Blizzard Watch 4, NWS Anchorage, AK

AREA FORECAST DISCUSSION
SOUTHCENTRAL AND SOUTHWEST ALASKA
500 AM AKST FRI FEB 5 2010

.ANALYSIS AND UPPER LEVELS...UPPER TROUGH EXTENDS FROM POLAR VORTEX
DOWN THROUGH SOUTHWEST ALASKA AND THE ALASKA PENINSULA. VERY COLD AIR
HAS ACCOMPANIED THIS TROUGH...AND TEMPERATURES WELL BELOW ZERO ARE
COMBINING WITH GUSTY WINDS TO PRODUCE WIND CHILLS AS LOW AS 40 BELOW
OVER THE KUSKOKWIM VALLEY AND DELTA. A WEAK SURFACE LOW HAS MOVED
INLAND NEAR PRINCE WILLIAM SOUND AND IS PRODUCING AN AREA OF LIGHT
SNOW FROM THE SOUTHERN SUSITNA VALLEY TO ANCHORAGE AND INTO PRINCE
WILLIAM SOUND. A STRONGER LOW APPROACHING THE EASTERN GULF IS MOVING
TO THE NORTHWEST AND WILL BE THE MAIN CHALLENGE IN THE MORNING
FORECAST PACKAGE AS IT RACES RAPIDLY TO THE NORTHERN GULF.

.MODEL DISCUSSION...ALL MAJOR MODELS ARE CONSISTENT WITH THIS FIRST
BIG STORM THAT WILL IMPACT SOUTHCENTRAL TONIGHT THROUGH SATURDAY
MORNING. BEGINNING SUNDAY NIGHT THE MODEL CONSENSUS BEGINS TO RAPIDLY
FALL APART. THE MODELS AGREE THAT A LARGE LOW COMPLEX SOMEWHERE SOUTH
OF COLD BAY WILL BECOME NEARLY STATIONARY...BUT DO NOT AGREE ON THE
DETAILS OF NUMEROUS SIGNIFICANT SHORTWAVES THAT ROTATE THROUGH THE
GULF DURING THE WEEK. GFS IS A BIT OF AN OUTLIER TRYING TO DEVELOP A
STRONG LOW IN THE EASTERN GULF ON MONDAY....WHEREAS THE ECMWF (AND
HPC AS WELL) LEAN MORE TOWARDS A MORE FLUID SITUATION WITH NUMEROUS
EMBEDDED WAVES PINWHEELING AROUND THE COMPOSITE LOW CENTER. AT THIS
TIME THE EC/HPC SOLUTION SEEMS MORE LIKELY...AND SINCE INHERITED
FORECASTE ALREADY REFLECTED THIS PHILOSOPHY...HAVE OPTED TO LEAVE
EXTENDED FORECAST UNCHANGED FROM YESTERDAY. OVERALL...FORECAST
CONFIDENCE IS ABOVE AVERAGE IN THE SHORT TERM...BUT SLIGHTLY BELOW
AVERAGE FOR THE MID AND LONG RANGE.

.SHORT TERM FORECAST...
.SOUTHCENTRAL...FIRST BIG QUESTION OF THE DAY IS HOW MUCH SNOW WILL
FALL IN THE COOK INLET AREA THIS MORNING. WEAK LOW THAT IS
DISSIPATING IN THE VICINITY OF PRINCE WILLIAM SOUND IS CONTINUING TO
CREATE SNOW ON BOTH SIDES OF THE CHUGACH RANGE. THIS PROCESS IS
LIKELY BEING SIGNIFICANTLY ENHANCED BY THE WARM SOUTHEASTERLY FLOW
OVERRUNNING THE COLD AIR ALREADY IN PLACE AT THE LOW LEVELS. MOST
LOCATIONS SHOULD SEE NO MORE THAN ONE INCH OF SNOW. HOWEVER IF ONE OF
THE MODERATE SNOW BANDS BECOMES STATIONARY...ALL BETS ARE OFF AND
SEVERAL INCHES OF SNOW COULD FALL. LATE THIS AFTERNOON ALL EYES TURN
TO MORE SIGNIFICANT STORM CURRENTLY IN THE SOUTHEAST GULF. AS THIS
LOW RACES TO THE NORTHWEST SNOW WILL AGAIN OVERSPREAD THE ENTIRE
REGION. THE BREAK BETWEEN THIS FIRST AND SECOND SYSTEM WILL LIKELY BE
ONLY A FEW HOURS...AND ALONG COASTAL AREAS THERE MAY NOT BE ANY BREAK
AT ALL. THE LOW WILL MOVE INLAND TONIGHT IN THE VICINITY OF (OR
PERHAPS JUST SOUTH OF) SEWARD ON THE KENAI PENINSULA. THE EXACT TRACK
OF THE LOW WILL MAKE A LARGE DIFFERENCE FOR SNOWFALL TOTALS IN THE
WESTERN KENAI AND ANCHORAGE AREAS. IF THE LOW IS FURTHER
NORTH...DOWNSLOPING OVER THE REGION WILL BE MINIMIZED AND SNOWFALL
TOTALS WILL INCREASE. CONVERSELY...A MORE SOUTHERLY TRACK WOULD ALLOW
DOWNSLOPING TO INCREASE AND SIGNIFICANTLY DECREASE SNOW AMOUNTS ON
THE LEE SIDE OF THE KENAI AND CHUGACH MOUNTAINS. THE FORECAST THIS
MORNING WILL OPT FOR A MIDDLE-OF-THE-ROAD SOLUTION...BRINGING
MODERATE SNOW TO THE AREA. MEANWHILE...SNOW WILL BEGIN IN VALDEZ AND
THE REST OF PRINCE WILLIAM SOUND THIS AFTERNOON. UP TO A FOOT OF SNOW
WILL FALL OVERNIGHT IN VALDEZ. WITH PLENTY OF COLD AIR AND WINDS
INCREASING IN THOMPSON PASS...EXPECT BLIZZARD CONDITIONS TO DEVELOP
THIS EVENING AND PERSIST THROUGH SATURDAY MORNING. FINAL AREA TO NOTE
WILL BE THE COPPER RIVER DELTA...WHERE ARCTIC AIR IN PLACE WILL ALLOW
PERIODS OF OVERRUNNING TO OCCUR THROUGH SATURDAY AFTERNOON. SNOWFALL
ACCUMULATIONS SHOULD REMAIN BELOW ADVISORY CRITERIA.

.SOUTHWEST...FORECAST MUCH MORE STRAIGHT-FORWARD WEST OF THE ALASKA
RANGE. PARTLY CLOUDY SKIES WITH CONTINUED COLD NORTHEASTERLY FLOW
WILL PREVAIL TODAY. THE LOW IN COOK INLET TONIGHT WEAKENS RAPIDLY AS
IT ATTEMPTS TO CROSS THE ALASKA RANGE...AND WILL PRODUCE SOME SNOW
SHOWERS IN THE KUSKOKWIM VALLEY DURING THE DAY TOMORROW. IN THE
BERING...COLD NORTH/NORTHEAST FLOW WITH SNOW SHOWERS WILL CONTINUE
THROUGH THE WEEKEND.

.LONG TERM FORECAST...WE ARE TRANSITIONING TO A MUCH MORE ACTIVE
WEATHER PATTERN THAN THE LAST FEW WEEKS. WIDESPREAD SOUTHERLY FLOW IN
THE GULF WILL ALLOW TEMPERATURES TO WARM UP BY MIDWEEK. INDIVIDUAL
SHORTWAVES EMBEDDED IN SOUTHEAST FLOW IN THE GULF ARE IMPOSSIBLE TO
TIME THIS FAR OUT...BUT EACH WAVE WILL BRING RAIN AND SNOW TO THE
COAST...WITH PRECIPITATION LESS LIKELY INLAND.

.AER/ALU...WATCH/WARNING SUMMARY
PUBLIC...BLIZZARD WARNING 131
MARINE...GALE 120 127 160 172 175 176
         HEAVY FREEZING SPRAY 150 155 160 165 179 185
FIRE WEATHER...NONE

AMD FEB 10