FXAK68 PAFC 132132
AFDAFC

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION 
NATIONAL WEATHER SERVICE ANCHORAGE AK
1230 PM AKST WED FEB 13 2013

.ANALYSIS AND UPPER LEVELS...HIGHLY AMPLIFIED FLOW IS PRODUCING
STRONG BAROCLINIC ZONES AND DEVELOPMENT OF STRONG LOW PRESSURE
SYSTEMS AT THE SURFACE. A COLD UPPER LOW OVER THE NORTHERN BERING 
IS LIFTING NORTHWARD TOWARD THE BERING STRAIT WHILE A SHORT-WAVE
TROUGH DIGS SOUTHEASTWARD ACROSS THE CENTRAL BERING AND ALEUTIANS.
DEEP COLD AIR MOVING ACROSS THE WARMER WATERS OF THE BEING SEA IS
GENERATING WIDESPREAD SNOW SHOWERS AND GUSTY NORTHERLIES. A SECOND
SHORTWAVE TROUGH ON THE EAST SIDE OF THE LOW CENTER IS LIFTING
NORTHWARD ACROSS THE SOUTHERN MAINLAND THIS MORNING PRODUCING AREAS
OF SNOW OR SNOW SHOWERS.  

MEANWHILE ANOTHER TROUGH IS SITTING BENEATH THE BERING ONE OVER THE
NORTH-CENTRAL PACIFIC. A STRONG VERTICALLY STACKED LOW WELL SOUTH 
OF THE CENTRAL ALEUTIANS IS DROPPING SOUTHWARD AS TROUGH CONTINUES 
TO AMPLIFY. HOWEVER...AN OCCLUDED FRONT EXTENDS NORTHEASTWARD FROM
THE LOW CENTER TO A WEAK TRIPLE POINT LOW SOUTH OF THE ALASKA
PENINSULA. THESE ARE PIVOTING NORTHWESTWARD HELPING TO TIGHTEN THE
PRESSURE AND THERMAL GRADIENTS RESULTING IN VERY STRONG WINDS OUT
AHEAD OF THE FRONT. THE DUTCH HARBOR AREA OBSERVED A FEW WIND GUSTS
TO AROUND 50 KTS THIS MORNING. A WARM FRONT EXTENDING EASTWARD FROM
THE TRIPLE POINT IS MOVING NORTHWARD INTO THE GULF HEADED TOWARD
SOUTHCENTRAL ALASKA.    

A STRONG WAVE ROUNDING THE BASE OF THE NORTH PACIFIC TROUGH HAS
INDUCED CYCLOGENESIS EAST OF THE MAIN LOW CENTER. THIS NEWLY FORMED
LOW IS DEEPENING AS IT TRACKS NORTHWARD TOWARD THE WESTERN GULF. 

.MODEL DISCUSSION...THE 12Z NAM SOLUTION QUICKLY BECOMES OUT OF 
PHASE WITH SYNOPTIC FEATURES COMPARED WITH OTHER MODELS AND 
THEREFORE WILL BE DISREGARDED. THE GFS/ECMWF/EKMET/GEMGLB ALL 
SHOW A SIMILAR SYNOPTIC SCALE SOLUTION WHICH IS SIMILAR TO 
YESTERDAY. THERE ARE MINOR DIFFERENCES WHICH WILL CERTAINLY AFFECT
SOME OF THE DETAILS OF THE FORECAST...BUT FORECAST CONFIDENCE IS
RISING. BASED ON THESE LATEST SOLUTIONS THE MOST SIGNIFICANT CHANGES
TO EXISTING FORECASTS WILL BE TO INCREASE WINDS ASSOCIATED WITH THE
DEVELOPING PACIFIC LOW AND COLD ADVECTION ON THE BACK SIDE OF IT 
AND INCREASE POPS. MORE SPECIFICALLY IT IS LOOKING VERY FAVORABLE 
FOR A SNOW EVENT IN ANCHORAGE AND THE MAT VALLEY BY FRIDAY.    

.SHORT TERM FORECAST...

SOUTHCENTRAL ALASKA...THE WARM FRONT MOVING INTO THE GULF THIS
MORNING WILL QUICKLY TRACK TO THE NORTH GULF COAST LATE THIS
AFTERNOON AND THEN WEAKEN AS IT IS FORCED INLAND TONIGHT BY 
STRONG SOUTHERLY FLOW ALOFT. LOW LEVEL EASTERLY FLOW AHEAD OF 
THE FRONT WILL DOWNSLOPE THE TYPICAL AREAS WITH THE BULK OF 
PRECIP FALLING ALONG THE GULF COAST AND THE WEST SIDE OF COOK 
INLET AND THE SUSITNA VALLEY. AIR MASS OUT AHEAD OF THE WARM FRONT 
IS COLD ENOUGH TO PRODUCE MOSTLY SNOW IN THE PRINCE WILLIAM SOUND
AREA...BUT MUCH WARMER AIR WILL MOVE IN BEHIND THE FRONT AND WITH 
THE NEXT SYSTEM MOVING OUT OF THE PACIFIC INTO THE WESTERN GULF ON
THURSDAY. PRECIP WILL INITIALLY BE CONFINED TO THE SAME LOCATIONS 
AS TONIGHTS SYSTEM. HOWEVER...THE LOW WILL BE WEAKENING AS IT MOVES
INTO THE GULF SO THAT BY THE TIME IT TRACKS TO THE EASTERN KENAI
PENINSULA/WESTERN PRINCE WILLIAM SOUND THURSDAY EVENING DOWNSLOPING
WINDS WILL HAVE ENDED AND LIGHT PRECIP WILL BEGIN TO SPREAD INLAND.
THE BEST PERIOD FOR SNOW ACCUMULATION (ESPECIALLY FOR ANCHORAGE AND
THE MAT VALLEY) WILL BE FRIDAY AS UPPER TROUGH CURRENTLY OVER THE
CENTRAL BERING APPROACHES SOUTHCENTRAL AND COLD AIR ADVECTION BEGINS.

THE ONLY THING WORKING AGAINST A REALLY BIG SNOW EVENT IS THAT 
ARCTIC AIR WILL BE COMING IN FROM THE WEST. THE ALASKA RANGE REALLY
SLOWS DOWN THE STRENGTH OF THE ADVECTION SO THAT IT MORE SLOWLY
FILTERS INTO THE AREA. STILL..THERE IS POTENTIAL FOR ADVISORY LEVEL
SNOW AMOUNTS (6 INCHES OR GREATER) IN THE ANCHORAGE AREA.   

SOUTHWEST ALASKA...THE WARM FRONT CURRENTLY SOUTH OF THE ALASKA
PENINSULA WILL MOVE NORTHWARD INTO BRISTOL BAY THIS AFTERNOON AND
EVENING. PRECIP WITH THIS FRONT WILL BE LIGHT. HOWEVER...AS THE
TROUGH CROSSING THE BERING GETS CLOSER THERE WILL BE LIFT GENERATED
FROM THE EAST AND FROM THE WEST WHICH WILL SET UP A BAND OF SLIGHTLY
HEAVIER SNOW FROM DILLINGHAM AREA NORTHWARD TO BETHEL AND ANIAK. 
THIS BAND OF SNOW WILL REMAIN IN PLACE UNTIL THE UPPER TROUGH MOVES
THROUGH SOUTHWEST ALASKA THURSDAY AFTERNOON AND EVENING AT WHICH
POINT THE SNOW WILL SPREAD NORTH AND EASTWARD. A DEEP LAYER OF 
ARCTIC AIR WILL MOVE BEHIND THIS TROUGH AND DRY THINGS OUT ON 
FRIDAY. THIS WILL CAUSE SURFACE TEMPERATURES TO PLUMMET BELOW 
ZERO FRIDAY INTO FRIDAY NIGHT.

BERING SEA/ALEUTIANS...THE UPPER TROUGH OVER THE CENTRAL BERING 
WILL CONTINUE EASTWARD ACROSS THE EASTERN BERING/ALEUTIANS AND
SOUTHERN ALASKA PENINSULA. THIS WILL PUSH THE REMNANTS OF THE
OCCLUDED FRONT (CURRENTLY JUST SOUTH OF THE EASTERN ALEUTIANS 
AND ALASKA PENINSULA) OFF TO TEH SOUTH AND EAST. COLDER AIR ALONG
WITH SNOW SHOWERS AND GUSTY WINDS WILL MOVE IN. IN PARTICULAR...
EXPECT VERY STRONG WINDS ALONG THE SOUTHERN ALASKA PENINSULA WHERE
THE PRESSURE GRADIENT WILL BE TIGHTEST ON THE BACK SIDE OF PACIFIC
LOW MOVING INTO THE WESTERN GULF. BY THE TIME BERING TROUGH ARRIVES
TO THE SOUTHERN ALASKA PENINSULA EARLY THURSDAY MORNING EXPECT 
STEADY SNOW ASSOCIATED WITH TODAYS FRONT TO BE TAPERING OFF. IF 
THERE WAS STEADY SNOW STILL IN PLACE THEN THERE WOULD LIKELY BE 
A BLIZZARD. CURRENT THINKING IS THAT PRECIP WILL BE SHOWERY WITH 
JUST BRIEF PERIODS OF LOW VISIBILITY IN BLOWING SNOW.
MEANWHILE...BACK OVER THE CENTRAL TO EASTERN BERING WIDESPREAD 
SNOW SHOWERS AND GUSTY WINDS WILL PERSIST UNTIL COLD AIR ADVECTION
DIMINISHES THURSDAY AFTERNOON AND EVENING. ANOTHER MAJOR STORM WILL
MOVE OUT OF THE NORTHWEST PACIFIC THURSDAY AND FRIDAY TRACKING SOUTH
OF THE ALEUTIAN CHAIN. WITH ARCTIC AIR IN PLACE AS IT FIRST ARRIVES
EXPECT SNOW AND BLOWING SNOW ALONG THE ALEUTIANS.

.LONG TERM FORECAST...ACTIVE PATTERN WILL CONTINUE THROUGH THIS
WEEKEND WITH A STORM TRACK FROM THE ALEUTIAN CHAIN TO THE GULF OF
ALASKA. MODELS INDICATE A SHIFT IN THE LONG-WAVE NEXT WEEK WITH AN
UPPER RIDGE BUILDING OVER MAINLAND ALASKA AND A DEEP TROUGH MOVING
INTO THE BERING. THIS WILL SHIFT THE FOCUS OUT WEST FOR THE 
BEGINNING OF NEXT WEEK. AFTER A COUPLE REALLY GOOD SHOTS OF SNOW
SOUTHCENTRAL WILL SETTLE DOWN FOR A FEW DAYS INTO A CALM AND COLD
REGIME. 

.AER/ALU...WATCH/WARNING SUMMARY
PUBLIC...NONE.
MARINE...GALE WARNING...120 125 130 132 140 150 155 160 165 
                        170 171 172 175 176.  
FIRE WEATHER...NONE.

SEB FEB 13