Gale Warning 266, NWS Anchorage, AK

AREA FORECAST DISCUSSION
SOUTHCENTRAL AND SOUTHWEST ALASKA
100 PM AKST SAT FEB 28 2009

.ANALYSIS AND UPPER LEVELS...A STRONG UPPER RIDGE REMAINS CENTERED
OVER THE NORTH PACIFIC NEAR 170 DEGREES WEST LONGITUDE AND 40 DEGREES
NORTH LATITUDE. A POSITIVELY TILTED NOSE OF THIS RIDGE EXTENDS ACROSS
THE GULF OF ALASKA TO THE YUKON. THE INFLUENCE OF THIS RIDGE EXTENDS
NORTHWARD TO THE BERING SEA. A DEEP UPPER TROUGH IS CENTERED THE
CHUKOTSK PENINSULA WITH A STRONG UPPER JET EXTENDING OUT OF ASIA
ALONG THE BASE OF THE TROUGH AND THEN UP AND ACROSS THE BERING
SEA...HELPING TO FLATTEN OUT THE TOP OF THE RIDGE. FINALLY...A
CUT-OFF LOW IS NEARLY STATIONARY ALONG 140 DEGREES WEST SOUTH OF
THE PANHANDLE. SYSTEMS CROSSING OVER THE RIDGE HAVE BEEN DROPPING
INTO THIS TROUGH.

THE PRIMARY WEATHER PRODUCER TODAY IS A SHORT-WAVE TROUGH MOVING
ONSHORE OVER WESTERN ALASKA AND A SURFACE FRONT OVER THE WESTERN
GULF. WIDESPREAD SNOW IS FALLING ACROSS SOUTHCENTRAL ALASKA UNDER
BROAD SOUTHWEST FLOW ALOFT. EMBEDDED UPPER WAVES MOVED ACROSS THE
REGION OVERNIGHT PROVIDING SOME ENHANCED UPPER LIFT. THE SNOW
LIGHTENED UP OVER THE WESTERN KENAI...ANCHORAGE...NORTH COOK INLET
REGION FOR SEVERAL HOURS...BUT HAS REDEVELOPED AND INTENSIFIED
AROUND NOON AS THE UPPER TROUGH BEGINS TO IMPACT THE AREA. THERE
HAS BEEN A LOW LEVEL SOUTHEAST FLOW AHEAD OF THE SURFACE FRONT WHICH
HAS REALLY KEPT ACCUMULATION LIGHT ACROSS ANCHORAGE...BUT THE FRONT
IS CURRENTLY MOVING BY AND THIS FLOW IS WEAKENING. A SURFACE LOW HAS
ALSO DEVELOPED ALONG THE FRONT NORTHEAST OF KODIAK...WHICH WILL ALSO
CONTRIBUTE TO A SHIFT IN LOW LEVEL FLOW.

THE COLDEST TEMPERATURE IN THE ANCHORAGE SOUNDING THIS MORNING WAS
ABOUT -7 DEGREES CELSIUS...SO THE AIRMASS IS QUITE COLD. THIS IS
REFLECTED OBSERVED SNOW DENSITY RATIOS FROM AROUND THE REGION WHICH
HAVE RANGED FROM 30:1 TO 50:1. BECAUSE OF THIS SNOW IS PILING UP
QUICKLY IN SOME AREAS...WITH 5 TO 8 INCHES OF SNOW REPORTED FROM
WASILLA TO WILLOW AND NORTHWARD TO CHULITNA. DUE TO RECENT
INTENSIFICATION OF SNOW AND THE UPPER TROUGH STILL BACK OVER WESTERN
ALASKA DECIDED TO ISSUE WINTER WEATHER ADVISORIES FOR SNOW FOR
ZONES 101 111 AND 145...FOR UP TO 8 INCHES ADDITIONAL ACCUMULATION.

.MODEL DISCUSSION...MODEL SOLUTIONS ARE IN UNUSUALLY GOOD AGREEMENT
THROUGH THE END OF NEXT WEEK. FORECASTS IN THE MID TO LONG-TERM
WILL BE UPDATED TO BETTER REFLECT THE AGREED UPON SOLUTION. ONE
NOTEWORTHY DEFICIENCY IN MODEL GUIDANCE THIS MORNING IS THE
UNDERFORECASTING OF QPF. THE SNOW IS MORE WIDESPREAD AND ACCUMULATING
MORE THAN MODELS INDICATE.

.SHORT TERM FORECAST...

SOUTHCENTRAL ALASKA...THE UPPER TROUGH OVER WESTERN ALASKA WILL DIG
SOUTHEASTWARD AS IT REACHES SOUTHCENTRAL THIS EVENING...WITH THE
BULK OF ENERGY DROPPING INTO THE GULF AND HELPING TO FROM AN UPPER
LOW. THE TROUGH AXIS WILL MOVE EAST OF ANCHORAGE AND THE MAT-SU
SHORTLY AFTER 9 PM. UNTIL THEN LIFT ASSOCIATED WITH THE TROUGH AS
WELL AS COLD ADVECTION BEHIND THE SURFACE FRONT WILL MAINTAIN STEADY
SNOW ACROSS THE REGION. SNOWFALL AMOUNTS WILL LIKELY HAVE TO BE
INCREASED EVEN FURTHER THAN THEY HAVE ALREADY. BASED ON RADAR
TRENDS...WITH THE BACK EDGE OF SNOW BUILDING WESTWARD AND A LARGE
AREA OF MODERATE SNOW...SOME AREAS COULD SEE ANOTHER FOOT OF SNOW
ON TOP OF WHAT HAS ALREADY FALLEN. SIMILARLY...THE REST OF
SOUTHCENTRAL WILL ALSO SEE SIGNIFICANT ACCUMULATION...ALTHOUGH SNOW
WILL QUICKLY LIGHTEN UP OVERNIGHT INTO SUNDAY MORNING AS THE BULK
OF ENERGY DROPS INTO THE GULF. COLD ADVECTION ALONG WITH A STRONG
UPPER JET ON THE BACK SIDE OF THE TROUGH WILL PRODUCE THE USUAL
STRONG WINDS ALONG THE GULF COASTAL WATERS TONIGHT AND SUNDAY. THE
UPPER RIDGE WILL BUILD NORTHWARD A BIT BEHIND THIS TROUGH AND
PROVIDE DRY CONDITIONS ACROSS SOUTHCENTRAL THROUGH AT LEAST MID WEEK.

SOUTHWEST ALASKA...UPPER WAVES DROPPING INTO THE BACK SIDE OF
THE UPPER TROUGH WILL CONTINUE TO PRODUCE A FEW SNOW SHOWERS OVER
SOUTHWEST ALASKA THROUGH TONIGHT. THERE WILL BE A BRIEF BREAK BEFORE
A FRONT MOVING ACROSS THE UPPER RIDGE REACHES THE COAST SUNDAY NIGHT.
IT WILL BE ELONGATING AND WEAKENING AS IT APPROACHES...SO AT THIS
POINT DONT EXPECT ANY SIGNIFICANT WINDS OR SNOW.

BERING SEA/ALEUTIANS...A DEVELOPING SYSTEM SOUTHWEST OF THE ALEUTIANS
WILL RIDE OVER TOP OF THE RIDGE ADVECTING LOTS OF WARM AIR ALONG
WITH IT. THIS AIR WILL EVENTUALLY CROSS THE ALASKA PENINSULA AND
HEAD BACK SOUTHWARD ON THE EAST SIDE OF THE RIDGE. THE PRESSURE
GRADIENT WILL AGAIN BE VERY TIGHT BETWEEN THE VERY STRONG HIGH AND
MODERATE STRENGTH LOW...SO EXPECT ANOTHER BOUT OF GALE FORCE WINDS
AS THE SYSTEM MOVES ACROSS THE ALEUTIANS AND BERING SEA TONIGHT
THROUGH MONDAY.

.LONG TERM FORECAST...A STRONGER SYSTEM DEVELOPS IN THE BERING
SEA TUESDAY AND THIS PUSHES UP A HIGH AMPLITUDE RIDGE OVER ALASKA
TUESDAY NIGHT AND WEDNESDAY. SIGNIFICANT WEATHER WILL REACH
SOUTHWEST ALASKA PROBABLY EARLY WEDNESDAY AND SOUTH CENTRAL
WEDNESDAY NIGHT.

THE BASIC PATTERN OF TROUGH TO THE NORTHWEST AND RIDGE TO THE
SOUTHEAST WILL CONTINUE THROUGH LATE NEXT WEEK AND POSSIBLY LONGER.

.AER/ALU...WATCH/WARNING SUMMARY
PUBLIC...NONE.
MARINE...STORM WARNING...130.
         GALE WARNING...120 121 129 132 150 170 172 175 176 179 180
                        185.
         HEAVY FREEZING SPRAY WARNING...130 138 140 141 185.
FIRE WEATHER...NONE.

SEB FEB 09