIC
LOW IS THEN DEPICTED MOVING TO AROUND 450 MILES SOUTHWEST OF ATTU IN
THE 965 TO 970 MB RANGE TUE MORNING...THEN MOVING TO AROUND 300 MILES
SOUTH OF ATTU ON WED. THE ECMWF HAS THE DEEPEST PRESSURE IN THE
SOLUTION ENVELOPE...THOUGH THE GFS AND NAM HAVE NOW TRENDED STRONGER
WITH THE SYSTEM AS WELL.

FURTHER EAST...THERE IS DECENT AGREEMENT ON ENERGY FROM THE GULF
SYSTEM PHASING WITH THE INTERIOR POLAR VORTEX OVERNIGHT TONIGHT TO
EARLY MON MORNING. THE NAM IS A HAIR FURTHER EAST WITH THE GULF LOW
UPPER SUPPORT...BUT OVERALL STILL WITHIN TOLERANCE ESPECIALLY WHEN
ONE LOOKS AT THE SURFACE FEATURE. MODEL SURFACE LOW POSITIONS ARE
CLUSTERED OVER WESTERN PRINCE WILLIAM SOUND JUST EAST OF SEWARD...AND
REMAINING NEARLY STATIONARY INTO TUE. COLD ADVECTION BLEEDING ACROSS
COOK INLET AND THE GULF ARE HANDLED SIMILARLY AS WELL.

.SHORT TERM FORECAST...
SOUTHCENTRAL/GULF...
STEADY PRECIP ACROSS PRINCE WILLIAM SOUND GETS A REINFORCING BOOST AS
THE GULF LOW WRAPS NORTH...SO LITTLE CHANGE IN THE FORECAST EXPECTED
OTHER THAN AREAS SEEING RAIN SHOULD START MIXING WITH SNOW AGAIN.
ELSEWHERE ACROSS THE SOUTHERN MAINLAND...FORECAST CONFIDENCE HIGH
ENOUGH TO POST WINTER WEATHER ADVISORIES FOR SNOW FOR THE
ANCHORAGE...MATANUSKA AND SUSITNA VALLEYS. ALOFT...THE PHASING VORTEX
AND GULF SYSTEMS RESULT IN A NEGATIVELY TILTED UPPER TROUGH WHICH IS
THEN VERY SLOW TO CLEAR THROUGH THE REGION. AS SUCH...EXPECTING A
LONG DURATION SNOW EVENT. COLD ADVECTION SWEEPING EAST WILL COOL AND
SATURATE THE VERTICAL COLUMN BY THE EARLY MORNING HOURS AND PRODUCE
AN APPRECIABLE -12 TO -18 C SNOW GROWTH LAYER. STRONG LIFT ALSO WILL
BE COINCIDING WITH THIS AND SHOULD LEAD TO PERIODS OF MODERATE TO
HEAVY SNOW. THERE IS AMPLE COLD AIR WITH THE POLAR VORTEX...AND
EVENTUALLY THE VERTICAL TEMPERATURE PROFILE WILL COOL BELOW THE RANGE
OF MAXIMIZED DENDRITIC GROWTH...SO SNOW INTENSITY/ACCUMULATION SHOULD
BEGIN TO LIGHTEN MONDAY EVENING. RESIDUAL CYCLONIC FLOW ALOFT KEEPS
THE THREAT OF AT LEAST SOME LIGHT SNOW AROUND INTO EARLY TUE. COLD
ADVECTION WRAPPING UP THE RELATIVELY WARMER KACHEMAK BAY WATERS MAY
INTRODUCE THE THREAT OF SOME LOCALLY REDUCED VISIBILITY IN BLOWING
SNOW ON MON...THOUGH AVAILABLE TRANSPORTABLE SNOW MAY NOT BE
SUFFICIENT TO WARRANT AN ADVISORY/WARNING. AS THE PRECIP BEGINS TO
LIGHTEN...THE COLD AIR SWINGING EAST WILL ALSO LEAD TO STRONG NORTH
TO WEST GAP WINDS THROUGH FAVORED AREAS SUCH AS TURNAGAIN
ARM...SEWARD AND WHITTIER.

ACROSS KODIAK ISLAND...BRISK NORTH TO WEST WINDS IN THE LOW'S WAKE
ARE ANTICIPATED TONIGHT THROUGH MON...WITH SOME SOME WEAKENING AS THE
LOW TO THE NORTH FILLS AND THE RESULTING PRESSURE GRADIENT RELAXES.
OVERALL HOWEVER A DRYING TREND IS ANTICIPATED IN OFF-SHORE FLOW.

SOUTHWEST/AKPEN...
THE VORTEX SWINGING DOWN BRINGS A REINFORCING SHOT OF COLD AIR TO THE
SW MAINLAND. MODELS SHOW A HINT OF INCREASING RH IN THE LOWER LAYERS
WITH SOME INCREASED CYCLONIC FLOW ALOFT WHICH MAY LEAD TO A BIT MORE
CLOUDS...BUT BY AND LARGE CONDITIONS WILL REMAIN DRY. SOME WIND
CHILLS 20 TO 35 BELOW IN LOCALLY BREEZY AREAS ARE STILL POSSIBLE. THE
PRESSURE GRADIENT DOES RELAX WITH TIME AS THE REGION WILL BE NESTLED
WITH SURFACE HIGH PRESSURE IN BETWEEN THE RESIDUAL GULF OVER
SOUTHCENTRAL AND BROAD LOW PRESSURE OVER THE WESTERN CHAIN/BERING.

BERING SEA/ALEUTIANS...
SENSIBLE WEATHER IMPACTS TO REVOLVE AROUND THE SERIES OF LOWS
TRACKING THROUGH THE CHAIN...THE PACIFIC STORM ON TUE BEING THE
STRONGER OF THE TWO SYSTEMS. THE THREAT OF GALES EXISTS WITH BOTH
SYSTEMS...BUT A BIT MORE WIDESPREAD WITH THE SECOND ONE.

.LONG TERM FORECAST...
MODELS SHOW THE COLD MAINLAND VORTEX RETREATING TO NORTHERN INTERIOR
AK AND POINTS NORTHEAST LATE WEEK WITH WEAK UPPER-RIDGING BUILDING IN
BEHIND IT. THIS SHOULD BRING SOME MODERATION TO THE
TEMPERATURES...BUT WITH THE FLOW STILL SOMEWHAT
SOUTHERLY/ONSHORE...STILL KEEPING WITH A CLOUDY PATTERN WITH SOME
CHANCES FOR PRECIP. THE WESTERN PART OF THE AOR WILL STILL BE UNDER
THE INFLUENCE OF AN UPPER-LOW WHICH SLOWLY TRACKS THROUGH THE
BERING/CHAIN...KEEPING THINGS UNSETTLED.

.AER/ALU...WATCH/WARNING SUMMARY
PUBLIC...WINTER WEATHER ADVISORY FOR SNOW...101 111 145
MARINE...STORM...130
         GALE...120 129 132 136 137 138 140 170 172 175 176 150 351
                352 411 412 413 414
         HEAVY FREEZING SPRAY...129 130 132 136 137 138 140 150 165
                                179 141 411 412 413 414
FIRE WEATHER...NONE

MALOY