Winter Weather Advisory 9, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
517 AM AKDT TUE MAR 15 2016

.ANALYSIS AND UPPER LEVELS...
THE FAIRLY QUITE PATTERN THAT MUCH OF THE STATE HAS BEEN ENJOYING
IS STARTING TO CHANGE. EVIDENCE OF THIS CAN BE SEEN ON SATELLITE
THIS MORNING. A FAIRLY IMPRESSIVE UPPER LEVEL LOW IS DROPPING FROM
THE BROOKS RANGE AND STARTING TO IMPINGE ON THE ALASKA RANGE. OVER
THE NORTHERN GULF AND NEAR THE COAST...MOISTURE IS STARTING TO
CONSOLIDATE AND INSTABILITY CAN BE SEEN WITH THE SHOWERS SAMPLED
BY THE MIDDLETON RADAR. MEANWHILE...OVER THE EASTERN ALEUTIANS
AND THE ALASKA PENINSULA...SEVERAL WEAK LOWS CONTINUE TO FILL
ALONG A DECAYING THERMAL BOUNDARY. FURTHER WEST...ONE OF THE
STRONGER LOWS THAT WE HAVE SEEN IN THE PAST SEVERAL WEEKS IS
APPROACHING THE WESTERN ALEUTIANS. MODELS PROG THE CURRENT CENTRAL
PRESSURE IN THE UPPER 960MB-RANGE. WITH THE AID OF A 150KT JET AT
300MB...THE LOW WILL CONTINUE TO DEEPEN INTO THE 950S. A SNAPSHOT
OF THE JET SHOWS IT DRIVING FROM THE SOUTH AND INTO THE WESTERN
BERING...IT THEN TURNS QUICKLY BACK TO A NORTHERLY DIRECTION
DIVING BACK INTO THE NORTH PACIFIC. FROM THERE IT RESUMES THE
FAMILIAR ZONAL POSITION SOUTH OF THE GULF OF AK THAT IT HAS
OCCUPIED FOR MUCH OF THIS WINTER.

&&

.MODEL DISCUSSION...
THE POTENTIAL FOR SNOW ACROSS SOUTH CENTRAL TONIGHT CONTINUES TO
BE THE MOST NOTABLE MODEL DISCREPANCY. WE HAVE BEEN WATCHING THIS
EVENT TAKE SHAPE OVER THE PAST FEW DAYS...BUT EVEN ONLY 24 HOURS
OUT...FINER-SCALE FEATURES ARE YET TO BE RESOLVED. ALL OF THE
MODELS SHOW THE SYNOPTIC-SCALE TROUGH DROPPING ACROSS THE ALASKA
RANGE AND INTO THE SUSITNA VALLEY. THEY ALL ALSO DEPICT A CLOSED
LOW DEVELOPING SOMEWHERE OVER THE NORTHERN COOK INLET. BUT WHERE
THIS LOW SETS UP WILL BE CRITICAL TO WHERE THE BEST CONVERGENCE
WILL BE. THE OTHER ELEMENT AT PLAY IN THIS WILL BE INSTABILITY.
MODELS ALL SHOW A RELATIVELY MOIST AND SOMEWHAT UNSTABLE AIRMASS
CREEPING OVER THE CHUGACH MOUNTAINS AND RIDING UP AND OVER THE
COLDER AIR DIGGING IN FROM THE NORTHWEST. SO WHILE IT IS LIKELY
THERE WILL BE A BANDED ELEMENT TO THE PRECIPITATION TONIGHT INTO
EARLY WED...IT REMAINS TO BE SEEN EXACTLY WHERE THE HEAVIEST BANDS
WILL SET UP. THAT SAID...THE HIGHER RESOLUTION MODELS WERE RELIED
UPON IN MAKING THIS FORECAST.

OUT WEST...THE NAM WAS UTILIZED FOR THE FIRST 18 HOURS OF THE
FORECAST. AFTER THAT...A BLEND OF THE ECMWF AND THE GFS WERE USED
TO HANDLE THE STORM-FORCE FRONT ENCROACHING UPON THE WESTERN
BERING.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (DAYS 1 AND 2)...
THE PRIMARY FORECAST CHALLENGE THROUGH WED WILL BE THE INTENSITY
AND PLACEMENT OF ACCUMULATING SNOW FROM THE ANCHORAGE BOWL TO THE
MATANUSKA AND SUSITNA VALLEYS. THE MAIN FORCING IS COMING FROM A
UPPER LOW DIGGING SOUTH INTO WESTERN ALASKA. AS THIS UPPER CENTER
DOES SO IT WILL SET UP BROAD SOUTHERLY FLOW ACROSS SOUTHCENTRAL.
IT APPEARS AT THIS TIME THAT THE MAIN FOCUS OF SNOW WILL BE OVER
THE SUSITNA VALLEY AND ALONG THE TALKEETNA MOUNTAINS PRIMARY DUE
TO WEAK ISENTROPIC LIFT AND OROGRAPHIC ENHANCEMENT. MODEL QPF
(PRECIPITATION) GUIDANCE SEEMS A LITTLE LOW...BUT FEEL A MORE
AGGRESSIVE SNOW FORECAST IS WARRANTED THIS ISSUANCE...THOUGH WILL
KEEP SNOW TOTALS BELOW ADVISORY LEVELS FOR THIS PACKAGE ISSUANCE.
MODEL GUIDANCE DIFFERS CONSIDERABLY WITH THE RELATIVE PLACEMENT OF
SNOW ACROSS THE ANCHORAGE BOWL AND MATANUSKA VALLEY TONIGHT AND
WED. THAT SAID...WILL GO A LITTLE CONSERVATIVE FOR SNOW AMOUNTS
DUE TO MODEL UNCERTAINTY...BUT WILL INCREASE SNOW PROBABILITIES AS
A 500 MB JET MOVES OVER THE REGION AND PROVIDES FAVORABLE LIFT
LATE TONIGHT AND WED. UPPER SUPPORT WEAKENS WED NIGHT...SO SHOULD
SEE DECREASING CHANCES OF ACCUMULATING SNOW.

.SHORT TERM FORECAST SOUTHWEST ALASKA (DAYS 1 AND 2)...
THE SOUTHWEST MAINLAND LOOKS TO REMAIN FAIRLY DRY THROUGH THE
NEXT SEVERAL DAYS WITH THE EXCEPTION OF SOME SCATTERED SNOW
SHOWERS ALONG AND NEAR THE AHKLUN MOUNTAINS. SOME WEAK LIFT AHEAD
OF AN UPPER LEVEL TROUGH WILL INTERACT WITH SOME WEAK INSTABILITY
AND MID-LEVEL MOISTURE WHICH WILL HELP PRODUCE THE AFOREMENTIONED
SHOWERS. SOME OF THESE SHOWERS COULD DEVELOP AWAY FROM THE
MOUNTAINS WITH SOME FAVORABLE LAPSE RATES ASSOCIATED WITH A MUCH
COLDER AIR MASS DROPPING DOWN FROM THE ARCTIC. SPEAKING OF THIS
COLDER AIR MASS...AN UPPER LEVEL TROUGH WILL CONTINUE TO DIG SOUTH
ACROSS SOUTHWEST ALASKA AND WILL BE SLOW TO EXIT THE REGION.
COLDER AND DRIER AIR WILL BE ADVECTED SOUTH RESULTING IN CLEARING
SKIES AS WELL AS MUCH COLDER TEMPERATURES. FOR THE FIRST TIME IN
QUITE A WHILE...MOST (IF NOT ALL) OF SOUTHWEST ALASKA WILL SEE
BOTH HIGH AND LOW TEMPERATURES BELOW NORMAL. THESE DRY AND COOL
CONDITIONS LOOK TO PERSIST THROUGH MUCH OF THE WEEK.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (DAYS 1 AND 2)...
A BROAD UPPER LEVEL TROUGH CONTINUES TO BRING SNOW SHOWERS TO THE
PRIBILOF ISLANDS AND EASTERN ALEUTIANS THIS MORNING. THIS TROUGH
WILL AMPLIFY THROUGHOUT THE DAY AS HIGH PRESSURE BEGINS TO QUICKLY
BUILD OVER THE WESTERN BERING SEA. AS THE TROUGH AMPLIFIES...THE
BULK OF THE UPPER LEVEL ENERGY WILL PUSH SOUTH INTO THE NORTH
PACIFIC WHICH WILL ALLOW SNOW SHOWERS TO TAPER OFF FROM NORTH TO
SOUTH DURING THE DAY TODAY. A STORM FORCE FRONT WILL ENTER THE
WESTERN BERING SEA THIS AFTERNOON THAT WILL BE ASSOCIATED WITH A
STRENGTHENING LOW MOVING NEAR KAMCHATKA. THE FRONT LOOKS TO
QUICKLY ELONGATE ON WEDNESDAY AS IT CLASHES WITH THE BUILDING
RIDGE OVER THE CENTRAL BERING SEA. THIS WILL KEEP THE STRONGEST
WINDS CONFINED TO THE WESTERN BERING SEA AND AS THE FRONT MOVES
AWAY FROM THE PARENT LOW...A DECREASE IN WIND INTENSITY WILL ALSO
BE OBSERVED.

&&

.LONG TERM FORECAST (DAYS 3 THROUGH 7)...
AN UPPER LEVEL TROUGH WILL CONTINUE TO SWING THROUGH THE
SOUTHCENTRAL ALASKA REGION BEFORE LIFTING NORTHWARD TOWARDS THE
BROOKS RANGE BY FRIDAY. THE BULK OF THE REMAINING PRECIPITATION
WILL BE CONFINED TO THE EASTERN PRINCE WILLIAM SOUND AND THE
COPPER RIVER BASIN AS THE WAVE MOVES THROUGH THE EASTERN GULF.
MODELS HAVE NOW STARTED TO COME INTO BETTER AGREEMENT ON HOW THE
PATTERN WILL EVOLVE TOWARDS THE WEEKEND...AS THE DEPARTING TROUGH
IS REINFORCED BY SHORTWAVE ENERGY DROPPING OUT OF THE ARCTIC AND
THEN RETROGRADES BACK OVER THE CENTRAL MAINLAND AS A DEEP CLOSED
UPPER LOW. THIS SCENARIO WOULD BRING GENERALLY COOL AND DRY
CONDITIONS TO THE SOUTHERN MAINLAND THROUGH THE WEEKEND...WITH THE
CHANCE FOR A FEW SHOWERS PERSISTING ACROSS SOUTHCENTRAL AK IN THE
PRESENCE OF DEEP CYCLONIC FLOW.

OUT WEST...THE STRONG FRONT POISED TO ENTER THE WESTERN ALEUTIANS
TUESDAY NIGHT WILL BE IMPEDED IN ITS EASTWARD PROGRESS BY AN UPPER
LEVEL HIGH AMPLITUDE RIDGING DEVELOPING OVER THE EASTERN HALF OF
BERING AS THE MAINLAND TROUGH LIFTS TO THE NORTH DURING THE MIDDLE
OF THE WEEK. THIS WILL YIELD GENERALLY BENIGN CONDITIONS ACROSS
THE BERING FOR THE END OF THE WEEK AS THE FRONT DISSIPATES...WITH
THE BEST CHANCE FOR PRECIPITATION SHIFTING TO THE ALASKA PENINSULA
AND THE EASTERN ALEUTIANS ON THE PERIPHERY OF A DEEP NORTH PACIFIC
SYSTEM WELL TO THE SOUTH.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...STORM WARNING...178.
         GALE WARNING...173 174 175 176 177.
         HEAVY FREEZING SPRAY WARNING...160 179 180 181 185.

&&

SYNOPSIS AND MODEL DISCUSSION...MO
SOUTHCENTRAL ALASKA...BC
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...MC
LONG TERM...CB