Gale Warning 198, NWS Anchorage, AK

AREA FORECAST DISCUSSION
SOUTHCENTRAL AND SOUTHWEST ALASKA
140 PM AKST WED FEB 18 2009

.ANALYSIS AND UPPER LEVELS...

IN THE UPPER LEVELS...
AN UPPER LEVEL LOW OVER EASTERN SIBERIA ACTS AS A PIVOT POINT FOR
WAVES MOVING ACROSS THE BERING. THE FIRST TROUGH EXTENDS SOUTH INTO
THE CENTRAL ALEUTIANS AND IS MOVING EAST. THE SECOND WEAKER WAVE IS
DEVELOPING SOUTH AND WEST OF THE KAMCHATKA PENINSULA AND WILL BE THE
FOCUS FOR THE NEXT SYSTEM MOVING INTO THE AREA EARLY THIS WEEKEND. AN
UPPER LEVEL RIDGE OVER THE EASTERN MAINLAND EXTENDS SOUTHWEST INTO
THE GULF AND CONTINUES WEAKEN AND MOVE TO THE EAST.

AT THE SURFACE...
HIGH PRESSURE OVER THE INTERIOR WITH A NORTHEAST TO SOUTHWEST AXIS
EXTENDING OVER THE GULF CONTINUES TO WEAKEN AND SLIDE TO THE EAST. A
973 MB SURFACE LOW CONTINUES TO FILL AND MOVE FURTHER NORTH OVER THE
BERING. A STRONG WEATHER FRONT EXTENDS FROM THIS LOW NORTHEAST
TOWARDS NOME AND THEN SHARPLY DIVES SOUTH THROUGH NORTON SOUND AND
INTO THE CENTRAL ALEUTIANS.

.MODEL DISCUSSION...
MODELS ALL SEEM TO INITIALIZE WELL IN THE SHORT TERM AND HANDLE THE
FRONTAL PASSAGE WITH THE SAME SPEED AND INTENSITY THROUGH
SOUTHCENTRAL THURSDAY. ALL MODELS BUILD IN A WEAK SURFACE LOW OVER
EASTERN PRINCE WILLIAM SOUND THURSDAY NIGHT AS THE FRONT MOVES
THROUGH AND WEAKENS. THE SURFACE LOW THEN MOVES SOUTHEAST OVER THE
PANHANDLE WHERE IT WILL DISSIPATE BY SATURDAY MORNING. AS THE SURFACE
TROUGH SHIFTS EASTWARD TODAY ALL MODELS BUILD IN A TRANSIENT RIDGE
OVER THE CENTRAL ALEUTIANS AND INTO THE AKPEN EARLY THIS WEEKEND.
BEYOND THAT MODELS BEGIN TO DIVERGE AS THEY ARE HAVING A DIFFICULT
TIME HANDLING WAVES MOVING WITH THE PROGRESSIVE SOUTHWEST FLOW.

.SHORT TERM FORECAST...
.SOUTHCENTRAL...
A TRICKY FORECAST IN STORE FOR SOUTHCENTRAL WITH A COMPLEX SYSTEM
MOVING INTO SOUTHWEST ALASKA TODAY. PRECIPITATION WILL SPREAD OVER
THE AREA TONIGHT AS WARM AIR ALOFT OVERRUNS THE COLD AIR AT THE
SURFACE. THE RESULT WILL BE A WIDE RANGE OF PRECIPITATION TYPE...FROM
SNOW TO RAIN WITH THE POSSIBILITY OF FREEZING RAIN AND SLEET IN MANY
AREAS. ANCHORAGE ESPECIALLY WILL BE TRICKY BECAUSE OF THE STRONG
SOUTHEASTERLY WIND COMING OUT OF TURNAGAIN ARM. SOUTHEAST WINDS WILL
INCREASE ALONG THE ARM AND MOVE ACROSS THE ANCHORAGE BOWL.
TYPICALLY...THIS WARMS TEMPERATURES UP ENOUGH TO CHANGE PRECIP TYPE
TO RAIN BUT MANY OTHER VARIABLES WILL FACTOR INTO THE FORECAST
OVERNIGHT AND TOMORROW. EVEN THOUGH THERE IS A SUBTROPICAL FETCH OF
MOISTURE...SURFACE TEMPERATURES HAVE REMAINED NEAR FREEZING ALONG THE
WEST COAST. THE DEEP MOISTURE ASSOCIATED WITH THE SYSTEM IS ALSO
HELPING TO COOL THE COLUMN UNTIL IT IS NEARLY ISOTHERMAL...A GOOD
SETUP FOR HEAVY SNOWFALL. SO WHERE TEMPERATURES REMAIN BELOW FREEZING
LIKE THE NORTHERN AND WESTERN SUSITNA VALLEY WE ARE EXPECTING HEAVY
SNOWFALL. COASTAL SECTIONS WILL LIKELY SEE A RAIN AND SNOW MIX WHILE
INTERIOR SECTIONS LIKE ANCHORAGE AND THE MATANUSKA VALLEY WILL SEE A
MIX.

OUR CURRENT THINKING IS THAT PRECIP WILL BEGIN AS A RAIN SNOW MIX BUT
AS THE INTENSITY INCREASES...PRECIP WILL CHANGE OVER TO SNOW. THE
INTENSITY WILL HELP EVAPORATIVE COOLING RESULTING IN A NEAR
ISOTHERMAL SOUNDING. AT THIS POINT...WE CAN NOT RULE OUT THE
POSSIBILITY OF A PERIOD OF LIGHT FREEZING RAIN BUT WITH SOUTHEAST
WINDS EXPECT THAT TO BE A VERY SHORT LIVED EVENT IF IT DOES HAPPEN.
WILL TREND THE SNOW AMOUNTS UP BUT STILL NOT CONFIDENT ENOUGH TO PUT
SIGNIFICANT SNOW FALL TOTALS IN ANCHORAGE AND THE MATANUSKA VALLEY.

A WEAK SURFACE LOW WILL DEVELOP THURSDAY AFTERNOON JUST SOUTH OF
SEWARD...CUTTING OFF THE SOUTHEASTERLY FLOW THROUGH TURNAGAIN ARM.
THIS IS A RESULT OF COLD AIR ADVECTING ACROSS THE REGION. LIGHT SNOW
WILL REMAIN A POSSIBILITY UNTIL THE UPPER TROUGH SWINGS THROUGH LATE
THURSDAY NIGHT.

AS THE TROUGH SWINGS THROUGH...EXPECTING STRONG WINDS THROUGH THE
NORMAL CHANNELED TERRAIN LIKE SEWARD...WHITTIER...AND THOMPSON PASS.
IN ADDITION...WILL HAVE TO CONSIDER NEAR HIGH WIND WARNING CRITERIA
ACROSS KODIAK ON FRIDAY. COLD AIR ADVECTION WILL BE SUPPORTED BY A
150 KT UPPER LEVE JET WHICH WOULD HELP BRING SOME STRONGER GUSTS DOWN
TO THE SURFACE. AGAIN...WILL BE BUMPING WINDS UP ACROSS KODIAK ISLAND
BUT NOT CONFIDENT ENOUGH TO PUT OUT ANY WATCHES OR WARNINGS AT THIS
TIME.

.SOUTHWEST...
ACTIVE WEATHER IS OUT WEST THIS MORNING WITH NUMEROUS WINTER WEATHER
ADVISORIES FOR SNOW AND FREEZING RAIN...PLUS A BLIZZARD WARNING FOR
THE KUSKOKWIM DELTA. TEMPERATURES HAVE BEEN VERY SLOW TO WARM UP
RESULTING IN AN EXTENDED PERIOD OF BLIZZARD CONDITIONS ALONG THE
DELTA COASTLINE. INLAND...FREEZING RAIN MIXED WITH SLEET IN BETHEL
WILL CONTINUE SPREADING EAST OVERNIGHT. MCGRATH IS ALREADY SEEING
LIGHT SNOW ASSOCIATED WITH THE WARM FRONT THIS AFTERNOON. THE UPPER
KUSKOKWIM WILL SEE A BREAK BEFORE THE HEAVIER PRECIPITATION MOVES
OVER THE AREA OVERNIGHT. WILL LIKELY SEE A HEAVY MIX OF FREEZING
RAIN...SLEET...AND SNOW FROM BRISTOL BAY TO THE UPPER KUSKOKWIM. 18Z
SOUNDINGS FROM KING SALMON AND BETHEL SHOW THE WARM NOSE ALOFT WHICH
WILL RESULT IN THE CONTINUED MIX OF PRECIPITATION. THANKS TO THE TWO
WSOS FOR GETTING THE 18Z SOUNDINGS UP SO FAST.

COLD AIR WILL MOVE ACROSS THE AREA LATE TONIGHT CHANGING THE MIX OF
PRECIPITATION BACK TO ALL SNOW BEFORE TAPERING OFF. SNOW WILL LINGER
THROUGH THURSDAY NIGHT UNTIL THE UPPER TROUGH MOVES THROUGH. HIGH
PRESSURE WILL QUICKLY BUILD OVER THE AREA BUT WILL ONLY SEE A BRIEF
BREAK BEFORE THE NEXT SYSTEM IMPACTS THE COAST ON SATURDAY.

.BERING SEA/ALEUTIANS...
STRONG SOUTHEASTERLY WINDS OVER BRISTOL BAY AND THE ALASKA PENINSULA
WILL SUBSIDE AS THE FRONT MOVES INLAND TONIGHT. BEHIND THE
FRONT...WIDESPREAD SNOW SHOWERS WILL PERSIST UNDER THE BROAD UPPER
TROUGH. SURFACE HIGH PRESSURE WILL BUILD ON THURSDAY NIGHT AND
AMPLIFY AHEAD OF ANOTHER DEVELOPING LOW SOUTH OF THE WESTERN
ALEUTIANS. THIS LOW WILL FOLLOW THE SAME PATH AS THE PREVIOUS
SYSTEMS...MOVING RAPIDLY TOWARD THE ANCHOR LOW NEAR THE KAMCHATKA
PENINSULA WHILE THE ASSOCIATED FRONTAL BOUNDARY SPREADS GALE AND
STORM FORCE WINDS ACROSS THE ALEUTIANS OVER THE NEXT SEVERAL DAYS.

.LONG TERM FORECAST...
COPIED FROM PREVIOUS FORECAST DISCUSSION...

A STRENGTHENING RIDGE WILL BUILD OVER SOUTHWESTERN ALASKA
FRIDAY...MOVING ACROSS THE INTERIOR THROUGH THE WEEKEND. MEANWHILE
SOUTHERLY FLOW WILL ENCOMPASS THE ALEUTIANS...WITH A SERIES OF
SURFACE LOWS EXPECTED TO MOVE FROM THE NORTH PACIFIC INTO THE BERING
SEA. GENERALLY EXPECT COOLER AND MORE TRANQUIL WEATHER ACROSS THE
MAINLAND...WITH CONTINUED UNSETTLED WEATHER ACROSS THE ALEUTIANS FOR
THE WEEKEND.

.AER/ALU...WATCH/WARNING SUMMARY
PUBLIC...WINTER STORM WARNING...145
         FREEZING RAIN ADVISORY...161
         WINTER WEATHER ADVISORY FOR SNOW AND FREEZING RAIN...151
MARINE...GALE WARNING...130 132 138 150 155 172 175 176 180 185
FIRE WEATHER...NONE

AB/SSS FEB 09