Heavy Freezing Spray Warning 84, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
210 PM AKST TUE FEB 21 2012

.ANALYSIS AND UPPER LEVELS...
MOST OBVIOUS FEATURE ON SATELLITE IMAGERY EARLY THIS AFTERNOON IS AN
IMPRESSIVE PLUME OF MOISTURE ASSOCIATED WITH AN ACTIVE JET STREAM
SOUTH OF OUR DOMAIN. THE JET STREAM IS SENDING ABUNDANT MOISTURE
TOWARD THE NORTHWEST CONUS WITH POTENTIAL FOR FLOODING AND AVALANCHE
DANGER. A BROAD UPPER TROF IS POSITIONED OVER ALASKA HELPING TO STEER
THE JET STREAM SOUTH OF OUR AREA OF RESPONSIBILITY. MULTIPLE WEAK
IMPULSES ROTATING AROUND THE BROAD CYCLONIC CIRCULATION CONTINUES TO
BRING LIGHT SNOW TO MANY REGIONS WITH A FEW BREAKS IN THE CLOUDS
BETWEEN THE DISTURBANCES.

AT THE SURFACE...ALTHOUGH IT IS NOT OBVIOUS ON THE SATELLITE...A FEW
OBSERVATIONS IN THE NORTHERN GULF POINT TOWARD A REMNANT SURFACE LOW
BETWEEN MIDDLETON ISLAND AND MONTAGUE ISLAND. COUPLED WITH A WEAK
UPPER LEVEL WAVE...THIS SURFACE LOW IS RESPONSIBLE FOR THE LIGHT SNOW
AROUND SOUTHCENTRAL ALASKA THIS MORNING. ANOTHER WEAK LOW IS APPARENT
ON SATELLITE OVER THE EASTERN BERING SEA APPROACHING THE KUSKOKWIM
DELTA. HIGH PRESSURE IS IN CONTROL OF THE WESTERN BERING SEA AHEAD OF
THE NEXT SYSTEM THAT IS DEVELOPING SOUTH OF THE KAMCHATKA PENINSULA.

.MODEL DISCUSSION...
IN THE EAST...THE MODELS INITIALIZE WELL ON THE MAIN SYNOPTIC
FEATURES AT 18Z AND ARE VERY CONSISTENT AS WELL AS IN EXCELLENT
AGREEMENT. MINIMAL CHANGES WILL BE MADE TO THE AFTERNOON FORECAST
PACKAGE. THE FOCUS WILL BE INCREASING THE PROBABILITY OF
PRECIPITATION OVER CERTAIN REGIONS AS CONFIDENCE INCREASES WITH THE
CURRENT PATTERN.

.SHORT TERM FORECAST...
SOUTHCENTRAL...
LIGHT SNOW FALLING ACROSS SOUTHCENTRAL ALASKA THIS MORNING IS
LINGERING AND A BIT MORE STEADY THEN PREVIOUS ANTICIPATED. THE
SHORTWAVE TROF LIFTING NORTH APPEARS TO BE A BIT MORE AGGRESSIVE THAN
PREVIOUS MODELS HAD INDICATED. ALTHOUGH CLEARING CONTINUES OVER THE
SOUTHERN KENAI PENINSULA...THE ANCHORAGE BOWL AND NORTHWARD IS STILL
STRUGGLING TO BREAK OUT OF THE LOWER CEILINGS AND VIS RESTRICTIONS IN
SNOW.

THE SHORTWAVE IS CURRENTLY POSITIONED OVER ANCHORAGE AND WILL MOVE
NORTH THIS EVENING SO DO EXPECT SOME CLEARING LATE THIS AFTERNOON OR
EVENING OVER ANCHORAGE WHILE THE MATSU VALLEY...SUSITNA VALLEY...AND
COPPER RIVER BASIN ALL MAINTAIN THE MARGINAL CEILINGS AND OCCASIONAL
LIGHT SNOW.

SOUTHWEST FLOW ALOFT AND EMBEDDED IMPULSES WILL LIKELY RESULT IN
MARGINAL CEILINGS AND FLURRIES RETURNING TO THE WESTERN KENAI
PENINSULA AND ANCHORAGE BOWL LATER TONIGHT FOLLOWED BY ANOTHER
CLEARING PERIOD WEDNESDAY AFTERNOON. THE SURFACE LOW IN WESTERN
PRINCE WILLIAM SOUND WILL GO THROUGH CYCLES OF STRENGTHENING AND
WEAKENING AS IT MEANDERS AROUND THE NEXT 48 HOURS. IT IS NOT UNTIL
THURSDAY AFTERNOON THAT FLOW ALOFT BECOMES MORE WEST TO NORTHWESTERLY
WHICH WILL HELP START TO DRY THE REGION OUT AND USHER THE REMNANT LOW
SOUTHEAST AND OUT OF OUR DOMAIN.

A SHARP RIDGE OF HIGH PRESSURE BUILDS OVER THE REGION THURSDAY NIGHT
BRINGING WHAT LOOKS TO BE VERY PLEASANT WEATHER BY FRIDAY.

SOUTHWEST...
THE WEAK LOW THAT IS APPROACHING THE KUSKOKWIM DELTA WILL MOVE INLAND
OVERNIGHT AND WEAKEN OVER THE MIDDLE YUKON VALLEY THROUGH THURSDAY
AFTERNOON. AS A RESULT...ONSHORE FLOW WILL CONTINUE FROM BRISTOL BAY
UP TO THE KUSKO DELTA...ALLOWING FOR LOW LEVEL MOISTURE IN THE FORM
OF LIGHT SNOW AND FOG TO INVADE THE REGION. INTERIOR SECTIONS WILL
SEE BREAKS IN THE CLOUDS...ESPECIALLY DURING THE AFTERNOON AND
EVENINGS...BUT MARGINAL TO IFR CONDITIONS CAN BE EXPECTED FOR MOST OF
THE COASTAL AREAS THROUGH WEDNESDAY.

THE AFOREMENTIONED RIDGE OF HIGH PRESSURE SHOVES THE AREA OF LOW
PRESSURE FAR ENOUGH INLAND THAT SNOW FINALLY TAPERS OFF THURSDAY.
OUTSIDE OF THE CHANCE OF VALLEY FOG...CONDITIONS WILL DRASTICALLY
IMPROVE THURSDAY NIGHT AND FRIDAY UNDER THE INFLUENCE OF THE HIGH
PRESSURE. THE NEXT STORM SYSTEM APPROACHES FRIDAY EVENING WITH THE
CHANCE OF BLIZZARD CONDITIONS RETURNING TO THE COAST.

BERING SEA/ALEUTIANS...
SHOWERS ASSOCIATED WITH UPPER LEVEL DISTURBANCES ARE THE MAIN WEATHER
MAKER OUT WEST WHILE HIGH PRESSURE BUILDS OVER THE WESTERN BERING.
THESE LIGHT TO MODERATE COLD AIR ADVECTION SHOWERS WILL CONTINUE OVER
THE EASTERN BERING THROUGH WEDNESDAY UNTIL THE RIDGE MOVES OVERHEAD.

FURTHER WEST...A LARGE AND VERY COMPLEX LOW DEVELOPS SOUTHWEST OF THE
WESTERN ALEUTIANS. THE MODELS ARE INDICATING THAT THIS LOW WILL
RAPIDLY OCCLUDE WITH NUMEROUS CENTERS ROTATING AROUND THE BROAD
CYCLONIC CIRCULATION. THE ASSOCIATED STRONG GALE FORCE FRONT WILL
MARCH ACROSS THE WESTERN AND CENTRAL ALEUTIANS THROUGH THURSDAY...BUT
WILL HAVE A HARD TIME PROGRESSING MUCH FASTER GIVEN THE HIGH
AMPLITUDE RIDGE OBSTRUCTING ITS FORWARD PROGRESS. THE LOW AND
ASSOCIATED FRONT GREATLY DIMINISH IN STRENGTH AND SIZE BY THE END OF
THE WEEK...LEAVING WIDESPREAD RAIN AND SNOW SHOWERS ACROSS THE
BERING.

.AER/ALU...WATCH/WARNING SUMMARY
PUBLIC...NONE
MARINE...GALE...172 175 176 411 413
         HEAVY FREEZING SPRAY...160
FIRE WEATHER...NONE

AB FEB 12