Winter Weather Advisory 30, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
200 PM AKDT THU MAR 22 2012

.ANALYSIS AND UPPER LEVELS...THE FOCUS TODAY IS A RAPIDLY DEEPENING
LOW OVER THE WESTERN BERING SEA. THIS SYSTEM IS TRACKING NORTHWARD
UP THE WEST SIDE OF A HIGH AMPLITUDE RIDGE CENTERED OVER THE EASTERN
BERING SEA. UPPER LEVEL JET DYNAMICS ARE STRONG AND ARE HELPING TO
PUMP LOTS OF MOISTURE AND WARM AIR NORTHWARD. THIS IS CREATING SOME
VERY STRONG TEMPERATURE GRADIENTS AS WARM SUBTROPICAL AIR MOVES INTO
AN ENVIRONMENT WHICH HAS BEEN DOMINATED BY COLD ARCTIC AIR IN RECENT
DAYS. TEMPERATURES HAVE CLIMBED INTO THE MID 40S IN THE CENTRAL
ALEUTIANS IN THE WARM SECTOR...WHILE OUT AHEAD OF THE WARM FRONT
BLIZZARD CONDITIONS ARE BEING OBSERVED ON THE PRIBILOF ISLANDS WITH
TEMPERATURES IN THE TEENS TO LOWER 20S. THEREFORE...EXPECT A
DRAMATIC WARM UP IN TEMPERATURES AND TRANSITION IN PRECIP TYPE FROM
SNOW TOWARD MIXED PRECIP OR RAIN AS THIS SYSTEM PROGRESSES EASTWARD
ACROSS THE BERING SEA.

DOWNSTREAM OF THE UPPER RIDGE CLEAR AND COLD CONDITIONS DOMINATE
MAINLAND ALASKA. THERE ARE SOME LOCALIZED GAP WINDS ALONG THE GULF
COAST DUE TO A TIGHT PRESSURE GRADIENT CAUSED BY A SYSTEM JUST
SOUTH OF THE GULF OF ALASKA. THIS LOW IS TRACKING AWAY FROM THE
AREA SO WINDS WILL DIMINISH AS WE HEAD THROUGH THIS AFTERNOON.

.MODEL DISCUSSION...THE NAM IS AN OUTLIER WITH TRACK OF TRIPLE
POINT LOW WHICH FORMS NEAR KODIAK ISLAND FRIDAY AFTERNOON...TAKING
IT ALONG THE NORTH GULF COAST. PREFER THE MORE SOUTHERLY TRACK
WHICH HAS BEEN CONSISTENTLY DEPICTED BY THE GFS AND ECMWF. THE
TRACK OF THIS LOW WILL DETERMINE HOW STRONG THE WINDS WILL BE
THROUGH TURNAGAIN ARM AND PORTAGE VALLEY. AND MORE GENERALLY
ACROSS THE KENAI MOUNTAINS. THIS IN TURN AFFECTS HOW STRONG THE
DOWNSLOPE DRYING AFFECT WILL BE ON THE WEST SIDE OF THE KENAI
AND CHUGACH MOUNTAINS.

.SHORT TERM FORECAST...

SOUTHCENTRAL ALASKA...CONDITIONS WILL RAPIDLY HEAD DOWNHILL FRIDAY
AS FRONT MOVING ACROSS THE BERING TODAY MOVES INTO THE WESTERN GULF.
OVERRUNNING OUT AHEAD OF THE FRONT WILL SPREAD LIGHT SNOW INTO
THE COOK INLET REGION FRIDAY MORNING SPREADING NORTH AND EASTWARD
FRIDAY AFTERNOON. A TRIPLE POINT LOW WILL THEN FORM NEAR KODIAK
ISLAND LATE FRIDAY AFTERNOON AND MOVE OUT INTO THE GULF FRIDAY NIGHT.
THIS WILL CHANGE THE WHOLE DYNAMIC OF THE SYSTEM BY CAUSING FLOW
OVER SOUTHCENTRAL TO GRADUALLY BACK TOWARD THE NORTH AND CUT OFF
OR AT A MINIMUM LIGHTEN UP THE SNOWFALL.

THE GREATEST UNCERTAINTY IN THE FORECAST IS WITH THE STRENGTH OF
EASTERLY FLOW ACROSS THE KENAI PENINSULA AND ANCHORAGE...AND THE
MORE SPECIFIC FORECAST THROUGH PORTAGE VALLEY AND TURNAGAIN ARM.
THERE WILL CERTAINLY BE A PRESSURE GRADIENT WIND AS THE FRONT NEARS.
THE BIGGER QUESTION IS THE TRACK OF THE LOW AND WHETHER STRONG
EASTERLY WINDS ON THE NORTH SIDE OF THE LOW WILL MAKE IT TO TURNAGAIN
ARM AND THE KENAI PENINSULA OR REMAIN FURTHER SOUTH. RIGHT NOW WE
THINK THEY WILL REMAIN FURTHER SOUTH. BASED ON THIS MORE SOUTHERLY
TRACK IT STILL APPEARS THERE IS JUST ENOUGH WIND TO CAUSE BLIZZARD
CONDITIONS THROUGH PORTAGE VALLEY AND EASTERN TURNAGAIN ARM...SO
WILL ISSUE A BLIZZARD WATCH FOR FRIDAY NIGHT. FOR ANCHORAGE...THE
MAT VALLEY...AND WESTERN KENAI PENINSULA...AFTER THE INITIAL
OVERRUNNING SNOW EXPECT THE EASTERLY FLOW TO CUT SNOW OFF BY FRIDAY
EVENING. WITH THE SURFACE LOW AND MOST OF THE UPPER LEVEL ENERGY
BECOMING CENTERED OVER THE GULF FRIDAY NIGHT INTO SATURDAY...SNOW
WILL GRADUALLY END ACROSS THE SOUTHCENTRAL. A VERY WEAK SHORT-WAVE
TROUGH WILL CROSS SOUTHCENTRAL SATURDAY NIGHT...PERHAPS CAUSING SOME
LIGHT SNOW BUT WITH LITTLE OR NO ACCUMULATION.

A WEAK UPPER RIDGE WILL MOVE IN BEHIND THIS SYSTEM BRINGING A RETURN
TO DRY CONDITIONS. IT IS NOTABLE THAT THERE WON'T BE ANY COLD AIR
MOVING BACK IN BEHIND THIS SYSTEM. THEREFORE...EXPECT TEMPERATURES
TO BE WARMER THAN THEY HAVE BEEN RECENTLY...ESPECIALLY DURING THE
DAYTIME.

SOUTHWEST ALASKA...BLIZZARD WARNINGS ARE ALREADY IN EFFECT ALONG
THE KUSKOKWIM DELTA COAST AND WESTERN CAPES OF THE BRISTOL BAY ZONE
AS THE FRONT REACHES THE COAST THIS EVENING. EVEN THOUGH THE FRONT
WILL WEAKEN AS IT MOVES ONSHORE IT LOOKS STRONG ENOUGH TO CAUSE
BLOWING SNOW OVER OTHER PARTS OF BRISTOL BAY...PRIMARILY FROM
DILLINGHAM NORTHWARD. WITH STRONG DYNAMICS AND LOTS OF MOISTURE
TO WORK WITH ALSO EXPECT ADVISORY LEVEL SNOW AMOUNTS FOR THOSE
SAME AREAS OF BRISTOL BAY AND APPROACHING THOSE LEVELS IN THE
SOUTHERN KUSKOKWIM VALLEY. THEREFORE...PLAN TO ISSUE A WINTER
WEATHER ADVISORY FOR SNOW AND BLOWING SNOW FOR DILLINGHAM NORTHWARD
AND CONSIDER AN ADVISORY FOR THE SOUTHERN KUSKOKWIM VALLEY. THE
FRONT WILL BECOME OCCLUDED BEFORE IT REACHES MOST OF SOUTHWEST
ALASKA...SO PRECIP WILL REMAIN ALL SNOW. THE ONE EXCEPTION WILL BE
THE AREA FROM KING SALMON SOUTHWARD ALONG THE ALASKA PENINSULA WHERE
THE COMBINATION OF WARMER AIR AND STRONG SOUTHEAST WINDS MAY RESULT
IN A PERIOD OF MIXED PRECIP OR ALL RAIN DURING THE DAY FRIDAY. THIS
SYSTEM WILL EXIT TO THE EAST FRIDAY NIGHT INTO SATURDAY WITH HIGH
PRESSURE MOVING BACK AND PROVIDING DRY CONDITIONS FOR THE REST OF
THE WEEKEND. THERE IS NO SURGE OF ARCTIC AIR LIKE OFTENTIMES OCCURS
BEHIND THESE SYSTEMS...SO DON'T EXPECT TEMPERATURES TO BE AS COLD
AS THEY HAVE BEEN RECENTLY.

BERING SEA/ALEUTIANS...AS FRONT PROGRESSES EASTWARD THIS AFTERNOON
AND EVENING WARMER AIR WILL MAKE IT TO THE PRIBILOF ISLANDS AND
TO THE SOUTHERN ALASKA PENINSULA CAUSING A CHANGE TO MIXED
PRECIPITATION OR RAIN. THERE LIKELY WILL BE A PERIOD OF FREEZING
RAIN AS THE WARM AIR RIDES OVER TOP OF THE VERY COLD AIR NEAR THE
SURFACE. HOWEVER...DO NOT EXPECT THIS TO BE LONG LIVED OR CAUSE
ANY SIGNIFICANT PROBLEMS. THE SYSTEM WILL BEGIN TO OCCLUDE TONIGHT
AND ARCTIC AIR WILL MOVE BACK IN TO THE WESTERN TO CENTRAL BERING
AND ALEUTIANS BY LATE TONIGHT AND THE EASTERN BERING AND ALASKA
PENINSULA FRIDAY MORNING. THE CURRENT SYSTEM WILL CAUSE THE
UPPER LEVEL PATTERN TO BECOME MUCH MORE PROGRESSIVE. EXPECT A
PACIFIC STORM TO TRACK TO THE CENTRAL TO EASTERN ALEUTIANS FRIDAY
NIGHT AND SATURDAY WITH ANOTHER SYSTEM TO FOLLOW ALONG THE WESTERN
ALEUTIANS ON SUNDAY.

.LONG TERM FORECAST...ZONAL FLOW WILL SET UP IN THE WAKE OF THE
CURRENT SYSTEM AND KEEP STORMS ALONG THE SOUTHERN TIER OF THE
FORECAST AREA...ALONG THE ALEUTIANS AND INTO THE GULF OF ALASKA.
MODELS DO INDICATE A DEEPENING UPPER LOW IN THE GULF AS WE HEAD
THROUGH NEXT WEEK...WHICH MEANS SOMETHING COULD ALWAYS ROTATE
NORTHWARD AND AFFECT SOUTHCENTRAL. OTHERWISE...CONDITIONS LOOK
DRY FOR THE ALASKA MAINLAND.

.AER/ALU...WATCH/WARNING SUMMARY
PUBLIC...BLIZZARD WARNING 155 161 195.
MARINE...STORM WARNING...185.
         GALE WARNING...120 130 132 155 160 165 170 175 176 179 180.
         HEAVY FREEZING SPRAY WARNING 130.
FIRE WEATHER...NONE.

SEB MAR 12