Blizzard and Ice Storm in Alaska, November 20 to 23, 2013

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
150 PM AKST WED NOV 20 2013

.ANALYSIS AND UPPER LEVELS...THE TROUGH WHICH HAD BEEN DROPPING
SOUTHWARD ACROSS MAINLAND ALASKA HAS STALLED OVER THE SOUTHCENTRAL
AND SOUTHWEST ALASKA. THE PRIMARY CAUSE OF THIS IS A SHORT-WAVE
OVER THE NORTHERN BERING SEA WHICH IS DIGGING SOUTHWARD AND CAUSING
FLOW OUT AHEAD OF IT TO BACK TOWARD THE SOUTH AND ALLOW TROPICAL
MOISTURE ASSOCIATED WITH A SYSTEM OVER THE NORTH PACIFIC TO BEGIN
TO FLOW NORTHWARD. THE INITIAL FOCUS AND IMPACTS OF THIS SYSTEM WILL
BE A WARM FRONT WHICH IS VERY SLOWLY LIFTING NORTHWARD INTO THE GULF
OF ALASKA. SNOW AND BLOWING SNOW HAS DEVELOPED ACROSS KODIAK ISLAND
AND WINDS HAVE BEGUN TO STRENGTHEN ACROSS THE GULF WATERS.

.MODEL DISCUSSION...MODELS CONTINUE TO BE SIMILAR WITH THE BIG
PICTURE OVER THE NEXT 48 HOURS AND FORECAST CONFIDENCE IS INCREASING
IN WIDESPREAD IMPACTS FROM WINTRY PRECIPITATION AND WINDS AS THIS
STORM MOVES THROUGH. THERE ARE STILL SOME TIMING DIFFERENCES WITH
THE WARM FRONT WHICH WILL DETERMINE ARRIVAL OF SNOW ACROSS THE
SOUTHERN MAINLAND TOMORROW. THE MODELS CONTINUE TO TREND FASTER
AND THE LATEST FORECAST WILL REFLECT THAT TREND. THE MOST
SIGNIFICANT DIFFERENCE IS WITH THE TRACK OF THE MAIN LOW MOVING UP
FROM THE PACIFIC THURSDAY AND INTO SOUTHCENTRAL/SOUTHWEST ALASKA
FRIDAY. SMALL DIFFERENCES IN THE TRACK OF THIS LOW WILL HAVE MAJOR
IMPACT ON THE FORECASTS FOR ANCHORAGE AND THE MATANUSKA VALLEY -
PRIMARILY DUE TO THE STRENGTH OF TURNAGAIN ARM WINDS AND
CROSS-BARRIER FLOW AND THE AFFECT THAT HAS ON QPF IN THESE AREAS.
THE GFS/ECMWF HAVE BEEN THE MOST CONSISTENT OVER THE PAST 24 HOURS
WITH A LOW TRACK ACROSS THE WESTERN KENAI PENINSULA/COOK INLET AND
JUST WEST OF ANCHORAGE.

.SHORT TERM FORECAST...BASED ON THE INCREASED CONFIDENCE IN THE
IMPACTS ACROSS THE SOUTHERN MAINLAND WILL ISSUE SOME SORT OF WINTER
WEATHER ADVISORY FOR A MAJORITY OF SOUTHCENTRAL AND SOUTHWEST ALASKA
WITH THE AFTERNOON FORECAST PACKAGE. AS THE WARM FRONT APPROACHES
THURSDAY AFTERNOON AND EVENING THE MAIN IMPACTS WILL BE SNOW AND
BLOWING SNOW. AS IT THEN PROGRESSES INLAND THURSDAY NIGHT AND FRIDAY
THERE WILL BE A TRANSITION FROM SNOW TO FREEZING RAIN OVER MANY
INLAND LOCATIONS AND FROM SNOW TO PLAIN RAIN ALONG THE GULF COAST.

AS MENTIONED IN THE MODEL DISCUSSION THE MOST CHALLENGING ASPECT
OF THE FORECAST WILL BE IN THE POPULATION CENTERS OF ANCHORAGE AND
PALMER/WASILLA. THERE IS FAIRLY HIGH CONFIDENCE THAT THE INITIAL
OVERRUNNING WILL PRODUCE SOME SNOW ACCUMULATION THURSDAY AFTERNOON
INTO EVENING. THE BIG QUESTION THEN...BASED ON LOW TRACK...IS HOW
STRONG DO THE WINDS GET ACROSS WEST TURNAGAIN ARM AND THE HIGHER
ELEVATIONS OF THE CHUGACH AND KENAI MOUNTAINS. IN GOING WITH THE
GFS/ECMWF MODEL SOLUTION OF TRACKING THE LOW TO THE BARREN ISLANDS
AND PASSING JUST WEST OF ANCHORAGE AS IT CONTINUES NORTHWARD EXPECT
THESE WINDS WON'T BECOME STRONG ENOUGH TO PRODUCE SIGNIFICANT
DOWNSLOPE DRYING. THEREFORE...AS WARMER AIR MOVES OVERHEAD THURSDAY
NIGHT INTO FRIDAY MORNING EXPECT AREAS OF FREEZING RAIN TO DEVELOP.
IF THE TRACK OF THE LOW ENDS UP BEING A LITTLE FARTHER WEST AND
STRONGER TURNAGAIN WINDS DEVELOP THEN THERE WOULD LIKELY BE A FAIRLY
DECENT PERIOD WITHOUT PRECIPITATION...LIMITING THE IMPACT OF FREEZING
PRECIPITATION AND ICE ACCUMULATION.

FORECAST CONFIDENCE INCREASES ONCE AGAIN ON THE BACK SIDE OF THE
SYSTEM AS A COLD FRONT RAPIDLY MOVES THROUGH IN THE LOW LEVELS
COINCIDENT WITH A POTENT SHORT-WAVE IN THE UPPER LEVELS. THIS WILL
RESULT IN A BAND OF MODERATE TO HEAVY SNOW. BASED ON CURRENT TIMING
IT LOOKS LIKE THIS BAND COULD CAUSE TRAVEL DIFFICULTIES FROM
ANCHORAGE TO THE MAT-SU DURING THE LATE FRIDAY AFTERNOON/EARLY
EVENING COMMUTE.

.LONG TERM FORECAST...THE UPPER LEVEL PATTERN LOOKS LIKE IT WILL
REMAIN IN FLUX AS WE HEAD THROUGH THE WEEKEND AND INTO NEXT WEEK.
THE LATEST MODELS ARE STARTING TO CONVERGE ON MORE SIMILAR SOLUTIONS
IN THIS TIME FRAME. THE GENERAL THEME IS THAT THE SOUTHERN STREAM
FLOW WILL DOMINATE MEANING THE SOUTHERN MAINLAND WILL BE RIGHT ON
THE BORDER BETWEEN THE ARCTIC AIR MASS AND WARMER AIR TO THE
SOUTH...WHICH WOULD TEND TO KEEP MOST PRECIPITATION IN THE FORM
OF SNOW.

.AER/ALU...WATCH/WARNING SUMMARY
PUBLIC...NONE.
MARINE...STORM WARNING...130 131 138.
         GALE WARNING...119 120 125 132 139 150 155 160 172.
         HEAVY FREEZING SPRAY WARNING...139 140 160 180 181 185.
FIRE WEATHER...NONE.

SEB NOV 13