Winter Storm Watch 4, NWS Seattle, WA

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
945 AM PST MON JAN 23 2012...CORRECTED TYPO 4TH PARAGAPH

.SYNOPSIS...TODAY WILL BE MAINLY DRY IN BETWEEN WEATHER SYSTEMS. WET
WEATHER WILL RETURN TUESDAY THROUGH THURSDAY. HIGH PRESSURE WILL
GIVE DRIER WEATHER AGAIN ON FRIDAY. A COLD FRONT WILL ARRIVE
SATURDAY OR SATURDAY NIGHT.

&&

.SHORT TERM...LATEST RADAR LOOP INDICATES SOME SHOWER ACTIVITY
SKIRTING THE N COAST AND FAR N INTERIOR THIS MORNING. IR IMAGERY
SHOWS THE BRIGHTER CLOUD TOPS WITH HEAVIER SHOWERS MOVING N INTO
VANCOUVER ISLAND AND SRN B.C. WITH A FEW OF THESE WEAKER SHOWERS
PASSING THROUGH FAR NRN AREAS OF WRN WA. THE REST OF WRN WA IS
EXPERIENCING MOSTLY CLEAR SKIES LEADING SOME A DROP IN TEMPS TO NEAR
OR JUST ABOVE FREEZING AROUND WRN WA INCLUDING THE PUGET SOUND
REGION. THESE NEAR FREEZING TEMPERATURES WILL CAUSE ANY WET PAVEMENT
TO FORM ICE AND CAUSE HAZARDOUS TRAVEL THE NEXT COUPLE HOURS. A
SHORT TERM FORECAST WAS SENT TO ADDRESS THIS. WILL ALSO PLAN ON A
QUICK UPDATE TO KEEP THE MENTION OF ISOLATED SHOWERS UP NORTH TODAY
AND REMOVE THE REST OF THE PRECIP FROM CENTRAL/SRN AREAS.

THE NEXT VIGOROUS STORM SYSTEM WILL BEGIN TO IMPACT WRN WA STARTING
TONIGHT AND CONTINUING THROUGH WEDNESDAY NIGHT. EXPECT A COMBINATION
OF RAIN...STRONG WINDS...AND SNOW IN THE MOUNTAINS. SNOW LEVELS MAY
RISE ENOUGH SO THAT LOCALLY HEAVY RAIN FALLS IN THE OLYMPICS AND
PORTIONS OF THE CASCADES...ESPECIALLY SRN AND CENTRAL PORTIONS. SEE
HYDRO DISCUSSION BELOW.

MODELS APPEAR SLOWER THIS MORNING WITH THE ONSET OF PRECIPITATION
WITH THE NAM-12 AND NEW 12Z GFS SHOWING RAIN TO BEGIN ON THE COAST
AROUND MIDNIGHT AND SPREAD INLAND SLOWLY. SOME ADJUSTMENTS TO THE
FORECAST TIMING WILL HAVE TO BE MADE TODAY TO PUSH BACK THE ONSET OF
RAINFALL. THE WINDS WILL PICK UP ALONG THE COAST AFTER MIDNIGHT AND
EARLY EARLY TUESDAY MORNING FOR THE INTERIOR...INITIALLY FROM THE
ADMIRALTY INLET AREA NWD ACROSS THE NW INTERIOR. SE WINDS SHOULD
EASILY REACH WIND ADVISORY CRITERIA...SUSTAINED NEAR 30 MPH WITH
GUSTS TO 45-50 MPH. WINDS WILL PERSIST THROUGH THE DAY TUESDAY. THE
FRONT WILL PUSH THROUGH EARLY TUESDAY EVENING. MODELS INDICATE A
SHARP RISE IN PRESSURES AND A KPDX-KBLI GRADIENT NEAR +12MB. THIS
SUPPORTS WINDS ADVISORY WINDS TO EXPAND ACROSS THE REST OF THE
PUGET SOUND REGION AND SW INTERIOR.

MOUNTAIN SNOW WILL BE TRICKY DUE TO SNOW LEVEL ISSUES AND ALSO
MODELS SHIFTING THE HEAVIEST PRECIPITATION A BIT FURTHER S. IT LOOKS
LIKE THE N CASCADES HAVE A DELAY GETTING COLD AIR IN WITH NOT MUCH
SNOW LEVEL RISE EXPECTED THERE. WITH OVER 1 INCH LIQUID...I SUSPECT
I MAY NEED WINTER STORM WATCH OR WARNING FOR THE N CASCADES. NEED TO
LOOK AT THAT MORE THIS MORNING. THE CENTRAL CASCADES WILL PROBABLY
HAVE SNOW LEVELS RISE FASTER ABOVE 5-6K FT WHICH MAY PRECLUDE THE
NEED FOR NO MORE THAN AN ADVISORY. THE PASSES WILL PROBABLY HAVE
SOME E WIND DRAINAGE FOR THE A PORTION OF THE EVENT BEFORE GOING TO
RAIN.

RAIN TAPERS TO SHOWERS ACROSS THE LOWLANDS TUESDAY NIGHT AND
WEDNESDAY BUT SNOW WILL PERSIST IN THE MOUNTAINS. DECENT ONSHORE
FLOW AND COLDER AIR BEHIND THE FRONT SHOULD DROP SNOW LEVELS DOWN AT
LEAST THE MID/HIGHER PASSES. EXPECT ADDITIONAL SNOWFALL.

ANOTHER WEAK SURFACE LOW TRACKS THROUGH THE AREA LATE
WEDNESDAY/WEDNESDAY NIGHT WITH WILL BRING RAIN AND MORE MOUNTAIN
SNOW TO THE AREA. EXPECT A PERIOD OF DIFFICULT TRAVEL THROUGH THE
PASSES.  CONDITIONS SHOULD DRY OUT THURSDAY AS HIGHER PRESSURE
BUILDS OVER THE AREA.  MERCER

.LONG TERM...PREVIOUS DISCUSSION...A COLD FRONT WILL SAG THROUGH THE
AREA EARLY THURSDAY WITH RAIN CHANGING TO DECREASING SHOWERS. FRIDAY
IS LOOKING DRY WITH AN UPPER RIDGE OVER THE AREA. THE FORECAST STILL
HAS A CHANCE OF SHOWERS FOR FRIDAY. LATER SHIFTS MAY WANT TO DROP
THIS IF MODELS REMAIN CONSISTENT SHOWING THE UPPER RIDGE OVER THE
AREA. A COLD FRONT WILL BRING ANOTHER ROUND OF RAIN LATE SATURDAY.
LONG RANGE MODELS DIVERGE ON SUNDAY. THE GFS SHOWS A RIDGE
REBUILDING FOR DRY WEATHER. THE ECMWF IS MORE ZONAL AND WET.
SCHNEIDER

&&

.HYDROLOGY...PREVIOUS DISCUSSION....A WET PERIOD IS EXPECTED FOR
WESTERN WASHINGTON TUESDAY THROUGH EARLY THURSDAY. MOISTURE WITH
SUBTROPICAL ORIGINS WILL MOVE INTO THE AREA TUESDAY. WHILE DETAILS
DIFFER...MODELS GENERALLY AGREE THAT THERE WILL BE SOME LESSENING OF
THE PRECIPITATION LATE TUESDAY NIGHT AND WEDNESDAY MORNING THEN A
REINFORCING SHOT LATER WEDNESDAY BEFORE EVERYTHING TAPERS ON
THURSDAY. IT ALSO LOOKS LIKE THE HEAVIER RAINS WILL IMPACT THE
SOUTHERN PARTS OF THE FORECAST AREA AS THE BRUNT OF THE STORMS ARE
DIRECTED INTO NORTHERN OREGON. SNOW LEVELS OVER THE SOUTHERN HALF OF
THE FORECAST AREA WILL RISE TO 5000 OR 6000 FEET TUESDAY AFTERNOON
AND TUESDAY NIGHT AND THEN FALL TO BETWEEN 3000 AND 4000 FEET ON
WEDNESDAY.

AT THIS TIME IT APPEARS THE RAINS AND RISING SNOW LEVELS ON TUESDAY
AND WEDNESDAY WILL LEAD TO A THREAT OF MINOR RIVER FLOODING FOR SOME
OF THE RIVERS OF SOUTHWEST WASHINGTON WEDNESDAY OR THURSDAY...
PRIMARILY THE SKOKOMISH AND CHEHALIS RIVERS. OTHER RIVERS IN
SOUTHWEST WASHINGTON COULD SEE FLOODING IF THE RAINS ARE HEAVIER
THAN CURRENTLY EXPECTED. A FLOOD POTENTIAL OUTLOOK IS CURRENTLY IN
EFFECT TO ADDRESS THIS THREAT. SCHNEIDER

&&

.AVIATION...WLY FLOW ALOFT AND LOW LEVEL ONSHORE FLOW BUT THERE WERE
AREAS OF CLEARING OVERNIGHT AND IT LOOKS LIKE THERE WILL BE PARTLY
SUNNY SKIES TODAY. A WEAK SYSTEM AND SHOWERS WERE CLIPPING THE NW
COAST THIS MORNING. THE AIR WILL STABILIZE THROUGH TONIGHT AHEAD OF
THE WARM ADVECTION YOU CAN SEE OFFSHORE OUTSIDE OF 130W ASSOCIATED
WITH THE NEXT FRONTAL SYSTEM. SEE THE TAFS FOR DETAILS. 19

KSEA...THE PUGET SOUND AREA HAS BEN GETTING A DOWNSLOPE CLEARING
AREA BEHIND THE OLYMPICS WITH THE WLY FLOW ALOFT. AS THE AIR MASS
STABILIZES AND THE FLOW WEAKENS BKN CIGS MAY SET UP OVER THE COURSE
OF THE DAY BUT THE NEXT SYSTEM IS STILL OUTSIDE OF 130W THIS MORNING
AND SKIES THROUGH THIS EVENING SHOULD AVERAGE JUST PARTLY CLOUDY...A
NICE BREAK IN THE WEATHER THRU THIS EVENING. 19

&&

.MARINE...SEAS HAVE SUBSIDED AND THERE WILL BE A BREAK IN THE
WEATHER THROUGH THIS EVENING. THE NEXT SYSTEM IS ON TAP FOR TUESDAY
AND GALES SHOULD RETURN TO THE COASTAL WATERS OVERNIGHT
TONIGHT...AND PROBABLY THE NRN INLAND WATERS AS WELL. THE 12Z NAM
HAS A 972MB LOW CENTER AT 50N/140W WITH THE WARM FRONT MOVING INTO
THE WATERS LATE THIS EVENING...THE LOW DEEPENS TO 966MB AS IT TRACKS
JUST NORTH OF THE CHARLOTTES BY DAYBREAK TUE...SO THE GRADIENT IS
PRETTY STRETCHED OUT THRU THE B.C. AND WA WATERS BUT STILL
SUFFICIENT FOR GALES COASTAL WATERS AND EAST ENTRANCE TO THE STRAIT.
NO BIG CHANGES TO THE FCST THIS MORNING. 19

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...SMALL CRAFT ADVISORY COASTAL WATERS AND WESTERN STRAIT.
    .SMALL CRAFT ADVISORY FOR ROUGH BAR GRAYS HARBOR BAR.
    .GALE WATCH LATE TONIGHT THROUGH TUESDAY AFTERNOON COASTAL
     WATERS AND ENTRANCES TO THE STRAIT OF JUAN DE FUCA.

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