Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 232204
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
300 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...A 5590 METER UPPER LOW IS OVER THE WASHINGTON OFFSHORE
WATERS NEAR 47N/126W THIS AFTERNOON. WIDESPREAD SHOWERS HAVE BEEN
FALLING ACROSS WESTERN WASHINGTON. STORM TOTAL RAINFALL EXCEEDS AN
INCH IN SOME AREAS -- MAINLY OVER THE WEST PART OF THE PUGET SOUND
REGION...WEST SLOPES OF THE CASCADES...AND SOUTHWEST INTERIOR. THE
DAILY RAINFALL RECORD FOR JULY 23 HAS ALREADY BEEN BROKEN AT
SEATAC...BELLINGHAM...AND HOQUIAM. THE THUNDERSTORMS THAT DEVELOPED
EARLY THIS MORNING MOVED NORTHEAST...AND AT LEAST IN OUR FORECAST
AREA THEY HAVE BEEN LIMITED TO THE CASCADE CREST SINCE LATE MORNING.

THE UPPER LOW WILL CONTINUE TRACKING EASTWARD...MOVING INTO WESTERN
WASHINGTON THIS EVENING AND LIFTING INTO SOUTHERN BRITISH COLUMBIA
LATE TONIGHT. WIDESPREAD SHOWERS WILL CONTINUE...BUT RAINFALL RATES
WILL GENERALLY DECREASE OVERNIGHT. ANOTHER 0.25 TO 0.50 INCH IS
LIKELY OVER MUCH OF THE LOWLANDS...WITH 1 TO 2 INCH AMOUNTS COMMON
OVER THE MOUNTAINS. THE SNOW LEVEL WILL BE AROUND 7000 FT.

THERE IS A CHANCE OF THUNDERSTORMS THROUGH TONIGHT...AS THE AIR MASS
IS STILL MOIST AND UNSTABLE WITH DIFFLUENT SOUTHERLY FLOW ALOFT. IT
IS A BIT SURPRISING THAT MORE THUNDERSTORMS HAVE NOT DEVELOPED YET
OVER OUR AREA...WESTERN OREGON...OR OFFSHORE OF THE PACIFIC
NORTHWEST. THE AIR MASS SHOULD BE AT ITS MOST UNSTABLE THROUGH THIS
EVENING THOUGH...AND THE SMALL COMMA IN THE VICINITY OF THE UPPER
LOW ITSELF COULD HELP RENEW CONVECTIVE ACTIVITY.

THE FLOW ALOFT WILL VEER TO WESTERLY AGAIN THURSDAY AS THE UPPER LOW
TRACKS INTO SOUTHERN ALBERTA. THE AIR MASS WILL STABILIZE AND BEGIN
TO DRY...BUT MOIST ONSHORE LOW LEVEL FLOW WILL CONTINUE AND A PUGET
SOUND CONVERGENCE ZONE WILL PROBABLY DEVELOP. ON THE WHOLE SHOWERS
WILL DECREASE THURSDAY...WITH SOME SUNBREAKS LIKELY OVER THE
LOWLANDS.

WESTERLY FLOW ALOFT WILL CONTINUE OVER THE FORECAST AREA THURSDAY
NIGHT AND FRIDAY...BUT UPPER HEIGHTS WILL START TO RISE AS A RIDGE
BUILDS NORTHWARD ALONG THE ROCKY MOUNTAINS FROM THE FOUR CORNERS
HIGH. THERE WILL LIKELY BE SOME MARINE LAYER CLOUDS TO START THE
DAY...BUT THEY WILL GIVE WAY TO MOSTLY SUNNY SKIES AT LEAST OVER THE
INTERIOR.

THE UPPER RIDGE WILL BUILD FURTHER FRIDAY NIGHT AND SATURDAY...WITH
500 MB HEIGHTS CLIMBING TO AROUND 5820 METERS. ONSHORE FLOW WILL
WEAKEN. SATURDAY SHOULD BE WARMER AND SUNNY WITH SOME NIGHT AND
MARINE LAYER CLOUDS MAINLY ALONG THE COAST.   MCDONNAL

.LONG TERM...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...THERE HAVE BEEN 23 JULY DAYS WITH 0.50 INCHES OF RAIN OR
MORE SINCE RECORDS STARTED AT SEA-TAC IN 1945. ONLY 3 OF THOSE DAYS
HAVE BEEN SINCE 2000...0.65 INCHES ON JULY 16TH 2001....0.60 INCHES
ON JULY 20TH 2012 AND 0.58 INCHES ON JULY 7TH 2002. THE RECORD FOR
THE WETTEST JULY DAY AT SEA-TAC IS 0.85 INCHES ON JULY 13TH 1981.
THE WETTEST JULY DAY ON RECORD IN SEATTLE IS 0.98 INCHES ON JULY 5TH
1897 WHEN RECORDS WERE KEPT AT THE FEDERAL BUILDING DOWNTOWN. FELTON

&&

.AVIATION...AN UPPER LEVEL LOW JUST OFF THE COAST WILL MOVE INLAND
THIS EVENING THEN NORTHEAST TONIGHT. STRONG SOUTHERLY FLOW ALOFT
WILL BECOME LIGHT TONIGHT...THEN STRONG NORTHWEST LATE. AT THE
SURFACE MODERATE ONSHORE FLOW WILL DEVELOP TONIGHT AND CONTINUE
THURSDAY. AIR MASS UNSTABLE AND MOIST ALL LEVELS.

SATELLITE IMAGERY SHOWS SOME EASTWARD MOVEMENT OF THE MAIN WEATHER
SYSTEM AND THERE HAS BEEN SOME CLEARING ON THE SOUTH COAST. THIS
WILL CONTINUE INTO THE EVENING. SOME RETURN FLOW SHOWERS ARE LIKELY
ON THURSDAY ESPECIALLY OVER NORTHERN SECTIONS. CEILINGS HAVE BEEN
ALL OVER THE PLACE TODAY BUT CURRENTLY ARE MOSTLY VFR. SHOULD BE
MOSTLY VFR GOING FORWARD WITH OCCASIONAL EXCURSIONS TO MVFR IN
SHOWERS. GENERAL IMPROVING TREND WILL BEGIN ABOUT MID DAY THURSDAY.

KSEA...VFR CEILINGS EXCEPT MVFR IN SHOWERS. AN ISOLATED THUNDERSTORM
IS JUST BARELY POSSIBLE THIS EVENING. IMPROVING TREND AFTER ABOUT
20Z THURSDAY. SOUTHWEST WIND 10G20 KT TONIGHT AND THURSDAY. CHB

&&

.MARINE...A SURFACE TROUGH OFF THE COAST THIS AFTERNOON WILL WEAKEN
AND MOVE INLAND TONIGHT. IT SHOULD STILL HAVE ENOUGH PUNCH TO BRING
MODERATE ONSHORE FLOW TONIGHT RESULTING IN SMALL CRAFT ADVISORY
SOUTHWEST OR WEST WINDS IN MOST ZONES INCLUDING PUGET SOUND. ONSHORE
FLOW BECOMES STRONGER THURSDAY. FOR NOW HAVE INDICATED 20-30 KT
WESTERLIES IN THE STRAIT BUT GALES SEEM POSSIBLE THERE ON THURSDAY.

AFTER THURSDAY THE NORMAL SUMMER PATTERN RETURNS WITH ONSHORE FLOW
STRONG ENOUGH FOR SMALL CRAFT ADVISORY WESTERLIES IN THE STRAIT AT
TIMES. CHB

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...SMALL CRAFT ADVISORY ALL WATERS EXCEPT CENTRAL AND NORTH INNER
COAST.

&&

$$

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





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