Snow Advisory 25, NWS Anchorage, AK

AREA FORECAST DISCUSSION
SOUTHCENTRAL AND SOUTHWEST ALASKA
200 PM AST WED DEC 19 2007

.ANALYSIS...
A LARGE LOW PRESSURE SYSTEM OVER THE WESTERN ALEUTIANS ASSOCIATED
WITH A CLOSED UPPER LEVEL AND 100 KT JET TO STRETCH A MODERATE FRONT
ACROSS THE CENTRAL BERING TODAY. STRONG WINDS AND MODERATE
PRECIPITATION WRAPPED AROUND THE LOW IN THE WESTERN ALEUTIANS. A WEAK
LOW IN THE NORTHERN GULF WITH SOME SNOW SHOWERS. COLD CORE NATURE OF
THIS LOW MAKING MOST OF THE PRECIPITATION QUITE CONVECTIVE. IN OTHER
AREAS...COLD AIR IN PLACE OVER THE MAINLAND AND EASTERN BERING AND
WESTERN GULF. WITH THE LOW IN THE GULF AND COLD AIR INLAND...STRONG
PRESSURE GRADIENT ALONG THE GULF COAST. THIS IS REINFORCED BY A SHORT
WAVE MOVING THROUGH SOUTH CENTRAL TODAY. THEREFORE...SEEING MODERATE
TO STRONG GAP WINDS ONCE AGAIN TODAY.

.UPPER LEVELS AND MODEL DISCUSSION...
COLD CORE UPPER LOW NEAR THE BROOKS RANGE MOVING VERY LITTLE OVER THE
NEXT 24 HOURS THEN RETROGRADES INTO THE BERING STRAIT FRIDAY.
COMBINED WITH AN UPPER LOW IN THE YUKON...A BROAD TROUGH IS OVER THE
GULF. THIS LOW IN THE YUKON TO LEAVE BRINGING A RIDGE THURSDAY TO THE
GULF AHEAD OF A LOW TO ROTATE INTO THE AREA AROUND THE TROUGH DIGGING
IN THE EASTERN BERING. A FEW SYSTEMS TO ROTATE AROUND THIS LARGE
SCALE TROUGH OVER THE NEXT WEEK. THE EXACT POSITIONING OF EACH WAVE
ROTATING AROUND THIS LOW IS STILL A BIT UNCERTAIN ESPECIALLY AFTER
SUNDAY. THEREFORE...THE DETAILS OF STRENGTH WITH EACH SYSTEM IN THE
MID RANGE HAS BEEN MODERATED DUE TO UNCERTAINTY. LOOKING AT THE
CONSISTENCY OF THE EC OVER THE PAST COUPLE DAYS AND THE FACT THAT THE
12Z GFS HAS NOW LEANED TO THIS SOLUTION THROUGH DAY 4...WE HAVE
RELOADED THE FORECAST TO THE 12Z GFS SOLUTION THROUGH DAY 7.

IN THE SHORT TERM...THE UPPER LOW OVER THE WESTERN ALEUTIANS TO MOVE
ALONG THE ALEUTIAN CHAIN TO COLD BAY FRIDAY. MEANWHILE...A WAVE
UNDERNEATH THIS LOW TO RIDE ALONG THE JET TO ROTATE A LOW INTO THE
GULF THURSDAY NIGHT/FRIDAY. THE UPPER LOW TO ROTATE TO THE NORTH GULF
COAST FRIDAY NIGHT THEN INLAND. THE COLD BAY LOW TO MOVE INTO THE
WESTERN GULF SATURDAY WITH A RIDGE PUSHING OVER THE WESTERN BERING.

.SHORT TERM FORECAST...
BERING AND ALEUTIANS AND WESTERN ALASKA...
LOW PRESSURE SYSTEM IN THE WESTERN ALEUTIANS WITH A STRONG FRONT
EXTENDING INTO CENTRAL BERING. THE COLD AIR TO THE NORTH OF THIS
FRONT WILL CLASH AND CREATE BLIZZARD CONDITIONS TO THE PRIBILOF
ISLANDS. HAVE SLOWED THE START TIME OF THIS WARNING BASED ON
SATELLITE AND START OF HEAVIER SNOWFALL. THE FRONT WILL STRETCH TO
THE ALASKA PENINSULA AND BRISTOL BAY THURSDAY. THE COLD AIR IN PLACE
OVER THE MAINLAND NOT GOING TO BE REPLACED WITH THE FRONT OVER THE
KUSKOKWIM VALLEY AND DELTA. THE LOW WILL MOVE THROUGH THE CENTRAL
ALEUTIANS THURSDAY AND TO COLD BAY FRIDAY. MOST PRECIP WILL NOT MOVE
INTO THE KUSKOKWIM VALLEY AND DELTA. BEHIND THE LOW...STRONG COLD AIR
TO PUSH ACROSS THE BERING WITH SNOW SHOWERS AND STRONG WINDS NEAR THE
LOW IN THE EASTERN BERING.

SOUTH CENTRAL ALASKA...
CURRENTLY...LOW IN THE NORTHERN GULF WITH A SHORT WAVE TROUGH MOVING
THROUGH THE AREA. COLD AIR ALSO IN PLACE OVER THE MAINLAND...THIS HAS
PLACED A TIGHT PRESSURE GRADIENT ALONG THE COAST. THEREFORE...STRONG
GAP WINDS EXPERIENCED THROUGH THE NORTH GULF COAST AND KENAI
PENINSULA. THIS LOW TO MOVED OUT TONIGHT WITH WINDS DISSIPATING.
MEANWHILE...A FRONT WILL WORK TO KODIAK WITH WINDS PICKING UP FROM
THE SOUTHEAST ACROSS THE GULF THURSDAY. THE FRONT TO MAKE ITS WAY TO
THE GULF COAST THURSDAY AFTERNOON. GOOD DOWNSLOPE PATTERN FOR THIS
FRONT...SO PRECIP WILL BE LIMITED IN THE MATANUSKA VALLEY AND
ANCHORAGE BOWL THURSDAY. ANOTHER LOW TO MOVE NORTH THROUGH THE GULF
THURSDAY NIGHT/FRIDAY TO CONCENTRATE THE LOW AND PULL PRECIP AWAY
FROM THE INLAND MORE. WINDS WILL INCREASE AS THE LOW MOVES TO GULF
COAST FRIDAY AFTERNOON. THE LOW TO WEAKEN THROUGH FRIDAY NIGHT WITH
PRECIP WANING.

.LONG TERM FORECAST...
GENERALLY...BEYOND SATURDAY THE LARGE SCALE TROUGH TO REMAIN OVER THE
BERING WITH SOUTHWEST FLOW OVER SOUTH CENTRAL AND COLD NORTHERLY AIR
MASS OVER THE BERING. A FEW LOW PRESSURE SYSTEMS TO ROTATE AROUND
THIS TROUGH FROM FRIDAY THROUGH THURSDAY. THE TIMING AND EXACT
POSITION OF THESE SYSTEMS IS STILL UNCERTAIN BETWEEN THE EC AND GFS.

.AER/ALU...WATCH/WARNING SUMMARY
PUBLIC...BLIZZARD WARNING 195.
MARINE...GALE WARNING ZONES 121 129 130 165 170 171 172 176 179 180
          185
         HEAVY FREEZING SPRAY WARNING ZONES 121 125 129 130 138 140
          141 150 160 179 180 185
FIRE WEATHER...NONE

JN DEC 07