Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 222154
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
300 PM PDT TUE JUL 22 2014

.SYNOPSIS...AN OFFSHORE UPPER LOW WILL PRODUCE SOME SHOWERS OVER
WESTERN WASHINGTON TONIGHT. THE LOW WILL BRING WIDESPREAD SHOWERY
PRECIPITATION AND A CHANCE OF THUNDERSTORMS AS IT MOVES INLAND
WEDNESDAY AND WEDNESDAY NIGHT...WITH MORE THAN A HALF INCH OF RAIN
IN AREAS. SHOWERS WILL TAPER OFF THURSDAY AS THE LOW MOVES EAST.
HIGH PRESSURE WILL BRING SUNNY WARMER WEATHER FRIDAY THROUGH MONDAY.

&&

.SHORT TERM...A 5560 METER UPPER LOW IS OFF THE COAST OF BRITISH
COLUMBIA NEAR 50N/131W AND DIGGING SLOWLY SOUTHEAST. A SHORTWAVE IN
THE SOUTHEAST QUADRANT OF THE LOW PRODUCED SHOWERS OVER THE SOUTH
PART OF THE WASHINGTON CASCADES THIS MORNING BEFORE MOVING
NORTHEAST. HOWEVER MID LEVEL MOISTURE AND INSTABILITY HAVE PRODUCED
A FEW MORE SHOWERS OVER THE WESTERN WASHINGTON INTERIOR LOWLANDS IN
THE AFTERNOON...AND THE PARTIAL CLEARING WE WERE EXPECTING HAS NOT
MATERIALIZED.

THE UPPER LOW WILL CONTINUE TRACKING SOUTHEAST TONIGHT...REACHING
THE WASHINGTON OFFSHORE WATERS LATE TONIGHT. THEN IT WILL SWING
EASTWARD TOWARD WASHINGTON ON WEDNESDAY AND MOVE INLAND WEDNESDAY
NIGHT. THIS FEATURE WILL BRING WIDESPREAD SHOWERY PRECIPITATION TO
THE FORECAST AREA -- ESPECIALLY WEDNESDAY AND WEDNESDAY NIGHT --
WITH MODELS IN BROAD AGREEMENT THAT MUCH OF THE WESTERN WASHINGTON
LOWLANDS WILL RECEIVE MORE THAN A HALF INCH RAIN...WITH UP TO 1 INCH
ALONG THE COAST...AND UP TO 2 INCHES IN THE MOUNTAINS. THE SNOW
LEVEL WILL MAINLY BE 8000 TO 9000 FT.

THERE WILL PROBABLY BE ISOLATED THUNDERSTORMS AS WELL. THE AIR MASS
WILL BECOME INCREASINGLY UNSTABLE...WITH MODERATELY STRONG AND
WEAKLY DIFFLUENT SOUTHERLY FLOW ALOFT. THE DIFFICULTY IS PINNING
DOWN THE TIMING AND AREA OF SPECIFIC THREAT. ISOLATED THUNDERSTORMS
DEVELOPING OVER THE OREGON CASCADES THIS AFTERNOON COULD SPREAD
NORTH TO OUR CASCADES AS EARLY AS THIS EVENING...AND THE
THUNDERSTORM THREAT WILL BECOME WIDESPREAD ESPECIALLY WEDNESDAY
AFTERNOON AND EVENING. A JET STREAK JUST TO OUR SOUTHEAST WEDNESDAY
AFTERNOON COULD ALSO ENHANCE THE CONVECTIVE ACTIVITY.

THE UPPER LOW WILL LIFT NORTHEAST ACROSS ALBERTA ON THURSDAY. SOME
SHOWERS WILL LINGER OVER WESTERN WASHINGTON THURSDAY...BUT MOST OF
THE LOWLANDS SHOULD BE DRY BY THURSDAY AFTERNOON WITH PARTLY SUNNY
SKIES.

AN UPPER RIDGE WILL START BUILDING NORTHWARD FROM THE FOUR CORNERS
HIGH AGAIN ON FRIDAY...FOR DRY AND WARMER WEATHER WITH MORE
SUNSHINE.   MCDONNAL

.LONG TERM...THE UPPER RIDGE WILL CONTINUE TO BUILD WITH ITS AXIS
ALONG THE ROCKY MOUNTAINS OVER THE WEEKEND. BY SUNDAY...THE 500 MB
HEIGHTS OVER WESTERN WASHINGTON WILL BE AROUND 5850 METERS...AND
HIGHS OVER MUCH OF THE WESTERN WASHINGTON LOWLANDS SHOULD BE WELL
INTO THE 80S. ONSHORE FLOW WILL BE WEAK...AND THE NIGHT AND MORNING
MARINE LAYER CLOUDS SHOULD BE LIMITED MOSTLY TO THE COAST. THE GFS
STARTS TO WEAKEN THE RIDGE WITH THE ARRIVAL OF AN UPPER TROUGH ALONG
THE BRITISH COLUMBIA COAST AROUND TUESDAY...IMPLYING A MARINE PUSH
FOR TUESDAY NIGHT. THE ECMWF LOOKS AT LEAST A DAY LATER WITH THAT.
MCDONNAL

&&

.CLIMATE...UNUSUAL RAIN EVENT FOR LATE JULY POSSIBLE ON WEDNESDAY.
THE RECORD RAINFALL FOR JULY 23RD AT SEATTLE IS 0.54 INCHES SET IN
1949. THE SECOND HIGHEST RAINFALL TOTAL FOR JULY 23RD IN SEATTLE
INCLUDING THE FEDERAL BUILDING RECORDS WHICH GO BACK TO 1891 IS 0.06
INCHES IN 1959...1952 AND 1912. THERE IS THE POTENTIAL ON WEDNESDAY
FOR THE DAILY PRECIPITATION RECORD IN SEATTLE...AS WELL AS OTHER
LOCATIONS IN WESTERN WASHINGTON...TO BE BROKEN. FELTON

&&

.AVIATION...UPPER LEVEL TROUGH OFFSHORE WITH SOUTHWESTERLY FLOW
ALOFT. AIR MASS WILL BE STABLE OVER MUCH OF WESTERN
WASHINGTON...EXCEPT FOR THE SOUTHEASTERN CORNER OF THE CWA WHICH MAY
SEE SOME WEAK INSTABILITY THIS AFTERNOON AND EVENING.

MID AND HIGH LEVEL CLOUDS CURRENTLY COVERING MUCH OF THE STATE WITH
MANY TERMINALS REPORTING BKN TO OVC. SOME AREAS ALONG THE COAST LOOK
TO QUALIFY MORE AS SCT TO BKN AS THERE APPEAR TO BE SOME BREAKS IN
THE CLOUDS. SHOWERS ASSOCIATED WITH THE APPROACHING LOW HAVE BEEN
PRETTY SCATTERED...WITH CURRENT RADAR SHOWING ACTIVITY NORTH AND
EAST OF MOST TERMINAL SITES. STILL NOT EXPECTING THIS ACTIVITY TO
BECOME MORE WIDESPREAD UNTIL TOMORROW MORNING. CIGS UNDER RAINFALL
HAVE BEEN FALLING TO DOWN AROUND 3000 TO 4000 FT...OTHERWISE CIGS
NOT EXPECTED TO FALL OUT OF VFR UNTIL TOMORROW MORNING WITH MAIN
SHOWER THREAT.

KSEA...MOSTLY MID LEVEL CLOUDS THIS AFTERNOON AND OVERNIGHT WITH
CIGS EXPECTED TO REMAIN VFR. CIGS EXPECTED TO FALL INTO MVFR LATE
TOMORROW MORNING ALONG WITH MAIN PRECIP THREAT. LIGHT AND VARIABLE
WINDS EXPECTED TO TURN NORTHWESTERLY 4 TO 8 KNOTS THIS EVENING
BEFORE RETURNING TO LIGHT AND VARIABLE OVERNIGHT. SMR

&&

.MARINE...A WEAK 1015 MB SURFACE LOW WILL MOVE INTO THE COASTAL
WATERS TONIGHT AND REMAIN THROUGH WEDNESDAY MORNING. THIS PATTERN
WILL CREATE LIGHT PRESSURE GRADIENTS OVER THE WATERS TODAY THROUGH
EARLY WEDNESDAY. PRESSURE GRADIENTS INCREASING MID DAY WEDNESDAY
INTO WEDNESDAY NIGHT WITH SMALL CRAFT ADVISORY WINDS DEVELOPING OVER
MOST OF THE WATERS WEDNESDAY NIGHT OR THURSDAY THEN EASING THURSDAY
NIGHT. THE ONSHORE FLOW PATTERN WILL RETURN FRIDAY AND INTO THE
WEEKEND WITH SMALL CRAFT ADVISORY WESTERLIES IN THE CENTRAL AND
EASTERN STRAIT LATE IN THE DAY. FELTON/SMR

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...NONE.

&&

$$

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





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