Gale Warning 895, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION...UPDATE
NATIONAL WEATHER SERVICE ANCHORAGE AK
1030 AM AKST THU NOV 24 2011

.MORNING UPDATE...
CHALLENGING FORECAST THIS MORNING WITH SNOW TAPERING OFF OVER THE
KENAI PENINSULA AND NOW WEST ANCHORAGE. A VERY COMPLEX LOW PRESSURE
SYSTEM IN THE NORTHERN GULF COUPLED WITH SUBTLE UPPER LEVEL FEATURES
ARE RESULTING IN SNOW OVER PORTIONS OF SOUTHCENTRAL ALASKA. THE SNOW
ADVISORY THAT WAS ISSUED FOR ANCHORAGE REMAINS IN EFFECT...ALTHOUGH
SNOW HAS SUDDENLY TAPERED OFF AND CLEAR SKIES ARE INVADING FROM THE
WEST. THIS IS A RESULT OF THE UPPER LEVEL TROF TILTING SLIGHTLY TO
THE EAST AND CUTTING OFF OUR SOURCE OF MOISTURE.

THE CHALLENGE REVOLVES AROUND THE MULTIPLE SURFACE LOWS SPINNING UP
IN THE NORTHERN GULF. THE MAIN LOW IS CURRENTLY LOCATED JUST SOUTH OF
MIDDLETON ISLAND WITH A SECONDARY LOW TO THE EAST. THIS MAIN LOW IS
SLOWLY PROGRESSING TO THE NORTH AND NORTHWEST...WHICH OVERTIME WILL
PUT IT IN A VERY FAVORABLE POSITION FOR THE DEFORMATION ZONE TO
REDEVELOP OVER THE ANCHORAGE BOWL. THE 12Z NAM AND NOW THE LOCAL WRF
MODEL INDICATE THIS WILL HAPPEN EARLY THIS AFTERNOON...WITH SNOWFALL
INTENSITY INCREASING AFTERNOON THROUGH EARLY EVENING. FOR THAT
REASON...DESPITE THE CLEARING TRENDS IN WEST ANCHORAGE...WE ARE GOING
TO STICK WITH THE ADVISORY FOR NOW.

IF THE LOW DOES NOT PROGRESS TO THE NORTH AND WEST AS THE HIGHER
RESOLUTION MODELS ARE SUGGESTING...THE FORECAST WILL CHANGE GREATLY
THIS AFTERNOON WITH PARTLY SUNNY SKIES PREVAILING AND PATCHY FOG
DEVELOPING OVERNIGHT.

PREVIOUS FORECAST DISCUSSION
--------------------------------------

.ANALYSIS AND UPPER LEVELS...
A COMPLEX UPPER PATTERN EXISTS OVER THE GREATLAND THIS MORNING. A
LARGE UPPER LEVEL LOW IS SPREAD OUT OVER MUCH OF THE STATE WITH
SEVERAL LOW CENTERS. ROTATING AROUND THE TROUGH ARE A HANDFUL OF LOW
AMPLITUDE SHORTWAVE TROUGHS. THE JET STREAM LIES WELL TO THE SOUTH
KEEPING THE STAGNANT LONG WAVE PATTERN IN PLACE. A KAMCHATKA UPPER
LOW IS DRAPING A FRONT JUST WEST OF THE ALEUTIANS. AHEAD OF THAT
SYSTEM LIES A BUILDING RIDGE ACROSS THE BERING. AT THE SURFACE A
FILLING SURFACE LOW LIES IN THE GULF. THE REST OF THE STATE REMAINS
UNDER A RELATIVELY WEAK PRESSURE GRADIENT KEEPING WINDS LIGHT FOR THE
MAINLAND.

ISSUES IN THE FORECAST WILL ARISE FROM THE MODELS TRYING TO RESOLVE
THE INDIVIDUAL SHORTWAVES ROTATING AROUND THE BROADER LARGE SCALE
TROUGH. ADDING TO THE COMPLEXITY OF THE SITUATION IS A LARGER
AMPLITUDE SHORTWAVE IN THE NORTHERN PACIFIC AND THE AFOREMENTIONED
WESTERN BERING FRONT PUSHING OVER THE ALEUTIANS.

.MODEL DISCUSSION...
AS PREVIOUSLY MENTIONED...GUIDANCE SEEMS TO STRUGGLE ON THE EXACT
TIMING AND PLACEMENT OF EACH INDIVIDUAL WAVE BUT GLOBAL AND MESOSCALE
MODELS ALIKE DO SHOW A SIMILAR PATTERN THROUGH THE WEEKEND. WHILE THE
LOW AMPLITUDE WAVES WILL GIVE THE MODELS PROBLEMS ALL GUIDANCE PHASES
THESE WAVES INTO TWO TROUGHS...THE FIRST A STRONGER SYSTEM PUSHING
INTO THE PANHANDLE...THE FOLLOWING WAVES ON THE BACKSIDE OF THE
TROUGH LOOK TO PHASE WITH THE NORTHERN PACIFIC SHORTWAVE TO FORM ONE
CONTINUOUS TROUGH FROM THE SOUTHCENTRAL DOWN THROUGH THE
MID-LATITUDES. THESE INTERACTIONS ARE COMPLEX AND WILL DEFINITELY
NEED TO BE MONITORED.

.SHORT TERM FORECAST...
SOUTHCENTRAL ALASKA...MID AND UPPER LEVEL LOW CENTERED FROM THE
NORTHWESTERN GULF UP THROUGH THE KENAI PENINSULA AND THE
ALASKA/ALEUTIAN RANGE WILL STREAM MOISTURE UP THROUGH THE ANCHORAGE
BOWL AND MATANUSKA VALLEY TODAY BRINGING CONTINUOUS LIGHT SNOW
THROUGHOUT THE DAY AND INTO FRIDAY EVENING. EXPECT SNOW TO TAPER OFF
BY EARLY SATURDAY MORNING. THE NEXT SYSTEM...A TRIPLE POINT LOW...
WILL ROLL UP INTO THE GULF LATE SUNDAY AND EARLY MONDAY BRINGING
ANOTHER CHANCE FOR LIGHT SNOW.

SOUTHWEST ALASKA...BENIGN WEATHER PATTERN AND COLD DRY CONDITIONS
WILL CONTINUE FOR THE SOUTHWEST MAINLAND. A FRONT WILL APPROACH WEST
COAST ON SUNDAY BUT LOOKS TO STAY JUST OFFSHORE. BEST CHANCE FOR SNOW
OVER THE AKPEN.

BERING SEA AND ALEUTIANS...WIDESPREAD SNOW SHOWERS FOR MOST OF THE
BERING AND BRISTOL BAY. A FRONT WILL TRAVERSE THE CHAIN THROUGHOUT
THE WEEKEND BRINGING WIDESPREAD PRECIPITATION. BEHIND THE FRONT
WESTERLY FLOW WILL PREVAIL WITH SNOW SHOWERS.

.LONG TERM FORECAST...
EXTENDED FORECAST BEGINS TO GET TRICKY AS THE ENTRENCHED PATTERN OF
THE PAST WEEK TRANSITIONS TO AN ACTIVE SOUTHERLY STORM TRACK INTO
MAINLAND ALASKA.

.AER/ALU...WATCH/WARNING SUMMARY
PUBLIC...WINTER WEATHER ADVISORY 101
MARINE...GALE 120 130 150 155 165 170 172 175 176 179 185 411 412 413
414 HEAVY FREEZING SPRAY 121 127 130 138 150 160 165 180 185
FIRE WEATHER...NONE

MTL NOV 11