Gale Warning 907, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
1220 PM AKST SAT NOV 27 2010

.ANALYSIS AND UPPER LEVELS...WATER VAPOR IMAGERY SHOWS A HIGH
AMPLITUDE SHORT-WAVE TROUGH FROM THE CENTRAL TO EASTERN ALEUTIANS
SOUTHWARD ACROSS THE NORTH CENTRAL PACIFIC COUCHED IN BETWEEN TWO
SHORT-WAVE RIDGES. A MATURE SURFACE LOW IS TRACKING UP THE EAST SIDE
OF THIS TROUGH. ANOTHER MUCH WEAKER LOW AMPLITUDE SHORT-WAVE TROUGH
IS JUST OFF THE WESTERN ALASKA COAST AND BEGINNING TO AMPLIFY AS IT
MOVES ONSHORE. WITH ORIGINS IN THE ARCTIC THERE IS A LOT OF COLD AIR
BEHIND THIS TROUGH. AT THE SURFACE THERE IS A WEAK LOW OVER SOUTHWEST
ALASKA TRACKING TOWARD THE GULF. WITH A DEEP LAYER OF WESTERLY FLOW
AND WEAK DYNAMICS NOT MUCH LOW LEVEL MOISTURE OR PRECIPITATION IS
MAKING IT ACROSS THE MOUNTAINS TO SOUTHCENTRAL. OBSERVATIONS AND
RADAR IMAGERY SHOW SOME SHOWERS OVER THE NORTHERN GULF AND SOUTHERN
KENAI PENINSULA WITH JUST FLURRIES OR ISOLATED SNOW SHOWERS FURTHER
NORTH. LASTLY...A MODERATE LOW OVER THE NORTHWEST PACIFIC IS TRACKING
UP THE WEST SIDE OF THE SHORT-WAVE RIDGE JUST TO THE EAST. WITH
PROGRESSIVE UPPER LEVEL FLOW ALL OF THESE WAVES ARE HEADING EASTWARD.

.MODEL DISCUSSION...NAM QPF OUTPUT FOR SOUTHCENTRAL TONIGHT INTO
SUNDAY IS NOTABLY HIGHER THAN OTHER SOLUTIONS EVEN THOUGH THE
HANDLING OF MAJOR FEATURES LOOKS VERY SIMILAR. THE 18Z NAM CAME IN
WITH VALUES MUCH CLOSER TO THE OTHER MODELS WHICH GIVES CONFIDENCE
IN THE LOWER AMOUNTS. OTHER THAN THAT...MODELS ARE ALL IN REASONABLE
AGREEMENT WITH HANDLING OF SYNOPTIC FEATURES THROUGH THE NEXT FEW
DAYS DISPLAY GOOD RUN-TO-RUN CONSISTENCY...SO FORECAST CONFIDENCE
IS ABOVE AVERAGE.

.SHORT TERM FORECAST...

SOUTHCENTRAL ALASKA...UPPER LEVEL TROUGH WILL CONTINUE TO AMPLIFY
AS IT MOVES FROM SOUTHWEST ALASKA TONIGHT TO SOUTHCENTRAL SUNDAY
RESULTING IN LIGHT SNOW ACROSS MUCH OF THE REGION. 12Z ANCHORAGE
SOUNDING SHOWS TEMPERATURES OF -10 TO -20 DEGREES FROM 850MB TO
600MB. THESE TEMPERATURES SUPPORT GROWTH OF DENDRITES AND
PLATES...WHICH ARE TYPES OF SNOW CRYSTALS...AND THESE GENERALLY
RESULT IN HIGHER SNOW TO LIQUID RATIOS ON THE ORDER OF 25:1 OR
HIGHER. THEREFORE...EVEN WITH LOW LIQUID VALUES THIS FAVORS HIGHER
SNOW ACCUMULATION THAN MIGHT OTHERWISE BE EXPECTED. THE UPPER
TROUGH WILL CONTINUE TO DIG DOWN INTO THE GULF DURING THE DAY
SUNDAY AND MUCH COLDER AND DRIER AIR WILL QUICKLY USHER IN.
THEREFORE...THIS WILL BE A BRIEF DURATION SNOW EVENT AND SNOW
AMOUNTS WILL BE LIGHT IN SPITE OF THE HIGHER SNOW TO LIQUID RATIOS.

A WIND EVENT WILL DEVELOP ACROSS SOUTHCENTRAL AND THE GULF SUNDAY
NIGHT AND MONDAY AS:

1) ARCTIC AIR PLUMMETS SOUTHWARD

2) PACIFIC LOW MOVES INTO THE EASTERN GULF AND STRENGTHENS DUE
TO AMPLIFYING UPPER TROUGH AND COLD AIR FEEDING INTO THE SYSTEM.

3) ADDITIONAL WEAK UPPER WAVES DROP DOWN ACROSS WESTERN PORTIONS
OF SOUTHCENTRAL ENHANCING SUBSIDENCE WHICH ALLOWS HIGHER LEVEL
WINDS TO MIX DOWN TOWARD THE SURFACE.

4) RIDGE BUILDS INTO EASTERN BERING WITH UPPER JET DIVING SOUTHWARD
ACROSS SOUTHWEST ALASKA AND THE WESTERN GULF PROVIDING ADDITIONAL
SYNOPTIC SUBSIDENCE OVER WESTERN SOUTHCENTRAL.

AT THIS MOMENT IT LOOKS LIKE THE PEAK OF THE WIND EVENT FROM THE
MAT-SU TO ANCHORAGE TO THE WESTERN KENAI PENINSULA WILL BE LATE
SUNDAY NIGHT INTO MONDAY WHEN SURFACE PRESSURE GRADIENT IS TIGHTEST
AND WHEN UPPER LEVEL SUPPORT IS AT ITS MAXIMUM. WILL HAVE TO WAIT
UNTIL GET CLOSER TO THE EVENT TO SEE HOW FAVORABLE STABILITY IS FOR
MOUNTAIN WAVE FORMATION. FOR NOW IT LOOKS LIKE WINDS WILL REMAIN
BELOW WARNING CRITERIA. AREAS TO THE EAST WHICH TYPICALLY GET WINDS
IN THIS SCENARIO...SUCH AS THOMPSON PASS/VALDEZ AND THE COPPER RIVER
DELTA WILL BE A LITTLE DELAYED WITH MAXIMUM WINDS EXPECTED MONDAY
INTO MONDAY NIGHT. THESE AREAS WILL NOT SEE THE SAME UPPER LEVEL
SUPPORT AS TO THE WEST...BUT THEY WILL BE CLOSER IN PROXIMITY TO
SURFACE LOW OVER THE GULF AND OBSERVE A TIGHTER PRESSURE GRADIENT.
THEREFORE...THERE IS STILL POTENTIAL THAT WARNING LEVEL WINDS WILL
BE REACHED. SINCE THIS IS THE FOURTH PERIOD OF THE FORECAST AND IT
LOOKS BORDERLINE WILL NOT ISSUE A WATCH YET...BUT NEXT SHIFT WILL
HAVE TO RE-EVALUATE THIS POTENTIAL.

WIND STORM WILL GRADUALLY DIE DOWN TUESDAY AND WEDNESDAY AS GULF
LOW WEAKENS AND EXITS TO THE SOUTH AND EAST. THAT WILL LEAVE US
WITH CLEAR AND COLD CONDITIONS ACROSS THE REGION.

SOUTHWEST ALASKA...SURFACE LOW WILL EXIT TO THE GULF THIS AFTERNOON
AND UPPER LEVEL TROUGH WILL CROSS TONIGHT RESULTING IN CLEARING
SKIES AND FALLING TEMPERATURES. NORTHERLY WINDS WILL KICK UP IN
SOME PLACES AS COLD ADVECTION KICKS IN...PRIMARILY OVER THE BRISTOL
BAY REGION AND ALONG THE ALASKA PENINSULA. THE UPPER LEVEL SUPPORT
MENTIONED ABOVE WILL REALLY ENHANCE WINDS THROUGH GAPS OF THE ALASKA
PENINSULA SO LATEST FORECAST WILL BUMP UP FORECAST SPEEDS. UNDER THE
INFLUENCE OF UPPER LEVEL RIDGE COLD AND DRY CONDITIONS WILL DOMINATE
THE REGION THROUGH AT LEAST MONDAY. COASTAL AREAS WILL THEN HAVE A
CHANCE OF PRECIP AS BERING SYSTEMS APPROACH FROM THE WEST ALTHOUGH
THEY WILL BE VERY WEAK BY THE TIME THEY MAKE IT OVER THE RIDGE.

BERING SEA/ALEUTIANS...A SERIES OF LOWS WILL TRACK FROM THE
NORTHWEST PACIFIC INTO THE BERING SEA THROUGH EARLY NEXT WEEK.
EACH OF THESE WILL HELP MAINTAIN RIDGE DOWNSTREAM SO THAT THEY
WILL WEAKEN AS THEY PROGRESS EASTWARD INTO THE RIDGE.

.LONG TERM FORECAST...A MEAN RIDGE WILL PERSIST OVER THE EASTERN
BERING THROUGH MID TO LATE NEXT WEEK AND KEEP ANY STORMS FROM
PUSHING INLAND ACROSS SOUTHERN ALASKA. THEREFORE...COLD AND DRY
CONDITIONS WILL PERSIST. MEDIUM RANGE MODELS HINT AT A CHANGE
LATE NEXT WEEK INTO THE WEEKEND AS A DEEP TROUGH DROPS INTO THE
BERING SEA AND EVENTUALLY TOWARD THE GULF. HOWEVER...THERE IS
DISAGREEMENT ON THE DETAILS OF WHETHER THIS COMES CLOSE ENOUGH
TO GREATLY AFFECT THE MAINLAND. PATTERN OVER THE WESTERN BERING
AND ALEUTIANS WILL REMAIN ACTIVE ALTHOUGH THERE DOESN'T APPEAR
TO BE ANY PARTICULARLY STRONG STORMS ON THE WAY.

.AER/ALU...WATCH/WARNING SUMMARY
PUBLIC...NONE.
MARINE...STORM WARNING...127 130.
         GALE WARNING...120 121 125 126 128 129 132 138 140 150 155
                        160 165 175 176 185.
FIRE WEATHER...NONE.

SEB NOV 10