Brisk Wind Advisory 20, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION...CORRECTED
NATIONAL WEATHER SERVICE ANCHORAGE AK
1225 PM AKST THU FEB 21 2013

CORRECTED MARINE HEADLINES AT BOTTOM

.ANALYSIS AND UPPER LEVELS...A LONG-WAVE TROUGH ENCOMPASSES ALL OF
THE ALASKA REGION WITH THE EXCEPTION OF A BLOCKING HIGH CENTERED
OVER THE CHUKCHI SEA. A STRONG SUBTROPICAL JET EXTENDS ACROSS THE
NORTH PACIFIC PASSING JUST SOUTH OF THE ALEUTIANS AND GULF OF ALASKA.
THIS HAS RESULTED IN RAPID MOVEMENT OF DEEP LOWS ACROSS THE SOUTHERN
TIER OF THE FORECAST AREA. ONE SUCH LOW IS PASSING JUST SOUTH OF
THE ALASKA PENINSULA AND KODIAK ISLAND THIS MORNING. AS HAS BEEN
THE CASE WITH SYSTEMS PRECEDING THIS ONE IT IS SMALL AND COMPACT.
THIS PRODUCES RAPID PRESSURE FALLS AHEAD OF THE SYSTEM AND RAPID
PRESSURE RISES BEHIND IT. THE PRESSURE RISES ARE LEADING TO STRONG
GUSTY WINDS IN THE WAKE OF THE LOW.

MEANWHILE...A SHORT-WAVE TROUGH OVER THE NORTHERN GULF IS LIFTING
NORTHWARD HELPING TO ADVECT LOW LEVEL MOISTURE INLAND RESULTING
IN AREAS OF LIGHT SNOW OVER SOUTHCENTRAL. A SEPARATE OUT OF PHASE
SHORT-WAVE TROUGH OVER SOUTHWEST ALASKA IS ALSO LIFTING NORTHWARD...
BUT BEGINNING TO STALL THANKS TO ANOTHER SHORT-WAVE DROPPING DOWN
FROM NORTHWEST ALASKA. THE AREA IN BETWEEN THE TWO IS OBSERVING
SNOW...FROM THE KUSKOKWIM DELTA TO THE KUSKOKWIM VALLEY. A COUPLE
OTHER SHORT-WAVES CROSSING THE BERING SEA AND ALEUTIANS ARE
PRODUCING SHOWERS AND LOCALLY GUSTY WINDS.

THE NEXT MAJOR STORM SYSTEM IS JUST BEGINNING TO DEEPEN SOUTH OF
THE ALEUTIANS.

.MODEL DISCUSSION...MODELS HAVE COME INTO MUCH BETTER AGREEMENT
WITH THE MANY WEAK SHORT-WAVES TRANSITING THE REGION OVER THE
NEXT DAY OR SO. THERE ARE HOWEVER MUCH MORE SIGNIFICANT DIFFERENCES
WITH THE EVOLUTION OF THE DEVELOPING NORTH PACIFIC LOW. OVER THE
PAST 24 HOURS THE ECMWF HAS SHIFTED EAST CLOSER TO WHERE ALL OTHER
AVAILABLE MODELS HAD THE LOW WHILE THE GFS AND NAM HAVE SHIFTED
WESTWARD TO THE OLD POSITION OF THE ECMWF. FOR THE 12Z MODEL CYCLE
THE FASTER (FARTHER EAST) ECMWF TRACK IS SUPPORTED BY THE
GEMGLB/UKMET AS WELL AS ECMWF ENSEMBLE AND OTHER GLOBAL ENSEMBLES.
INHERITED FORECAST IS IN BETWEEN THE ECMWF AND GFS SOLUTIONS...SO
PLAN TO BLEND TOWARD THE ECMWF RECOGNIZING THERE IS STILL QUITE
A BIT OF UNCERTAINTY. IN ADDITION...MODELS ARE TRENDING TOWARD
DEVELOPMENT OF A TRIPLE POINT LOW IN BRISTOL BAY WHICH CHANGES
THE IMPACT OF THIS SYSTEM ON SOUTHCENTRAL. FOR NOW WILL TRY AND
FOCUS ON THE HIGHER PROBABILITY ASPECTS OF THE FORECAST AND
BROAD-BRUSH THE REST.

.SHORT TERM FORECAST...

SOUTHCENTRAL ALASKA...EXPECT AREAS OF SNOW TO PERSIST THROUGH FRIDAY
AS WEAK UPPER LEVEL SHORT-WAVES MOVE THROUGH THE AREA. GENERALLY
EXPECT LIGHT SNOW WITH MINOR ACCUMULATION. THE STRONGEST AND MOST
FOCUSED VERTICAL MOTION WILL BE DURING THE DAY FRIDAY AS A SHORT-WAVE
AMPLIFIES AS IT TRACKS FROM SOUTHWEST ALASKA TO SOUTHCENTRAL THANKS
TO A BUILDING UPSTREAM RIDGE (AHEAD OF THE PACIFIC STORM). SOME WEAK
COLD ADVECTION WILL ACCOMPANY THIS SHORT-WAVE AND PRODUCE SNOW MAINLY
ON THE WEST SIDE OF THE CHUGACH AND TALKEETNA MOUNTAINS FROM
ANCHORAGE TO THE MAT VALLEY AND EASTERN SUSITNA VALLEY. MEANWHILE...
STRONG WINDS WILL DEVELOP OVER THE GULF AND KODIAK ISLAND WATERS IN
THE WAKE OF THE LOW CURRENTLY PASSING SOUTH OF KODIAK. INITIALLY LOW
LEVEL FORCING WILL DOMINATE WITH A TIGHT PRESSURE GRADIENT/PRESSURE
RISES/AND WEAK COLD AIR ADVECTION. AN UPPER TROUGH WILL THEN MOVE
THROUGH FRIDAY MORNING AND PRODUCE STRONG DOWNWARD MOTION WHICH WILL
HELP MIX DOWN STRONGER WINDS FROM ALOFT...IN EFFECT MAINTAINING THE
STRONG WINDS AT THE SURFACE THROUGH FRIDAY AFTERNOON.

THERE WON'T BE MUCH OF A BREAK BEFORE FRONTAL SYSTEM ASSOCIATED
WITH DEEP LOW MOVING INTO THE BERING SEA ARRIVES IN THE GULF AND
SOUTHCENTRAL FRIDAY NIGHT AND SATURDAY. AT THIS POINT THERE IS QUITE
A BIT OF UNCERTAINTY IN THE DETAILS OF THIS SYSTEM...BUT GENERALLY
EXPECT A STRONG FAST MOVING FRONT TO QUICKLY WEAKEN AS IT CROSSES
THE GULF AND REACHES SOUTHCENTRAL. THE TRIPLE POINT LOOKS LIKE IT
WILL SPEED QUICKLY TOWARD SOUTHEAST ALASKA...WHICH MEANS AIR MASS
WILL REMAIN COLD ENOUGH OVER SOUTHCENTRAL TO PRODUCE PRIMARILY SNOW.

SOUTHWEST ALASKA...THE SHORT-WAVE WHICH HAS BEEN LIFTING NORTHWARD
THIS MORNING WILL STALL THIS AFTERNOON AND THEN SLIDE EASTWARD
TOWARD SOUTHCENTRAL TONIGHT. THIS WILL BRING AN END TO MOST OF THE
SNOW ACROSS THE REGION. THERE MAY BE SOME LINGERING SHOWERS AS ONE
FINAL UPPER LEVEL WAVE MOVES THROUGH ON FRIDAY...ESPECIALLY ON THE
WES SIDE OF THE ALASKA AND ALEUTIAN RANGES AND ALONG THE ALASKA
PENINSULA. THE FRONTAL SYSTEM FROM THE STRONG LOW OUT WEST WILL
ARRIVE FRIDAY NIGHT INTO SATURDAY. THERE IS QUITE A BIT OF
UNCERTAINTY AS TO LOCATION AND EXTENT OF TRIPLE POINT LOW
DEVELOPMENT. THERE IS POTENTIAL FOR A BRIEF PERIOD OF LOW
VISIBILITIES IN SNOW AND BLOWING SNOW AS THIS FIRST ARRIVES...BUT
WILL NEED TO WAIT UNTIL HAVE A BETTER IDEA OF HOW STRONG WINDS WILL
BE BEFORE CONSIDERING WHETHER ANY ADVISORIES OR WARNINGS WILL BE
NEEDED. IT DOES LOOK LIKE EVERYTHING WILL ME MOVING FAIRLY QUICKLY
AND WEAKENING AS IT ARRIVES...SO THE DURATION OF THIS STORM SHOULD
BE FAIRLY SHORT.

BERING SEA/ALEUTIANS...THERE IS FAIRLY HIGH PROBABILITY THAT WINDS
ALONG THE ALEUTIANS WILL REACH HIGH WIND WARNING THRESHOLDS ON THE
SOUTH SIDE OF THE STORM AS IT MOVES INTO THE SOUTHERN BERING AND
RAPIDLY OCCLUDES. HOWEVER...UNCERTAINTY IN THE TRACK OF THIS SYSTEM
MAKES IT DIFFICULT TO PINPOINT WHERE THE STRONGEST WINDS WILL BE.
THEREFORE...PLAN TO STICK WITH THE CURRENT HIGH WIND WATCH FOR THE
CENTRAL ALEUTIANS AND MAKE THE WARNING DECISION AFTER SEEING THE
00Z MODEL CYCLE. THE OTHER POTENTIAL HAZARDS ARE A BLIZZARD ON THE
PRIBILOF ISLANDS AS THE OCCLUDED FRONT ARRIVES TOMORROW AFTERNOON
OR EVENING AND A BLIZZARD OVER THE WESTERN ALEUTIANS ON THE BACK
SIDE OF THE LOW. SINCE SYSTEM POSITION WILL BE KEY TO WHETHER THESE
HAPPEN OR NOT...WILL HOLD OFF ON ISSUING ANY ADDITIONAL WATCHES FOR
NOW. THIS SYSTEM WILL ULTIMATELY TRACK EASTWARD ACROSS THE SOUTHERN
BERING AS WE HEAD THROUGH THE WEEKEND WITH ARCTIC AIR DIVING
SOUTHWARD BEHIND THE LOW AND PRODUCING SNOW SHOWERS AND
GUSTY WINDS.

.LONG TERM FORECAST...THE UPPER LEVEL FLOW LOOKS LIKE IT WILL REMAIN
PROGRESSIVE THROUGH THE EXTENDED FORECAST PERIOD. A PERSISTENT
STRONG SUBTROPICAL JET OVER THE NORTH PACIFIC WILL CONTINUE TO
GENERATE DEEP LOWS TRACKING ALONG THE SOUTHERN TIER OF THE FORECAST
AREA FROM THE ALEUTIANS TO THE GULF OF ALASKA. THIS WILL GENERALLY
KEEP TEMPERATURES ON THE COLD SIDE ACROSS THE BERING
SEA/ALEUTIANS/SOUTHWEST ALASKA WHILE SOUTHCENTRAL WILL LIKELY
REMAIN NEAR NORMAL.

.AER/ALU...WATCH/WARNING SUMMARY
PUBLIC...HIGH WIND WATCH 187.
MARINE...HURRICANE FORCE WIND WARNING...172
         STORM WARNING...170 175
         GALE WARNING...120 130 132 137 150 155 160 165 171 179 185
FIRE WEATHER...NONE.

SEB FEB 13