Storm Warning 6, NWS Seattle, WA

AREA FORECAST DISCUSSION...CORRECTED
NATIONAL WEATHER SERVICE SEATTLE WA
330 PM PDT THU APR 1 2010

CORRECTED THE AVIATION...MARINE...AND WATCH/WARNING/ADVISORY
SEGMENTS.

.SYNOPSIS...A STRONG LATE SEASON STORM SYSTEM DEVELOPING OVER THE
EASTERN PACIFIC TONIGHT WILL MOVE INTO VANCOUVER ISLAND JUST OFF THE
NORTHWEST TIP OF THE OLYMPIC PENINSULA FRIDAY AFTERNOON.  THIS
SYSTEM WILL DRIVE A STRONG FRONT ACROSS THE AREA FRIDAY PRODUCING
VERY WINDY CONDITIONS ACROSS MUCH OF WESTERN WASHINGTON.  COOL AND
UNSTABLE AIR WILL INVADE THE REGION BEHIND THIS SYSTEM FRIDAY NIGHT
AND SATURDAY.  ANOTHER...WEAKER SYSTEM WILL APPROACH THE WEST COAST
LATER SUNDAY AND LINGER OVER THE REGION THROUGH MONDAY. A SERIES OF
WEATHER SYSTEMS WILL CONTINUE THE UNSETTLED WEATHER PATTERN THROUGH
THE MIDDLE OF NEXT WEEK.

&&

.SHORT TERM...LATEST RADAR LOOP INDICATES SCATTERED SHOWER ACTIVITY
CONTINUING ACROSS MUCH OF WRN WA LATE THIS AFTERNOON. THE ATMOSPHERE
REMAIN MOIST AND SOMEWHAT UNSTABLE BUT DIURNAL INSTABILITY WILL WANE
OVER THE NEXT FEW HOURS. WILL LEAVE POPS IN THE CHANCE CATEGORY WITH
THE EXPECTATION THAT SHOWERS WILL START TO DECREASE IN COVERAGE.

WV AND IR SATELLITE IMAGERY SHOWS THE NEXT STRONG LOW PRESSURE
SYSTEM BEGINNING TO DEVELOP OUT NEAR 150 W AND 45 N. A VERY STRONG
190+ KT JET WILL SPUR A SURFACE LOW TO DEEPEN OVERNIGHT. THE JET
WILL WEAKEN SOMEWHAT BUT THE FLOW WILL BUCKLE PUTTING THE PAC NW IN
THE STORM TRACK. ALL MODELS AGREE THAT A DEEPENING LOW WILL TRACK
INLAND SOMEWHERE BETWEEN THE N WA COAST AND THE CENTRAL VANCOUVER
COAST FRIDAY AFTERNOON. THE ECMWF/NAM/AND MOST GFS ENSEMBLE MEMBERS
TRACK A SUB 980 MB SURFACE LOW INTO S/CENTRAL VANCOUVER ISLAND. THIS
SCENARIO WOULD LIKELY PRODUCE THE STRONGEST AND MOST WIDESPREAD
WINDS ACROSS WRN WA. THE OPERATIONAL GFS AND MM5 ARE WEAKER AND
TRACK A BROADER SURFACE LOW ACROSS THE OLYMPIC PENINSULA WITH A
SOMEWHAT LESS FAVORABLE TRAJECTORY. BASED ON FORECAST SURFACE
GRADIENTS AND THE MEAN MODEL SURFACE LOW TRACK...IT SEEMS PRUDENT TO
CONVERT THE HIGH WIND WATCHES TO A WARNINGS FOR THE
COAST...STRAIT...AND N INTERIOR SINCE THESE LOCATIONS WILL PROBABLY
VERIFY AT LEAST IN GUSTS IN EITHER OUTCOME. AS FOR THE GREATER PUGET
SOUND REGION AND SW INTERIOR...THE EXACT DEPTH AND LANDFALL LOCATION
WILL MAKE A BIGGER DIFFERENCE ON WIND SPEEDS. THESE LOCATIONS COULD
END UP LOWER END ADVISORY OR POSSIBLY LOW END HIGH WIND CRITERIA.
SINCE THE STRONGER WINDS HOLD OFF HERE UNTIL FRIDAY AFTERNOON I HAVE
OPTED TO LEAVE THE WATCH IN PLACE AND LET THE NEXT SHIFT LOOK AT
ADDITIONAL MODEL RUNS BEFORE MAKING A DECISION. WILL WORD THE
STATEMENT TO LEAN TOWARD ADVISORY LEVEL WINDS FOR NOW.

THIS SYSTEM WILL ALSO PACK A GOOD DEAL OF MOISTURE AND STRONG SLY
FLOW BECOMING WLY BEHIND THE FRONT FRIDAY AFTERNOON. SLOPES THAT DO
WELL IN S AND W FLOW REGIMES WILL SEE THE HEAVIEST AMOUNT OF
PRECIPITATION. SNOW LEVELS WILL REMAIN FAIRLY LOW...GENERALLY IN THE
2000-2500 FOOT RANGE. THE HEAVIEST SNOW IN THE PASSES WILL LIKELY
OCCUR LATER FRIDAY AFTERNOON AND EVENING AS STRONG POST FRONTAL
WESTERLIES WILL BE IDEAL FOR OROGRAPHIC ENHANCEMENT. THE MOUNTAINS
AND PASSES SHOULD RECEIVE 1 TO 2 FEET OF SNOW BY THE END OF THE
EVENT SATURDAY MORNING...WITH LOCALLY 3 FEET POSSIBLE IN THE HIGHER
ELEVATIONS FAVORED BY BOTH S AND W FLOW SUCH AS MT BAKER AND
PARADISE. IN ADDITION...VERY STRONG WEST WINDS OF 40 MPH WITH GUSTS
TO 60 MPH ARE EXPECTED TO DEVELOP IN THE MOUNTAINS AND PASSES FRIDAY
AFTERNOON AND EVENING PRODUCING BLOWING AND DRIFTING SNOW. THIS WILL
CREATE REDUCED VISIBILITY AND HAZARDOUS DRIVING CONDITIONS OVER THE
PASSES. THE WINTER STORM WATCH WILL BE CONVERTED TO A WARNING THIS
AFTERNOON.

SNOW WILL FIRST TAPPER OFF TO SHOWERS OVER THE OLYMPICS FRIDAY NIGHT
THEN ACROSS THE CASCADES BY MIDDAY SATURDAY AS THE FLOW WEAKENS. A
DEEPENING TROUGH OFFSHORE SATURDAY NIGHT WILL FORCE SOME WEAK
RIDGING OVER WA ALLOWING PRECIP TO BECOME JUST SCATTERED SHOWERS.
MODELS SLOW THE EWD PROGRESS OF THE LOW ON SUNDAY BUT BRING IT JUST
CLOSE ENOUGH TO THE COAST TO WARRANT CHANCE POPS ACROSS THE AREA.

.LONG TERM...THE COOL AND UNSETTLED WEATHER PATTERN WILL CONTINUE
THROUGH MUCH OF NEXT WEEK. MODELS SWING A LOW IN ACROSS THE AREA ON
MONDAY WITH A BETTER CHANCE OF LIGHT RAIN AND MOUNTAIN SNOW. COOL
AND MOIST NW FLOW IS LEFT IN ITS WAKE WHICH EXTENDS UPSTREAM TO THE
GULF OF ALASKA. THIS PATTERN IS PRETTY GOOD FOR PRODUCING
OROGRAPHICALLY ENHANCED SNOWFALL IN THE MOUNTAINS WITH SNOW LEVELS
REMAINING DOWN TO PASS LEVELS. A SERIES OF WEAK SYSTEM WILL
TRANSVERSE WRN WA THROUGH MID WEEK WITH SNOW AT TIMES IN THE
MOUNTAINS AND UNSEASONABLY LOW SNOW LEVELS.  MERCER

&&

.HYDRO...A STRONG FRONTAL SYSTEM WILL BRING HEAVY RAIN AND SNOW TO
THE OLYMPICS AND CASCADES LATER TONIGHT THROUGH FRIDAY. SNOW LEVELS
WILL REMAIN FAIRLY LOW...AROUND 2000 TO 2500 FEET WHICH WILL
MITIGATE THE RISK OF FLOODING IN MOST BASINS. THE STRONG SLY FLOW
OUT AHEAD OF THE LOW WILL FAVOR HEAVY PRECIPITATION OVER THE S
FACING SLOPES OF THE OLYMPICS WHERE MODELS ARE PRODUCING 2-3 INCHES
QPF. AT THIS TIME...THE ONLY RIVER AT SLIGHT RISK FOR FLOODING IS
THE SKOKOMISH RIVER SINCE PART OF THE BASIN LIES BELOW THE SNOW
LEVEL. HOWEVER...THE LATEST HYDRO MODELS DO NOT BRING THE SKOKOMISH
RIVER TO FLOOD STAGE. NO PRODUCTS WILL BE ISSUED AT THIS TIME.
CONTINUE TO MONITOR FORECASTS AND ANY FLOOD STATEMENTS. NO FLOODING
IS EXPECTED ON OTHER RIVERS INCLUDING THE GREEN.  MERCER

&&

.AVIATION...DELAYED.

&&

.MARINE...
A DEVELOPING SURFACE LOW WELL OFF THE OREGON COAST WILL DEEPEN
OVERNIGHT AND RACE NE TOWARD THE COASTAL WATERS. THIS LOW IS
EXPECTED TO DEEPEN TO NEAR 975 MB BEFORE MOVING ACROSS VANCOUVER
ISLAND EARLY FRI AFTERNOON. THE ASSOCIATED STRONG...COLD FRONT WILL
SWEEP ACROSS THE COASTAL WATERS FRI MORNING...AND THE INTERIOR IN
THE AFTERNOON. STRONG ONSHORE FLOW WILL OCCUR BEHIND THE FRONT FRI
NIGHT...BUT WILL WEAKEN ON SATURDAY.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...WINTER STORM WARNING IN EFFECT FOR THE OLYMPICS FROM MIDNIGHT
     TONIGHT THROUGH MIDNIGHT FRIDAY NIGHT.

     WINTER STORM WARNING IN EFFECT FOR THE CASCADES FROM FRIDAY
     MORNING THROUGH SATURDAY MORNING.

     HIGH WIND WARNING IN EFFECT FOR THE COAST...STRAIT OF JUAN
     DE FUCA...NORTH INTERIOR FROM ADMIRALTY INLET NORTHWARD...AND
     THE SOUTHWEST INTERIOR /VICINITY OF LAKE LAWRENCE/.

     HIGH WIND WATCH IN EFFECT FOR THE REMAINDER OF THE PUGET
     SOUND REGION AND LOWER CHEHALIS VALLEY.

PZ...A STORM WARNING IS IN EFFECT FOR THE COASTAL WATERS FOR FRI.

     GALE WARNING IN EFFECT FOR THE REMAINING WATERS THRU FRI NIGHT.

     A SMALL CRAFT ADVISORY FOR ROUGH GRAYS HARBOR BAR CONDITIONS IS
     IN EFFECT THRU SATURDAY AFTERNOON.

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