Heavy Snow in Washington, April 11 to 12, 2009

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
335 PM PDT SAT APR 11 2009

.SYNOPSIS...CLOUDS OUT AHEAD OF THE NEXT PACIFIC FRONTAL SYSTEM WILL
SPREAD OVER WESTERN WASHINGTON THIS EVENING WITH RAIN DEVELOPING
AFTER MIDNIGHT. HEAVIER RAIN AND BREEZY CONDITIONS ARE EXPECTED ON
SUNDAY WITH THE FRONT MOVING THROUGH WESTERN WASHINGTON LATE IN THE
DAY. A COOL AND SHOWERY PATTERN IS EXPECTED FOR THE FIRST HALF OF
NEXT WEEK. A RIDGE OF HIGH PRESSURE WILL BUILD OVER WESTERN
WASHINGTON IN THE MIDDLE OF THE WEEK FOR SHORT SPELL OF DRIER AND
WARMER WEATHER.

&&

.SHORT TERM...SATELLITE IMAGERY SHOWS WHATS IS LEFT OF THE
DISSIPATING FRONT ALONG THE COAST THIS AFTERNOON. WSR-88D SHOWING
JUST SOME ISOLATED SHOWERS OVER THE AREA AT 22Z. NEXT SYSTEM TO MOVE
INTO THE AREA CURRENTLY BACK NEAR 135W WITH THE CLOUDS OUT AHEAD OF
THE WARM FRONT ALREADY INSIDE OF 130W. WARM FRONT MOVING INTO
WESTERN WASHINGTON SUNDAY MORNING WITH THE RAIN OUT AHEAD OF THE
FRONT SPREADING ACROSS THE AREA AFTER MIDNIGHT. TRAILING COLD FRONT
ARRIVING LATER IN THE DAY WITH THE CURRENT TIMING BRINGING THE FRONT
TO THE COAST AROUND 21Z...PUGET SOUND AROUND 00Z AND INTO THE
CASCADES BY 03Z MONDAY. WITH WESTERN WASHINGTON IN THE WARM SECTOR
ON SUNDAY SNOW LEVELS GOING UP TO AROUND 5000 FEET. THIS SYSTEM HAS
SOME JUICE TO IT WITH THE NEW PW IMAGERY CONTINUING TO SHOW 1.5
INCHES IN THE CORE OF THE STORM. 850 WINDS INCREASING OVERNIGHT AND
BY SUNDAY MORNING WILL BE IN THE 40-50 KNOT RANGE OUT OF THE
SOUTHWEST SO EXPECT HEAVY PRECIPITATION TOTALS ALONG THE SOUTHWEST
SLOPES OF THE OLYMPICS...2 TO 4 INCHES BY 00Z MONDAY. DECENT
PRECIPITATION AMOUNTS IN THE CASCADES AS WELL WITH THE SNOW LEVEL
RISING UP TO 5000 FEET PRECIPITATION IN THE PASSES WILL BE IN THE
FORM OF RAIN ALL DAY WITH THE EXCEPTION OF THE EARLY MORNING HOURS
WHERE THE PRECIPITATION COULD BRIEFLY BE IN THE FORM OF SNOW. FOR
THE LOWLANDS A WET AND BREEZY EASTER. BEHIND THE COLD FRONT COLD
UPPER LEVEL TROUGH MOVING INTO THE AREA ON MONDAY. MODEL 850 MB
TEMPS DOWN TO -6C MONDAY AFTERNOON WITH 500 MB TEMPS -36C. LIFTED
INDEXES RANGING FROM NEAR 0 TO -2 WITH CONVECTIVE TEMPERATURES IN
THE UPPER 40S. GIVEN THESE VARIABLES HAVE PUT A CHANCE FOR
THUNDERSTORMS IN ALL OF THE ZONES MONDAY AFTERNOON WITH THE CHANCE
EXTENDING INTO MONDAY EVENING. SNOW LEVELS WILL DROP TO AS LOW AS
1500 FEET ON MONDAY. TUESDAY WILL BE TRANSITION DAY WITH THE UPPER
LEVEL TROUGH DIGGING SOUTH INTO THE GREAT BASIN. THE AIR MASS WILL
STILL BE UNSTABLE WITH LIFTED INDEXES REMAINING NEAR ZERO ANY PLENTY
OF LOW LEVEL MOISTURE AROUND TO KEEP SHOWERS IN THE FORECAST. PUGET
SOUND CONVERGENCE ZONE ALSO POSSIBLE ON TUESDAY. FELTON

.LONG TERM...EXTENDED MODELS SHOWING GOOD CONSISTENCY IN THE FIRST
HALF OF THE EXTENDED WITH AN UPPER LEVEL RIDGE BUILDING OVER THE
AREA ON WEDNESDAY. RIDGE SHIFTS EAST ON THURSDAY. TIMING OF THE NEXT
SYSTEM VARYING FROM RUN TO RUN. LATEST GFS HAS THE FRONT MOVING INTO
WESTERN WASHINGTON THURSDAY NIGHT WHILE THE 12Z ECMWF STILL HAS THE
FRONT HOLDING OFF UNTIL FRIDAY. ECMWF THEN BUILDS A STRONG RIDGE
OVER THE AREA FOR NEXT WEEKEND WHILE THE GFS IS MUCH WEAKER WITH THE
RIDGE AND BRINGS ANOTHER FRONT THROUGH THE AREA ON SAT. WITHOUT ANY
MODEL CONSENSUS WILL STAY WITH THE CURRENT FORECAST OF CHANCE POPS
FOR FRIDAY AND SATURDAY. FELTON

&&

.AVIATION...A FLAT UPPER LEVEL RIDGE WITH W FLOW ALOFT AND A WEAK
SURFACE RIDGE WILL BE OVER W WA THIS EVENING...THEN SHIFT EAST AFTER
MIDNIGHT AS A FRONTAL SYSTEM APPROACHES THE WA COAST. THE AIR MASS
OVER W WA REMAINS SOMEWHAT MOIST BUT WITH VFR CONDITIONS MOST AREAS.
EXPECT CIGS TO FILL IN AND LOWER AS MOISTURE FROM THE APPROACHING
FRONTAL SYSTEM ARRIVES. RAIN SHOULD REACH PUGET SOUND AFTER 10Z. THE
COLD FRONT WILL CROSS W WA SUNDAY AFTERNOON.

KSEA...WILL KEEP CIGS VFR THIS EVENING...GENERALLY 050 OR HIGHER.
OBS TO THE SW SHOW SOME LOWER CIGS BKN025-040 THAT MAY WORK BRIEFLY
UP OVER KSEA. EXPECT LOWEST CIGS AND MAIN PRECIP SUNDAY FROM LATE
MORNING THROUGH MID AFTERNOON AS THE FRONT IS MOVING THROUGH THE
AREA. S FLOW CONTINUES THROUGH SUNDAY. 2-6 THOUSAND FOOT WINDS WILL
INCREASE TO S-SW 35-50 KT BETWEEN 15Z-21Z THEN SHIFT W AND DECREASE
BY 00Z. SW WIND OVER THE OLYMPIA ARRIVAL GATE WILL BE 40-50 KT
DURING THE SAME PERIOD. KAM

&&

.MARINE...THE WEAK SURFACE RIDGE OVER W WA THIS EVENING WILL MOVE
EAST TONIGHT AS A FRONTAL SYSTEM APPROACHES THE COAST. LOW REACHES N
VANCOUVER ISLAND ABOUT 18Z SUNDAY NEAR 1000 MB. COLD FRONT CROSSES
THE COASTAL WATERS DURING THE MORNING AND THE INLAND WATERS DURING
THE AFTERNOON. WENT AHEAD AND RAISED THE GALE WARNING FOR THE COAST
AND EAST STRAIT AND N INLAND WATERS THOUGH THE GALE MIGHT BE
MARGINAL. MESOSCALE GUIDANCE WAS JUST UP TO 35 KT OR JUST BELOW. SCA
UP FOR ADJOINING W ENTRANCE AND ADMIRALTY INLET. WILL LET THE
MIDSHIFT HANDLE THE DROP OFF FROM GALE THROUGH SCA WINDS SUNDAY
AFTERNOON. STRONGEST WINDS STILL EXPECTED AHEAD OF THE COLD FRONT.
W PUSH BEHIND THE FRONT LATE SUNDAY SHOULD GENERATE SCA WINDS
THROUGH THE STRAIT. ONSHORE FLOW CONTINUES THROUGH
TUESDAY...ALTHOUGH THE UPPER LEVEL TROUGH CROSSING THE AREA MONDAY
AFTERNOON WILL SHIFT THE WINDS AROUND A BIT. STILL LOOKS LIKE A
SURFACE RIDGE WILL BE OVER THE AREA WEDNESDAY/THURSDAY. KAM

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...SMALL CRAFT ADVISORY WEST ENTRANCE AND ADMIRALTY INLET.
    .GALE WARNING COASTAL WATERS...EAST ENTRANCE TO THE
     STRAIT...AND NORTHERN INLAND WATERS.

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