Winter Weather Advisory 13, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
135 PM AKDT MON APR 4 2011

.ANALYSIS AND UPPER LEVELS...PROGRESSIVE UPPER LEVEL FLOW IS
RESULTING IN RAPIDLY CHANGING WEATHER ACROSS THE FORECAST AREA.
WHILE ONE MATURE LOW EXITS TO THE EASTERN GULF A 120KT+ SUBTROPICAL
JET ALONG AND SOUTH OF THE ALEUTIANS IS PUSHING EVERYTHING QUICKLY
EASTWARD. A VERY WEAK TROUGH EXTENDING FROM THE KUSKOKWIM VALLEY
TO WESTERN GULF IS WEAKENING AS IT LIFTS NORTHEASTWARD OVER TOP
OF A SHORT-WAVE RIDGE. THERE IS JUST ENOUGH VERTICAL MOTION WITH
THIS TROUGH TO PRODUCE SOME LIGHT RAIN AND SNOW ACROSS PORTIONS OF
SOUTHCENTRAL ALASKA. AT THE SURFACE THERE IS A WEAK LOW OVER
INTERIOR ALASKA AND A BUILDING RIDGE ALONG THE NORTH GULF COAST.
THIS IS PRODUCING LOCALIZED SOUTHEAST WINDS WHICH WILL HELP LIMIT
PRECIPITATION REACHING THE GROUND ON THE LEE SIDE OF THE MOUNTAINS.

OUT WEST IS AN ELONGATED UPPER LOW OVER THE BERING SEA WITH A
SHORT-WAVE OVER THE EASTERN BERING RAPIDLY TRACKING TOWARD
SOUTHWEST ALASKA. AT THE SURFACE THERE IS A CORRESPONDING COLD
OCCLUSION WITH STRONG WINDS AND MORE UNSTABLE CONDITIONS BEHIND
THE FRONT.

.MODEL DISCUSSION...NAM IS A LITTLE FASTER WITH MOVEMENT OF
UPPER TROUGH CURRENTLY OVER EASTERN BERING AS IT REACHES
SOUTHCENTRAL EARLY TUESDAY MORNING. THIS WILL AFFECT TIMING OF
WHAT IS EXPECTED TO BE A QUICK SHOT OF SNOW IN ANCHORAGE AND
VICINITY...WITH THE DIFFERENCE BEING EITHER BEFORE OR DURING
THE EARLY MORNING RUSH HOUR TRAFFIC. WILL EVALUATE 18Z RUNS
BEFORE FINALIZING TIMING. NAM IS ALSO AN OUTLIER WITH SPEED OF
MAJOR PACIFIC LOW TUESDAY NIGHT THROUGH THURSDAY SO PREFER AVERAGE
POSITION OF GFS/ECMWF. OTHERWISE...MODELS ARE IN GOOD AGREEMENT
THE NEXT FEW DAYS...ESPECIALLY CONSIDERING THAT TIMING IS USUALLY
A PROBLEM IN THESE FAST FLOW SCENARIOS.

.SHORT TERM FORECAST...

SOUTHCENTRAL ALASKA...AS UPPER LEVEL SHORT-WAVE RIDGE MOVES OVERHEAD
THIS AFTERNOON THE SURFACE RIDGE ALONG THE NORTH GULF COAST WILL
STRENGTHEN AND SOUTHEAST FLOW ACROSS THE COASTAL RANGE WILL INCREASE.
THIS WILL PRODUCE STRONGER DOWNSLOPE WINDS AND DRY OUT THE LEE SIDE
OF THE MOUNTAINS INCLUDING ANCHORAGE. THE BERING SEA TROUGH WILL
BECOME NEGATIVELY TILTED AND SLOW DOWN AS IT MOVES INTO SOUTHCENTRAL
OVERNIGHT. EXPECT PRECIPITATION TO FORM AHEAD OF IT BUT THE SOUTHEAST
FLOW WILL KEEP THINGS MOSTLY DRY FROM ANCHORAGE TO THE MAT-SU UNTIL
THE TROUGH IS RIGHT OVERHEAD. THERE IS SOME UNCERTAINTY ABOUT WHETHER
THIS FIRST TROUGH REACHES ANCHORAGE. LOOKS TO BE CLOSE ENOUGH TO
PRODUCE A QUICK SHOT OF PRECIPITATION IN THE EARLY MORNING. SURFACE
TEMPERATURES ARE FAIRLY WARM RIGHT NOW SO EVEN WITH SLIGHT COOLING
ALOFT PRECIP-TYPE COULD INITIALLY BE A MIX OF RAIN AND SNOW. IN ANY
CASE...ANY SNOW ACCUMULATION THAT DOES OCCUR WILL BE LIGHT. CHANCE
OF PRECIPITATION WILL INCREASE THROUGH THE DAY WEDNESDAY AS A SECOND
TROUGH CATCHES UP TO THE FIRST AND COLD ADVECTION ENSUES. SURFACE
LOW OVER WEST PRINCE WILLIAM SOUND WILL ALSO PROVIDE SOME MOISTURE
ADVECTION. THEREFORE...PLAN ON INCREASING POPS AND ADDING
IN SNOW ACCUMULATION FOR LATE TUESDAY THROUGH WEDNESDAY MORNING.
THIS SYSTEM HAS THE POTENTIAL TO PRODUCE SEVERAL INCHES OF SNOW
FROM THE KENAI PENINSULA TO ANCHORAGE AND THE MAT-SU. HOWEVER...THE
DETAILS OF THE UPPER LEVELS ARE STILL IN QUESTION SO WILL BE
CONSERVATIVE WITH AMOUNTS UNTIL CONFIDENCE IN A GIVEN SOLUTION
INCREASES. CONDITIONS WILL DRY OUT WEDNESDAY BEHIND THE TROUGH...BUT
FRONT ASSOCIATED WITH A VERY STRONG BERING SYSTEM WILL ARRIVE BY
THURSDAY.

SOUTHWEST ALASKA...AS SURFACE FRONT AND UPPER TROUGH MOVE ONSHORE
THIS AFTERNOON AND EVENING SNOW WILL SPREAD INLAND. COLD ADVECTION
COMBINED WITH A SECONDARY TROUGH DROPPING DOWN FROM THE NORTH ON
TUESDAY WILL PRODUCE SNOW SHOWERS AND GUSTY WINDS...ESPECIALLY
ALONG THE COAST. AFTER A BREAK LATE TUESDAY NIGHT INTO WEDNESDAY
RAPIDLY INTENSIFYING LOW WILL MOVE INTO BERING WITH A STRONG FRONT
LIKELY IMPACTING THE SOUTHWEST COAST BY WEDNESDAY AFTERNOON.
TEMPERATURES SUPPORT ALL SNOW AS PRECIPITATION FIRST BEGINS...BUT
STRONG WARM ADVECTION WILL CHANGE PRECIP TO MOSTLY RAIN WEDNESDAY
NIGHT. THERE COULD BE SOME BRIEF BLOWING SNOW ISSUE IN THE KUSKOKWIM
DELTA AT THE ONSET. AT THIS POINT DON'T THINK IT WILL BE OF A LONG
ENOUGH DURATION TO WARRANT A BLIZZARD BUT WILL RE-EVALUATE THIS AS
WE GET CLOSER TO THE EVENT.

BERING SEA/ALEUTIANS...A WEAK SURFACE LOW OVER THE NORTHWEST
PACIFIC WILL RAPIDLY INTENSIFY TUESDAY WITH AN AMPLIFYING UPPER
TROUGH AND STRONG UPPER DIVERGENCE BETWEEN TWO JET STREAKS. THIS
WILL BE A VERY LARGE SYSTEM...AFFECTING THE ENTIRE BERING SEA
REGION BY WEDNESDAY AFTERNOON. PLAN TO MAX WINDS OUT AT STORM FORCE
FOR NOW BUT IF MODELS ARE CONSISTENT IN NEXT MODEL CYCLE WILL HAVE
TO INCREASE SOME AREAS TO HURRICANE FORCE AND CONSIDER ISSUING HIGH
WIND WATCHES OR BLIZZARD WATCHES ALONG THE ALEUTIANS. LOTS OF ARCTIC
AIR WILL FEED IN BEHIND THE SYSTEM AS IT TRACKS TOWARD THE BERING
STRAIT LATE IN THE WEEK. THUS...STRONG WINDS AND SNOW SHOWERS WILL
DOMINATE FOR A FEW DAYS.

.LONG TERM FORECAST...MAJOR STORM IN THE BERING MID WEEK WILL
CONTINUE TO AFFECT MUCH OF ALASKA ALL THE WAY INTO THE WEEKEND.
LOOKS LIKE UPPER LEVEL PROGRESSIVE PATTERN WILL CONTINUE AFTER
THIS WITH ANOTHER STORM MOVING IN FROM THE NORTHWEST PACIFIC
LATE IN THE WEEK.

.AER/ALU...WATCH/WARNING SUMMARY
PUBLIC...NONE.
MARINE...GALE WARNING...130 132 150 155 160 165 170 171 172 175
                        176 179 180.
         HEAVY FREEZING SPRAY WARNING...160 165 185.
FIRE WEATHER...NONE.

SEB APR 11