Winter Storm Watch 9, NWS Seattle, WA

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
300 AM PDT TUE MAR 13 2012

.SYNOPSIS...A COLD UPPER TROUGH WILL BRING RAIN AND SNOW SHOWERS TO
WESTERN WASHINGTON TODAY WITH A FEW THUNDERSTORMS. ANOTHER VIGOROUS
FRONTAL SYSTEM WILL MOVE THROUGH THE AREA WEDNESDAY...FOLLOWED
QUICKLY BY YET ANOTHER WEDNESDAY NIGHT AND THURSDAY...FOR WET AND
OCCASIONALLY WINDY WEATHER AND LOTS MORE SNOW IN THE MOUNTAINS.
SHOWERY WEATHER...AND SOME SUN BREAKS...ARE LIKELY THIS WEEKEND.

&&

.SHORT TERM...THE BACK SIDE OF THE FRONTAL BAND THAT MOVED INTO THE
REGION MONDAY IS SLOWLY SHIFTING EASTWARD ACROSS THE AREA. THIS
FEATURE HAS GRADUALLY BECOME MORE NORTH-SOUTH ORIENTED...WITH ITS
EASTWARD PROGRESS SLOWED ACCORDINGLY...AS THE DEEP UPPER TROUGH
OFFSHORE HAS BEEN DIGGING SOUTHEAST. A COLD AIR MASS MOVED INTO
WESTERN WASHINGTON BEHIND THE FRONT. BY 5 PM MONDAY THE FREEZING
LEVEL AT QUILLAYUTE -- ON THE NORTH COAST -- HAD FALLEN TO
1100 FT...IMPLYING THE SNOW LEVEL IS JUST A FEW HUNDRED FEET ABOVE
SEA LEVEL. NOT SURPRISINGLY THERE IS A MIX OF RAIN AND SNOW SHOWERS
ACROSS THE FORECAST AREA EARLY THIS MORNING WITH TEMPERATURES IN THE
LOWER TO MID 30S.

THE DEEP UPPER TROUGH OFFSHORE ALONG 130W EARLY THIS MORNING WILL
MOVE ACROSS WESTERN WASHINGTON TODAY. SATELLITE IMAGERY SHOWS PLENTY
OF CONVECTION IN THE TROUGH AND LIGHTNING DETECTION EQUIPMENT SHOWS
ISOLATED THUNDERSTORMS OVER THE PACIFIC NORTHWEST OFFSHORE WATERS...
AND ALL THAT WILL BE MOVING INLAND WITH THE UPPER TROUGH TODAY. IN
GENERAL PRECIPITATION AMOUNTS WILL NOT BE VERY HEAVY...BUT THERE
WILL PROBABLY BE SOME RANDOM BRIEF BURSTS IN HEAVIER SHOWERS OR
THUNDERSTORMS.

THE SNOW LEVEL WILL BE WITHIN A FEW HUNDRED FEET OF SEA LEVEL THIS
MORNING...RISING TO AROUND 1000 FT LATE TODAY. TEMPERATURES ARE TOO
MARGINAL AND ROADWAYS TOO WET FOR MUCH IF ANY SNOW TO STICK THERE...
BUT LOCAL ACCUMULATIONS OF AN INCH OR TWO ARE POSSIBLE ON OTHER
SURFACES. THOSE MINOR ACCUMULATIONS WILL BE MORE LIKELY AWAY FROM
BODIES OF WATER...ON THE HIGHER TERRAIN OF THE LOWLANDS...AND OF
COURSE IN HEAVIER SHOWERS...MAYBE IN A LOOSELY ORGANIZED PUGET SOUND
CONVERGENCE ZONE.

THE UPPER TROUGH WILL MOVE EAST OF THE CASCADES TONIGHT AND SHOWERS
WILL DECREASE...BUT THE COLD AIR MASS WILL STAY WITH US.

THE NEXT FRONTAL SYSTEM UPSTREAM...CURRENTLY DEVELOPING OUT NEAR
150W...WILL MOVE THROUGH THE FORECAST AREA WEDNESDAY. THIS SYSTEM IS
FAIRLY VIGOROUS FOR MID MARCH BUT IT WILL NOT BE AS STRONG...WITH
RESPECT TO WINDS AND MOUNTAIN SNOW...AS THE MONDAY SYSTEM. IT WILL
LIKELY BRING WIND ADVISORY LEVEL WINDS TO THE COASTAL AND NORTHWEST
INTERIOR ZONES. ONE CONCERN HOWEVER IS THAT THE AIR MASS WILL BE
COLD ENOUGH FOR PRECIPITATION TO START AS SNOW OVER THE
LOWLANDS...AND MINOR LOCAL ACCUMULATIONS ARE AGAIN POSSIBLE. THE
SNOW LEVEL WILL RISE TO 2000 TO 3000 FT LATE WEDNESDAY.

YET ANOTHER VIGOROUS FRONTAL SYSTEM...THIS ONE JUST BEGINNING TO
TAKE SHAPE JUST BEYOND THE DATELINE...WILL MOVE THROUGH WESTERN
WASHINGTON WEDNESDAY NIGHT AND THURSDAY. ITS POST-FRONTAL TROUGH
WILL MOVE INTO THE REGION THURSDAY NIGHT. THE WINDS WILL PROBABLY BE
SOMEWHAT LESS WITH THIS FRONT...BUT PRECIPITATION WILL BE HEAVIER
AND THE SNOW LEVEL WILL GRADUALLY RISE TO AROUND 4500 FT.
PRECIPITATION TOTALS IN THE MOUNTAINS FOR THE 48-HOUR PERIOD FROM
EARLY WEDNESDAY MORNING THROUGH LATE THURSDAY NIGHT -- WHICH
INCLUDES BOTH STORMS -- WILL BE IN THE 2 TO 5 INCH RANGE. I WILL
ISSUE A WINTER STORM WATCH FOR THIS.   MCDONNAL

.LONG TERM...FOR THE LAST FEW DAYS THE LONGER RANGE MODELS HAVE BEEN
IN ROUGH AGREEMENT THAT THERE WILL BE A TRANSITION FROM THE VERY
STORMY PATTERN THIS WEEK TO A COOL AND SHOWERY BUT DRIER PATTERN
OVER THE WEEKEND. THE RECENT RUNS SHOWS THIS TRANSITION OCCURRING A
BIT MORE SLOWLY...WITH FRIDAY NOW LOOKING MORE WET THAN PREVIOUSLY
EXPECTED AS A SHORTWAVE TROUGH MOVES THROUGH. THE CHANGE I HAVE MADE
IS TO BOOST POPS FOR FRIDAY TO LIKELY. IT STILL LOOKS LIKE A LARGER
UPPER TROUGH WILL MOVE OVER THE REGION THIS WEEKEND...AND IT WILL BE
COOL WITH MORE SCATTERED SHOWERS AND PROBABLY SOME SUN BREAKS.
MCDONNAL

&&

.HYDROLOGY...AS DISCUSSED ABOVE THE PRECIPITATION FROM THE WEDNESDAY
AND THURSDAY STORMS WILL BE SUBSTANTIAL...WITH GENERALLY 2 TO 5
INCHES IN THE MOUNTAINS DURING A 48-HOUR PERIOD. THE HEAVIEST
AMOUNTS WILL OCCUR OVER THE SOUTH SLOPES OF THE OLYMPICS...AND THE
MEAN AREAL PRECIPITATION OVER THE SKOKOMISH RIVER BASIN WILL LIKELY
EXCEED 3 INCHES. EVEN WITH RELATIVELY LOW SNOW LEVELS DURING MOST OF
THE EVENT...THIS COULD BE ENOUGH TO DRIVE THIS UNIQUELY FLOOD-PRONE
RIVER TO FLOOD STAGE ON THURSDAY.

FLOODING IS NOT EXPECTED ON ANY OTHER RIVER IN THE FORECAST AREA
DURING THE NEXT 7 DAYS.   MCDONNAL

&&

.AVIATION...A LARGE DEEP UPPER LEVEL TROF ALONG THE B.C. COAST WILL
MAINTAIN MOIST SW FLOW ALOFT OVER W WA TODAY. THE BAND OF
PRECIPITATION AHEAD OF THE TROF CONTAINS A MIX OF RAIN AND SNOW...
WITH SOME LOCATIONS REPORTING ALL SNOW. SOMETIME THIS MORNING THE
STEADY PRECIPITATION WILL CHANGE TO SHOWERS. SNOW SHOWERS CAN STILL
BE EXPECTED THROUGH THIS MORNING. THIS AFTERNOON SHOULD BE MAINLY
RAIN SHOWERS BUT FROZEN PRECIPITATION OF SOME SORT COULD BE MIXED
IN. ISOLATED THUNDERSTORMS ARE POSSIBLE. CIGS WILL VARY DEPENDING ON
PRECIP INTENSITY AND TYPE.

KSEA...CIGS WILL BE MAINLY IN THE MVFR RANGE...THOUGH PERIODS OF IFR
CIGS ARE POSSIBLE ESPECIALLY IN SNOW SHOWERS. STEADY WET SNOW WILL
PROBABLY FALL THROUGH 12Z. AFTER THAT IT SHOULD TURN MORE SHOWERY...
WITH SOME COMBINATION OF RAIN AND/OR SNOW. A HEAVY SHOWER COULD GIVE
UP TO 1 INCH OF SLUSHY SNOW. SHOWERS WILL STICK AROUND MOST OF THE
DAY. 13

&&

.MARINE...CURRENT WINDS AND FCST GRADIENTS ARE SUCH THAT SMALL CRAFT
ADVISORY WINDS ARE EXPECTED AT TIMES OVER ALL WATERS TODAY. SWELL IN
THE COASTAL WATERS WILL REMAIN ABOVE 10 FEET THROUGH TONIGHT.

ANOTHER WEAKER SYSTEM WILL SEND ABOUT A 980 MB LOW CURVING NE TOWARD
THE CENTRAL B.C. COAST TONIGHT AND WEDNESDAY...WHILE THE ASSOCIATED
COLD FRONT REACHES THE WA COAST EARLY WEDNESDAY MORNING. A STRONGER
970 MB LOW WILL CURVE SLIGHTLY FARTHER NORTH...TOWARD HAIDA GWAII
WEDNESDAY NIGHT/THURSDAY MORNING...WITH THAT COLD FRONT REACHING THE
WA COAST THURSDAY MORNING. AFTER A PERIOD OF MODERATE ONSHORE FLOW
FRIDAY...ANOTHER WEAKER LOW COULD REACH THE AREA SATURDAY. KAM/13

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...WINTER STORM WATCH FOR THE CASCADES AND OLYMPICS WEDNESDAY
     THROUGH THURSDAY.

PZ...SMALL CRAFT ADVISORY ALL WATERS.

     SMALL CRAFT ADVISORY FOR SEVERE/ROUGH GRAYS HARBOR BAR.

&&

YOU CAN SEE AN ILLUSTRATED VERSION OF THE FORECAST DISCUSSION AT
WWW.WEATHER.GOV/SEATTLE/GAFD/LATEST_WEBAFD.HTML.

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE SEATTLE WA
416 AM PDT TUE MAR 13 2012

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0413 AM     SNOW             E ISSAQUAH              47.53N 122.04W
03/13/2012  M7.5 INCH        KING               WA   NWS EMPLOYEE

            OVERNIGHT SNOWFALL TOTAL OBSERVED AT ELEVATION 1375 FT.

&&

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE SEATTLE WA
431 AM PDT TUE MAR 13 2012

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0431 AM     SNOW             4 WNW ENUMCLAW          47.23N 122.06W
03/13/2012  M4.0 INCH        KING               WA   TRAINED SPOTTER

&&