Heavy Freezing Spray Warning 3, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
1220 PM AKST THU JAN 5 2012

.ANALYSIS AND UPPER LEVELS...AN ELONGATED UPPER LOW IS CENTERED
JUST EAST OF SAINT LAWRENCE ISLAND WITH TROUGH AXIS EXTENDING
SOUTHWARD ACROSS THE EASTERN BERING SEA. MULTIPLE EMBEDDED
SHORT-WAVES ARE MOVING THROUGH THE TROUGH AND ROTATING NORTHWARD
ACROSS MAINLAND ALASKA. ONE WAVE IS MOVING NORTHWARD ACROSS
SOUTHCENTRAL THIS MORNING WITH A BAND OF MODERATE TO HEAVY SNOW
RIGHT OUT AHEAD OF IT. WATER VAPOR IMAGERY SHOWS THERE IS SOME
PACIFIC MOISTURE FEEDING INTO THE REGION AHEAD OF THIS WAVE. A
WEAK SURFACE LOW HAS ALSO DEVELOPED JUST NORTH OF KODIAK ISLAND
AND IS TRACKING NORTHWARD ACROSS THE KENAI PENINSULA. TIGHT PRESSURE
GRADIENTS AHEAD OF THIS LOW ARE CAUSING LOCALIZED STRONG WINDS AND
BLOWING SNOW THROUGH MOUNTAIN GAPS. A COLD FRONT EXTENDS SOUTHWARD
FROM THIS LOW ACROSS THE WESTERN GULF WITH COLD ADVECTION ACROSS
THE ALASKA PENINSULA BEGINNING TO KICK UP GAP WINDS.

A MATURE MODERATE STRENGTH LOW OVER THE NORTHEAST PACIFIC IS
TRACKING NORTHEASTWARD TOWARD THE EASTERN GULF. THIS LOW IS HELPING
TO BLOCK SOME OF THE PACIFIC MOISTURE FROM REACHING SOUTHCENTRAL...
BUT AS THE LOW GETS FURTHER NORTH EXPECT SOME OF THIS MOISTURE
TO BE PULLED BACK UP TOWARD SOUTHCENTRAL TONIGHT INTO FRIDAY.

A WEAK LOW IS PASSING JUST NORTH OF THE EASTERN ALEUTIANS AND
SOUTHERN ALASKA PENINSULA...ENHANCING WINDS AND SNOW SHOWERS.
TYPICAL COLD ADVECTION SNOW SHOWERS DOMINATE THE REST OF THE
BERING SEA REGION.

.MODEL DISCUSSION...MODELS CONTINUE TO STRUGGLE WITH RUN-TO-RUN
CONSISTENCY. THE FORECAST PROBLEM OF THE DAY IS DETERMINING HOW
FAR WEST DEEP PACIFIC MOISTURE WILL MAKE IT AS UPPER TROUGH MOVES
ACROSS SOUTHCENTRAL TONIGHT THROUGH FRIDAY MORNING. THE 12Z GFS
IS SLOWEST WITH THE UPPER TROUGH AND FARTHEST WEST WITH THE
MOISTURE...AS IT BRINGS A WELL DEFINED SURFACE LOW INTO PRINCE
WILLIAM SOUND AT 12Z FRIDAY. THIS SCENARIO WOULD BRING MUCH MORE
SNOW TO ANCHORAGE AND THE MATANUSKA VALLEY. THE UKMET HAS A SIMILAR
SOLUTION TO THE GFS. MEANWHILE...THE NAM IS BY FAR THE FASTEST OF
ANY OF THE MODELS WITH UPPER TROUGH MOVEMENT SO CONSIDER IT AN
OUTLIER. PLAN TO USE THE MIDDLE GROUND SOLUTION OF THE
ECMWF/GEMGLB...WHICH ALSO HAPPENS TO BE THE MOST CONSISTENT
SOLUTION AND NOT TOO FAR FROM THE INHERITED FORECAST. THE ECMWF
IS ALSO VERY CONSISTENT WITH MAJOR PACIFIC LOW MOVING INTO THE
GULF AND EVENTUALLY PRINCE WILLIAM SOUND THIS WEEKEND.

.SHORT TERM FORECAST...

SOUTHCENTRAL ALASKA...BASED ON THE ECMWF SOLUTION THE DEEPEST
PACIFIC MOISTURE WILL REMAIN FROM PRINCE WILLIAM SOUND AND
EASTWARD. THIS SHOULD KEEP ANCHORAGE AND THE MATANUSKA VALLEY
JUST BELOW ADVISORY SNOW AMOUNTS...BUT A SLIGHT SHIFT WESTWARD
WOULD DRAMATICALLY AFFECT SNOWFALL AMOUNTS. BASED ON THE STRENGTH
OF UPPER TROUGH AND ACCOMPANYING COLD ADVECTION PLAN TO UP CURRENT
FORECAST AMOUNTS TO JUST BELOW ADVISORY CRITERIA. FOR EASTERN
PRINCE WILLIAM SOUND AND PORTIONS OF THE COPPER RIVER BASIN EXPECT
PERIODS OF HEAVY SNOW UNTIL THE UPPER TROUGH PASSES THROUGH ON
FRIDAY. LOW LEVEL COLD ADVECTION PRIOR TO UPPER TROUGH PASSAGE WILL
LEAD TO DRIER AND DRIER SNOW WHICH WILL RAPIDLY ACCUMULATE. BASED
ON THIS WILL HAVE TO CONSIDER A HEAVY SNOW WARNING FOR EASTERN
PRINCE WILLIAM SOUND.

AS THIS SYSTEM EXITS DURING THE DAY FRIDAY A STORM WWILL BE
DEVELOPING OVER THE NORTH CENTRAL PACIFIC ALONG THE UPPER JET.
AS IT STRENGTHENS DOWNSTREAM RIDGE WILL BUILD AHEAD OF IT AND
CAUSE IT TO TURN NORTHWARD INTO THE WESTERN GULF FRIDAY NIGHT
AND SATURDAY. THE EXACT TRACK OF THIS LOW WILL BE KEY TO WEATHER
ON KODIAK ISLAND AND THE DIFFERENCE BETWEEN CHANGING TO RAIN AND
STAYING ALL SNOW WITH LIKELY BLIZZARD CONDITIONS. THIS STORM WILL
IMPACT ALL OF SOUTHCENTRAL WITH WIDESPREAD SNOWFALL OVER THE
WEEKEND. OTHER POTENTIAL IMPACTS WILL BE WORKED OUT THE NEXT
24 HOURS AS WE LOCK IN ON THE TRACK AND STRENGTH OF THIS SYSTEM.

SOUTHWEST ALASKA...THE LOW JUST NORTH OF THE SOUTHERN ALASKA
PENINSULA WILL DISSIPATE AS IT CROSSES THE PENINSULA TONIGHT.
WITH LITTLE MOISTURE TO WORK WITH OVER SOUTHWEST ALASKA...EXPECT
LITTLE IMPACT FROM UPPER TROUGH AS IT ROTATES ACROSS THE REGION
TONIGHT. BEST CHANCE OF SNOW WOULD BE ALONG AND NEAR THE ALASKA
RANGE. MEANWHILE CLEAR SKIES AND COLD ADVECTION BEHIND THE TROUGH
MEANS EXTREMELY COLD TEMPERATURES WILL SETTLE BACK IN ACROSS THE
REGION. THE PACIFIC STORM WILL TRACK FAR ENOUGH NORTH AND WEST TO
BRING SNOW AND WIND TO BRISTOL BAY...ESPECIALLY ALONG THE ALASKA
PENINSULA. THEREFORE...PLAN TO ISSUE A BLIZZARD WARNING FOR SOUTH
OF KING SALMON FRIDAY NIGHT INTO SATURDAY AND UPGRADE TO A BLIZZARD
WARNING FOR THE SOUTHERN ALASKA PENINSULA. IF MODELS TREND ANY
FARTHER NORTHWEST WITH MOISTURE OTHER AREAS COULD BE AFFECTED BY
SNOW AND BLOWING SNOW AS WELL.

BERING SEA/ALEUTIANS...RIGHT NOW IT APPEARS THAT THE BULK OF STEADY
SNOW WITH DEVELOPING PACIFIC LOW WILL STAY JUST TO THE SOUTH OF THE
EASTERN ALEUTIANS FRIDAY AND FRIDAY NIGHT. THERE WILL DEFINITELY BE
STRONG NORTHERLY WINDS...SO IF STEADY SNOW DOES MAKE IT IN THERE A
BLIZZARD WILL ENSUE. WILL KEEP A CLOSE EYE ON THIS AS WE GET CLOSER
TO THE EVENT. FOR THE REST OF THE BERING SEA AND ALEUTIANS COLD
ADVECTION SNOW SHOWERS WILL PERSIST.

.LONG TERM FORECAST...UPPER LEVEL LOW WILL RETROGRADE BACK TOWARD
EASTERN RUSSIA TONIGHT AND FRIDAY WHICH THEN ALLOWS THE NEXT PACIFIC
STORM TO TRACK FARTHER NORTH THIS WEEKEND. THIS IS THE BEGINNING OF
A LONG-WAVE PATTERN CHANGE. AS WE HEAD THROUGH NEXT WEEK MODELS
INDICATE VERY PROGRESSIVE UPPER LEVEL FLOW WITH THE STORM TRACK
SHIFTING NORTHWARD TOWARD ALASKA. AN EVEN BIGGER CHANGE MAY BE IN
STORE AT THE END OF NEXT WEEK AS MODELS SHOW PACIFIC STORMS
BEGINNING TO TRACK INTO THE BERING SEA...WHICH COULD EVENTUALLY
LEAD TO A WARM-UP OVER MAINLAND ALASKA FURTHER DOWN THE ROAD.

.AER/ALU...WATCH/WARNING SUMMARY
PUBLIC...BLIZZARD WATCH 181.
MARINE...STORM WARNING...155.
         GALE WARNING...120 130 132 138 140 150 160 165 170 171.
         HEAVY FREEZING SPRAY WARNING...121 129 130 132 138 140 141
                                        160 165 179 185.
FIRE WEATHER...NONE.

SEB JAN 12