Winter Storm Warning 19, NWS Seattle, WA

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
930 AM PDT TUE MAR 24 2009

.SYNOPSIS...ONSHORE FLOW WILL GIVE SHOWERS AND MOUNTAIN SNOW SHOWERS
TO THE AREA TODAY. HEAVY SNOW WILL DEVELOP IN THE MOUNTAINS LATE
THIS AFTERNOON AND CONTINUE THROUGH WEDNESDAY MORNING AS A
DISTURBANCE ASSOCIATED WITH A STRONG JET STREAM MOVES ACROSS THE
AREA. HIGH PRES WILL BUILD IN FROM THE NORTH GIVING DRY BUT COOL
CONDITIONS TO THE AREA LATER WED INTO THU. A SERIES OF DIFFICULT TO
TIME SYSTEMS MOVING ALONG IN WESTERLY FLOW ALOFT WILL GIVE GENERALLY
UNSETTLED CONDITIONS TO THE AREA FRI THROUGH THE WEEKEND.

&&

.SHORT TERM...A STRONG 155 KT 300 MB JET EXTENDING FROM JUST ESE OF
KODIAK ISLAND TO THE QUEEN CHARLOTTE SOUND IS GENERALLY AIMED AT NW
WA. AS THE JET STREAK MOVES IN...UPPER LEVEL TROUGHING WILL
GRADUALLY DEVELOP OVER THE INTERMOUNTAIN WEST. IN THE SHORT TERM THE
CONCERN IS HEAVY MOUNTAIN SNOW AS A STRONG OROGRAPHIC FLOW OF COOL
MOIST AND WEAKLY STRATIFIED AIR GETS RAMMED UP AGAINST THE TERRAIN.
IT IS DIFFICULT TO TELL EXACTLY WHERE ONE SYSTEM ENDS AND THE NEXT
STARTS...BUT MESO MODELS ARE FORECASTING A GRADUAL RAMP UP IN
SNOWFALL RATES IN THE MOUNTAINS BEGINNING LATE THIS AFTERNOON
...SHORTLY BEFORE 00Z... AND CONTINUING UNTIL 18Z WED. AS A
RESULT...WILL CHANGE THE WINTER STORM WATCH IN THE CASCADES TO A
WARNING BEGINNING 21Z THIS AFTERNOON. SNOWFALL AMOUNTS IN THE
OLYMPICS ARE LOOKING LESS HEAVY...SO WILL CONVERT THE WATCH TO A
WINTER WEATHER ADVISORY THERE. THE CENTRAL CASCADE PASSES LOOK LIKE
THEY WILL GET HAMMERED OVERNIGHT. DURING THE 18 HOUR PERIOD FROM 00Z
THIS AFTERNOON THROUGH 18Z WED...SNOQUALMIE AND STEVENS COULD SEE 18
TO 24 INCHES...WITH OCCASIONAL RATES OF 2-4 INCHES PER HOUR DURING
THE LATE NIGHT PERIOD. SNOW LEVELS WILL AVERAGE 2500 FT
TONIGHT AND 2000 FT WED...THOUGH THE NAM12 IS A BIT WARMER THAN THE
GFS GUIDANCE.

IN THE LOWLANDS...PRECIPITATION WILL BE MOST LIKELY COAST AND
FOOTHILLS WITH INTERIOR VALLEYS SEEING SOME PRECIPITATION TONIGHT
AND WED ESPECIALLY WITH A SOUTHWARD MOVING FRONT/CONVERGENCE ZONE
MIDDAY WED. THERE WILL BE QUITE A BIT OF RAIN SHADOW EFFECT OFF THE
OLYMPICS AND VANCOUVER ISLAND WITH SUCH STRONG WNW/NW FLOW
ALOFT...SO PRECIPITATION WILL BE GENERALLY BE CONFINED TO THE FRONT
AND OR CONVERGENCE ZONE.

CONDITIONS WILL DRY FROM THE NORTH WEDNESDAY AFTERNOON WITH DRIER
SUBSIDING AIR FILTERING IN WED NIGHT AND THU. SOME LOWER
CLOUDS...BELOW A DEVELOPING LOW LEVEL INVERSION...WILL AFFECT THE
LOWLAND VALLEYS LATE WED NIGHT AND THU MORNING AND THIS WILL KEEP
TEMPERATURES UP 2-3 DEG FROM PREVIOUSLY EXPECTED VALUES.

UPDATES WILL BE MADE FOR HEADLINES AND MINOR TEMPERATURE CHANGES WED
NIGHT. ALBRECHT

.LONG TERM...PREVIOUS LONG TERM DISCUSSION FOLLOWS...THE BRIEF UPPER
RIDGE BREAKS DOWN FRIDAY AND A BROAD AREA OF HEIGHT FALLS SPREADS
INTO THE REGION THRU FRI NIGHT. THE CHANCE OF RAIN IN THE FORECAST
WONT BE ENOUGH FOR FRI NIGHT AND SATURDAY WHICH LOOKS LIKE THE BEST
BET FOR PRECIP. ECMWF ALSO HAS ANOTHER UPPER TROF DEVELOPING OVR
PACNW THRU SAT NIGHT. THEN UPPER RIDGING SUNDAY COULD GIVE A DECENT
DAY...ALTHOUGH HEIGHTS DONT RISE MUCH AT ALL ON GFS. ANOTHER
SHORTWAVE IN FAST ZONAL FLOW IS INTO THE AREA MONDAY IN GFS...JUST
DIRTY RIDGE ON EURO. ON BALANCE LOOKS LIKE WET IN THE MOUNTAINS THRU
THE PERIOD BUT MAY NOT BE SO BAD IN THE LEE OF THE OLYMPICS. A
LITTLE ON THE COOL SIDE IN THE EXTENDED. A BIT OF A MEAN RIDGE
POSITION ARND 145W...AND THAT ALLOWS PLENTY OF ENERGY TO SLIP OVER
THE RIDGE AND DIG SHORTWAVES INTO THE PACNW IN FAST UPPER FLOW...A
GOOD PSCZ PATTERN AS WELL W/ PERIODS OF SNOW IN THE MTNS A GOOD BET.

&&

.AVIATION...ONSHORE FLOW TODAY WITH A MOIST AND STABLE AMS. A MIX OF
IFR/LOW MVFR CIGS SHOULD IMPROVE SLIGHTLY TO MOSTLY MVFR 1500-2500
FT AFTER 21Z. HOWEVER...SOME SPOTS MAY STAY BELOW 1K FT. CIGS WILL
PROBABLY DROP BACK TO MOST IFR BY 06Z AS ONSHORE FLOW INCREASES
AHEAD OF A FRONTAL SYSTEM.

KSEA...SHOWERS THROUGH TONIGHT...WITH STEADY RAIN INCREASING LATE
TONIGHT AND WED MORNING. CIGS MAY IMPROVE TO LOW MVFR 1-2K FT AFTER
21Z BUT ONLY ABOUT A 50/50 CHANCE.  IFR RE-DEVELOPING THIS EVENING
AS ONSHORE FLOW INCREASES AHEAD OF A FRONT.  SSW WIND 6-10 KT RISING
TO 8-12 KT THIS AFTERNOON. A NLY WIND SHIFT IS EXPECTED BEHIND
A FRONT MIDDAY WED. DTM

&&

.MARINE...SMALL CRAFT ALL WATERS. ONSHORE FLOW WILL STRENGTHEN
THROUGH TONIGHT. A FRONT WILL DROP SOUTHEAST THROUGH THE WATERS ON
WED WITH A CHANGE TO NLY OR OFFSHORE PRESSURE GRADIENTS. DTM

&&

.AVALANCHE...STRONG WEST CREST LEVEL WINDS AND HEAVY SNOWFALL SHOULD
BE SEEN NEAR AND WEST OF THE CASCADE CREST TUESDAY AFTERNOON INTO
WEDNESDAY MORNING. WIND VARIATIONS...AND SNOWFALL INTENSITIES AND
ACCUMULATIONS SHOULD BE SUFFICIENT TO CAUSE STORM CYCLE WEAK
LAYERS...SOFT SLAB CONDITIONS...AND LIKELY NATURAL OR TRIGGERED
AVALANCHES DURING THE STORM. AN AVALANCHE WARNING HAS BEEN ISSUED BY
THE NW WEATHER AND AVALANCHE CENTER FOR TUESDAY NIGHT TO WEDNESDAY
MORNING. PLEASE SEE NWAC.US FOR DETAILS. FERBER

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...WINTER STORM WARNING IN EFFECT FROM 21Z THIS AFTERNOON
      THROUGH MIDDAY WED FOR THE WEST SLOPES OF THE CASCADES AND
      PASSES.
    .WINTER WEATHER ADVISORY IN EFFECT FROM 21Z THIS AFTERNOON
      THROUGH MIDDAY WED FOR THE OLYMPIC MOUNTAINS.
    .AVALANCHE WARNING FOR THE CASCADES.
PZ...SMALL CRAFT ADVISORY ALL WATERS.
    .SMALL CRAFT ADVISORY FOR ROUGH BAR GRAYS HARBOR BAR.

&&

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