Heavy Freezing Spray Warning 239, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
200 PM AKST THU DEC 20 2012

.MODEL DISCUSSION...
GUIDANCE IN GOOD AGREEMENT THROUGH 60 HOURS. GENERALLY THE FORECASTS
INCORPORATE THE NAM FOR ITS BETTER HANDLING OF TERRAIN RELATED/SMALL
SCALE FEATURES. AFTER 60 HOURS THERE STARTS TO BE SOME DIVERGENCE ON
HOW THE MODELS HANDLE THE UPPER LEVEL LOW MOVING ACROSS SOUTHERN
ALASKA...AND THE ASSOCIATED SURFACE LOW(S) OVER THE GULF. THE GFS
TENDS TO BE SLOWER WITH THE UPPER LOW WHILE THE NAM AND ECMWF ARE A
LITTLE FASTER. AS THE LOW MOVES WEST AND BEGINS TO SPLIT...THE GFS
IS PUSHING MORE OF THE ENERGY INTO THE NORTHERN BRANCH OF THE SPLIT
WITH THE NAM AND ECMWF PUTTING MORE ENERGY IN THE SOUTHERN BRANCH.
THEREFORE CONFIDENCE IS NOT TOO HIGH AT THIS POINT IN THE
FORECAST...BUT THERE ARE PLENTY OF MODEL RUNS AHEAD FOR THIS TO SORT
ITSELF OUT.

.ANALYSIS AND UPPER LEVELS...
THERE IS A HUGE BLOCKING RIDGE BETWEEN 150 AND 160 DEGREES WEST
LONGITUDE. THIS RIDGE IS NEARLY FULL-LATITUDE...IT STRETCHES FROM
THE TROPICS TO 80 DEGREES NORTH. THERE IS AN UPPER LEVEL LOW OVER
THE SOUTHEAST GULF OF ALASKA THAT IS CONTINUING TO MOVE SOUTH. A
WEAK UPPER LEVEL TROUGH STRETCHES NORTH FROM THIS LOW INTO THE
ARCTIC OCEAN NEAR MACKENZIE BAY. THERE IS A LARGE UPPER LEVEL LOW
CENTERED OVER THE KAMCHATKA PENINSULA WITH STRONG SOUTHERLY FLOW
OVER THE CENTRAL AND WESTERN BERING SEA. NORTHERLY FLOW IS OVER MOST
OF SOUTHERN ALASKA.

.SHORT TERM FORECAST...
THE OVER-ALL PATTERN WILL REMAIN IN PLACE INTO FRIDAY. THE UPPER
LEVEL RIDGE WILL SLOWLY GET SQUEEZED FRIDAY AND THEN WILL "PINCH
OFF" WITH A CLOSED HIGH MOVING INTO THE ARCTIC OCEAN NEAR POINT
BARROW SATURDAY MORNING. AS THIS HAPPENS AN UPPER LEVEL LOW WILL
DEVELOP OVER THE YUKON TERRITORY FRIDAY NIGHT AND MOVE INTO SOUTHERN
ALASKA BY SATURDAY AFTERNOON. THIS LOW WILL THEN SPLIT ON SUNDAY
WITH SOME OF THE ENERGY MOVING NORTHWEST INTO WESTERN ALASKA...AND
SOME MOVING SOUTHEAST INTO THE GULF OF ALASKA. THE MODELS AGREE
THERE WILL BE A SPLIT...BUT DISAGREE ON HOW EXACTLY THIS WILL
PROGRESS.

SOUTH CENTRAL ALASKA...STRONG WINDS OVER FAVORED AREAS WILL CONTINUE
INTO FRIDAY WITH THE STRONG PRESSURE GRADIENT AND GOOD UPPER LEVEL
SUPPORT. WINDS WILL DIMINISH ON FRIDAY AS THE SURFACE GRADIENT
RELAXES. AS THE UPPER LOW MOVES INTO SOUTHERN ALASKA LATER SATURDAY
INTO SUNDAY...CHANCES OF PRECIPITATION WILL RETURN TO THE AREA.

SOUTHWEST ALASKA...WILL REMAIN DRY AND SEASONABLY COLD INTO SUNDAY
AS THE RIDGE STUBBORNLY HOLDS BACK ANY INTRUSION OF MOISTURE FROM
THE WEST. BY SUNDAY AFTERNOON THERE IS A SLIGHT CHANCE THAT
PRECIPITATION WILL MOVE IN FROM THE EAST AS THE UPPER LOW SLIDES
INTO THE AREA.

ALEUTIANS AND BERING SEA...SOUTHERLY FLOW ALOFT WILL CONTINUE TO
BRING PRECIPITATION INTO THE AREA THROUGH FRIDAY (MOSTLY IN THE FORM
OF RAIN). PRECIPITATION WILL THEN SLACK OFF FRIDAY NIGHT THROUGH
SUNDAY AS A WEAK UPPER LEVEL RIDGE BUILDS INTO THE BERING SEA.

.LONG TERM FORECAST...
THE EXTENDED FORECAST IS A BIT UNCERTAIN DUE TO MODEL DISAGREEMENT.
BUT WITH THE LACK OF A STRONG BLOCKING RIDGE...THERE IS SOME CHANCE
OF PRECIPITATION FOR MOST AREAS THROUGH THE PERIOD.

.AER/ALU...WATCH/WARNING SUMMARY
PUBLIC...HIGH WIND WARNING 111 131 135.
MARINE...GALE WARNING 127 128 130 170 172 175 176 179 180 185.
         HEAVY FREEZING SPRAY 121 125 126 127 128 129 130 132 138
             140 141 160.
FIRE WEATHER...NONE.

LUDWIG DEC 12