Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 211740
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
940 AM PST WED JAN 21 2015

.SYNOPSIS...AN UPPER LEVEL RIDGE OVER WESTERN WASHINGTON THIS
MORNING WILL DEPART INTO EASTERN WASHINGTON THIS AFTERNOON. AN
APPROACHING WARM FRONT WILL CAUSE CLOUDS TO INCREASE TODAY...WITH
RAIN SPREADING ONTO THE COAST AND INLAND STARTING EARLY THIS
EVENING. ADDITIONAL FRONTS WILL MAINTAIN A WET PATTERN THROUGH
SATURDAY. AN UPPER LEVEL RIDGE WILL BRING MORE SUNSHINE AND MILD
WEATHER SUNDAY AND MONDAY.

&&

.SHORT TERM...HAVE UPDATED THE FORECASTS THIS MORNING TO SPEED UP
THE ARRIVAL OF RAIN...WITH RAIN REACHING THE COAST AROUND SUNSET
THEN SPREADING INLAND OVER THE COURSE OF TONIGHT.

AN UPPER LEVEL RIDGE AXIS OVER WRN WA THIS MORNING IS CURRENTLY
MOVING INTO ERN WA...BRINGING A CHANGE TO MOISTER SW FLOW ALOFT OVER
THE FCST AREA. HIGH CLOUDS ARE ALREADY STARTING TO INCREASE...AND
RADAR ECHOES ARE ALREADY SHOWING UP WELL OFF THE WA COAST THIS
MORNING. THE 12Z KUIL SOUNDING SHOWED A DRY AIR MASS BELOW 700
MB...BUT THIS WILL MOISTEN UP AS DEEPER MOISTURE OFF THE COAST
SPREADS IN...AND AS WARM ADVECTION LIFT TAKES ITS TOLL. THE 12Z GFS
SHOWS RAIN REACHING THE COAST AS EARLY AS 4 PM TODAY...AND THE 15Z
HRRR IS SIMILAR. FELT THE NEED TO UPDATE THE FORECAST TO SPEED UP
THE INITIAL ARRIVAL OF RAIN BY ABOUT 4-6 HOURS FROM THE PREVIOUS
FORECAST. RAIN WILL EVER-SO-SLOWLY SPREAD INLAND THIS EVENING...THEN
ACCELERATE INLAND OVERNIGHT.

THURSDAY WILL FEATURE SLOPPY OVERRUNNING RAINS...BROUGHT ABOUT BY A
WEAK STRETCHED-OUT SHORTWAVE AND WARM ADVECTION LIFT. THE 12Z GFS
AND 12Z NAM NOW SHOW A RELATIVE LULL OR LIGHTENING OF THE RAINFALL
ON THU AFTN...AS THE EXIT OF THE SHORTWAVE LESSENS THE AMOUNT OF
LIFT. ANOTHER WEAK SHORTWAVE WILL MOVE ACROSS DURING THE DAY ON
FRI...KEEPING LIGHT TO MODERATE RAIN IN THE FORECAST. SNOW LEVELS
WILL BE QUITE HIGH OVER THE NEXT COUPLE OF DAYS...RANGING FROM
4000-6000 FEET ON THURSDAY...THEN RISING EVEN MORE ON FRI INTO THE
7000-8000 FT RANGE.     HANER

REMAINDER FROM PREVIOUS DISCUSSION...ANOTHER WARM FRONT WILL CLIP
WESTERN WA LATE THURSDAY NIGHT AND FRIDAY FOR MORE RAIN...MAINLY
ALONG THE COAST AND IN THE MOUNTAINS. SNOW LEVELS WILL REMAIN HIGH
WITH THIS SYSTEM...6000-7000 FT. THE COLD FRONT WILL STALL OVER
SOUTHERN B.C. FRI NIGHT THEN SLIDE S/SE INTO WESTERN WA ON SAT.
STRONG WESTERLY FLOW ALOFT WILL FOCUS HEAVY PRECIP ALONG THE WEST
SLOPES OF THE OLYMPICS AND CASCADES AND MODELS SHOW 1-3 INCHES OF
RAIN WITH THIS EVENT. SNOW LEVELS ARE STILL VERY HIGH DURING THIS
PERIOD. SHARP RISES ARE EXPECTED ON THE RIVERS FLOWING OFF THE
OLYMPICS AND NORTH CASCADES BUT FLOODING IS NOT EXPECTED WITH THESE
AMOUNTS. PRECIP WILL START TO TAPER OFF SAT AFTERNOON AND EVENING AS
AN UPPER LEVEL RIDGE REBUILDS OVER THE PAC NW. 33

.LONG TERM...FROM PREVIOUS DISCUSSION...THE LATEST GFS AND ECMWF
SOLUTIONS DEFLECT ALL MOISTURE AWAY FROM WESTERN WA EARLY NEXT WEEK
AS A STRONG UPPER LEVEL RIDGE BUILDS OVER THE PAC NW. SO LOOK FOR
DRIER WEATHER AHEAD...ALTHOUGH WILL STILL SEE MORNING LOW CLOUDS AND
FOG. A WEAK SYSTEM MAY BRING A FEW LIGHT SHOWERS BY WED AS THE RIDGE
SHIFTS INLAND. 33

&&

.HYDROLOGY...FLOODING IS UNLIKELY OVER THE NEXT SEVEN DAYS.

&&

.AVIATION...AN UPPER RIDGE OVER THE REGION WILL WEAKEN AND MOVE E.
NLY FLOW ALOFT WILL BECOME WLY. CONTD LOW LEVEL OFFSHORE FLOW.
LOCALIZED VSBYS AND CIGS BLO 1SM AND 500 FT...RESPECTIVELY...WILL
DISSIPATE BY 2000 UTC...2100 UTC AT THE LATEST. EXPECT -RA TO
GRADUALLY OVERSPREAD THE AREA FROM THE W BEGINNING LATER THIS
AFTERNOON.

KSEA...SHALLOW FOG PATCHES SHOULD NOT HAVE AN IMPACT ON THE
TERMINAL THIS MORNING. VFR CONDITIONS ARE ANTICIPATED TO PERSIST.

&&

.MARINE...
PERSISTENT LOW PRES OFFSHORE AND HIGHER PRES E OF THE CASCADES
WILL RESULT IN E OR SE FLOW THROUGH THU. THE FLOW WILL BECOME
SOUTHERLY OR ONSHORE ON FRI. MEANWHILE...A WEAK COLD FRONT WELL
OFFSHORE WILL DISSIPATE AS IT MOVES ACROSS THE COASTAL WATERS
TONIGHT. A SLIGHTLY STRONGER FRONTAL SYSTEM WILL AFFECT THE AREA
ON FRI.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...SMALL CRAFT ADVISORY IN EFFECT TIL NOON TODAY FOR THE
     NEARSHORE WATERS FROM CAPE FLATTERY TO JAMES ISLAND AND THE
     WEST ENTRANCE TO THE STRAIT OF JUAN DE FUCA.

&&

$$

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YOU CAN SEE AN ILLUSTRATED VERSION OF THIS FORECAST DISCUSSION AT
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