High Wind Watch 1, NWS Seattle, WA

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
300 PM PDT TUE MAR 19 2013

.SYNOPSIS...A STRONG PACIFIC STORM SYSTEM WILL BRING
RAIN...WIND...AND HEAVY MOUNTAIN SNOW TO THE AREA TONIGHT THROUGH
THURSDAY NIGHT. COLDER AIR WILL ARRIVE WEDNESDAY NIGHT AND THURSDAY
WITH SHOWERY CONDITIONS AND LOW SNOW LEVELS. WEAK HIGH PRESSURE
COULD BRING A DRYING TREND TOWARD THE WEEKEND.

&&

.SHORT TERM...RADAR INDICATES RAIN IS STARTING TO INCREASE ACROSS
THE AREA THIS AFTERNOON AS EXPECTED. THE CENTER OF THE STORM IS
STILL DEVELOPING OFFSHORE APPROACHING 130 W AND ABOUT 45 N TRACKING
TOWARD THE NE. WATER VAPOR IMAGERY TELLS THE STORY SHOWING COLD AIR
DUMPING SOUTH FROM THE GULF OF ALASKA MEETING UP WITH THE LOW
PRESSURE SYSTEM THAT DEVELOPED ALONG THE SUBTROPICAL JET. THE SYSTEM
IS NOW IN THE PROCESS OF PHASING WITH THE COLDER NORTHERN STREAM
WHICH WILL RESULT IN FURTHER DEEPENING AND INTENSIFICATION OF THE
SYSTEM. THERE ARE NO GOOD OBS OFFSHORE TO VERIFY THE LOW DEPTH BUT
THE SYSTEM APPEARS TO BE DEVELOPING AS MODELS PREDICTED.

MODELS ARE IN FAIR AGREEMENT ON TRACKING A ROUGHLY 985 MB SURFACE
LOW INTO VANCOUVER ISLAND BY LATE TONIGHT/EARLY WEDNESDAY MORNING.
SE GRADIENTS WILL INCREASE RAPIDLY AHEAD OF THE LOW TONIGHT WITH
WINDY CONDITIONS ALONG THE COAST/N INTERIOR. THE WRF-GFS SHOWS SOME
30 KT WINDS ALONG THE COAST AND PARTS OF THE N INTERIOR SO I DECIDED
TO ISSUE A WIND ADVISORY FOR THOSE AREAS FROM LATE THIS EVENING
THROUGH WEDNESDAY MORNING. AS THE LOW MAKES LANDFALL IT WILL HEAD
EWD WITH THE BENT-BACK FEATURE SWINGING INTO WRN WA DURING THE
AFTERNOON. THE GFS IS THE MOST AGGRESSIVE WITH SLY GRADIENT UP TO
+16 MB PDX-KBLI. OTHER MODELS SUCH AS THE NAM-12/CANADIAN/UKMET ARE
IN THE MIDDLE WITH A PEAK AROUND +12 MB AND THE ECMWF IS SHOWING
ABOUT +10 MB. THE FORECAST IS BASED TOWARD THE NAM SOLUTION AND
CLOSER TO THE MORE RELIABLE ECMWF. THE BENT-BACK STILL LOOKS WELL
DEVELOPED WITH UNSTABLE CONDITIONS HELPING TO TRANSPORT THE 50-60 KT
WINDS AT 925 MB TOWARD THE SURFACE. SINCE THE ECMWF IS USUALLY THE
SUPERIOR MODEL IT WILL NOT BE DISCOUNTED EITHER.  DECIDED TO LIMIT
THE HIGH WIND WATCH TO THE N INTERIOR DUE TO THE STRONG WLY SURGE
DOWN THE STRAIT AND POSSIBLE BRIEF NW SURGE DOWN THE STRAIT OF
GEORGIA. A WIND ADVISORY WILL LIKELY BE NEEDED FOR THE REMAINDER OF
THE AREA WITH A SW SURGE UP PUGET SOUND LIKELY WEDNESDAY AFTERNOON
AND WLY ALONG THE COAST. GUSTS COULD REACH 45 TO 50 MPH. AREAS UNDER
THE HIGH WIND WATCH FROM ADMIRALTY INLET NWD MAY SEE GUSTS TO 60
MPH. WINDS WILL SLOWLY EASE LATE WEDNESDAY EVENING BUT REMAIN BREEZY
INTO THURSDAY.

SNOW IS EXPECTED IN THE MOUNTAINS BUT THE SNOW LEVELS WILL BE VERY
TRICKY. WARMER AIR IS ALREADY INFILTRATING WRN WA WITH EVIDENCE OF
WARMING EVEN AT RATHER HIGH ELEVATIONS SUCH AS HURRICANE RIDGE ON
THE OLYMPICS. TEMPERATURES AT SNOQUALMIE PASS ARE ALSO ON THE RISE
THE PAST HOUR. EARLIER THOUGHT WAS THAT COLDER AIR WOULD REMAIN
TRAPPED FOR A WHILE IN SNOQUALMIE BUT THIS IS LOOKING HIGHLY
UNCERTAIN. THE N CASCADES WILL STAY IN THE COLDER AIR LONGER THIS
EVENING BUT EVEN STEVENS PASS MAY SEE SOME CHANGE OVER TO RAIN LATER
TONIGHT. WILL LEAVE THE WARNING UP AS POST FRONTAL FLOW WILL BRING A
MUCH BETTER SHOT OF SNOW LATER WEDNESDAY AND WEDNESDAY NIGHT WHICH
IS COVERED BY THE WARNING. SNOW LEVELS WILL PLUMMET TO BELOW 3000
FEET WEDNESDAY AFTERNOON WITH HEAVY SNOW LIKELY IN THE PASSES
THROUGH WEDNESDAY NIGHT. AMOUNTS WILL RANGE FROM A FOOT TO 2 FEET AT
THE HIGHER ELEVATIONS BY THURSDAY MORNING. MODELS TRENDED MUCH
COLDER TODAY FOR THE THURSDAY/THURSDAY NIGHT PERIOD AND INDICATE
ANOTHER DISTURBANCE COMING THROUGH IN NW FLOW. SNOW RATIOS WILL BE
HIGH IN THE COLDER AIR SO ANOTHER FOOT OR MORE OF SNOW COULD FALL.
THE WARNING MAY NEED TO BE EXTENDED AT SOME POINT.

SNOW LEVELS WILL HOVER NEAR 500 TO 1000 FEET THURSDAY MORNING TROUGH
FRIDAY MORNING. A RAIN/SNOW MIX COULD FALL AT TIMES DOWN TO NEAR SEA
LEVEL. THE FOOTHILLS AND A FEW SPOTS ABOVE 500 FEET COULD SEE LOCAL
LIGHT ACCUMULATIONS. WILL NEED TO WATCH THE CONVERGENCE ZONE DURING
THE PERIOD AS CONDITIONS WILL BE FAVORABLE FOR DEVELOPMENT. WITH 850
MB TEMPS NEAR -7C AND 500 MB TEMPS DOWN TO -35C...ANY CONVERGENCE
ZONE COULD PRODUCE LOCALIZED ACCUMULATION. LOCALLY HIGHER AMOUNTS
COULD ALSO FALL FROM STEVENS PASS TO SNOQUALMIE PASS.

.LONG TERM...THE LARGE TROUGH WILL SHIFT EAST ON SATURDAY WITH HIGH
PRESSURE ALOFT JUST OFF THE PAC NW. THIS SHOULD PROVIDE MOSTLY DRY
CONDITIONS BUT MODELS STILL SHOW SOME SPOTTY LIGHT QPF AROUND THE
AREA. DIURNAL SHOWER ACTIVITY IS COMMON THIS TIME OF YEAR SO
EXPECTED SOME SHOWERS IN THE AFTERNOON/EVENING. ANOTHER SYSTEM COULD
APPROACH THE AREA EARLY NEXT WEEK BUT MODELS ARE NOT IN AGREEMENT.
STUCK WITH CLIMATOLOGY AND THE CHANCE OF SHOWERS FOR NOW. M

MERCER

&&

.HYDROLOGY...MODELS INDICATE ABOUT 2-3 INCHES LIQUID OVER THE
OLYMPICS AND A LITTLE LESS OVER THE CASCADES. SINCE SNOW LEVELS WILL
BE RELATIVELY LOW...3000-4000 FEET AND LOWER UP NORTH....LITTLE
IMPACT IS EXPECTED ON RIVERS. ONLY THE LOWEST RIVER BASINS WILL SEE
RISES. WILL NEED TO WATCH THE CHEHALIS AND POSSIBLY THE SKOKOMISH
RIVER BUT NO FLOODING IS FORECAST. BY THURSDAY PRECIPITATION WILL BE
SHOWERY. NO FLOODING IS EXPECTED OUT TO 7 DAYS.

&&

.AVALANCHE...A STRONG STORM WILL BRING HEAVY RAIN AND SNOW...VERY
STRONG WINDS AND RISING FREEZING LEVELS TO THE OLYMPICS, CASCADES
AND MT HOOD AREA OVERNIGHT AND EARLY WEDNESDAY. THESE CONDITIONS
WILL RESULT IN A SIGNIFICANTLY INCREASING AVALANCHE DANGER WITH VERY
DANGEROUS AVALANCHE CONDITIONS DEVELOPING OVERNIGHT AND EARLY
WEDNESDAY. AS A RESULT AN AVALANCHE WARNING WILL BE ISSUED FOR THE
OLYMPICS, WA CASCADES NEAR AND WEST OF THE CREST AND THE MT HOOD
AREA. AN EXPECTED SHIFT TO WEST FLOW BEHIND THE FRONTAL PASSAGE
WEDNESDAY MORNING MAY CAUSE A CHANGE FROM SNOW TO RAIN NEAR STEVENS
AND SNOQUALMIE PASSES WHICH MAY IN TURN CAUSE A RAPID INCREASE IN
DANGER.

TRAVEL IN AVALANCHE TERRAIN IS NOT RECOMMENDED TUESDAY NIGHT AND
WEDNESDAY

LESS EXPECTED SNOWFALL ALONG THE CASCADE EAST SLOPES SHOULD CAUSE
LESS THAN AVALANCHE WARNING CRITERIA TO BE REACHED, HOWEVER AN
ELEVATED AVALANCHE DANGER IS EXPECTED.

FOR COMPLETE FORECAST DETAILS VISIT WWW.NWAC.US
KRAMER/NORTHWEST WEATHER AND AVALANCHE CENTER

&&

.AVIATION...SOUTHWEST FLOW ALOFT AND MOISTURE AT ALL LEVELS WILL
INCREASE THIS EVENING AS RAIN SPREADS INLAND. A WARM FRONT WILL MOVE
OVER WESTERN WA TONIGHT AND THE COLD FRONT WILL PUSH TO THE COAST
AROUND THE MIDDLE OF THE MORNING WEDNESDAY AND THROUGH THE INTERIOR
OF WESTERN WA EARLY WED AFTERNOON. A 985 MB LOW WILL MOVE TO
NORTHERN OR CENTRAL VANCOUVER ISLAND BY DAYBREAK WED AND THEN
ONSHORE SLY AND THEN WLY PRESSURE GRADIENTS WILL STRENGTHEN AS THE
LOW MOVES INLAND OVER B.C. AND HIGHER PRES BUILDS IN FROM THE SW.
CIGS AND AREAS OF LOW VSBY WILL DEVELOP TONIGHT WITH THE WARM FRONT
AND THEN RAIN WILL TURN TO SHOWERS WITH RATHER BLUSTERY CONDITIONS
ON WED. THERE IS A CHANCE OF THUNDERSTORMS AS THE COLD AIR ALOFT
MOVES IN LATER WED AND BEYOND. 19

KSEA...CURRENT TAF IS IN THE BALLPARK AND NO BIG CHANGES EXPECTED
FOR THE 00Z TAF. THE EASTERLY GAP WINDS ARE NOT PUSHING OUT ANYWHERE
NEAR DOWNTOWN SEATTLE SO LLWS IS NOT LIKELY...AND THE CROSS CASCADE
GRADIENT IS ONLY ABOUT -6MB THIS AFTERNOON WHICH IS NOT STRONG. 19

&&

.MARINE...A 985 MB LOW WILL REACH NORTHERN OR CENTRAL VANCOUVER
ISLAND BY DAYBREAK WEDNESDAY. THE ASSOCIATED FRONTAL SYSTEM WILL
MOVE THROUGH THE AREA TONIGHT AND WEDNESDAY. OFFSHORE EASTERLY FLOW
TODAY WILL SWITCH TO SOUTHERLY FLOW TONIGHT AND STRONG WESTERLY FLOW
WILL DEVELOP OVER THE COASTAL WATERS BY ABOUT MID WED MORNING AND
PUSH THROUGH THE STRAIT MIDDAY. BY EVENING A NW WIND ARND 30KT
SHOULD SHOOT DOWN ADMIRALTY INLET AND A PSCZ WIND PATTERN WILL SET
UP. IN FACT THE PSCZ WIND PATTERN WILL BE IN PLACE THU AND FRI AS
WELL. THE ONSHORE FLOW WILL EASE THU AND FRI.

SEAS WILL RISE TO AROUND 20FT BY ABOUT WED EVENING OVER THE COASTAL
WATERS AND A HIGH SURF ADVISORY WILL VERY LIKELY BE ISSUED TONIGHT
FOR LATER WED AND WED NIGHT. 19

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...WINTER STORM WARNING OLYMPICS AND NORTH AND CENTRAL
     CASCADES THROUGH WEDNESDAY NIGHT.

     WIND ADVISORY FOR THE NORTH AND CENTRAL COAST AND THE NORTH
     INTERIOR LATE THIS EVENING THROUGH WEDNESDAY MORNING.

     HIGH WIND WATCH FOR THE NORTH INTERIOR FROM LATE WEDNESDAY
     MORNING THROUGH WEDNESDAY EVENING.

     FLOOD WATCH FOR MASON COUNTY.

PZ...GALE WARNING ALL WATERS EXCEPT A GALE WATCH FOR PUGET SOUND AND
     HOOD CANAL.

&&

WWW.WEATHER.GOV/SEATTLE

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