Blizzard Watch 27, NWS Anchorage, AK

AREA FORECAST DISCUSSION
SOUTHCENTRAL AND SOUTHWEST ALASKA
1230 PM AKST WED DEC 10 2008

.ANALYSIS AND UPPER LEVELS...A LARGE BROAD TROUGH EXTENDS FROM
SIBERIA SOUTHEASTWARD TO SOUTHWEST ALASKA. THERE IS A STRONG
CLOSED HIGH TO THE NORTHEAST OVER THE BEAUFORT SEA BLOCKING EASTWARD
PROGRESSION OF THIS TROUGH...SO IT IS BEGINNING TO LIFT NORTHWARD.
WEAK UPPER LEVEL WAVES EMBEDDED WITHIN THE TROUGH CONTINUE TO ROTATE
ACROSS THE EASTERN BERING SEA NORTHEASTWARD ACROSS MAINLAND
ALASKA...PRODUCING AREAS OF LIGHT SNOW. THE SURFACE LOW THAT HAD
BEEN SITTING NEAR SEWARD ALL NIGHT HAS FINALLY MOVED INLAND. THE
ARCTIC FRONT IS DRAPED ALONG COOK INLET AND SHELIKOF STRAIT SLOWLY
MOVING EASTWARD AS LOW PRESSURE PROGRESSES INLAND. COLD ADVECTION
IS PRODUCING THE USUAL GUSTY WINDS THROUGH KAMISHAK BAY/BARREN
ISLANDS REGION AND INTO COOK INLET. A RIDGE IS BUILDING OVER THE
WESTERN BERING AHEAD OF ANOTHER TROUGH SWINGING OUT OF EASTERN
ASIA...PUSHING A STRONG FRONT TOWARD THE WESTERN ALEUTIANS.

.MODEL DISCUSSION...MODELS INITIALIZED WELL AND ARE IN EXCELLENT
AGREEMENT THROUGH THE WEEKEND. SOME FINE-TUNING OF FORECAST WILL
BE NECESSARY FOR TIMING AND LOCATION OF UPPER LEVEL SHORT-WAVES
AND RESULTANT PRECIPITATION AS WELL AS STRENGTH OF COLD ADVECTION
WINDS.

.SHORT TERM FORECAST...

SOUTHCENTRAL ALASKA...THE UPPER TROUGH WILL GRADUALLY PROGRESS
EASTWARD WHILE IT CONTINUES TO WEAKEN AND LIFT TO THE NORTH. THE
REMNANTS OF THE TROUGH WILL MOVE ACROSS SOUTHCENTRAL ALASKA THURSDAY
NIGHT. UNTIL THEN THE THREAT OF LIGHT SNOW OR SNOW SHOWERS WILL
CONTINUE...ESPECIALLY IN THE UPSLOPE AREAS ALONG THE MOUNTAINS.
SNOW ACCUMULATION WILL GENERALLY BE VERY LIGHT. A WEAK SURFACE HIGH
WILL BUILD FROM BRISTOL BAY TO THE KUSKOKWIM VALLEY TONIGHT CAUSING
LOW LEVEL FLOW OVER WESTERN SOUTHCENTRAL TO GRADUALLY SHIFT TO WEST
LATE TONIGHT INTO THURSDAY MORNING. THIS WILL ADVECT DRIER LOW LEVEL
AIR TOWARD ANCHORAGE AND LIKELY HELP CUT OFF PRECIPITATION FOR ALL
BUT RIGHT ALONG THE MOUNTAINS. WITH COLD ADVECTION THE FLOW IS
EXPECTED TO REMAIN STRONG ENOUGH TO PREVENT FORMATION OF
FOG...ALTHOUGH LOCALIZED FOG IS CERTAINLY STILL A POSSIBILITY.
INSTEAD...WOULD EXPECT MOISTURE TO BE BANKED UP AGAINST THE
MOUNTAINS AND MAINTAIN A STRATUS DECK. THE SURFACE HIGH WILL
STRENGTHEN AS THE UPPER RIDGE CURRENTLY BUILDING OVER THE WESTERN
BERING MOVES TO SOUTHWEST ALASKA THURSDAY NIGHT. AS THIS RIDGE
BUILDS INTO SOUTHCENTRAL A COMBINATION OF A TIGHT PRESSURE GRADIENT
AND COLD ADVECTION WILL PRODUCE THE USUAL GAP WINDS ALONG THE GULF
COASTAL WATERS. WITH UPPER RIDGE OVERHEAD IT LOOKS DRY HEADING INTO
THE WEEKEND.

SOUTHWEST ALASKA...AS THE UPPER TROUGH SWINGS NORTHEASTWARD TONIGHT
INTO THURSDAY STEADIER AND MORE WIDESPREAD SNOW OR SNOW SHOWERS CAN
BE EXPECTED...ESPECIALLY ALONG THE KUSKOKWIM DELTA. THIS TROUGH WILL
EXIT SOUTHWEST ALASKA LATE THURSDAY AS UPPER RIDGE CROSSING THE
BERING SEA APPROACHES. THE UPPER PATTERN WILL REMAIN PROGRESSIVE
HEADING TO THE WEEKEND SO UPPER RIDGE WILL QUICKLY MOVE EASTWARD AND
A STRONG FRONT WILL MOVE ACROSS SOUTHWEST ALASKA LATE THURSDAY NIGHT
THROUGH FRIDAY. UPPER RIDGE WILL BUILD AGAIN OVER THE WEEKEND AHEAD
OF ANOTHER BERING SEA SYSTEM...SO LOOKS DRY FOR THE WEEKEND WITH
SEASONABLE TEMPERATURES.

BERING SEA/ALEUTIANS...A MEAN RIDGE WILL SET UP FROM THE EASTERN
BERING TO MAINLAND ALASKA THROUGH THE WEEKEND WITH UPPER JET EXITING
EAST ASIA AND RIDING NORTHEASTWARD ACROSS THE BERING SEA TO NORTHERN
ALASKA. THUS...LOWS WILL STAY MAINLY NORTH OF THE FORECAST AREA AND
MOVE QUICKLY TOWARD THE ARCTIC. HOWEVER...STRONG FRONTAL SYSTEMS
ASSOCIATED WITH THESE LOWS WILL MOVE ACROSS THE BERING SEA ONE AFTER
ANOTHER WITH RAPIDLY CHANGING CONDITIONS.

.LONG TERM FORECAST...MEDIUM RANGE MODELS INDICATE THE STRONG UPPER
RIDGE IN PLACE OVER MAINLAND ALASKA SUNDAY WILL PROGRESS EASTWARD
AND FLATTEN AS STORMS MOVE OVER-TOP OF IT. THUS...SYSTEMS MOVING
ACROSS THE BERING SEA WILL BEGIN TO IMPACT MAINLAND ALASKA AS WE
HEAD THROUGH NEXT WEEK. THE PROGRESSIVENESS OF THE HEMISPHERIC
PATTERN MAKES IT DIFFICULT TO PINPOINT THE WHERE AND WHEN OF
PARTICULAR SYSTEMS.

.AER/ALU...WATCH/WARNING SUMMARY
PUBLIC...NONE.
MARINE...STORM WARNING...175 176.
         GALE WARNING...130 155 160 165 170 171 172 179 180 185.
         HEAVY FREEZING SPRAY WARNING...185.
FIRE WEATHER...NONE.

SEB DEC 08