Heavy Freezing Spray Warning 182, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
1240 PM AKST WED DEC 21 2011

.ANALYSIS AND UPPER LEVELS...A LOOK AT THE UPPER LEVELS THIS MORNING
SHOWS A NEW LONG-WAVE PATTERN TAKING SHAPE ACROSS ALASKA AND THE
NORTH PACIFIC. THE UPPER LOW WHICH HAS BEEN ANCHORED OVER THE BERING
SEA THE PAST COUPLE WEEKS CONTINUES TO DISSIPATE. MEANWHILE...THE
MAJOR STORM WHICH MOVED ACROSS THE MAINLAND YESTERDAY AND LAST
NIGHT HELPED TO FLATTEN OUT DOWNSTREAM RIDGE WHICH HAS BEEN
QUASI-STATIONARY NEAR THE ALASKA/YUKON BORDER. A MAJOR STORM OVER
THE NORTH PACIFIC WILL HELP KICK THIS RIDGE EASTWARD TONIGHT AND
THURSDAY. IN THE FINAL PIECE OF THE PATTERN CHANGE A RIDGE IS
BEGINNING TO BUILD OVER THE FAR NORTHWEST PACIFIC AHEAD OF A
TROUGH DIGGING OUT OF ASIA. ALL OF THESE FACTORS WILL CAUSE THE
LONG-WAVE PATTERN TO PROGRESS EASTWARD TONIGHT AND FRIDAY WITH A
WEAK RIDGE SETTING UP OVER THE BERING AND A TROUGH OVER MAINLAND
ALASKA. THIS WILL BRING A DRAMATIC CHANGE TO THE WEATHER ACROSS
THE STATE.

AS FOR ONGOING WEATHER THIS MORNING...THE REMNANTS OF YESTERDAY'S
STORM ARE HEADING INLAND THROUGH THE INTERIOR TO THE YUKON. CYCLONIC
FLOW WITH EMBEDDED WAVES CONTINUES OVER THE SOUTHERN MAINLAND. THIS
IS COMBINING WITH LOW LEVEL INSTABILITY TO PRODUCE SCATTERED SHOWERS.

GUSTY SOUTH WINDS ALSO PERSIST ACROSS MUCH OF THE REGION THIS
MORNING...COINCIDENT WITH PRESSURE RISES IN THE WAKE OF THE STORM.
THESE WINDS WILL QUICKLY TREND DOWNWARD AS WE HEAD INTO THE
AFTERNOON HOURS.

A STRONG LOW HAS DEVELOPED OVER THE NORTH PACIFIC. IT IS TRACKING
EASTWARD THIS MORNING BUT EXPECT IT TO BEGIN TO CURVE NORTHWARD
TOWARD THE GULF OF ALASKA DUE TO THE INFLUENCE OF THE DOWNSTREAM
RIDGE.

.MODEL DISCUSSION...THE ECMWF IS VERIFYING BEST WITH THE NORTH
PACIFIC LOW AND SHOWS THE BEST RUN-TO-RUN CONSISTENCY SO IS THE
PREFERRED SOLUTION. ALL MODELS DEVELOP A TRIPLE POINT LOW OVER THE
SOUTHERN GULF WITH THE MAIN LOW PASSING TO THE SOUTH. THE ECMWF/UKMET
MODELS BACK THIS LOW INTO PRINCE WILLIAM SOUND THURSDAY EVENING
WHILE THE NAM/GFS TRACK IT INLAND EAST OF CORDOVA. CLIMATOLOGY
SUPPORTS THE LOW MOVING INTO THE SOUND.

ECMWF IS ALSO FAIRLY CONSISTENT WITH TRACK OF A LOW SOUTH OF THE
ALEUTIAN CHAIN TONIGHT AND THURSDAY. SOLUTIONS FOR NEXT LOW MOVING
INTO THE GULF LATE IN THE WEEK ARE ALL OVER THE PLACE SO WILL STICK
WITH INHERITED FORECAST FOR NOW.

.SHORT TERM FORECAST...

SOUTHCENTRAL ALASKA...AFTER A BRIEF REPRIEVE THIS AFTERNOON THROUGH
TONIGHT WITH HIGH PRESSURE MOVING OVERHEAD THE PACIFIC SYSTEM WILL
BEGIN TO AFFECT SOUTHCENTRAL THURSDAY MORNING. THE PRESSURE GRADIENT
WILL TIGHTEN ALONG THE GULF COAST AS THE LOW APPROACHES CAUSING GAP
WINDS TO INCREASE ESPECIALLY IN THE PRINCE WILLIAM SOUND REGION.
MOISTURE WILL OVER-RUN SOUTHCENTRAL FROM EAST TO WEST DURING THE DAY
THURSDAY...BUT WITH UNCERTAINTY IN THE LOW TRACK AS IT REACHES THE
NORTH GULF COAST IT MAKES IT DIFFICULT TO SAY WHERE THE WESTERN EDGE
WILL SET UP. THERE IS HIGHER CONFIDENCE THAT SNOW WILL OVERSPREAD
THE COPPER RIVER BASIN AND ALL OF PRINCE WILLIAM SOUND. THE GAP WINDS
NEAR WHITTIER AND THROUGH THOMPSON PASS WILL CREATE POTENTIAL FOR
BLIZZARD CONDITIONS DEPENDING ON THE STRENGTH OF WIND AND INTENSITY
OF SNOW...SO WILL ISSUE BLIZZARD WATCHES FOR THOSE LOCATIONS. THE
COPPER RIVER BASIN WILL SEE WIDESPREAD SNOW THURSDAY AND THURSDAY
NIGHT NO MATTER WHICH OF THE LOW TRACKS IS CORRECT...SO PLAN TO
ISSUE A WINTER WEATHER ADVISORY FOR SNOW. IF THE LOW DOES MAKE IT
ALL THE WAY INTO PRINCE WILLIAM SOUND SNOW WILL SPREAD OVER THE
KENAI AND CHUGACH MOUNTAINS TO ANCHORAGE AND THE MAT VALLEY...BUT
UNCERTAINTY IN THIS IS HIGHER. FOR NOW WILL GO WITH LIGHT SNOW
ACCUMULATION THURSDAY NIGHT. AS THIS SYSTEM MOVES INTO THE GULF
COLDER AIR WILL BEGIN TO FILTER FROM SOUTHWEST ALASKA INTO THE
WESTERN GULF SUPPORTING STRONG WINDS ACROSS PORTIONS OF THE WESTERN
GULF COASTAL WATERS AND BEGINNING TO COOL SURFACE TEMPERATURES
AS THIS AIR FILTERS ACROSS SOUTHCENTRAL ALASKA.

WITH THE LONG-WAVE PATTERN CONTINUING TO SHIFT EASTWARD THE NEXT
SYSTEM MOVING OUT OF THE PACIFIC AT THE END OF THE WEEK WILL
LIKELY TAKE A MORE EASTWARD TRACK AS IT MOVES INTO THE GULF OVER
THE WEEKEND. THIS WILL ALLOW MUCH COLDER TO DROP DOWN ACROSS
MAINLAND ALASKA AND SETTLE INTO SOUTHCENTRAL NEXT WEEKEND.

SOUTHWEST ALASKA...A FEW ISOLATED SHOWERS WILL LINGER ACROSS
SOUTHWEST ALASKA AS WEAK UPPER LEVEL FEATURES MOVE THROUGH TONIGHT
AND THURSDAY. HOWEVER...THE TREND THROUGH THE REST OF THE WEEK WILL
BE DRIER AND MUCH COLDER AS AN ARCTIC LOW DROPS INTO THE REGION AS
WE HEAD INTO THE WEEKEND.

BERING SEA/ALEUTIANS...THE STORM TRACK WILL REMAIN SOUTH OF THE
ALEUTIANS WITH THE ALEUTIAN CHAIN AND ALASKA PENINSULA RIGHT ALONG
THE PERIPHERY. AS ARCTIC LOW DROPS DOWN ACROSS MAINLAND ALASKA THIS
WEEKEND ARCTIC AIR WILL SPREAD SOUTHWARD ACROSS THE BERING SEA
PRODUCING SOME SNOW SHOWERS AND GUSTY WINDS. THE COLD ADVECTION
WILL BE WEAK...SO DON'T EXPECT WINDS TO BE VERY STRONG.

.LONG TERM FORECAST...ARCTIC LOW WILL SET UP SHOP OVER THE SOUTHERN
MAINLAND THROUGH AT LEAST THE MIDDLE OF NEXT WEEK RESULTING IN BELOW
NORMAL TEMPERATURES ACROSS THE REGION. THE STORM TRACK WILL REMAIN
SOUTH OF THE ALEUTIANS AND TOWARD SOUTHEAST ALASKA...SO THE ANCHORAGE
FORECAST AREA WILL REMAIN ON THE QUIET SIDE...ALTHOUGH THERE WILL
STILL BE SOME IMPACTS ALONG THE GULF COAST AND PORTIONS OF
SOUTHCENTRAL AS THESE STORMS PASS BY TO THE SOUTH AND EAST.

.AER/ALU...WATCH/WARNING SUMMARY
PUBLIC...BLIZZARD WATCH 125 131.
MARINE...GALE WARNING 120 130 132 155 170 172 185.
         HEAVY FREEZING SPRAY WARNING 185.
FIRE WEATHER...NONE.

SEB DEC 11