Heavy Freezing Spray Warning 99, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
212 PM AKDT WED MAR 30 2011

.ANALYSIS AND UPPER LEVELS...THE FOCUS TODAY IS ON ANCHORAGE AND
VICINITY WITH POTENTIAL FOR SIGNIFICANT SNOWFALL TONIGHT AND
THURSDAY. THE FEATURES OF CONCERN ARE ALL VISIBLE IN SATELLITE
IMAGERY THIS MORNING. THERE IS AN UPPER LOW OVER THE GULF OF ALASKA
WITH A SHORT-WAVE OVER THE NORTHEAST GULF ROTATING NORTHWESTWARD
TOWARD SOUTHCENTRAL ALASKA. THE BULK OF PRECIPITATION IS FALLING
ACROSS THE PRINCE WILLIAM SOUND REGION BUT RADAR IMAGERY SHOWS
MOISTURE ADVECTION WESTWARD ACROSS THE KENAI AND CHUGACH MOUNTAINS
TO THE COOK INLET CORRIDOR. MEANWHILE...A NORTH-SOUTH ORIENTED
UPPER TROUGH OVER WESTERN ALASKA IS MOVING EASTWARD TOWARD
SOUTHCENTRAL. SNOW IS FALLING JUST OUT AHEAD OF THE TROUGH FROM
THE ALASKA PENINSULA NORTHWARD TO ALONG THE WEST SIDE OF THE
ALASKA RANGE. A SEPARATE SHORT-WAVE TROUGH IS ENTERING NORTHWEST
ALASKA AND BEGINNING TO DIG SOUTHEASTWARD. MOISTURE FROM THE GULF
COMBINED WITH SYNOPTIC LIFT AHEAD OF UPPER TROUGHS MOVING IN FROM
THE WEST LOOKS LIKE IT WILL ALL COME TOGETHER OVER THE NEXT 24
HOURS TO PRODUCE SNOW OVER THE POPULATION CENTER OF SOUTHCENTRAL
ALASKA.

OUT WEST THERE IS A SHORT-WAVE RIDGE OVER THE EASTERN BERING AND
A CLOSED LOW JUST SOUTH OF THE WESTERN ALEUTIANS. THE SUBTROPICAL
JET IS SUPPRESSED WELL TO THE SOUTH SO THERE ARE CURRENTLY NO MAJOR
STORMS THREATENING ALASKA...ALTHOUGH THAT WILL CHANGE OVER THE NEXT
COUPLE DAYS.

.MODEL DISCUSSION...12Z NAM DID NOT DIG TROUGH CURRENTLY OVER
NORTHWEST ALASKA AS FAR SOUTH AS THE GFS/ECMWF/UKMET. HOWEVER THE
18Z RUN IS NOW ON BOARD WITH THIS SOLUTION WHICH IS KEY TO WHAT
LOOKS LIKE A TWO STAGE SNOW EVENT FOR ANCHORAGE AND THE MATANUSKA
VALLEY. THERE ARE OTHER MINOR DIFFERENCES AMONG THE MODELS WHICH
AFFECT THE EXACT TIMING OF SNOWFALL FOR THIS EVENT...BUT THE
SYNOPTIC SCALE FORCING MECHANISMS ARE ALL VERY SIMILAR. BEYOND
THIS EVENT...THE GFS IS TRENDING FASTER WITH MAJOR LOW MOVING
OUT OF THE PACIFIC LATE FRIDAY INTO SATURDAY...BUT PREFER TO HOLD
WITH THE MORE CONSISTENT ECMWF POSITION FOR NOW.

.SHORT TERM FORECAST...

SOUTHCENTRAL ALASKA...EXPECT THE FIRST PERIOD OF SNOW TO BEGIN
BY MID EVENING FROM ANCHORAGE TO THE MATANUSKA VALLEY AND UP THE
EASTERN SIDE OF THE SUSITNA VALLEY (MAINLY EAST OF THE PARKS
HIGHWAY) AS THE FIRST UPPER TROUGH ARRIVES FROM THE WEST. THE
SYNOPTIC LIFT LOOKS STRONG ENOUGH TO PRODUCE 2 OR 3 HOURS OF
MODERATE SNOW INTENSITY. THIS WILL HELP OVERCOME THE RELATIVELY
WARM (ABOVE FREEZING) SURFACE TEMPERATURES AT THE START SO THAT
SNOW WILL BEGIN TO ACCUMULATE NOT LONG AFTER SNOWFALL BEGINS.
THIS FIRST TROUGH WILL WASH OUT AS IT PASSES EAST OF THE ANCHORAGE
AREA AROUND 3AM SO SNOW SHOULD LIGHTEN UP FOR A WHILE. LIFT
ASSOCIATED WITH THE NEXT DIGGING TROUGH WILL THEN ARRIVE A FEW
HOURS LATER...LIKELY INTENSIFYING JUST IN TIME FOR THE MORNING
RUSH HOUR. IT APPEARS THAT THIS TROUGH WILL BE STRONGER THAN THE
FIRST ONE. HOWEVER...WITH ANCHORAGE RIGHT NEAR THE SOUTHERN END
OF THE TROUGH ANY SMALL CHANGE IN THE TROUGH POSITION COULD
DRAMATICALLY CHANGE RESULTANT SNOW ACCUMULATION. IF THE CURRENT
MODELS ARE CORRECT IN THEIR DEPICTION OF THIS TROUGH THEN SNOWFALL
WILL BE BRIEFLY HEAVY DURING THE MORNING HOURS AND SNOWFALL AMOUNTS
COULD BE MORE SIGNIFICANT THAN CURRENTLY ADVERTISED. COLD ADVECTION
WILL ALSO ACCOMPANY THIS TROUGH AND CONTRIBUTE TO LOW LEVEL VERTICAL
MOTION. WILL CONTINUE TO MONITOR NEW MODEL RUNS AND FINE-TUNE SNOW
AMOUNTS AS FORECAST CONFIDENCE INCREASES. IN ANY CASE...PLAN TO
ISSUE A WINTER WEATHER ADVISORY FOR ANCHORAGE AND THE MATANUSKA
VALLEY SINCE SNOW IS EXPECTED TO IMPACT THE MORNING RUSH HOUR. THE
UPPER TROUGH WILL MOVE THROUGH SOMETIME DURING THE AFTERNOON HOURS
SO MOST OF THE SNOW ACCUMULATION SHOULD BE OVER BY LATE AFTERNOON.
THE LOW LEVEL COLD ADVECTION COULD CONTINUE TO PRODUCE AREAS OF
LIGHT SNOW INTO THE EVENING HOURS.

ELSEWHERE ACROSS SOUTHCENTRAL EXPECT LOWER AMOUNTS OF PRECIPITATION
WITH BOTH TROUGH WEAKENING AS THEY PROGRESS EASTWARD ACROSS THE
REGION. A TRANSIENT RIDGE WILL THEN MOVE THROUGH THURSDAY NIGHT
AND FRIDAY BEFORE A MAJOR PACIFIC LOW BEGINS TO IMPACT THE AREA
OVER THE WEEKEND.

SOUTHWEST ALASKA...THE FIRST TROUGH WILL EXIT SOUTHWEST ALASKA
THIS EVENING SO SNOW WILL MOSTLY COME TO AN END ALONG THE ALASKA
AND ALEUTIAN RANGE. THE NEXT TROUGH MOVING DOWN FROM NORTHWEST
ALASKA COULD CONTINUE TO PRODUCE SOME LIGHT SNOW ALONG THE WEST
SIDE OF THE ALASKA RANGE THROUGH THE OVERNIGHT HOURS. OTHERWISE...
HIGH PRESSURE WILL BUILD IN FROM THE WEST AND PROVIDE DRY CONDITIONS
THROUGH FRIDAY. MAJOR PACIFIC STORM WILL THEN BRING A RETURN TO
WINDS AND PRECIPITATION AS WE HEAD INTO THE WEEKEND.

BERING SEA/ALEUTIANS...WITH JUST A FEW WEAK SURFACE LOWS IN THE
REGION CONDITIONS WILL REMAIN BENIGN UNTIL THE PACIFIC LOW
APPROACHES THE EASTERN ALEUTIANS AND ALASKA PENINSULA LATE FRIDAY.

.LONG TERM FORECAST...HIGH RESOLUTION NUMERICAL GUIDANCE FROM THE
GLOBAL MODELS (GFS AND ECMWF) ARE NOT DEALING WITH THE PATTERN IN
THE LONG TERM VERY WELL. THE HIGH RESOLUTION MODELS SEEM TO BE
STRUGGLING WITH FEATURES IN THE FAST FLOW OVER THE PACIFIC AND
THUS ARE PRODUCING SOMEWHAT DIFFERENT SOLUTIONS. THE RESPECTIVE
ENSEMBLE MEAN SOLUTIONS ARE CLOSER TO EACH OTHER AND SEEM TO
FILTER OUT SOME OF THE "NOISE" IN THE HIGH RESOLUTION RUNS.

CURRENT FORECAST STICKS PRETTY CLOSE TO THE 00Z GFS WHICH HAS AN
UPPER LEVEL LOW IN THE GULF ON SUNDAY THAT MOVES INTO BRITISH
COLUMBIA BY LATE MONDAY...THEN TRANSITIONS TO A BROAD UPPER TROUGH
OVER THE BERING SEA/WESTERN ALASKA AND A RIDGE OVER THE EASTERN
PACIFIC. THIS IS IN LINE WITH CURRENT HPC GUIDANCE AND THE ENSEMBLE
MEANS OF THE GFS AND ECMWF AND THUS IS PROBABLY REASONABLE.

.AER/ALU...WATCH/WARNING SUMMARY
PUBLIC...NONE.
MARINE...NONE.
FIRE WEATHER...NONE.

SEB/BL MAR 11

SPECIAL WEATHER STATEMENT
NATIONAL WEATHER SERVICE ANCHORAGE AK
1000 PM AKDT WED MAR 30 2011

AKZ101-111-311000-

...SIGNIFICANT SNOWFALL EXPECTED TONIGHT AND THURSDAY FROM
ANCHORAGE TO THE MATANUSKA VALLEY...

THE LATEST THINKING ON THE DEVELOPMENT OF THE SNOW TONIGHT IS THAT
THE MAIN ONSET OF THE SYSTEM WILL BE DELAYED UNTIL AFTER TONIGHT WITH
JUST SOME SCATTERED RAIN AND SNOW SHOWERS AROUND THE AREA FOR THE
REMAINDER OF THE EVENING. CONDITIONS BECOME MORE FAVORABLE FOR STEADY
SNOW AFTER MIDNIGHT AND 2 TO 4 INCHES BY THE BEGINNING OF RUSH HOUR
IS STILL REASONABLE. ANOTHER 2 TO 4 INCHES SHOULD FALL DURING THE DAY
THURSDAY WITH THE HEAVIEST AMOUNTS DURING THE MORNING. STORM TOTAL
ACCUMULATIONS OF SNOWFALL ARE EXPECTED TO RANGE BETWEEN 4 AND 8
INCHES OVER THE ANCHORAGE AND MATANUSKA VALLEY AREAS.

PLEASE STAY TUNED TO LOCAL MEDIA OR HTTP://PAFC.ARH.NOAA.GOV FOR
THE LATEST FORECASTS AND ADVISORIES ON THIS IMPENDING SNOWFALL
EVENT.

SPECIAL WEATHER STATEMENT
NATIONAL WEATHER SERVICE ANCHORAGE AK
1250 AM AKDT THU MAR 31 2011

AKZ101-111-311330-

...SIGNIFICANT SNOWFALL EXPECTED THIS MORNING FROM ANCHORAGE TO THE
MATANUSKA VALLEY...

THE COLD AIR HAS BEEN SLOW TO MOVE INTO SOUTHCENTRAL ALASKA. RAIN
WITH SOME SNOW MIXED IS CURRENTLY ACROSS MUCH OF THE ANCHORAGE BOWL
AND TO A LESSER EXTENT IN THE MATANUSKA VALLEY. TEMPERATURES SHOULD
SLOWLY DROP THROUGH THE REMAINDER OF TONIGHT WITH THE MIXED
PRECIPITATION CHANGING TO ALL SNOW EARLY THIS MORNING. TOTAL SNOW
ACCUMULATIONS OF 3 TO 6 INCHES IS EXPECTED THROUGH NOON TODAY OVER
THE ANCHORAGE AND MATANUSKA VALLEY AREAS.

PLEASE STAY TUNED TO LOCAL MEDIA OR HTTP://PAFC.ARH.NOAA.GOV FOR
THE LATEST FORECASTS AND ADVISORIES ON THIS IMPENDING SNOWFALL
EVENT.