Winter Storm Warning 1, NWS Juneau, AK

SOUTHEAST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE JUNEAU AK
602 AM AKST SAT JAN 3 2009

.SHORT TERM... MAP ANALYSIS: SURFACE TROUGH AND THE ASSOCIATED
UPPER LEVEL TROUGH MOVING ACROSS THE PANHANDLE EARLY SATURDAY
MORNING. WIDESPREAD SNOW SHOWERS CONTINUING TO MOVE ONSHORE BUT
WILL BE TAPERING OF FROM WEST TO EAST DURING THE DAY TIME. COLD
ARCTIC HIGH PRESS STILL DOMINATE OVER MOST OF THE INTERIOR
PORTIONS OF ALASKA AND NORTHWEST CANADA. A DEVELOP LOW ON THE
FRONTAL SOUTH OF THE ALEUTIANS...PRESENTLY SOUTHWEST OF DUTCH
HARBOR ABOUT 500 NM.

 STRATUS TO STRATOCUMULUS OVER THE MAIN PORTION OF THE GULF DUE
TO ALL THE COLD AIR THAT HAD MOVED OUT OF THE GULF FROM THE
INTERIOR PART OF THE STATE... A FEW BREAKS ARE EVIDENT CLOSER TO
THE PANHANDLE.

 FORECAST REASONING: THE LOW SOUTH OF THE ALEUTIANS IS EXPECTED
TO MOVE FAIRLY QUICKLY ACROSS THE GULF DEEPENING TO THE UPPER
960S BY SUNDAY MORNING. COLD AIR STILL ENTRENCHED OVER THE
FORECAST AREA SO WHEN THE BAND OF MOISTURE MOVES INTO THE AREA
LATE SATURDAY NIGHT INTO SUNDAY MORE SNOW IS EXPECTED. AT THE
MOMENT IT SEEMS BEST CHANCE OF PRECIPITATION WILL BE OVER THE
SOUTHER PORTION OF THE PANHANDLE. TRACK OF THE SYSTEM STILL
WAVERING A BIT... SO HAVE ISSUED A WATCH FOR HEAVY SNOW FOR
23-24-26>29 STARTING TONIGHT INTO SUNDAY. APPEARS THAT 27-28 MAY
START TO MIXING WITH OR CHANGE TO RAIN A LITTLE BEFORE THE LOW
MOVES ONSHORE OVER THE SOUTH CENTRAL PANHANDLE. GENERAL SNOW
ACCUMULATIONS FOR THIS EVENT LOOK TO BE ABOUT 10 TO 14 INCHES...
WITH 4 TO 8 FOR ZONES 27-28.

 WINDS OVER THE WHITE PASS AND INTO LYNN CANAL IN THE CAST OF THE
MARINE FORECASTS MAY DIMINISH SOME DURING THE DAY BUT ARE
EXPECTED TO INCREASE AGAIN TONIGHT SO WILL JUST LET THE CURRENT
WIND/WIND CHILL ADVISORY CONTINUE TO PERSIST RATHER THAN TO DO A
CANCEL/REISSUE THING WITH IN 6 TO 8 HOURS. GALES WITH FREEZING TO
HEAVY FREEZING SPRAY SHOULD EXPECTED THROUGH LYNN CANAL INTO
SUNDAY.

 IN COORDINATION WITH ANCHORAGE USED THE GFS AS MODEL OF CHOICE
INTO SUNDAY FOR ANY ADJUSTMENT. IT WAS IN FAIR AGREEMENT WITH
MOST OF THE OTHER MODELS EXCEPT THE 00Z NAM. OVERALL CONFIDENCE
IN THE SHORT TERM BETTER THAN AVERAGE BUT SNOW TOTALS AND
RAIN/SNOW LINE A DUE TO TRACK OF LANDFALL SUNDAY.

.LONG TERM...THE ARCTIC AIMASS ALREADY COVERING THE PANHANDLE
WILL PERSIST IN MOST OF THE PANHANDLE NEXT WEEK.  THE CHANGES
MADE FOR THE NEW LONG RANGE FORECAST ARE SMALL, AND THE CHANGE
TO A LARGE SCALE ZONAL FLOW IS SHOWN ON SEVERAL MODELS WITH
ONLY SMALL CHANGES FROM PRIOR RUNS.

THE ARCTIC AIRMASS ROLE WILL CYCLE BETWEEN TWO DIFFERENT
SENARIOS NEXT WEEK:

SCENARIO 1.  MARITIME AIMASSES WILL BE FORCED OVER IT, AND THE
RESULTING LIFTING, EXPANSION, AND THEREFORE COOLING WILL RESULT
IN SNOW FALLING THROUGH THE ARCTIC AIR AND ACCUMULATE.

SCENARIO 2.  COLDER EVENTS WHERE AN UNMODIFIED ARCTIC AIRMASS IS
SENT FROM THE CANADIAN INTERIOR THROUGH SE AK.

THE HEAVY SNOW WATCH ONSET IS DISCUSSED IN THE SHORT RANGE
SECTION ABOVE, AND IS SCENARIO 1.  THIS PROBABLE HEAVY SNOW
EVENT SHOULD PERSIST THROUGH SUN, IN THE CENTRAL AND SOUTHERN
ZONES AS LISTED BELOW.  THE ZONES 27 AND 28 HOWEVER, WILL
TEMPORARILY HAVE THE ARCTIC AIRMASS MODIFIED AND WEAKENED AND
THEREFORE PERMITTING RAIN TO BE AT LEAST MIXED IN SUNDAY.
THE ECMWF IS ACCEPTED ON HOW IT DEPICTS A ZONALLY TRACKING
POLAR FRONT ZONE WAVE TO MOVE EASTWARD THROUGH THE SRN
PANHANDLE SUN AND PROVIDE THIS TEMPORARY WARMING.

THE CONFIDENCE IS GOOD THAT A NEW ARCTIC OUTBREAK WILL SPREAD
S THROUGH THE PNHDL SUN NIGHT AND MONDAY, WITH THE PANHANDLE
NORTH OF FREDERICK SOUND.  THE PANHANDLE S OF FREDERICK SOUND
WILL CYCLE BETWEEN SCENARIO 1 AND A WEAK VERSION OF SCENARIO 2.

AS USUAL, WE HAVE LOWER CONFIDENCE THE FARTHER OUT THE FORECAST
GOES.  THE HYDROMETEOROLOGICAL PREDICTION CENTER HAS STATISTICAL
GUIDANCE THAT IS APPLIED TO THE FORECAST PACKAGE FOR THU ONWARD.
THE DETERMINISTIC MODELS CONTINUE TO DISPLAY SOLUTIONS THAT
ARE DIFFICULT TO RECONCILE.

&&

.AJK WATCHES/WARNINGS/ADVISORIES...
PUBLIC...WINTER STORM WATCH FROM LATE TONIGHT THROUGH SUNDAY AFTERNOON
     FOR AKZ023-024-026-029.
         WINTER STORM WATCH FROM LATE TONIGHT THROUGH SUNDAY MORNING FOR
     AKZ027-028.
         WINTER WEATHER ADVISORY UNTIL NOON AKST TODAY FOR AKZ017-025.
         WINTER WEATHER ADVISORY UNTIL 6 AM AKST EARLY THIS MORNING FOR
     AKZ021.
         WIND ADVISORY UNTIL 6 AM AKST SUNDAY FOR AKZ018.
         WIND CHILL ADVISORY UNTIL 6 AM AKST SUNDAY FOR AKZ018.
         WIND ADVISORY UNTIL NOON AKST TODAY FOR AKZ017.
MARINE...HEAVY FREEZING SPRAY WARNING FOR PKZ011-012.
         HEAVY FREEZING SPRAY WARNING FOR PKZ012.
         GALE WARNING FOR PKZ012.
         SMALL CRAFT ADVISORY FOR PKZ011-013-021-022-031>036-041>043-051-
     052.

BEZENEK/JBT