Heavy Snow Warning 3, NWS Anchorage, AK

AREA FORECAST DISCUSSION
SOUTHCENTRAL AND SOUTHWEST ALASKA
625 AM AST THU DEC 20 2007

.ANALYSIS AND UPPER LEVELS...
COLDEST MORNING OF THE SEASON SO FAR WITH TEMPERATURES BELOW ZERO
OVER A LARGE PORTION OF THE STATE. UPPER JET REMAINING WELL SOUTH OF
ALASKA ABOUT THE 40N PARALLEL WITH COLD CORE UPPER LOW OVER THE
WESTERN BROOKS RANGE ALLOWING COLD AIR TO POOL OVER THE INTERIOR AND
WESTERN CANADA. A VERTICALLY STACKED LOW NORTH OF THE CENTRAL
ALEUTIANS WILL SLOWLY DRIFT EASTWARD TODAY AS IT BEGINS TO WEAKEN. A
TRIPLE POINT LOW HAS DEVELOPED ALONG THE FRONTAL BOUNDARY JUST SOUTH
OF THE ALASKA PENINSULA AND ANOTHER LOW SOUTH OF THE GULF OF ALASKA
ESTIMATED TO BE AT ABOUT 983 MB WILL PLAY A LARGE ROLE IN SOUTH
CENTRAL FORECAST THE NEXT 24 HOURS.

.MODEL DISCUSSION...
GFS SHOWED SOME RUN TO RUN CONSISTENCY ALTHOUGH CONFIDENCE BEYOND 36
HOURS REMAINS LOW AS NOT SURE GFS OR ANY MODEL FOR THAT MATTER HAS A
GOOD HANDLE ON THE TRACK OF THE DEVELOPING LOW SOUTH OF THE GULF.
MADE VERY LITTLE CHANGES TO THE FORECAST FOR THESE REASONS.

.SHORT TERM FORECAST...
SOUTHCENTRAL...
A WEAK RESIDUAL LOW IS STILL HANGING ON SOUTH OF THE COPPER RIVER
DELTA IN EASTERN MARINE ZONE 120 EARLY THIS MORNING SPREADING
SCATTERED SNOW SHOWERS ALONG THE COAST AND NORTH INTO THE COPPER
RIVER BASIN. GAP WINDS ASSOCIATED WITH THE PRESSURE GRADIENT LIMITED
TO PASSAGE CANAL...RESURRECTION BAY...AND THOMPSON PASS. WINDS WILL
BE DIMINISHING THIS MORNING AS THE ORIENTATION OF THE ISOBARS CHANGES
WITH THE APPROACHING SYSTEM IN THE GULF. CIRRUS SHIELD ALREADY
REACHING KODIAK ISLAND AND WILL BE SPREADING NORTH QUICKLY THIS
MORNING. WITH EXCEPTIONALLY COLD AIRMASS IN PLACE...THINKING COASTAL
ZONES WILL SEE SIGNIFICANT SNOW FALL TOTALS FROM THIS SYSTEM WHILE
THE INTERIOR WILL REMAIN DOWNSLOPED. THE EXCEPTION WILL BE THE
SOUTHERN PART OF ZONE 121 INCLUDING HOMER WHERE A WINTER STORM WATCH
IS IN EFFECT STARTING THIS AFTERNOON. ALSO...WEST OF THE PARKS
HIGHWAY IN THE SUSITNA VALLEY WILL HAVE FAVORABLE LOWER AND MID LEVEL
WIND FLOW THAT WILL ALLOW FOR ADVISORY LEVEL SNOW FALL.
LIKEWISE...THE EASTERN KENAI WILL SEE HEAVY SNOW AS WARM AIR DOES NOT
MOVE INLAND SO ALL PRECIPITATION WILL REMAIN FROZEN. THE POSSIBILITY
OF BLOWING SNOW AS WELL THROUGH PORTAGE VALLEY AS THE PRESSURE
GRADIENT INCREASES OVERNIGHT. THERE IS UNCERTAINTY IN THE TRACK OF
THE DEVELOPING LOW ONCE IT ENTERS THE EASTERN GULF TONIGHT. THE GFS
STRENGTHENS THE LOW TO 976 MB COMPARED TO 988 MB AT THE SAME TIME FOR
THE NAM...THE DEEPER SOLUTION PULLS THE LOW A BIT FURTHER WEST.

PREVIOUSLY MENTIONED COLD CORE UPPER LOW WILL RETROGRADE TOWARD THE
SEWARD PENINSULA OVER THE WEEKEND WHICH ALLOWS FOR DEVELOPING LOWS IN
THE SOUTHERN GULF TO PUSH BACK TOWARD THE NORTHWEST...BRINGING THE
POSSIBILITY OF MORE SNOW SOUTHCENTRAL. STILL TOO MUCH UNCERTAINTY IN
THE FORECAST PAST 48 HOURS SO HAVE TRENDED THE FORECAST INSTEAD OF
MAKING ANY DRASTIC CHANGES.

WEST OF THE ALASKA RANGE...
NORTHEASTERLY WINDS DRAWING COLD AIR DOWN THE KUSKOKWIM VALLEY.
WEAKENING FRONT MOVING OVER THE ALASKA PENINSULA SPREADING SOME LIGHT
SNOW TOWARD KING SALMON WHILE THE UPPER KUSKOKWIM REMAINED CLEAR AND
COLD OVERNIGHT...NIKOLAI THE COLDEST IN OUR FORECAST DOMAIN AT -46.
THE LOW IN THE BERING SEA WILL SLOWLY WEAKEN AS IT MOVES TOWARD
BRISTOL BAY. COLD AIR WILL BRIEFLY MOVE FURTHER INLAND BEFORE IT IS
ADVECTED BACK SOUTH ONCE THE LOW MOVES INTO BRISTOL BAY. BLIZZARD
CONDITIONS OVER THE PRIBILOFS HAVE ENDED ALTHOUGH STRONG
NORTHEASTERLY WINDS AND INTERMITTENT SNOW WILL STILL LEAD TO SOME
PERIODS OF REDUCED VISIBILITY IN BLOWING SNOW.

.AER/ALU...WATCH/WARNING SUMMARY
PUBLIC...WINTER STORM WATCH 121
         SNOW ADVISORY 145.
MARINE...STORM WARNING ZONE 172.
         GALE WARNING ZONES 120 130 132 150 155 170 175 179 180 185.
         HEAVY FREEZING SPRAY WARNING ZONES 121 125 129 130 140
          141 160 179 180 185.
FIRE WEATHER...NONE.

AB DEC 07