Heavy Snow in Alaska, March 21, 2005

AREA FORECAST DISCUSSION
SOUTHCENTRAL AND SOUTHWEST ALASKA
500 PM AST SUN MAR 20 2005

.DISCUSSION...
PRIMARY FOCUS TODAY IS ON OUTFLOW WINDS IN SOUTHCENTRAL AND SOUTHWEST
MAINLAND ALASKA.

A LARGE 1052 MB HIGH OVER THE ARCTIC OCEAN WILL INTENSIFY TODAY TO
NEARLY 1060 MB. COUPLED WITH THIS LARGE HIGH TO THE NORTH...IS A
DEEPENING LOW IN THE NORTH CENTRAL GULF OF ALASKA.  THE RESULTANT
SURFACE PRESSURE GRADIENT IS STRONG THROUGH OUT MAINLAND AREAS OF THE
STATE... BRINGING STRONG NORTH TO NORTHEAST WINDS THROUGH TERRAIN
CHANNELED AREAS. THIS GRADIENT IS COUPLED WITH A PUSH OF COLD DENSE
AIR RAPIDLY ADVANCING FROM THE NORTHEAST.  THESE TWO COMPONENTS
(PRESSURE WITH COLD AIR ADVECTION) ALONE ARE ENOUGH TO WARRANT STRONG
ADVISORY LEVEL OR WARNING LEVEL WIND CONDITIONS ALONG THE NORTH GULF
COAST...KUSKOKWIM DELTA AND NORTHERN BRISTOL BAY COASTLINE.
STRONGEST WINDS ARE EXPECTED THROUGH THOMPSON PASS AND THE COPPER
RIVER DELTA...BOTH AREAS HAVE WARNINGS BEGINNING THIS AFTERNOON. THE
THIRD PRIMARY AREA OF CONCERN IS THE MATANUSKA VALLEY WITH HIGH
ADVISORY LEVEL WINDS.  THE ONE INGREDIENT NOT LINING UP FOR A MAJOR
BORA WIND EVENT IS IN THE UPPER LEVELS OF THE ATMOSPHERE. A SHORTWAVE
AT 500 MB IS ASSOCIATED WITH THE COLD AIR ADVECTION FROM THE
NORTHEAST. THIS INDUCES A NORTHWEST FLOW ACROSS THE SOUTHWESTERN
PORTION OF THE STATE AND THE WESTERN HALF OF SOUTHCENTRAL THIS
AFTERNOON. THIS COUNTERS THE NORTH GAP WINDS AND NORTHEAST
GEOSTROPHIC WINDS AT THE SURFACE. CONFIDENCE THAT WE WILL HIT WARNING
CRITERIA WOULD BE MUCH HIGHER IF THE UPPER LEVEL SUPPORT WOULD ALLOW
FOR A NORTH TO NORTHEAST ORIENTATION OF WINDS AT 500 MB.

THE PRIMARY DIFFERENCE WITH THIS FORECAST PACKAGE COMPARED TO THE
PREVIOUS PACKAGE IS THE DURATION OF THIS WIND EVENT. THE GULF LOW IS
NOW FORECAST TO MOVE OVER AFOGNAK ISLAND MONDAY MORNING... AND REMAIN
NEARLY STATIONARY THROUGH MONDAY NIGHT.  THIS INSTEAD OF TAKING A
TRACK RAPIDLY SOUTH.  THE RESULTANT CHANGE WILL MEAN A STRONGER
SURFACE PRESSURE GRADIENT OVER SOUTHCENTRAL THROUGH MONDAY EVENING...
AND THEREFORE LONGER DURATION OF ADVISORY LEVEL WINDS.

THE RECENT UNSEASONABLY WARM WEATHER IS ALSO PLAYING A ROLE IN THIS
FORECAST PACKAGE. THE SNOW PACK IS ALL BUT GONE FROM THE MAT-VALLEY
SOUTH.  BLOWING SAND GRAVEL AND DUST WILL BE A PROBLEM FOR AIR
QUALITY...HUMAN COMFORT... AND WILL REDUCE VISIBILITIES ESPECIALLY
NEAR DRY GLACIAL RIVER BEDS AND EXPOSED CONSTRUCTION AREAS.  FURTHER
NORTH THERE IS STILL A SIGNIFICANT SNOW PACK IN THE NORTHERN SUSITNA
VALLEY... HOWEVER RECENT RAINS HAVE CREATED A SOLID CRUST OVER THE
SNOW PACK.  THIS IS GOOD NEWS FOR THE FORECAST PACKAGE AS THE THREAT
OF BLOWING SNOW AND BLIZZARD CONDITIONS IS SIGNIFICANTLY REDUCED.

EXTENDED...
THE PATTERN BEGINS TO SHIFT ON TUESDAY AS THE ARCTIC RIDGE SLIDES
OVER THE YUKON TERRITORY AND BRITISH COLUMBIA. THIS WILL ALLOW FOR A
MORE SOUTHERLY PUSH THROUGH THE WORK WEEK. TEMPERATURES WILL BE BACK
ON THE RISE MID WEEK WITH A GREATER CHANCE OF PRECIPITATION AND CLOUD
COVER FROM THE ALASKA PENINSULA NORTH TO THE KUSKOKWIM VALLEY AND
EAST TO SOUTHEAST PRINCE WILLIAM SOUND.

.AER/ALU...WATCH/WARNING SUMMARY
PUBLIC...HIGH WIND WARNING...131 AND 135
         WIND ADVISORIES...101 111 125 141 145 155 161
MARINE...GALE WARNINGS...2B 2B1 3A 3C 4
         HEAVY FREEZING SPRAY...8
FIRE WEATHER...NONE

AMF MAR 05