Storm Warning 166, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
115 PM AKDT MON OCT 25 2010

.ANALYSIS AND UPPER LEVELS...A BROAD UPPER LEVEL TROUGH ENCOMPASSES
THE ALASKA REGION. THE MAIN FEATURE OF CONCERN TODAY IS A SHORT-WAVE
TROUGH ROTATING UP THE EAST SIDE OF AN UPPER LOW OVER BRISTOL BAY.
RADAR IMAGERY SHOWS A BAND OF PRECIPITATION JUST AHEAD OF THE
SHORT-WAVE FROM THE NORTH GULF COAST AND PRINCE WILLIAM SOUND TO
NORTHERN COOK INLET AND THE SOUTHERN SUSITNA VALLEY. THE AREA OF
PRECIPITATION IS SPREADING SLOWLY NORTHWARD WITH THE BACK EDGE OF
STEADY PRECIPITATION NOW MOVING THROUGH KENAI AND SEWARD. THE BACK
EDGE IS EXPECTED TO SLOW OR STALL THE NEXT FEW HOURS AS THE
SHORT-WAVE BEGINS TO ROTATE NORTHWESTWARD. PRECIPITATION TYPE
ACROSS SOUTHCENTRAL IS MAINLY IN THE FORM OF RAIN WITH THE EXCEPTION
OF THE FOOTHILLS OF THE ALASKA RANGE. HOWEVER...WEAK COLD ADVECTION
IN THE LOW LEVELS IS BEGINNING TO MOVE UP COOK INLET. THE 12Z
ANCHORAGE SOUNDING SHOWED A SIGNIFICANT WARM LAYER WITH FREEZING
LEVEL AROUND 4500 FEET AND MAXIMUM TEMPERATURE OF 6 DEGREES CELSIUS.
SURPRISINGLY...SNOW HAS ALREADY STARTED TO MIX IN WITH THE RAIN IN
ANCHORAGE. THIS HAS LED TO INCREASING CONFIDENCE IN SEEING THE FIRST
MEASURABLE SNOWFALL OF THE SEASON FOR ANCHORAGE AND PARTS OF THE
MATANUSKA AND SUSITNA VALLEYS LATER TODAY.

THE UPPER LEVEL SHORT-WAVE IS ALSO PRODUCING AREAS OF PRECIPITATION
BACK OVER SOUTHWEST ALASKA...PRIMARILY IN THE FORM OF SNOW. A CLOSED
UPPER LOW IS DRIFTING INTO BRISTOL BAY BRINGING IN SOME SHOWERS
BEHIND THE STEADY PRECIP ASSOCIATED WITH THE SHORT-WAVE.

CONDITIONS ARE FAIRLY BENIGN OVER THE BERING SEA AND ALEUTIANS WITH
JUST SOME SHOWERS AND LOCALLY GUSTY WINDS. HOWEVER...A MAJOR STORM
SOUTH OF THE KAMCHATKA PENINSULA IS HEADED NORTHEASTWARD INTO THE
WESTERN BERING WITH FRONTAL SYSTEM APPROACHING THE WESTERN ALEUTIANS.

.MODEL DISCUSSION...MODELS ARE IN EXCELLENT AGREEMENT THROUGH THE
NEXT FEW DAYS. THERE ARE SOME DIFFERENCES IN TIMING AND STRENGTH OF
FEATURES COMPARED TO CURRENT FORECASTS...SO THE NEW FORECASTS WILL
BE UPDATED TO REFLECT THIS. THE PRIMARY CHANGE IS FOR STRONGER WINDS
OUT WEST WITH THE FRONTAL SYSTEM AS IT CROSSES THE BERING SEA AND
ALEUTIANS THE THE NEXT COUPLE DAYS.

.SHORT TERM FORECAST...

SOUTHCENTRAL ALASKA...THE COMBINATION OF COLDER AIR MOVING UP THE
INLET AND COOLING OF THE VERTICAL COLUMN OF AIR DUE TO MELTING WILL
CHANGE PRECIPITATION FROM ANCHORAGE NORTHWARD TO ALL SNOW LATER
THIS AFTERNOON INTO EVENING. IT DOES LOOK LIKE THE SNOW WILL BE
SHORT-LIVED THOUGH AS THE SHORT-WAVE TROUGH ALOFT LIFTS NORTHWARD.
THE TIMING OF THE SNOW DOES LOOK LIKE IT WILL AFFECT RUSH HOUR
TRAFFIC IN ANCHORAGE WITH THE PRIME PERIOD OF SNOWFALL IN THE 3 PM
TO 6 PM TIME FRAME. THE TEMPERATURES OF PAVED SURFACES ARE STILL
QUITE WARM (AVERAGING IN THE UPPER 30S) SO IT WILL TAKE A WHILE
BEFORE SNOW STICKS ON THE ROADS.  WITH ALL OF THIS IN MIND EXPECT
SNOW ACCUMULATION OF AN INCH OR LESS AT SEA LEVEL WITH 2 INCHES
OR MORE AT HIGHER ELEVATIONS. THE SNOW WILL TAPER TO SHOWERS IN
ANCHORAGE BY MID EVENING WHILE THE STEADY PRECIP HEADS NORTHWARD
ACROSS THE MATANUSKA AND SUSITNA VALLEYS AS WELL AS HIGHER
ELEVATIONS OF THE COPPER RIVER BASIN. THE TROUGH WILL EXIT TO THE
NORTH ON TUESDAY BRINGING AN END TO PRECIPITATION ACROSS MOST OF
THE REGION. THE UPPER LEVEL LOW OVER BRISTOL BAY WILL MOVE TO KODIAK
ISLAND BY TUESDAY EVENING...SO SHOWERS ASSOCIATED WITH IT WILL MOVE
INTO THE GULF AFFECTING MAINLY THE GULF COAST AND KENAI PENINSULA.
FEW SHOWERS ARE EXPECTED TO MAKE IT INLAND. THE UPPER LOW WILL OPEN
INTO A TROUGH MID WEEK AND EXIT TO THE EAST. FAST ON ITS HEELS WILL
BE THE FRONT ASSOCIATED WITH MAJOR LOW MOVING INTO THE BERING SEA
THE NEXT COUPLE DAYS. THIS FRONT IS EXPECTED TO ARRIVE IN
SOUTHCENTRAL WEDNESDAY NIGHT INTO THURSDAY...INITIALLY PRODUCING
STRONG CHINOOK WINDS AND MAINLY RAIN. THERE WILL AGAIN BE POTENTIAL
FOR SNOW ON THE BACK SIDE OF THIS SYSTEM AS COLDER AIR MOVES BACK IN.

SOUTHWEST ALASKA...AS THE UPPER LEVEL SHORT-WAVE ROTATES
NORTHWESTWARD THIS AFTERNOON AND EVENING SNOW WILL SPREAD WESTWARD
ACROSS THE KUSKOKWIM DELTA. MEANWHILE...THE STEADY SNOW FURTHER
EAST OVER INLAND AREAS OF THE BRISTOL BAY ZONE WILL TAPER TO SHOWERS
AS THE UPPER LOW MOVES IN. COLDER ADVECTION WILL ENSURE THAT
PRECIPITATION TYPE WILL BE ALL SNOW TONIGHT. CONDITIONS WILL DRY OUT
DURING THE DAY TUESDAY AS THE UPPER LOW EXITS TOWARD KODIAK ISLAND.
HOWEVER...A MAJOR FRONT WILL ARRIVE IN SOUTHWEST ALASKA ON WEDNESDAY.
STRONG SOUTHEAST FLOW WILL DEVELOP AHEAD OF THE FRONT WHICH USUALLY
RESULTS IN DOWNSLOPE DRYING ALONG THE ALASKA AND ALEUTIAN RANGES.
IN THIS CASE...SOUTHWEST FLOW ALOFT AND STRONG VERTICAL MOTION
SHOULD BE STRONG ENOUGH TO COUNTERACT THIS EFFECT AND PRODUCE SOME
PRECIPITATION EVERYWHERE. WARM AIR WITH THE FRONT WILL PRODUCE
MAINLY RAIN OVER BRISTOL BAY WITH RAIN AND SNOW FURTHER NORTH.

BERING SEA/ALEUTIANS...A STRONG UPPER JET WILL CONTRIBUTE TO RAPID
STRENGTHENING OF LOW SOUTH OF KAMCHATKA AS IT TRACKS INTO THE
WESTERN BERING TUESDAY. EXPECT GALE TO STORM FORCE WITH THIS
LOW AS WELL AS ALONG THE FRONTAL SYSTEM AS IT TRACKS ACROSS THE
BERING AND ALEUTIANS TONIGHT THROUGH TUESDAY. STRONG WESTERLIES
WILL DOMINANT BEHIND THE FRONTAL AND LAST THROUGH THE END OF THE
WEEK. THIS WILL SET UP POTENTIAL FOR MINOR COASTAL FLOODING ALONG
THE SOUTHWEST COAST BEGINNING MID WEEK. AT THIS POINT COASTAL FLOOD
GUIDANCE IS INDICATING ONLY 1 TO 2 FEET ABOVE MEAN ASTRONOMICAL
TIDE...BUT WILL CONTINUE TO MONITOR THIS AS WE GET CLOSER.

.LONG TERM FORECAST...MAJOR LOW OVER THE NORTHERN BERING MID WEEK
WILL GRADUALLY DISSIPATE NEAR THE BERING STRAIT THROUGH THE END OF
THE WEEK. MODELS ALL INDICATE FORMATION OF A TRIPLE POINT LOW NEAR
THE KENAI PENINSULA THURSDAY WHICH WILL ENHANCE WINDS AND PRECIP IN
ITS VICINITY. COLD ADVECTION BEHIND THIS LOW AND FRONT LOOKS STRONGER
THAN WITH THE CURRENT UPPER LEVEL FEATURE...SO THERE A VERY GOOD
CHANCE OF SEEING SOME SNOW IN THE THURSDAY NIGHT TIME FRAME.
ANOTHER LOW WILL ENTER THE BERING/ALEUTIANS IN THE BROAD WESTERLIES
LATE THIS WEEK. HOWEVER...MODELS ARE INCONSISTENT IN THE TRACK AND
STRENGTH OF THIS LOW. THERE IS AGREEMENT ON A MAJOR LOW IN THE
GULF BY EARLY NEXT WEEK.

.AER/ALU...WATCH/WARNING SUMMARY
PUBLIC...NONE.
MARINE...STORM WARNING...185.
         GALE WARNING...172 175 176 179 180.
FIRE WEATHER...NONE.

SEB OCT 10