FXAK68 PAFC 212125 AFDAFC
SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION NATIONAL WEATHER SERVICE ANCHORAGE AK
1225 PM AKST THU FEB 21 2013
.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... 172 175. STORM WARNING... 170 176. GALE WARNING... 120 130 132 137 150 155 160 165 171 179 185. FIRE WEATHER... NONE.
SEB FEB 13