000
FXUS66 KSEW 120527
AFDSEW
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
918 PM PST THU MAR 11 2010...CORRECTED AVIATION SECTION
.SYNOPSIS...PERIODS OF RAIN WILL CONTINUE TONIGHT FROM A FRONT
STALLED OVER WESTERN WASHINGTON. MORE RAIN WILL FALL ON FRIDAY AS A
SURFACE LOW MOVES UP THE FRONT AND ACROSS THE COASTAL WATERS FRIDAY
MORNING. VERY WINDY CONDITIONS AND SHOWERS WILL DEVELOP BY FRIDAY
AFTERNOON AS HIGHER PRESSURE BUILDS NORTHWARD BEHIND THE LOW.
SHOWERS WILL CONTINUE THROUGH FRIDAY NIGHT AS AN UPPER LEVEL TROUGH
MOVES ACROSS THE AREA...THEN TAPER OFF SATURDAY AS THE TROUGH MOVES
EAST. A SLOW MOVING COLD FRONT WILL BRING A CHANCE OF RAIN TO THE
COAST AND NORTH INTERIOR SUNDAY...WITH RAIN SPREADING FURTHER INLAND
BY MONDAY.
&&
.SHORT TERM...THE REMAINS OF THIS MORNINGS COLD FRONT HAVE STALLED
OVER THE AREA. THE SURFACE FRONTAL BOUNDARY IS NO LONGER
APPARENT...BUT MOISTURE ABOUNDS IN THE BAROCLINIC ZONE. AREAS OF
RAIN WILL CONTINUE TO MOVE N ACROSS THE AREA TONIGHT. AT LEAST WINDS
HAVE EASED FOR THE TIME BEING AS THE FRONT HAS WEAKENED.
SATELLITE IMAGERY SHOWS UPPER LEVEL TROUGH CROSSING 140W HAS CAUSED
A WAVE TO DEVELOP ON THE S PART OF THE STALLED FRONT AS EXPECTED.
MODELS REMAIN CONSISTENT IN BRINGING THE WAVE NE ALONG THE FRONT
TONIGHT...WITH THE SURFACE LOW THAT FORMS MOVING UP ACROSS THE WA
COASTAL WATERS FRIDAY MORNING. EXPECT THE RAIN TO INCREASE AGAIN
LATER TONIGHT AND CONTINUE THROUGH SATURDAY MORNING AS THE WAVE
APPROACHES.
00Z UIL RAOB HAD THE FREEZING LEVEL AT 3200 FEET...AND THE
RAVENSDALE (CENTRAL CASCADES) PRECIP PROFILING RADAR SHOWS THE
MELTING LEVEL ABOUT 3500 FEET...WHILE THE WESTPORT PROFILING RADAR
HAS IT AROUND 3000 FEET. PASS REPORTS AND NWAC OBS SHOW IT STILL
SNOWING IN THE MOUNTAINS...AND 00Z NAM SHOWS ABOUT ANOTHER INCH OF
WATER EQUIVALENT FALLING OVER THE OLYMPICS AND CASCADE VOLCANOES
TONIGHT. NAM MAY BE ON THE HIGH SIDE SO WILL FIGURE ON ANOTHER 4 TO
8 INCHES OVERNIGHT. MAIN CENTRAL CASCADE PASSES WILL BE ON THE LOW
END OF THIS...WHICH SHOULD BE ENOUGH FOR ANOTHER 4-6 INCHES.
WILL KEEP THE WINTER STORM WARNING GOING THROUGH TONIGHT. THE
APPROACHING WAVE WILL PRODUCE ANOTHER PERIOD OF HEAVIER SNOW OVER
THE MOUNTAINS STARTING LATE TONIGHT AND MAINLY FRIDAY MORNING. THIS
COULD DROP ANOTHER 6 TO 12 INCHES OVER THE MOUNTAINS FRIDAY...WITH
THE HIGHEST ACCUMULATIONS OVER THE VOLCANOES. THIS IS BORDERLINE
BETWEEN AN ADVISORY AND ANOTHER WARNING. WILL LET THE MIDSHIFT MAKE
CALL AFTER THE 00Z WRF-GFS COMES IN. SNOW LEVELS WILL RISE TO AROUND
4000 FEET OVER THE CASCADES AND 3500 FEET OVER THE OLYMPICS.
00Z GFS/NAM AND 12Z ECMWF CONTINUE WITH THE SAME GENERAL IDEA FRIDAY
AFTERNOON...SHOWING STRONG PRESSURE RISES AND S PRESSURE GRADIENTS
BEHIND THE SURFACE LOW. CONSENSUS IS GOOD FOR VERY WINDY CONDITIONS
FRIDAY AFTERNOON...WITH GUSTS 35 TO 45 KNOTS POSSIBLE AS 925 MB
WINDS MIX DOWN IN THE UNSTABLE AIR MASS BEHIND THE LOW. THIS IS
STILL BELOW WIND ADVISORY CRITERIA...BUT IT WILL CREATE A STRONG
SURGE OF S WIND STARTING OVER THE SW INTERIOR AROUND MIDDAY...THEN
MOVING N ACROSS PUGET SOUND EARLY IN THE AFTERNOON...THEN THE NW
INTERIOR AROUND MID AFTERNOON. MODEL HAVE SOME DIFFERENCES REGARDING
THE SURFACE LOW POSITION AND THE ORIENTATION OF THE GRADIENT OVER
THE COASTAL WATERS...BUT ALL SOLUTIONS POINT TO ABOUT THE SAME
RESULT OVER LAND.
THE UPPER LEVEL TROUGH MOVES THROUGH FRIDAY NIGHT AND EAST OF THE
AREA ON SATURDAY...WITH SHOWERS TAPERING OFF SATURDAY.
GFS/ECMWF STILL DIFFER WITH HOW CLOSE THE NEXT FRONT GETS TO THE
COAST ON SUNDAY. GFS STILL SHOWS RAIN OVER THE COAST AND N INTERIOR
WHILE THE ECMWF KEEPS THE RAIN FURTHER OFFSHORE. KAM
.LONG TERM...PREVIOUS DISCUSSION FROM THE 4 PM AFD...IN A SIMILAR
VEIN AS SUNDAY...GFS AND CANADIAN KEEP UPPER RIDGE AXIS SLIGHTLY
FURTHER EAST AND ALLOW FRONTAL SYSTEMS TO IN SOUTHWEST FLOW TO COVER
MOST OF WESTERN WASHINGTON SUNDAY NIGHT THROUGH AT LEAST PART OF
TUESDAY. ECMWF CONTINUES TO STAND THE FRONT UP OFFSHORE...KEEPING
MOST OF THE AREA DRY. HAVE CONTINUED THE FORECAST TREND OF KEEPING
THE SOUND AND CENTRAL CASCADES DRY WITH A CHANCE OF RAIN ELSEWHERE.
NEW WAVE THAT DEVELOPS IN THE SOUTHWEST FLOW FINALLY LIFTS A LOW TO
OUR NORTH AND EAST BRINGING RAIN TUESDAY NIGHT FOLLOWED BY SHOWERS
ON WEDNESDAY. WESTERLY FLOW KEEP SHOWERS IN THE FORECAST WEDNESDAY
NIGHT AND THURSDAY. DAMICO
&&
.HYDROLOGY...LOW SNOW LEVELS HAVE REDUCED RUN OFF INTO THE RIVERS
WITH THIS EVENT. MAIN CONCERN HAS BEEN POTENTIALLY HEAVY RAIN OVER
THE S SLOPES OF THE OLYMPICS AND POTENTIAL FLOODING OF THE SKOKOMISH
RIVER. THE SKOKOMISH HAS HARDLY RESPONDED TO THE PRECIP THAT HAS
FALLEN SO FAR...AND IS LOW ENOUGH SO THAT WHAT FALLS FROM THE NEXT
WAVE LATE TONIGHT AND FRIDAY WILL NOT DRIVE IT TO FLOOD STAGE.
RECENT MODELS SHOW THE OLYMPICS SNOW LEVELS REMAINING BELOW 4000
FEET...RATHER THAN RISING HIGHER.
MODELS SHOW A SLOW MOVING FRONT AFFECTING THE AREA SUNDAY THROUGH
TUESDAY. THIS DOES NOT APPEAR TO BE A FLOODING THREAT AS MOST OF THE
INITIAL PRECIP HEADS NORTH TO BRITISH COLUMBIA...THEN WHEN THE FRONT
FINALLY MOVES INLAND THERE IS JUST A SHORT PERIOD OF MODERATE RAIN.
MODELS ARE LEANING TOWARD RIDGING OVER THE AREA FROM MID NEXT WEEK
INTO THE WEEKEND...WITH NO SIGNIFICANT PRECIP OVER W WA.
THE FLOODING RISK IS LOW FOR THE GREEN RIVER FOR THE NEXT 10 DAYS.
KAM
&&
.AVIATION...MODERATE SW FLOW ALOFT TONIGHT WILL BECOME STRONG SLY
FLOW ON FRIDAY AS AN UPPER LOW MOVES NORTH JUST OFFSHORE. THE
ATMOSPHERE WILL REMAIN MOIST AND STABLE...EXCEPT BECOMING SLIGHTLY
UNSTABLE FRIDAY AFTERNOON. STRONG WINDS ALOFT ARE EXPECTED TO MIX
DOWN TO THE SURFACE AT TIMES RESULTING IN PERIODIC GUSTS AND
CONVECTIVE TURBULENCE...ESPECIALLY 20Z-24Z. THERE MAY BE SOME
LOCALIZED LLWS AHEAD OF THE FRONT WHEN WINDS INCREASE ALOFT BUT DO
NOT MIX DOWN.
KSEA....WEAK DISTURBANCES ALOFT WILL MOVES ACROSS THE AREA THROUGH
FRIDAY MORNING PRODUCING LIGHT RAIN OR SHOWERS AT TIMES. CIGS WILL
GENERALLY REMAIN IN THE 3-5K FT RANGE BUT WILL LOWER TO MVFR AT
TIMES. A FRONT WILL LIFT NE ACROSS WA ON FRIDAY...PASSING THE KSEA
TERMINAL ARND 20Z FRI. THE FRONT WILL ALSO BE FOLLOWED BY A SLOW
DECREASING TREND IN CLOUD COVERAGE BUT A FEW SHOWERS FRIDAY
AFTERNOON.
SE WINDS 6-10 KT TONIGHT AND FRIDAY MORNING...BECOMING SW 20-25 KT
WITH GUSTS TO 35 KT AFTER 20Z. WINDS WILL EASE FRIDAY EVENING. DTM
&&
.MARINE...A LULL IN WINDS EXPECTED TONIGHT AND FRIDAY MORNING.
MODELS SHOW A SLIGHT INCREASE IN SLY GRADIENTS THROUGH AROUND
MIDNIGHT WHICH COULD PUSH THE INLAND WATERS CLOSE TO SMALL CRAFT
CRITERIA OVER THE NEXT FEW HOURS. GRADIENTS WILL RELAX
THEREAFTER...WITH WINDS BELOW 20 KT ALL WATERS UNTIL FRIDAY MORNING.
THE NEXT STRONG FRONT LIFTS NE ACROSS THE WATERS WITH STRONG WINDS
INCREASING FROM S TO N DURING THE DAY. GALES ARE LIKELY COAST AND W
ENTRANCE. SMALL CRAFT CONDITIONS DEVELOPING ELSEWHERE.
HOWEVER...LATEST MODELS RUNS SUGGEST A CHANCE OF GALES FOR A FEW
HOURS FRIDAY AFTERNOON ACROSS THE INLAND WATERS. WILL EVALUATE THIS
FURTHER AND MAY POSSIBLY CONVERT SOME INTERIOR SMALL CRAFT
ADVISORIES TO GALES FRIDAY AFTERNOON...MOSTLY LIKELY NORTH OF
PUGET SOUND.
LARGE SWELLS CONTINUE OVER THE COASTAL WATERS AND W ENTRANCE. LATEST
READINGS AT BUOY41 STILL INDICATE SEAS OF 22 FT. WAVE MODEL GUIDANCE
SUGGEST COMBINED SEAS WILL REMAIN ABOVE 20 FT THROUGH FRIDAY
MORNING...THEN BEGIN TO SUBSIDE BUT STILL REMAIN IN THE 12-15 FT
RANGE BY SATURDAY. DTM
&&
.AVALANCHE...PREVIOUS DISCUSSION...INCREASING AMOUNTS OF WIND DRIVEN
SNOW SHOULD BE DEPOSITED OVER A VARIETY OF EXISTING WEAK LAYERS OVER
AN OLD CRUST THURSDAY AS THE FIRST OF SEVERAL FRONTAL SYSTEMS MOVES
OVER THE REGION. ALONG WITH STRONG RIDGETOP WINDS AND SLOW WARMING
THIS WEATHER SHOULD PRODUCE A SIGNIFICANT INCREASE IN THE AVALANCHE
DANGER. ALTHOUGH A SLIGHT DECREASE IN PRECIPITATION RATES ARE
EXPECTED LATER THURSDAY AS THE FIRST STORM WEAKENS AND STALLS...A
SECONDARY FRONT SHOULD SPREAD FURTHER MODERATE TO HEAVY RAIN OR SNOW
NORTHWARD OVER THE REGION OVERNIGHT AND FRIDAY MORNING...ALONG WITH
ANOTHER SURGE OF STRONG WINDS AND FURTHER WARMING. THIS SHOULD ALLOW
DEVELOPING HIGH AVALANCHE DANGER TO SPREAD TO PROGRESSIVELY LOWER
ELEVATIONS. AS A RESULT OF THESE INCREASINGLY DANGEROUS AVALANCHE
CONDITIONS LIKELY OVER THE NEXT 24 TO 36 HOURS AN AVALANCHE WARNING
WILL BE ISSUED SHORTLY. BACK COUNTRY TRAVEL IN AVALANCHE TERRAIN IS
NOT RECOMMENDED. CONSULT NWAC.US FOR FURTHER DETAILS. MOORE
&&
.SEW WATCHES/WARNINGS/ADVISORIES...
WA...WINTER STORM WARNING THROUGH 6 AM FRIDAY MORNING FOR THE
OLYMPICS AND WEST SLOPES NORTH AND CENTRAL CASCADES AND PASSES.
.HIGH SURF ADVISORY 10 AM THIS MORNING THROUGH FRIDAY MORNING
FOR THE NORTH AND CENTRAL WASHINGTON COAST.
.AVALANCHE WARNING FROM 10 PM TONIGHT THROUGH NOON ON FRIDAY
MORNING FOR THE OLYMPICS AND CASCADES NEAR AND WEST OF THE
CREST.
PZ...SMALL CRAFT FOR HAZARDOUS SEAS COASTAL WATERS AND WEST ENTRANCE
TONIGHT.
.GALE WARNING FRIDAY COASTAL WATERS AND WEST ENTRANCE
FRIDAY MORNING THROUGH FRIDAY EVENING.
.SMALL CRAFT ADVISORY ALL OTHER WATERS EXCEPT CENTRAL STRAIT
THROUGH FRIDAY EVENING.
.SMALL CRAFT ADVISORY FOR SEVERE GRAYS HARBOR BAR
CONDITIONS.
$$
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