Freezing Rain Advisory 4, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
120 PM AKST SAT DEC 3 2011

.MODEL DISCUSSION...MODELS ARE GENERALLY IN GOOD AGREEMENT ON THE
SYNOPTIC FEATURES THROUGH 60-72 HRS...ESPECIALLY WITH FRONTAL TROUGH
MOVEMENT THROUGH SOUTHCENTRAL ALASKA. MODELS HOWEVER STRUGGLE WITH
MOISTURE ADVECTION INTO SOUTHCENTRAL THIS AFTERNOON AND THE NEXT
STORM CENTER TO ENTER THE BERING SEA TUE.

.ANALYSIS AND UPPER LEVELS...
.IN THE EAST...SHALLOW COLD AIR IS TRAPPED UNDER ABOVE FREEZING AIR
OVER THE ANCHORAGE BOWL AND MATANUSKA/SUSITNA VALLEY. MOISTURE MOVING
IN FROM THE GULF HAS PRODUCED LIGHT FREEZING RAIN OVER THE ANCHORAGE
BOWL AND POSSIBLY SOME ISOLATED AREAS OVER THE MATANUSKA AND SUSITNA
VALLEY. ABUNDANT MOISTURE ASSOCIATED WITH A DEEP SUBTROPICAL FETCH IS
SPREADING NORTH...BUT INITIALLY IS BEING MITIGATED WITH STRONG
DOWNSLOPE FLOW IN THE LOWER LAYERS. STRONG PRESSURE GRADIENTS ARE
EVIDENT ACROSS SOUTHCENTRAL ALASKA WITH WINDS APPROACHING WARNING
LEVELS ACROSS TURNAGAIN ARM AND HIGHER ELEVATIONS.

IN THE WEST...THE DOMINANT FEATURE CONTINUES TO BE THE LARGE LOW
NEARING SAINT MATTHEW ISLAND SUPPORTED BY A 150 KT 300 MB JET. AN
ASSOCIATED FRONT NEARING THE COAST AND AKPEN IS FOR THE MOST PART
KEEPING PRECIPITATION WITH THIS SYSTEM IS IN THE FORM OF RAIN.
SUBTROPICAL MOISTURE AND WARM ADVECTION HAS PUSHED WELL INTO THE
SOUTHWEST MAINLAND WITH WIDESPREAD SURFACE TEMPERATURES ABOVE
FREEZING AND EVEN 40S OVER EASTERN BRISTOL BAY AND LOWER
KUSKOKWIM VALLEY. A STRONG PRESSURE GRADIENT WITH THIS SYSTEM
HAS PRODUCED WIDESPREAD STORM FORCE WINDS THROUGH THE BERING
SEA...WITH WINDS PEAKING IN THE 65 TO 70 KT RANGE IN THE PRIBILOF
ISLANDS.

.SHORT TERM FORECAST...
.SOUTHCENTRAL ALASKA...
MOST OF THE ACTION ACROSS SOUTHCENTRAL WILL BE AHEAD OF AND WITH THE
PASSAGE OF THE FRONTAL TROUGH APPROACHING SOUTHCENTRAL. THE FRONT IS
EXPECTED TO MOVE INTO THE SUSITNA VALLEY AND EXTEND SOUTH THROUGH
COOK INLET TO JUST EAST OF KODIAK ISLAND SUN MORNING. THE FRONT THEN
MOVES INTO WESTERN CANADA SUN NIGHT. STRONG MOISTURE ADVECTION AND
WARMING WITH DEEP SOUTHERLY FETCH IN ADVANCE OF FRONT WILL CONTINUE
TO PRODUCE RAIN AND SNOW ACROSS MUCH OF SOUTHCENTRAL UNTIL FRONTAL
PASSAGE SUN. TRAPPED COLD AIR IN THE MATANUSKA VALLEY AND ANCHORAGE
BOWL WILL BE SCOURED OUT THIS EVENING WITH WARM ADVECTION AND
TURBULENT MIXING AND END THE FREEZING RAIN THREAT. TEMPERATURES WILL
WARM THROUGH THE NIGHT IN THE LOWER SUSITNA VALLEY WITH MIXED
PRECIPITATION LIKELY...THOUGH THE VALLEY NORTH AND WEST OF TALKEETNA
WILL REMAIN COLD ENOUGH FOR SNOW WITH SIGNIFICANT AMOUNTS EXPECTED
INTO SUN AFTERNOON. TEMPS WILL COOL OFF AND PRECIPITATION WILL BECOME
SHOWERY AFTER FRONTAL PASSAGE ON SUN. STRONG PRESSURE GRADIENTS
ACROSS SOUTHCENTRAL WILL CONTINUE TO PRODUCE VERY STRONG GAP FLOWS
WHICH WILL PEAK JUST PRIOR TO FRONTAL PASSAGE SUN MORNING. STRONG
PERPENDICULAR (SE) FLOW ACROSS THE CHUGACH COMBINED WITH A STRONG
STABLE LAYER AND INTENSE PRESSURE GRADIENTS WILL PRODUCE WARNING
LEVEL WINDS ACROSS THE ANCHORAGE HILLSIDE/TURNAGAIN ARM ALONG WITH
WIDESPREAD MOUNTAIN WAVE TURBULENCE. WINDS WILL DIMINISH
SIGNIFICANTLY ACROSS SOUTHCENTRAL AFTER FRONTAL PASSAGE SUN.

SOUTHWEST...
WIDESPREAD RAIN WITH SOME WET SNOW WILL CONTINUE INTO SUNDAY AS THE
FRONT DRAGS THROUGH. A REINFORCING SHOT OF COLD ADVECTION WILL FILTER
IN BEHIND THE FRONT AND ALLOW PRECIPITATION TO END AS ALL SNOW. SOUTH
TO EAST WINDS WILL BACK TO THE WEST CONCURRENT WITH FROPA...AND WIND
STRENGTH WILL SLOWLY DIMINISH AS THE LOW WEAKENS OVER NORTHERN AK
THROUGH MONDAY.

BERING SEA/ALEUTIANS...
THE STRONGEST WINDS ARE PEAKING OVER THE BERING SEA...AND WILL
DIMINISH FROM SOUTH TO NORTH AS THE LOW DEPARTS THE REGION THROUGH
SUNDAY. MORE SEASONABLY COOLER TEMPERATURES IN THE SYSTEM'S WAKE WILL
ALLOW FOR FOR SOME LINGERING SNOW SHOWERS. A FRONT ASSOCIATED WITH
THE NEXT PACIFIC LOW LOOKS TO BRING A SWATH OF STORM FORCE WINDS TO
THE WESTERN ALEUTIAN CHAIN LATE SUNDAY NIGHT INTO MONDAY...WITH SOLID
GALES ADVANCING TO THE CENTRAL CHAIN/BERING ON MONDAY. THE WINDS TO
BE ACCOMPANIED BY ANOTHER SLUG OF STEADY PRECIPITATION...MAINLY IN
THE FORM OF RAIN.

.LONG TERM FORECAST...
A JET STREAM TRACKING FROM THE ALEUTIAN CHAIN TOWARDS THE SOUTHERN
MAINLAND LOOKS TO KEEP THE PATTERN ACTIVE IN THE LONG TERM FORECAST
PERIOD.

.AER/ALU...WATCH/WARNING SUMMARY
PUBLIC...HIGH WIND WARNING...101
         FREEZING RAIN ADVISORY...101
         WINTER WEATHER ADVISORY FOR SNOW/FREEZING RAIN...145

MARINE...STORM...120 125 175 176 180 185 412 414
         GALE...129 130 132 150 160 165 170 172 179 351 352 411 413
         HEAVY FREEZING SPRAY...185

RMC/JAM DEC 11