Heavy Freezing Spray Warning 46, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
600 AM AKST MON JAN 31 2011

.ANALYSIS AND UPPER LEVELS...HIGHLY AMPLIFIED UPPER LEVEL PATTERN
CONTINUES TO BE IN PLACE OVER ALASKA WITH A STRONG RIDGE OVER THE
EASTERN PACIFIC EXTENDING FROM THE MIDLATITUDES NORTHWARDS INTO THE
ARCTIC. AN UPPER LEVEL TROUGH CENTERED OVER EASTERN SIBERIA COVERS
MUCH OF THE BERING SEA. A 115KT 250MB JET DELINEATES THE BOUNDARY
BETWEEN THESE TWO AIRMASSES AND IS ACTING TO STEER A SERIES OF UPPER
LEVEL WAVES...WITH SURFACE LOW REFLECTIONS...INTO THE GULF OF ALASKA
AND ULTIMATELY SOUTHCENTRAL. THE FIRST OF THESE FEATURES IS A LOW
THAT HAS MOVED FROM SHELIKOF STRAIT INTO THE BRISTOL BAY REGION
PUSHING A FRONT ONSHORE ACROSS THE NORTHERN GULF OF ALASKA. THIS
SYSTEM IS BRINGING SNOW TO THE USUAL UPSLOPE AREAS...WITH RAIN MIXING
IN FARTHER EAST AROUND CORDOVA. THE LOW HAS ALSO BROUGHT SNOW TO
SOUTHWEST ALASKA OVERNIGHT.

FARTHER WEST A RIDGE IS STARTING TO BUILD OVER THE EASTERN BERING
THOUGH THIS IS CURRENTLY BEING IMPINGED UPON BY A WEAK SHORTWAVE THAT
IS BRINGING AREAS OF CLOUDS AND SHOWERS TO THE CENTRAL AND EASTERN
ALEUTIANS. A SHARP UPPER LEVEL TROUGH LOCATED YET FARTHER WEST IS
ACTING TO SPIN UP A COMPACT...BUT IMPRESSIVE SURFACE LOW SOUTH OF THE
WESTERN ALEUTIANS.

.MODEL DISCUSSION...THE MODELS INITIALIZE REASONABLY CLOSE TO EACH
OTHER THIS MORNING...HOWEVER BY 36 HOURS SOME IMPORTANT DIFFERENCES
START TO CROP UP. THE ISSUE HAS TO DO WITH THE TRACK OF THE NEXT LOW
POISED TO MOVE INTO SOUTHCENTRAL ALASKA MONDAY NIGHT. ALL OF THE
MAJOR MODELS PROG A LOW TO MOVE INTO THE REGION TUE NIGHT INTO
WED...HOWEVER THERE ARE SUBTLE DIFFERENCES IN THE TRACK OF THE LOW
THAT COULD MAKE THE DIFFERENCE BETWEEN THE ANCHORAGE BOWL SEEING A
SIGNIFICANT SNOW EVENT OR PRECIP FALLING AS ALL RAIN. THE ECMWF AND
CANADIAN MODELS ARE TRACKING THE LOW WEST UP COOK INLET AND INTO THE
SUSITNA VALLEY...KEEPING THINGS WARM ENOUGH FOR RAIN WHILE THE NAM IS
THE FARTHER EAST OF THE MODEL SOLUTIONS...TRACKING THE LOW INTO
WESTERN PRINCE WILLIAM SOUND...A CLASSIC SNOW SETUP FOR ANCHORAGE.
THE GFS IS A LITTLE SLOWER IN TRANSITIONING THE LOW INTO THE
ANCHORAGE AREA...BUT OVERALL TRACKS THE LOW RIGHT OVER ANCHORAGE AND
THE MAT VALLEY...JUST FAR ENOUGH TO THE EAST TO KEEP THESE AREAS ON
THE COLD SIDE OF THE LOW. THE MORNING FORECAST PACKAGE FAVORS A MORE
EASTERLY SOLUTION...SIMILAR TO THE NAM AND GFS...WITH THE THINKING
THAT THE COLDER AIR IN PLACE SHOULD FAVOR A MORE EASTERLY TRACK. 12Z
PAFC SOUNDING DATA LENDS CREEDENCE TO THIS AS THE ECMWF 850MB TEMPS
ARE INITIALIZING ABOUT 2 DEGREES TOO WARM.

DIFFERENCES CONTINUE PAST 48 HOURS AS A SERIES OF LOWS MOVE UP INTO
THE GULF AND SOUTHCENTRAL HOWEVER THE NAM IS THE MODEL OF CHOICE IN
THE EAST. IT IS ALSO FAVORED IN THE WEST WHERE THERE IS A DIVERGENCE
OF TRACKS IN THE LOW MOVING INTO THE WESTERN ALEUTIANS MON NIGHT INTO
TUE MORNING. THE NAM SPLITS THE DIFFERENCE BETWEEN THE GFS AND ECMWF
AND AS SUCH SEEMS MOST PRUDENT FOR NOW.

CONFIDENCE IN THE EXTENDED FORECAST PAST 84 HOURS IS STILL VERY LOW
AS THERE ARE MAJOR DIFFERENCES BETWEEN THE MODELS.

.SHORT TERM FORECAST...

SOUTHCENTRAL...THE FRONT ALONG THE NORTH GULF COAST WILL CONTINUE TO
MOVE ONSHORE TODAY...BRINGING SNOW TO UPSLOPE AREAS ALONG THE NORTH
GULF COAST AND EASTERN SLOPES OF THE ALASKA RANGE AS WELL AS TO MUCH
OF SOUTHWESTERN ALASKA. PRECIP WILL TAPER OFF BRIEFLY THIS AFTERNOON
ACROSS SOUTHCENTRAL BEFORE THE ARRIVAL OF THE NEXT LOW STARTING THIS
EVENING...

AS MENTIONED ABOVE THERE IS STILL SOME UNCERTAINTY IN THE TRACK OF
THIS NEXT LOW BUT RIGHT NOW ALL OF THE INGREDIENTS...SOUTHERLY FLOW
BRINGING ABUNDANT MOISTURE WITH LITTLE DOWNSLOPING...A NEGATIVELY
TILTED UPPER LEVEL TROUGH PROVIDING LIFT...AND ENOUGH COLD AIR
ALREADY IN PLACE...ARE POINTING TOWARDS THE FIRST MAJOR SNOWFALL FOR
THE ANCHORAGE BOWL AND MAT-SU OF 2011. THIS LOW IS THE FIRST IN A
SERIES OF LOWS THAT WILL BE MOVING INTO SOUTHCENTRAL FROM THE
GULF...EACH BRINGING A POTENTIAL FOR SNOW AND RESULTING IN WHAT IS
LOOKING TO SHAPE UP INTO A VERY SNOWY WEEK ACROSS SOUTHCENTRAL. STAY
TUNED FOR MORE DETAILS...

SOUTHWEST AK...THE REMNANTS OF THE LOW MOVING NORTH ACROSS BRISTOL
BAY WILL BRING PERIODS OF SNOW TO MUCH OF SOUTHWEST ALASKA TODAY AND
INTO TONIGHT. AFTER THAT THOUGH MOST OF THE ACTION WILL BE CONFINED
TO SOUTHCENTRAL AS A RIDGE BUILDING OVER THE EASTERN BERING WILL
SERVE TO DRY THINGS OUT FOR THE MIDWEEK. A CHANCE OF SNOW CREEPS BACK
INTO THE FORECAST BY LATER IN THE WEEK AS RESIDUAL MOISTURE FROM THE
LOWS AFFECTING SOUTHCENTRAL SPILLS OVER THE ALASKA RANGE.

BERING SEA/ALEUTIANS...RELATIVELY QUIET ACROSS THE EASTERN ALEUTIANS
AND BERING AS A RIDGE BUILDS OVER THE AREA. TO THE WEST A COMPACT LOW
FORMING SOUTH OF THE WESTERN ALEUTIANS WILL MOVE NORTH INTO THE
WESTERN ALEUTIANS SPREADING SNOW AND GALES INTO THE WESTERN AND
CENTRAL ALEUTIANS. THE PROXIMITY OF THE WESTERN AND CENTRAL ALEUTIANS
TO THE PRIMARY UPPER LEVEL JET IN THE NORTH PACIFIC WILL KEEP THEM IN
LINE FOR A SERIES OF UPPER LEVEL WAVES TO BRUSH THE CHAIN...KEEPING
SNOW AND RAIN IN THE FORECAST FOR MUCH OF THIS WEEK.

.LONG TERM FORECAST...GIVEN THE LACK OF CONFIDENCE IN ANY SINGLE
MODEL SOLUTION...DETAILS ARE SUITABLY VAGUE AT THIS POINT...THOUGH
THE GENERAL INDICATIONS ARE THAT THE BROAD SYNOPTIC PATTERN WILL
LIKELY CONTINUE TO BE QUITE UNSETTLED AS THE RIDGE OVER THE EASTERN
PACIFIC IS NOT LIKELY TO BREAK DOWN ANYTIME SOON. THIS WILL KEEP MUCH
OF THE AOR IN THE BULLSEYE FOR THE NORTH PACIFIC STORM TRACKS THROUGH
THE WEEKEND AND INTO NEXT WEEK.

.AER/ALU...WATCH/WARNING SUMMARY
PUBLIC...NONE
MARINE...STORM WARNING...130
         GALE WARNING...120 125 155 160 175 176
FIRE WEATHER...NONE

WOOD JAN 11

SPECIAL WEATHER STATEMENT
NATIONAL WEATHER SERVICE ANCHORAGE AK
1050 AM AKST MON JAN 31 2011

AKZ101-111-121-125-131-135-141-145-010400-

...UNSETTLED WEATHER RETURNS TO SOUTHCENTRAL ALASKA THIS WEEK...

A SERIES OF LOW PRESSURE SYSTEMS WILL DEVELOP OVER THE NORTHERN
PACIFIC AND FOLLOW A SIMILAR PATH TOWARD SOUTHCENTRAL ALASKA. THE
FIRST LOW WILL ARRIVE LATER THIS EVENING AND SPREAD MOISTURE INLAND
OVERNIGHT. SNOW WILL MIX WITH OR CHANGE OVER TO RAIN ALONG THE GULF
COAST. INLAND...TEMPERATURES ARE EXPECTED TO REMAIN BELOW FREEZING SO
THAT ALL PRECIPITATION THAT FALLS WILL REMAIN IN THE FORM OF SNOW.

SNOWFALL TOTALS WILL VARY ACROSS THE REGION AS SUBSEQUENT LOWS TAKE
SLIGHTLY DIFFERENT PATHS DURING THE WEEK. HEAVIEST SNOW WILL BE AT
HIGHER ELEVATIONS AROUND PRINCE WILLIAM SOUND AND COASTAL AREAS LIKE
VALDEZ WHERE COLD AIR WILL REMAIN TRAPPED.

NO WATCHES, WARNINGS, OR ADVISORIES HAVE BEEN ISSUED AS OF MONDAY
MORNING BUT STAY TUNED AS THE SYSTEM EVOLVES AND SLIGHT CHANGES IN
THE TRACK OF THE DEVELOPING LOWS COULD LEAD TO LARGE CHANGES IN
FORECASTED SNOWFALL TOTALS.

THE LATEST INFORMATION CAN BE FOUND AT THE NATIONAL WEATHER SERVICE
FORECAST WEB PAGE... HTTP://PAFC.ARH.NOAA.GOV

AB JAN 10