Winter Weather Advisory 3, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
1225 PM AKST WED JAN 4 2012

.ANALYSIS AND UPPER LEVELS...COLD UPPER LEVEL LOW IS NEARLY
STATIONARY THIS MORNING BETWEEN THE YUKON DELTA AND SAINT LAWRENCE
ISLAND. WATER VAPOR IMAGERY SHOWS TWO VERY DISTINCT SHORT-WAVES
ROTATING AROUND THE SOUTH TO EAST SIDE OF THIS CENTER. ONE VERY
NARROW WAVE EXTENDS SOUTHWARD FROM THE WESTERN ALASKA INTERIOR
ACROSS THE KUSKOKWIM VALLEY TO KODIAK ISLAND AND SOUTHWARD. THIS
TROUGH CURRENTLY HAS A NEUTRAL TILT...BUT IS JUST SHOWING SIGNS
OF BECOMING NEGATIVELY TILTED AS IT LIFTS NORTHEASTWARD. THIS IS
HELPING PULL MOISTURE FROM THE NORTHEAST PACIFIC AND GULF
NORTH-NORTHWEST ACROSS SOUTHCENTRAL ALASKA. A WEAK LOW HAS BACKED
ITS WAY TO THE CAPE SUCKLING AREA AND SPREAD MODERATE TO HEAVY SNOW
ACROSS EASTERN PRINCE WILLIAM SOUND WITH LIGHTER SNOW TO THE NORTH
ACROSS THE COPPER RIVER BASIN. STRONG GAP WINDS ARE ALSO PRODUCING
BLIZZARD CONDITIONS THROUGH THOMPSON PASS. HOWEVER...PRESSURE
GRADIENTS HAVE BEGUN TO WEAKEN AS THIS LOW BACKS FURTHER TOWARD
PRINCE WILLIAM SOUND...SO WINDS ARE ON THE WAY DOWN. THERE IS STILL
SOME QUESTION ABOUT HOW FAR WEST THE MOISTURE WILL MAKE IT AS THE
UPPER TROUGH CONTINUES TO BECOME MORE AND MORE NEGATIVELY TILTED.
BASED ON RADAR AND SATELLITE TRENDS IT APPEARS THAT SNOW WILL REMAIN
NORTH AND EAST OF THE KENAI PENINSULA AND ANCHORAGE...BUT COULD
BRIEFLY MAKE IT INTO THE MAT-SU...ESPECIALLY ALONG THE TALKEETNA
MOUNTAINS.

THE SECOND UPPER SHORT-WAVE OF CONCERN IS MOVING EASTWARD ACROSS
THE EASTERN ALEUTIANS AND SOUTHERN ALASKA PENINSULA THIS MORNING.
IT IS MUCH STRONGER AND ACCOMPANIED BY A POTENT SURFACE LOW.
THIS SYSTEM IS CHARACTERIZED BY MODERATE TO HEAVY SNOW ALONG THE
COLD FRONT WITH A BREAK BEHIND THE FRONT UNTIL STRONG NORTHWESTERLIES
AND MODERATE TO HEAVY SNOW SHOWERS MOVE IN. IT IS PROGRESSING
EAST-NORTHEAST TOWARD BRISTOL BAY AND THE NORTHERN ALASKA PENINSULA.

THIS SYSTEM IS CAUSING NORTHERLY WINDS TO SLOWLY INCREASE ACROSS
THE KUSKOKWIM DELTA. WINDS ARE GENERALLY LESS THAN 15 MPH AT THE
MOMENT...BUT THESE WINDS ARE PRODUCING WIND CHILLS OF 40 BELOW TO
60 BELOW ZERO. WINDS WILL CONTINUE TO INCREASE AND AIR TEMPERATURES
SHOULD MODERATE SLIGHTLY...WHICH WILL KEEP WIND CHILLS IN THE 40
BELOW TO 50 BELOW RANGE.

.MODEL DISCUSSION...ECMWF INITIALIZED THE BEST ON FEATURES OF
CONCERN THIS MORNING WITH THE NAM A CLOSE SECOND. THE ECMWF HAS
BY FAR THE BEST HANDLE ON SNOW OVER EASTERN PRINCE WILLIAM SOUND
WITH OTHER MODELS BEING TOO FAR TO THE EAST. NAM/ECMWF ARE FAIRLY
CONSISTENT THROUGH THE SHORT-TERM AND ARE PREFERRED...BUT BASED ON
POOR RUN-TO-RUN CONSISTENCY IN ALL OF THE MODELS THE PAST FEW DAYS
CONFIDENCE IS BELOW AVERAGE.

.SHORT TERM FORECAST...

SOUTHCENTRAL ALASKA...UPPER TROUGH WILL LIFT NORTHEASTWARD ACROSS
SOUTHCENTRAL LATE THIS AFTERNOON THROUGH TONIGHT ENDING SNOW FROM
SOUTHWEST TO NORTHEAST. THE LOW HEADED FOR BRISTOL BAY WILL REMAIN
ON THE NORTH SIDE OF THE ALASKA PENINSULA...BUT AS IS TYPICAL IN
THESE CASES EXPECT A SECONDARY LOW TO DEVELOP ON THE SOUTH SIDE OF
THE ALASKA PENINSULA AND TRACK UP INTO COOK INLET ON THURSDAY. WITH
THIS TRACK STRONG SOUTHEASTERLIES WILL DEVELOP ACROSS THE KENAI AND
CHUGACH MOUNTAINS. THE NAM MODEL INDICATES THESE SOUTHEASTERLIES
WILL EXTEND THROUGH ABOUT THE FIRST 5000 FEET WITH SOUTHERLY FLOW
ABOVE THAT. THEREFORE...EXPECT SNOW TO FALL ON THE LEE SIDE OF THE
MOUNTAINS THURSDAY MORNING FROM THE WESTERN KENAI TO ANCHORAGE AND
PALMER...BUT SOUTHEASTERLIES WILL KEEP SNOW LIGHTER THAN IT
OTHERWISE WOULD BE. THERE WILL ALSO LIKELY BE A BRIEF PERIOD OF SNOW
BEHIND THE LOW THURSDAY AFTERNOON INTO EVENING. AS COLDER AIR WORKS
ITS WAY BACK UP THE INLET. BOTTOM LINE WITH THIS SYSTEM IS IT WILL
MOVE QUICKLY ACROSS SOUTHCENTRAL BRINGING GENERALLY LIGHT
ACCUMULATION AMOUNTS. THE EXCEPTION WILL ONCE AGAIN BE EASTERN
PRINCE WILLIAM SOUND WHERE UPSLOPE FLOW COMBINED WITH INSTABILITY
CREATED BY ARCTIC AIR MOVING OVER RELATIVELY WARMER WATERS OF THE
GULF WILL LEAD TO HEAVIER SNOWFALL.

AS ONE SHORT-WAVE IS MOVING ACROSS SOUTHCENTRAL THURSDAY ANOTHER
WILL MOVE INTO THE BACK SIDE OF THE TROUGH CAUSING AMPLIFICATION
OF THE TROUGH AS IT MOVES ACROSS THE GULF AND SOUTHCENTRAL THURSDAY
NIGHT. THE LAST COUPLE DAYS THE MODELS HAVE DEPICTED THIS AS A VERY
STRONG AND FOCUSED UPPER LEVEL FEATURE CAPABLE OF PRODUCING HEAVY
SNOW. HOWEVER...THE LATEST RUNS HAVE BACKED OFF A LITTLE ON THE
STRENGTH OF IT. THE OTHER FACTOR IN SNOW AMOUNTS IS THE AMOUNT OF
MOISTURE PULLED UP FROM THE PACIFIC. THERE IS A TREND TOWARD A
STRONGER LOW TRACKING FROM THE NORTHEAST PACIFIC TO EASTERN GULF
INSTEAD OF A MUCH WEAKER LOW MOVING UP TO PRINCE WILLIAM SOUND.
THIS STRONGER LOW WOULD STEAL A LOT OF THE MOISTURE THAT WOULD
OTHERWISE FLOW UP TO SOUTHCENTRAL. SO SNOW WILL DEFINITELY FALL
THURSDAY NIGHT ACROSS SOUTHCENTRAL...BUT THESE UNCERTAINTIES
MAKE IT VERY DIFFICULT TO NAIL DOWN SNOW AMOUNTS. BASED ON CURRENT
MODEL TRENDS WILL GO WITH FAIRLY LIGHT AMOUNTS FOR NOW.

SOUTHWEST ALASKA...THE ALASKA PENINSULA AND BRISTOL BAY REGION WILL
SEE THE BULK OF IMPACT AS SURFACE LOW MOVES ONSHORE LATE TONIGHT.
EXPECT SNOW AND BLOWING SNOW...BUT ANY REALLY LOW VISIBILITIES WILL
BE SHORT-LIVED AS THE LOW MOVES QUICKLY THROUGH. STRONG NORTHWEST
FLOW WITH MODERATE TO HEAVY SNOW SHOWERS WILL CONTINUE TO AFFECT
THE ALASKA PENINSULA TONIGHT THROUGH EARLY THURSDAY. WINDS WILL
THEN DIMINISH ALTHOUGH SNOW SHOWERS WILL PERSIST. THE KUSKOKWIM
VALLEY AND KUSKOKWIM DELTA WILL SEE AREAS OF LIGHT SNOW. THE MORE
SIGNIFICANT IMPACT THOUGH WILL CONTINUE TO BE LOW WIND CHILLS OVER
THE KUSKOKWIM DELTA. WINDS WILL LIGHTEN UP BY LATE THURSDAY MORNING
BRINGING THIS THREAT TO AN END. DRY AND COLD CONDITIONS WILL RESUME
OVER SOUTHWEST ALASKA BEHIND THIS SYSTEM. THAT MAY BE SHORT-LIVED
THOUGH AS A SYSTEM MOVING UP INTO THE GULF THIS WEEKEND COULD AFFECT
EASTERN PORTIONS OF SOUTHWEST ALASKA WITH WIND AND SNOW AND ONCE
AGAIN LEAD TO LOW WIND CHILLS ALONG THE KUSKOKWIM DELTA.

BERING SEA/ALEUTIANS...WINDS AND SNOW OVER THE EASTERN BERING AND
ALEUTIANS WILL DIMINISH RAPIDLY DURING THE EVENING HOURS. A MORE
TYPICAL SHOWERY PATTERN WILL THEN SETTLE IN. WEAK FEATURES
TRANSITING THE REGION WILL OCCASIONALLY BRING MORE INTENSE SNOW
SHOWERS AND STRONGER GUSTY WINDS. THE UPPER JET AND STORM TRACK
WILL SHIFT NORTHWARD TOWARD THE ALEUTIANS AND ALASKA PENINSULA...
SO THERE IS POTENTIAL FOR MORE SIGNIFICANT SNOW AND WINDS AS
SYSTEMS PASS JUST TO THE SOUTH.

.LONG TERM FORECAST...ECMWF/GFS HAVE BEEN VERY CONSISTENT IN
TRACKING A STRONG LOW FROM THE NORTH-CENTRAL PACIFIC ON FRIDAY
TO PRINCE WILLIAM SOUND SATURDAY NIGHT WITH STRONG COLD ADVECTION
BEHIND IT. A SYSTEM OF THIS SIZE AND STRENGTH HAS HIGH POTENTIAL
TO CREATE WIDESPREAD SIGNIFICANT WEATHER IMPACTS. AFTER THIS STORM
MODELS DIVERGE SIGNIFICANTLY AS LONG-WAVE PATTERN UNDERGOES A CHANGE.

.AER/ALU...WATCH/WARNING SUMMARY
PUBLIC...NONE.
MARINE...GALE WARNING...120 125 130 132 150 155 165 170 172.
         HEAVY FREEZING SPRAY WARNING...121 129 130 140 160 165
                                        170 179 185.
FIRE WEATHER...NONE.

SEB JAN 12