Heavy Snow in Washington, February 10, 2009

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
340 AM PST TUE FEB 10 2009

.SYNOPSIS...A WARM FRONT WILL OVERSPREAD WESTERN WASHINGTON TODAY
AND THEN SLOWLY DISSIPATE ON WEDNESDAY MORNING. TRANQUIL HIGH
PRESSURE WILL PREVAIL FROM LATE WEDNESDAY UNTIL SATURDAY.

&&

.SHORT TERM...SATELLITE IMAGERY SHOWS THE NEXT WEATHER SYSTEM
QUICKLY REACHING THE COAST. THE MAIN UPPER LOW WAS LOCATED NEAR
49N/138W AT 11Z AND WILL MOVE SOUTHEAST TOWARD SRN ORE THROUGH
TONIGHT. FIRST RADAR ECHOES ARE ALREADY PRESENT ALONG THE NORTH
COAST. PRECIP SHOULD SOON DEVELOP ALONG THE COAST AND SPREAD INTO
THE INTERIOR AROUND OR JUST AFTER SUNRISE. THE AIR MASS IS
SUFFICIENTLY COLD RIGHT NOW TO SUPPORT SNOW OVER NEARLY ALL OF WRN
WA AS EVIDENCED BY THE CONVERGENCE ZONE SNOW LAST EVENING AND 11Z
ASOS TEMPERATURES IN THE UPPER 20S AND LOWER 30S WITH DEWPOINTS
MAINLY IN THE 20S. WITH EASTERLY PRESSURE GRADIENTS AND SURFACE FLOW
INCREASING THIS MORNING...OROGRAPHIC ENHANCEMENT WILL TAKE PLACE
NEAR HOOD CANAL AND AROUND THE SOUTH PERIPHERY OF THE OLYMPICS TO
BRING HEAVY SNOW IN EXCESS OF 4 INCHES THERE. MODELS SHOW 1000-850
MB THICKNESSES REMAINING LOW ENOUGH FOR SNOW UNTIL EARLY AFTERNOON
ALONG THE COAST AND MID-AFTERNOON OVER MUCH OF THE INTERIOR. AFTER
THAT...A TRANSITION TO RAIN IS EXPECTED BELOW ABOUT 1000 FEET WITH
PRECIP INTENSITIES WEAKENING THIS EVENING.

AS FOR WARNINGS AND ADVISORIES...CONTINUED WITH THE PREVIOUSLY
ISSUED WARNINGS AS THE FOCUS OF HEAVIEST SNOW SHOULD BE FROM HOOD
CANAL TO THE CENTRAL COAST OF NORTHERN GRAYS HARBOR COUNTY.
ELSEWHERE...EXPANDED THE WINTER WEATHER ADVISORIES TO INCLUDE THE
WESTERN STRAIT (ALREADY HAD ONE OUT FOR THE NORTH COAST). ALSO CHOSE
TO ISSUE AN ADVISORY FOR THE SOUTHWEST INTERIOR AND SEATTLE/
BREMERTON AREAS. ALONG THE I-5 CORRIDOR OF KING...THURSTON AND LEWIS
COUNTIES...AN INCH OR TWO OF SNOW SHOULD BE EXPECTED WITH HEAVIER
AMOUNTS TO 4 INCHES WEST OF PUGET SOUND NEAR BREMERTON. OPTED TO
LEAVE THE TACOMA AREA OUT OF THE ADVISORY CONSIDERING THE SOUTHEAST
FLOW ALOFT WHICH COULD BRING JUST ENOUGH DOWNSLOPE WARMING FOR
MID-LATE MORNING TRANSITION TO RAIN...PLUS THEY WILL HAVE LOWER QPF
VALUES BECAUSE OF THE DOWNSLOPE EFFECTS. OTHERWISE...AREAS INCLUDING
EVERETT...MOUNT VERNON...BELLINGHAM...THE EAST LOWLANDS AND THE SAN
JUANS ARE NOT IN THE ADVISORIES...BUT SPOTTY ACCUMULATIONS UP TO 1
INCH ARE POSSIBLE EVEN IN THESE LOCATIONS WITH NO ADVISORY. WITH THE
MAIN UPPER SYSTEM PASSING BY TO OUR SOUTHWEST...THESE NORTHERN AND
EASTERN INTERIOR LOCATIONS WILL BE FURTHEST FROM THE ACTION WITH THE
LIGHTEST QPFS.

AS THE UPPER LOW AMBLES TOWARD THE SRN ORE COAST TONIGHT AND WED
MORNING...THE WARM FRONT THAT PIVOTS UP OVER WRN WA TODAY WILL LOSE
ITS SUPPORT AND DISSIPATE. THIS SHOULD BRING AND END TO PRECIP ON
WED...THOUGH SUFFICIENT MOISTURE WILL REMAIN FOR MOSTLY CLOUDY
SKIES. HIGH PRES WILL BE MORE FIRMLY IN CONTROL ON THU WITH DRY AND
PARTLY SUNNY WEATHER RESULTING.     HANER

.LONG TERM...YET ANOTHER UPPER LOW WILL MOVE SOUTHEAST FROM ABOUT
50N/140W TOWARD THE NRN CAL COAST ON THU AND FRI. THIS SYSTEM SHOULD
PASS SUFFICIENTLY FAR SOUTH AND WEST TO LEAVE THE INTERIOR LOWLANDS
WITH A DRY FORECAST THROUGH THIS PERIOD. THE FRINGE OF THIS SYSTEM
HAS PROMPTED A CHANCE OF PRECIP FOR THE COAST AND SOUTHWEST INTERIOR
ON FRI...BUT THESE PLACES COULD JUST AS EASILY STAY DRY. WEAK HIGH
PRES WILL REMAIN IN PLACE ON SAT WITH DRY AND COOL WEATHER. FROM
SUNDAY ON...MODELS SOLUTIONS DIVERGE QUICKLY. GIVEN THE LOW
CONFIDENCE IN THE OUTCOME OF THE WEATHER PATTERN FROM SUNDAY
ON...THE FORECAST BRINGS POPS BACK LATE IN THE WEEKEND WITH A
GRADUAL RAMPUP IN POPS CLOSER TO CLIMATOLOGY EARLY NEXT WEEK.   HANER

&&

.AVIATION...NW FLOW ALOFT THIS MORNING WILL SHIFT TO SOUTHERLY LATE
TODAY AS AN OFFSHORE FRONTAL SYSTEM APPROACHES THE REGION. AN
OCCLUDED FRONT WILL MOVE INLAND OVER SW WASHINGTON THIS AFTERNOON
AND EVENING THEN DISSIPATE. PRECIP IS EXPECTED TO SPREAD INTO THE
INTERIOR 13-16Z. PRECIP MAY BEGIN LARGELY AS SNOW...BUT BEST CHANCE
OF SIGNIFICANT ACCUMULATION APPEARS TO BE IN KITSAP/HOOD CANAL AREA.
MVFR CEILINGS TO PERSIST THROUGH THE DAY WITH LOCAL IFR
CEILINGS/VSBYS IN SNOW. PRECIP TYPE FOR THE AFTERNOON HOURS SHOULD
BE MOSTLY RAIN OR A RAIN/SNOW MIX AWAY FROM THE HOOD CANAL AREA.

KSEA...MVFR CEILINGS HANGING TOUGH THIS MORNING AND ARE EXPECTED TO
CONTINUE THROUGH THE DAY. PRECIP WITH NEXT SYSTEM SPREADS IN 14Z-17Z
THIS MORNING. WITH TEMPERATURES JUST ABOVE FREEZING AND DEWPOINTS IN
THE UPPER 20S...THERE IS SOME CONCERN THAT EVAPORATIVE COOLING COULD
INCREASE THE CHANCES OF A LIGHT SNOW ACCUMULATION...AN INCH OR
SO...WHEN THE PRECIP BEGINS. GIVEN THIS SCENARIO...I AM INCLINED TO
WORD THE FIRST 6 HOURS OF THE UPCOMING TERMINAL FORECAST AS MOSTLY
SNOW. GUIDANCE CONTINUES TO SUGGEST A TURNOVER TO RAIN FOR THE
AFTERNOON HOURS. THE SNOW MAY CREATE TEMPO IFR CEILINGS/VSBYS AND
THE 12-18Z PERIOD OF THE FORECAST WILL INDICATE THIS.

&&

.MARINE...GALES IN EFFECT FOR COASTAL WATERS AND WEST ENTRANCE TO
THE STRAIT AS STRONG OCCLUDED FRONT APPROACHES THE REGION. MUCH OF
THE NORTHERN INLAND WATERS...REMAINDER OF STRAIT...AND ADMIRALTY
INLET WILL SEE SMALL CRAFT ADVISORY WINDS WITH THIS SYSTEM. EAST
ENTRANCE TO THE STRAIT MAY FLIRT WITH NEAR GALE FORCE WIND LATER
THIS MORNING. PARENT LOW ASSOCIATED WITH THE FRONT SHIFTS SOUTHWARD
INTO THE OREGON OFFSHORE WATERS ON WEDNESDAY AND WEAKENS. THE STORM
TRACK SHIFTS SOUTH OF THE AREA FOR MUCH OF THE LATER HALF OF THE
WEEK...AS WEAK SURFACE HIGH PRESSURE SETTLES EAST OF THE CASCADES
PRODUCING LIGHT OFFSHORE FLOW AT TIMES. 27

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...WINTER STORM WARNING FOR HOOD CANAL...THE LOWER CHEHALIS
        VALLEY...THE CENTRAL COAST...AND THE OLYMPIC MOUNTAINS.
     WINTER WEATHER ADVISORY FOR THE NORTH COAST...WESTERN STRAIT
        OF JUAN DE FUCA...SOUTHWEST INTERIOR...THE SEATTLE/BREMERTON
        AREA...AND THE CENTRAL CASCADES.
PZ...GALE WARNING COAST AND WEST ENTRANCE FOR TUESDAY.
     SMALL CRAFT ADVISORY CENTRAL AND EAST STRAIT OF JUAN DE
        FUCA...NORTHERN INLAND WATERS...AND ADMIRALTY INLET.
     SMALL CRAFT ADVISORY FOR ROUGH BAR CONDITIONS GRAYS HARBOR BAR.

WEATHER.GOV/SEATTLE