Blizzard in Alaska, March 3, 2009

AREA FORECAST DISCUSSION
SOUTHCENTRAL AND SOUTHWEST ALASKA
500 AM AKST TUE MAR 3 2009

.MODEL DISCUSSION...

NAM/GFS ARE IN GOOD AGREEMENT THROUGH THE SHORT TERM...WITH JUST
MINOR DIFFERENCES IN STRENGTH OF THE BERING SEA LOW WEDNESDAY INTO
THURSDAY.  FOR THE EXTENDED PERIOD...SINCE ALL MODELS ARE IN POOR
AGREEMENT STUCK WITH PREVIOUS FORECASTS AND BLENDED SLIGHTLY TOWARDS
HPC GUIDANCE...ESPECIALLY TOWARDS THE END OF THE EXTENDED GRIDS.

.ANALYSIS AND UPPER LEVELS...

A WEAKENING TROUGH OF LOW PRESSURE IS EXTENDING FROM THE WESTERN
SEWARD PENINSULA THROUGH THE SOUTHWESTERN INTERIOR TO A WEAK 1006 MB
LOW CURRENTLY OVER THE AKPEN MOVING QUICKLY TOWARDS THE NORTHERN
GULF.  THIS WEAK LOW IS TRAVELING ALONG A WEAK PERSISTENT TROUGH THAT
HAS REMAINED OVER THE NORTHERN GULF FOR THE LAST SEVERAL DAYS NOW.
THIS TROUGH EXTENDS BACK TO THE SOUTH OF THE ALEUTIANS FROM THE AKPEN
LOW TO A 974 MB NORTH PACIFIC LOW THAT IS MOVING NORTH TOWARDS THE
BERING SEA DURING THE DAY TODAY.  A 1020 MB SURFACE RIDGE IS
CURRENTLY LOCATED JUST EAST OF THE ALCAN BORDER IN THE YUKON
TERRITORY. A WEAK TRANSIENT SURFACE HIGH IS LOCATED IN THE CENTRAL
AND WESTERN BERING SEA.

ALOFT...AN UPPER RIDGE PERSISTS ACROSS MOST OF THE SOUTHERN HALF OF
THE STATE...BUT IS FLATTENED BY A STRONG UPPER TROUGH MOVING EAST
FROM SIBERIA INTO THE NORTHERN HALF OF THE STATE.  THIS IS CREATING
FAST ZONAL FLOW ACROSS MOST OF THE MAINLAND TODAY AND IS CAUSING THE
LOW PRESSURE SYSTEM CURRENTLY OVER THE AKPEN TO REMAIN RATHER WEAK.
LIGHT PRECIPITATION ASSOCIATED WITH THIS LOW HAS ENCOMPASSED NEARLY
ALL OF SOUTHWEST ALASKA ALONG AND TO THE SOUTH OF THE KUSKOKWIM DELTA
AND IS ALSO BRINGING AREAS OF FREEZING DRIZZLE TO PARTS OF KODIAK
ISLAND.

.SHORT TERM FORECAST...

AS THE WEAK TROUGH AND LOW PRESSURE IN THE GULF AFFECTS SOUTHCENTRAL
TODAY...EXPECT A PERIOD OF LIGHT OVERRUNNING PRECIPITATION TO
DEVELOP...WITH SNOW ACCUMULATIONS RATHER LIGHT...GENERALLY LESS THAN
AN INCH.  THIS SYSTEM QUICKLY RACES EASTWARD THIS EVENING AS WEAK
HIGH PRESSURE AGAIN TEMPORARILY TAKES OVER MOST OF THE STATE.

STRONG WARM ADVECTION BEGINS ACROSS SOUTHWEST ALASKA WEDNESDAY
MORNING AS A STRONG NORTH PACIFIC LOW BEGINS TO MOVE ACROSS THE
ALEUTIAN CHAIN ABOUT HALFWAY BETWEEN ADAK AND SHEMYA...AND THEN MOVES
QUICKLY THROUGH THE BERING SEA TOWARDS THE BERING STRAIT BY THURSDAY
MORNING.  WITH SUCH A STRONG SYSTEM BOTH AT THE SURFACE AND
ALOFT...THIS SYSTEM WILL BRING GALE FORCE WINDS TO NEARLY ALL OF THE
BERING SEA AND STORM FORCE WINDS TO LOCATIONS NEAR THE TRACK OF THE
LOW.  THIS HAS NECESSITATED THE ISSUANCE OF A BLIZZARD WATCH FOR THE
WESTERN ALEUTIANS ON THE BACK SIDE OF THE LOW AS IT CROSSES THROUGH
THE CHAIN TONIGHT INTO THE DAY TOMORROW.  WINDS LOOK TO BE FAIRLY
STRONG IN THE KUSKOWKIM DELTA REGION ALSO AS PRECIPITATION MOVES
IN...BUT QUICKLY RISING TEMPERATURES IN THE STRONG WARM ADVECTION
REGIME WILL LIKELY KEEP VISIBILITIES FROM GETTING TOO LOW.
THEREFORE...WILL ISSUE A WINTER STORM WATCH FOR SNOW AND BLOWING SNOW
ALONG THE COAST. WITH SUCH STRONG WARM ADVECTION A CHANGEOVER TO
FREEZING RAIN FURTHER INLAND HAS ALSO BEEN ACCOUNTED FOR IN THE
WATCH.  SINCE THIS SYSTEM WILL BE LOADED WITH MOISTURE AND PLENTY OF
DYNAMICS ALOFT...PRECIPITATION AMOUNTS WILL BE QUITE ROBUST...AND
THIS LOOKS TO BE A MAJOR STORM FOR ALMOST THE ENTIRE STATE OVER THE
NEXT SEVERAL DAYS.

OVERRUNNING PRECIPITATION BEGINS FOR SOUTHCENTRAL LATE
WEDNESDAY...WITH STRONG SOUTHERLY FLOW AND STRONG MOISTURE ADVECTION
LIKELY RESULTING IN A FAVORABLE PERIOD FOR HEAVY SNOW IN THE SUSITNA
VALLEY LATE WEDNESDAY THROUGH THURSDAY NIGHT...WITH FAIRLY HIGH
PRECIPITATION AMOUNTS THROUGHOUT THE REGION.  WILL ALSO LIKELY SEE A
CHANGEOVER AT SOME POINT ON THURSDAY TO RAIN OR A MIX OF RAIN AND
SNOW AS FAR NORTH AS ANCHORAGE. PRECIPITATION DOES NOT APPEAR TO
FULLY END UNTIL THE STRONG UPPER TROUGH ASSOCIATED WITH THIS SYSTEM
MOVES THROUGH SOMETIME DURING LATE THURSDAY NIGHT...SO A PERIOD OF
COLD ADVECTION SNOW AT THE END APPEARS LIKELY FOR ANCHORAGE THURSDAY
NIGHT ESPECIALLY GIVEN THE SEPARATION OF THE SURFACE LOW/TROUGH THAT
FORMS IN THE GULF AND THE STRONG DYNAMICS ALOFT.

.LONG TERM FORECAST...

STRONG HIGH PRESSURE APPEARS TO BUILD IN BEHIND THIS SYSTEM ONCE
AGAIN FROM THE SOUTH AND WEST...LIKELY BRINGING MORE TRANQUIL WEATHER
CONDITIONS AT LEAST FOR A BRIEF PERIOD FROM FRIDAY NIGHT INTO
SATURDAY FOR THE ANCHORAGE AREA.  MODELS DIVERGE SIGNIFICANTLY FROM
THE WEEKEND ONWARD...BUT A CONTINUATION OF THE VERY ACTIVE WEATHER
PATTERN APPEARS LIKELY AT THIS TIME FOR MUCH OF THE STATE.

.AER/ALU...WATCH/WARNING SUMMARY
PUBLIC...WINTER STORM WATCH...155
         BLIZZARD WATCH...191
MARINE...GALE WARNING...160 170 179 180 185
         STORM WARNING...172 175 176
         HEAVY FREEZING SPRAY WARNING...121 185
FIRE WEATHER...NONE.

CC MAR 09