Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 240353
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
855 PM PDT WED JUL 23 2014

.SYNOPSIS...AN UPPER LOW WILL PRODUCE SHOWERS AND ISOLATED
THUNDERSTORMS ACROSS WESTERN WASHINGTON TONIGHT AS IT MOVES INLAND
FROM THE OFFSHORE WATERS. SHOWERS WILL DECREASE THURSDAY AS THE LOW
MOVES NORTHEAST INTO ALBERTA. HIGH PRESSURE WILL BUILD OVER THE
REGION ON FRIDAY...BRINGING SUNNY WARMER WEATHER THROUGH THE MIDDLE
OF NEXT WEEK.

&&

.SHORT TERM...AN UPPER LOW IS CURRENTLY MOVING INTO WESTERN
WASHINGTON. WIDESPREAD SHOWERS AHEAD OF THIS UPPER LOW FROM EARLIER
TODAY HAVE MOVED OFF TO THE NORTHEAST WITH ISOLATED TO SCATTERED
SHOWERS OVER THE AREA THIS EVENING. THE EXCEPTION IS OVER THE SOUTH
PART OF THE FORECAST AREA WHERE A MORE ORGANIZED BATCH OF SHOWERS
SOUTH OF THE UPPER LOW IS CURRENTLY MOVING ONSHORE. THE THUNDER
THREAT THIS EVENING WILL PROBABLY BE CONFINED TO THE SOUTHERN HALF
OF THE FORECAST AREA AND OVER THE CASCADES WHERE THERE WILL BE ADDED
LIFT AND THE AIR WILL BE MOST UNSTABLE. THERE COULD ALSO BE AN
INCREASE IN THE SHOWER ACTIVITY FOR A FEW HOURS THURSDAY MORNING AS
THE LOW MOVES EAST AND WRAP AROUND MOISTURE AFFECTS THE AREA.

THE UPPER LOW WILL MOVE INLAND LATER TONIGHT AND THURSDAY. SHOWERS
WILL TAPER AS THE LOW PULLS AWAY FROM THE AREA AND BECOME MOSTLY
CONFINED TO THE CASCADES BY AFTERNOON. AN UPPER RIDGE WILL SLOWLY
BUILD OVER THE PACIFIC NORTHWEST FRIDAY AND SATURDAY WHICH WILL
BRING A DRYING AND WARMING TREND. SCHNEIDER

.LONG TERM...PREVIOUS DISCUSSION...THE UPPER RIDGE WILL BUILD OVER
THE ROCKY MOUNTAINS THROUGH AT LEAST MONDAY...WITH 500 MB HEIGHTS
OVER WESTERN WASHINGTON CLIMBING TO AROUND 5880 METERS. SURFACE HIGH
PRESSURE OFFSHORE AND THERMALLY-INDUCED LOW PRESSURE EAST OF THE
CASCADES WILL MAINTAIN VERY WEAK ONSHORE FLOW. SOME MODEL RUNS --
MAINLY OF THE GFS -- HAVE SUGGESTED THAT THE TROUGH WILL WEAKEN WITH
A MARINE PUSH AROUND TUESDAY NIGHT. HOWEVER THE MOST RECENT RUNS
MAINTAIN THE UPPER RIDGE THROUGH MIDWEEK. BOTTOM LINE IS THIS WILL
BE A STRETCH OF SUNNY AND WARM WEATHER.   MCDONNAL

&&

.CLIMATE...0.74 INCHES OF RAIN HAS FALLEN SO FAR TODAY AT SEATAC
WHICH IS A RECORD FOR THE DATE. AS IT STANDS RIGHT NOW...THIS IS THE
FIFTH WETTEST JULY DAY AT SEATAC. THE ALL-TIME WETTEST DAY IN JULY
IS 0.85 INCHES SET ON JULY 13 1981. WITH MORE SHOWERS POSSIBLE...
THIS RECORD COULD STILL FALL BEFORE MIDNIGHT PST TONIGHT (1 AM PDT).
RECORDS AT SEATAC GO BACK TO 1945. THE ALL-TIME RECORD WETTEST JULY
DAY AT THE FEDERAL BUILDING IN DOWNTOWN SEATTLE IS 0.98 INCHES SET
ON JULY 5TH 1897. SCHNEIDER

&&

.AVIATION...THE UPPER LEVEL LOW OVER THE COASTAL WATERS WILL
CONTINUE TO MOVE INLAND THIS EVENING AND LIFT NORTH AND EAST INTO
EASTERN BRITISH COLUMBIA BY EARLY THURSDAY MORNING. THE AIR MASS
OVER WESTERN WASHINGTON REMAINS UNSTABLE AND MOIST...SUPPORTING
SHOWERS AND THUNDERSTORMS. EXPECT AN ADDITIONAL BOOST TO THAT
INSTABILITY AS THE UPPER LEVEL LOW MOVES INLAND LATER THIS EVENING.
CONDITIONS WILL GENERALLY REMAIN VFR TONIGHT WITH PERIODS OF MVFR
CONDITIONS ASSOCIATED WITH SHOWERS AS THE UPPER LEVEL LOW TRACKS
THROUGH THE AREA.

STRONG SOUTHERLY FLOW ALOFT WILL BECOME LIGHT TONIGHT AS THE UPPER
LEVEL LOW MOVES INLAND AND THEN STRONG NORTHWEST LATE AS IT LIFTS TO
THE NORTHEAST. AT LOW LEVELS...DEVELOPING MODERATE ONSHORE FLOW WILL
PERSIST AND STRENGTHEN THURSDAY. SOME RETURN FLOW SHOWERS ARE LIKELY
ON THURSDAY ESPECIALLY THE OVER NORTH WITH A GENERAL TREND TOWARDS
IMPROVEMENT DURING THE AFTERNOON.

KSEA...VFR CONDITIONS WILL PERSIST TONIGHT...EXCEPT MVFR IN SHOWERS.
ISOLATED THUNDERSTORMS REMAIN POSSIBLE THIS EVENING AS THE UPPER
LEVEL LOW MOVES THROUGH THE AREA. EXPECT SHOWERS TO LINGER THURSDAY
WITH AN IMPROVING TREND AFTER ABOUT 20Z. SOUTHWEST WINDS 10 KT
GUSTING TO 20 KT TONIGHT THROUGH THURSDAY MORNING.

&&

.MARINE...THE SURFACE LOW OVER THE COASTAL WATERS THIS EVENING WILL
WEAKEN AND MOVE INLAND. THE RESULTING MODERATE ONSHORE FLOW IS
PRODUCING SOUTHWEST TO WESTERLY WINDS RESULTING IN SMALL CRAFT
ADVISORY CONDITIONS FOR ALL WATERS INCLUDING PUGET SOUND. EXPECT THE
ONSHORE FLOW TO STRENGTHEN THURSDAY WITH 20-30 KT WESTERLIES LIKELY
IN THE STRAIT.

A MORE TYPICAL ONSHORE FLOW PATTERN WILL PERSIST THROUGH THE WEEKEND
WITH SMALL CRAFT ADVISORY WESTERLIES IN THE STRAIT AT
TIMES...ESPECIALLY DURING THE AFTERNOON AND EVENING HOURS.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...SMALL CRAFT ADVISORY ALL WATERS.

&&

$$

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







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