Gale Warning 309, NWS Anchorage, AK

AREA FORECAST DISCUSSION
SOUTHCENTRAL AND SOUTHWEST ALASKA
230 PM AKDT WED APR 14 2010

.ANALYSIS AND UPPER LEVELS...
NEUTRALLY TILTED TROUGH WITH AXIS AROUND 160W DRIVING THE WEATHER
OVER MUCH OF MAINLAND ALASKA TODAY. THE 105 KT JET EXITING THE
AMPLIFIED TROUGH IS TAPPING THE SUBTROPICAL MOISTURE AND DRIVING IT
NORTH TO THE NORTHEASTERN GULF OF ALASKA. STRONG SUBSIDENCE EVIDENT
ON THE BACK OF THE TROUGH. FURTHER WEST IS A DEEPENING AREA OF LOW
PRESSURE BEING FED BY AN IMPRESSIVE 180 KT JET. THE MOISTURE FETCH
WITH THIS SYSTEM IS VERY SUBSTANTIAL...AND CAN BE TRACED BACK TO
SOUTHERN JAPAN.

AT THE SURFACE...THE AREA OF LOW PRESSURE THAT BROUGHT MOST OF
SOUTHCENTRAL ALASKA SNOW OVERNIGHT AND THIS MORNING IS CURRENTLY
LOCATED JUST SOUTHWEST OF SEWARD AS IT FOLLOWS THE KENAI PENINSULA
COAST TOWARD PRINCE WILLIAM SOUND. HIGH PRESSURE IN CONTROL OF
SOUTHWEST ALASKA AND THE EASTERN BERING SEA. OUT WEST...A STRONG
FRONT IS PUSHING THROUGH THE WESTERN ALEUTIANS WITH WINDS REACHING 60
MPH AT SHEMYA THIS HOUR.

.MODEL DISCUSSION...
AGAIN...MODELS IN EXCELLENT AGREEMENT BUT HAVE BEEN PRETTY ERRATIC
THE LAST SEVERAL RUNS SO CONFIDENCE IS NOT HIGH. ALL MODELS
INDICATING SOME MINOR COLD AIR ADVECTION ON THE BACK SIDE OF THE
KENAI PENINSULA LOW AS IT WEAKENS AND MOVES INLAND. THE STRENGTH AND
TIMING OF THE FRONT AS IT TREKS EAST ACROSS THE ALEUTIANS AND INTO
THE WESTERN GULF IS STILL LOOKING GOOD SO NO MAJOR CHANGES WERE MADE
THROUGH 84 HOURS. A SLIGHT PREFERENCE TOWARD THE ECMWF BEYOND 84
HOURS AS MODELS DIVERGE ON THE PLACEMENT OF A LOW IN THE SOUTHERN
GULF OVER THE WEEKEND.

.SHORT TERM FORECAST...
.SOUTH CENTRAL ALASKA...SNOW CONTINUES TO FALL FROM THE UPPER KENAI
PENINSULA TO THE EASTERN SUSITNA VALLEY...AS WELL AS HEAVY RAIN AND
SNOW OVER EASTERN PRINCE WILLIAM SOUND. THE TRACK OF THE LOW THAT
DEVELOPED NEAR KODIAK ISLAND TOOK A MUCH FARTHER WEST TRACK THAN
EXPECTED...BRINGING THE ABUNDANT OVERRUNNING MOISTURE TO THE WESTERN
KENAI PENINSULA AND ANCHORAGE BOWL. AFTER A BRIEFLY DOWNSLOPING LATER
THIS MORNING ASSOCIATED WITH WARM AIR ALOFT ADVECTING OVER THE
REGION...COLD AIR IS BEGINNING TO MOVE UP THE INLET. THE RESULT IS
MORE LOW LEVEL CONVERGENCE OVER ANCHORAGE. TEMPERATURES ARE HOVERING
RIGHT AROUND FREEZING SO SNOW IS HAVING A DIFFICULT TIME ACCUMULATING
ON THE ROAD SURFACE...OTHERWISE SNOW TOTALS HAVE RANGED SO FAR FROM
AN INCH ON THE HILLSIDE TO NEARLY 7 INCHES IN PARTS OF NORTH
ANCHORAGE. SNOW WILL TAPER OFF LATER THIS EVENING AS THE LOW MOVES
INLAND AND DRIER AIR SLOWLY FILTERS ACROSS THE REGION. EXPECT SNOW TO
CONTINUE ALONG THE HILLSIDE UP TO PALMER THROUGH MOST OF THE NIGHT AS
FLOW TURNS SOUTHWEST TO WESTERLY AND THE UPSLOPE COMPONENT
CONTRIBUTES TO THE ADDITIONAL SNOWFALL.

HEAVY RAIN AND SNOW WILL CONTINUE ALONG THE MOUNTAINS EAST OF PRINCE
WILLIAM SOUND. THE LARGEST IMPACT IS ON THE ROAD SYSTEM THROUGH
THOMPSON PASS WHERE OVER TWO FEET OF SNOW HAS BEEN MEASURED SO FAR.
AT SEA LEVEL...PRECIP IS STILL HEAVY BUT FALLING IN THE FORM OF RAIN.
RATES WILL SLOWLY DIMINISH OVERNIGHT AS THE SUBTROPICAL FETCH OF
MOISTURE IS PINCHED OFF AND TRANSITIONS EAST.

HIGH PRESSURE BUILDS BRIEFLY THURSDAY AHEAD OF THE STRONG FRONT THAT
WILL MOVE INTO THE WESTERN GULF LATER THURSDAY. WILL BE INCREASING
THE SOUTHEASTERLY GAP WINDS WITH THIS SYSTEM WHICH WILL ALSO LEAD TO
A STRONG DOWNSLOPING SCENARIO...KEEPING ANCHORAGE AND THE LEE SIDE OF
THE MOUNTAINS MOSTLY DRY BUT CLOUDY THROUGH THE WEEKEND.

.SOUTHWEST ALASKA...
THE TROUGH AXIS IS LOCATED OVER THE ALASKA PENINSULA AND EXTENDS
NORTH ALONG THE BERING SEA COAST. THE CYCLONIC AND ONSHORE FLOW IS
LEADING TO WIDESPREAD STRATUS AND PERIODS OF LIGHT SNOW ALONG MUCH OF
SOUTHWEST ALASKA. CONDITIONS WILL IMPROVE AS THE TROUGH SHIFTS EAST
TONIGHT AND AN UPPER LEVEL RIDGE BRIEFLY MOVES OVER THE REGION. THE
FAIR WEATHER WILL NOT LAST LONG AS THE STRONG FRONT APPROACHES THE
COAST THURSDAY AFTERNOON. THE OCCLUDED FRONT WILL NOT ALLOW WARM AIR
TO PENETRATE TOO FAR NORTH SO PRECIPITATION ASSOCIATED WITH THE FRONT
WILL FALL MOSTLY AS SNOW AS THE FRONT MOVES INLAND THURSDAY NIGHT.
FOR THIS REASON...WILL BE ISSUING A BLIZZARD WATCH FOR THE KUSKOKWIM
DELTA. THE UPPER LEVEL TROUGH STALLS OUT AGAIN OVER THE REGION SO
MARGINAL CONDITIONS WILL PREVAIL THROUGH THE WEEKEND.

.ALEUTIANS AND BERING SEA...MAIN AREA OF CONCERN WILL BE ALONG THE
STRONG FRONTAL BOUNDARY CURRENTLY AFFECTING THE WESTERN ALEUTIANS.
WINDS ARE ALREADY STRONGER THAN FORECASTED SO MAY HAVE TO CONTINUE
THAT TREND AS THE FRONT PROGRESSES TO THE CENTRAL AND EVENTUALLY
EASTERN ALEUTIANS THROUGH THURSDAY. BLIZZARD CONDITIONS ALSO A
CONCERN FOR THE PRIBILOFS WHERE PRECIP WILL FALL IN THE FORM OF SNOW
AHEAD OF THE OCCLUSION AND WINDS WILL INCREASE TO GALE STRENGTH. A
BLIZZARD WATCH THAT WILL LIKELY HAVE TO BE UPGRADED TO WARNING LATER
THIS EVENING WILL BE ISSUED WITH THE AFTERNOON PACKAGE. THE
VERTICALLY STACKED LOW WILL FILL THROUGH THE WEEKEND AS IT DRIFTS TO
THE EASTERN BERING FRIDAY NIGHT AND EVENTUALLY ACROSS THE AKPEN
SATURDAY.

.AER/ALU...WATCH/WARNING SUMMARY
PUBLIC...NONE.
MARINE...GALE WARNING...155 160 165 170 171 172 175 176 179 180 185
FIRE WEATHER...NONE.

AB APR 10