Gale Warning 82, NWS Seattle, WA

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
1015 AM PST THU DEC 19 2013

.SYNOPSIS...COOL AND DRY NORTHERLY FLOW WILL PREVAIL OVER WESTERN
WASHINGTON TODAY...BUT HIGH CLOUDS WILL INCREASE THIS MORNING. A
WARM FRONT WILL SPREAD PRECIPITATION ACROSS THE AREA LATE TONIGHT
THROUGH FRIDAY. PRECIPITATION WILL LIKELY BEGIN AS SNOW WITH UP TO 3
INCHES ACCUMULATION POSSIBLE...BEFORE CHANGING TO RAIN SOMETIME
DURING THE MORNING. AFTER A MINOR LULL IN-BETWEEN SYSTEMS FRIDAY
NIGHT AND SATURDAY...ANOTHER WEAK WARM FRONT WILL BRING A LITTLE
RAIN ON SUNDAY...FOLLOWED BY A COLD FRONT ON MONDAY.

&&

.SHORT TERM...THE LARGE UPPER LEVEL RIDGE CENTERED OUT NEAR 140W HAS
PUT W WA UNDER COOL DRY NW FLOW ALOFT TODAY. EARLIER MODELS HAD
INDICATED A MOSTLY SUNNY DAY TODAY...BUT HIGH CLOUDS RACING SE AHEAD
OF AN APPROACHING WARM FRONT HAVE TROMPED ON THAT IDEA...AND WE WILL
HAVE LOTS OF HIGH CLOUDS OVER THE AREA TODAY WITH PARTLY TO MOSTLY
CLOUDS SKIES. AFTER MORNING LOWS IN THE MID 20S TO LOWER 30S THIS
MORNING...HIGHS WILL ONLY TOP OUT IN THE UPPER 30S TO NEAR 40 THIS
AFTERNOON.

THE BIG FORECAST PROBLEM IS STILL THE SNOW PROBLEM LATE TONIGHT AND
FRIDAY MORNING. THERE IS NO QUESTION THAT A WARM FRONT EMBEDDED IN
THE NW FLOW ALOFT WILL SPREAD PRECIPITATION ACROSS W WA LATER
TONIGHT THROUGH FRIDAY. CONFIDENCE IS HIGH THAT THE PRECIPITATION
WILL BEGIN ON THE N COAST AROUND OR A LITTLE BEFORE MIDNIGHT
TONIGHT...SPREADING EVENTUALLY OVER THE PUGET SOUND REGION AND THE
SW INTERIOR BETWEEN 1 AM AND 4 AM.

EVERYTHING ELSE RELATED TO SNOW VERSUS RAIN...SNOW
ACCUMULATIONS...AND START AND END TIMES...REMAINS IN THE HOLY MOLY
ZONE OF UNCERTAINTY. THE PREVIOUS AFD DISCUSSION LAID OUT THE PROS
AND CONS QUITE NICELY.

FACTORS FOR...THE COOL AND ESPECIALLY DRY AIR MASS IN PLACE OVER THE
AREA FAVOR PRECIPITATION BEGINNING AS SNOW. SURFACE DEW POINTS AND
SOUNDINGS SHOW A SIGNIFICANT LOW LEVEL LAYER OF DRY AIR THAT WILL
SUPPORT EVAPORATIVE COOLING OF THE AIR MASS AS PRECIPITATION FALLS
INTO IT TONIGHT. 1000MB-850MB THICKNESS VALUES BELOW 1300 METERS
INDICATE IT IS COLD ENOUGH FOR SNOW INITIALLY.

FACTORS AGAINST...EVEN THOUGH COOL AND DRY...THE AIR MASS REALLY IS
NOT THAT COLD. THAT MAKES THE POTENTIAL FOR SNOW EXTREMELY RELIANT
ON EVAPORATIVE COOLING TO KEEP THE AIR COLD ENOUGH...AND THIS IN
TURN IS DEPENDENT UPON PRECIPITATION RATES. LOW PRECIP RATES MEAN
LESS COOLING EFFECT AND THUS LESS SNOW ACCUMULATION. ALL MODELS SHOW
SOUTHERLY LOW LEVEL WINDS DEVELOPING AROUND MIDNIGHT OR A LITTLE
LATER AND THE WARMING CAUSED BY S FLOW IN MANY CASES CAN TAKE A BIG
BITE OUT OF SNOW AMOUNTS. NONE OF THE STATISTICAL GUIDANCE SHOW ANY
SNOW ACCUMULATION FOR THE AREA. THERE WILL BE SOME SHADOWING BY THE
OLYMPICS AND S VANCOUVER ISLAND IN THE NW FLOW ALOFT THAT SHOULD
DEPRESS PRECIP ACCUMULATIONS OVER THE CENTRAL AND S PUGET SOUND...SW
INTERIOR AND BELLINGHAM AREA.

THATS A BIG PARAGRAPH AGAINST AND A SMALL PARAGRAPH FOR. LOOKING AT
THE 12Z NAM AND 06Z ARW MODELS IT STILL LOOKS LIKE THE PRECIP WILL
START OUT AS SNOW...BUT ACCUMULATIONS WILL BE LESS AND THE
CHANGEOVER TO RAIN WILL OCCUR SOONER. POTENTIAL
ACCUMULATIONS...STILL SUBJECT TO CHANGE...WOULD BE ZERO TO 2 INCHES
FOR THE COAST...STRAIT...SW INTERIOR AND TACOMA AREA...ALL LEANING
CLOSER TO 0-1 THAN 2. THE SEATTLE AREA OVER TO HOOD CANAL ALSO LOOKS
LIKE THE 0-2 RANGE...BUT LEANING CLOSER TO 1 TO 2 INCHES. FROM
EVERETT UP TO BELLINGHAM AMOUNTS MAY BE IN THE 1-3 INCH RANGE WITH
THE SNOW HANGING ON LATER IN THE MORNING. ALL IS STILL UNCERTAIN.
SAY IT AGAIN.

THE RAIN SHOULD FINALLY TAPER OFF BY EARLY FRIDAY EVENING AFTER THE
TRAILING COLD FRONT MOVES SE ACROSS THE AREA.

SNOW AMOUNTS FOR THE CASCADES WILL HAVE TO BE CONSIDERED AS PRECIP
MAY BE HEAVY ENOUGH FOR A FOOT OR MORE OF SNOW THROUGH FRIDAY
AFTERNOON. RISING SNOW LEVELS MAY ALSO GENERATE SOME FREEZING RAIN
IN THE PASSES. SNOW LEVELS WILL RISE ABOVE THE LOWER CENTRAL CASCADE
PASSES BY EARLY EVENING.

A RELATIVE LULL IN THE WEATHER WILL OCCUR FRIDAY NIGHT AND SATURDAY
BEFORE THEN NEXT WEAKER WARM FRONT REACHES THE ARE LATE SATURDAY
NIGHT AND SUNDAY. KAM

.LONG TERM...PREVIOUS DISCUSSION FROM THE 3 AM AFD...A WARM FRONT
WILL BRING ANOTHER BATCH OF RAIN TO THE AREA SATURDAY NIGHT AND
SUNDAY...FOLLOWED BY A COLD FRONT MONDAY. DRY NORTHERLY FLOW ALOFT
WILL DEVELOP TUESDAY AND WEDNESDAY FOR MAINLY DRY
WEATHER...ESPECIALLY BY CHRISTMAS DAY. SCHNEIDER

&&

.HYDROLOGY...RIVER FLOODING IS NOT EXPECTED FOR THE NEXT 7 DAYS.

&&

.AVIATION...LIGHT NORTHWEST FLOW ALOFT TODAY WITH MID AND HIGH LEVEL
CLOUDS. A WARM FRONT WILL OVERRUN COLD AIR OVER WRN WA AFTER
MIDNIGHT. STRONG WESTERLY FLOW ALOFT BEHIND THE FRONT FRIDAY.
BECOMING MOIST AT ALL LEVELS AFTER MIDNIGHT THROUGH FRIDAY. STABLE
CONDITIONS. PRECIPITATION WILL BEGIN AS SNOW...FIRST ALONG THE
COAST...THEN SPREADING INLAND DURING THE PRE-DAWN HOURS FRIDAY.
ACCUMULATIONS UP TO 1 INCH EXPECTED ON THE COAST/STRAIT OF JUAN DE
FUCA/SW INTERIOR/TACOMA. FROM KSEA/KBFI NORTHWARD...1 TO 3 INCHES
ARE EXPECTED. KPAE MAY SEE 4 INCHES. SNOW WITH MVFR OR IFR CIGS/VIS
CHANGES TO RAIN BY LATE FRIDAY AFTERNOON THROUGHOUT THE AREA WITH
MVFR CONDITIONS STILL DOMINATING.

KSEA....VARIABLE OR S/SE WIND 5 KT OR LESS. MVFR AND OCCASIONAL IFR
CONDITIONS ARE EXPECTED AFTER 9Z FRIDAY NIGHT WITH LIGHT STEADY SNOW
DEVELOPING. SNOW ACCUMULATIONS OF 1 TO 3 INCHES ARE EXPECTED WITH A
COUPLE HOURS OF SLEET POSSIBLE PRIOR TO THE CHANGE OVER TO RAIN
AROUND 16-17Z (8-9 AM PST). RAIN WITH MVFR CONDITIONS THE REST OF
THE DAY.

&&

.MARINE...NORTHERLY FLOW WILL WEAKEN TODAY WITH DECREASING FLOW.
A PACIFIC FRONTAL SYSTEM WILL MOVE THROUGH THE FORECAST AREA FROM
THE NORTHWEST ON FRIDAY. IT SHOULD PRODUCE SMALL CRAFT ADVISORY
SOUTHERLY WINDS LATE TONIGHT THROUGH MIDDAY FRIDAY OVER ALL MARINE
ZONES EXCEPT THE CENTRAL STRAIT OF JUAN DE FUCA. IN ADDITION A GALE
WATCH HAS BEEN ISSUED FOR THE FOUR NORTHERN MOST COASTAL WATERS
ZONES.

POST-FRONTAL SMALL CRAFT ADVISORY WESTERLIES WILL DEVELOP LATE
FRIDAY AND CONTINUE FRIDAY NIGHT. WESTERLY GALES ARE POSSIBLE IN THE
CENTRAL AND EASTERN STRAIT OF JUAN DE FUCA.

A WEAK WARM FRONT WILL ARRIVE SUNDAY...FOLLOWED BY A WEAK COLD FRONT
ON MONDAY. DTM/MCDONNAL

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...WINTER STORM WATCH FOR MOST OF THE INTERIOR OF WESTERN
     WASHINGTON LATE TONIGHT AND FRIDAY MORNING.
PZ...GALE WATCH LATE TONIGHT AND FRIDAY MORNING FOUR NORTHERN
      MOST COASTAL WATERS ZONES.

&&

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