Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 221603
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
900 AM PDT TUE JUL 22 2014

.SYNOPSIS...AN OFFSHORE UPPER TROUGH APPROACHING WESTERN WASHINGTON
WILL PRODUCE SOME SHOWERS TODAY AND TONIGHT...MAINLY OVER THE
CASCADES. THE TROUGH WILL MOVE INLAND WEDNESDAY AND WEDNESDAY NIGHT
FOR WIDESPREAD RAIN AND A CHANCE OF THUNDERSTORMS. SHOWERS WILL
TAPER OFF THURSDAY AS THE TROUGH MOVES EAST. HIGH PRESSURE WILL
BUILD OVER THE AREA FRIDAY THROUGH MONDAY BRINGING SUNNY WEATHER.

&&

.SHORT TERM...A 5540 METER UPPER LOW IS OFF THE COAST OF BRITISH
COLUMBIA NEAR 51N/132W AND DIGGING SOUTHEAST. A SHORTWAVE IN THE
SOUTHEAST QUADRANT OF THE LOW IS CAUSING A BAND OF SHOWERS TO STREAM
NORTHEAST ACROSS THE PACIFIC NORTHWEST...WITH SHOWERS OVER THE
WASHINGTON CASCADES TO AROUND STEVENS PASS. THE WESTERN WASHINGTON
INTERIOR LOWLANDS IS WEST OF THE SHOWERS...BUT COVERED WITH MID AND
HIGH CLOUDS.

THE UPPER LOW WILL CONTINUE TRACKING SOUTHEAST TODAY AND TONIGHT...
MOVING INTO THE WASHINGTON OFFSHORE WATERS LATE TONIGHT. THE
SHORTWAVE PRODUCING THE SHOWERS WILL MOVE NORTHEAST QUICKLY
THOUGH...AND IN THE SHORT TERM WE SHOULD HAVE SOME CLEARING OVER THE
INTERIOR LOWLANDS TODAY. ON THE OTHER HAND THE AIR MASS WILL
CONTINUE TO DESTABILIZE...AND SHOWERS COULD DEVELOP THIS AFTERNOON
MAINLY AT THE COAST AND IN THE MOUNTAINS.

THE UPPER LOW WILL SWING EASTWARD TOWARD WASHINGTON ON WEDNESDAY AND
MOVE INLAND WEDNESDAY NIGHT. THIS FEATURE WILL BRING WIDESPREAD
SHOWERY PRECIPITATION TO THE FORECAST AREA...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...ESPECIALLY
WEDNESDAY AFTERNOON AND EVENING...AS THE AIR MASS DESTABILIZES
FURTHER. WE WILL ALSO HAVE MODERATELY STRONG AND WEAKLY DIFFLUENT
SOUTHERLY FLOW ALOFT...WITH A JET STREAK JUST TO OUR SOUTHEAST
WEDNESDAY AFTERNOON AS IT ROTATES THROUGH THE BASE OF THE UPPER LOW.

THE UPPER LOW WILL LIFT NORTHEAST ACROSS ALBERTA ON THURSDAY. SOME
SHOWERS WILL LINGER OVER WESTERN WASHINGTON THURSDAY -- OR WRAP
AROUND THE NORTHWEST SIDE OF THE UPPER LOW INTO THE FORECAST AREA.
BUT MOST OF THE LOWLANDS SHOULD BE DRY WITH PARTLY SUNNY SKIES IN
THE AFTERNOON.   MCDONNAL

.LONG TERM...HERE IS THE LONG TERM SECTION FROM THE EARLY MORNING
FORECAST DISCUSSION -- THE LONG RANGE FORECAST LOOKS DRY WITH A
WARMING TREND STILL IN THE CARDS. THE GFS/ECMWF BOTH SHOW GRADUALLY
RISING UPPER HEIGHTS AND WEAK ONSHORE OR NORTHWEST FLOW. THIS SHOULD
STEER ANY SYSTEMS WELL NORTH OF THE AREA. THE FOUR CORNERS HIGH
STRENGTHENS OVER THE WEEKENDS AND EXPANDS NORTH AND WESTWARD. 500 MB
HEIGHTS COULD REACH ABOVE 5990 MB BY MONDAY IF THE ECMWF IS CORRECT.
HIGH TEMPERATURES SHOULD EASILY REACH INTO THE 80S THIS WEEKEND AND
INTO MONDAY...AND POSSIBLY WARMER BY MONDAY. MERCER

&&

.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 THE SOUTHWESTERLY FLOW
ALOFT. AIR MASS CURRENTLY STABLE THIS MORNING BUT BECOMING SOMEWHAT
UNSTABLE BY LATE THIS AFTERNOON WITH INCREASING MID LEVEL MOISTURE
ESPECIALLY OVER SOUTHERN PORTIONS OF THE AREA.

MID AND HIGH LEVEL CLOUDS EMBEDDED IN THE SOUTHWESTERLY FLOW ALOFT
STRETCHING FROM SOUTHWESTERN WASHINGTON TO JUST SHORT OF KBLI.
STARTING TO SEE THESE MID TO HIGH LEVEL CLOUDS MOVE IN TO THE COAST
AND OLYMPIC PENINSULA AS WELL...SO WOULD NOT BE SURPRISED IF MUCH OF
THE AREA WAS BKN OR OVC BY AFTERNOON. SHOWERS ASSOCIATED WITH THE
APPROACHING LOW EXPECTED THIS EVENING AND TONIGHT...LIKELY LOWERING
CIGS DOWN TO THE 3500-5000 FOOT RANGE.

KSEA...MID AND HIGH LEVEL CLOUDS TODAY WITH LIGHT WINDS BECOMING
NORTHWESTERLY 4 TO 8 KNOTS IN THE AFTERNOON. CEILINGS LOWERING
TONIGHT BUT STILL REMAINING ABOVE 3500 FEET. FELTON/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 LATER WEDNESDAY INTO
WEDNESDAY NIGHT WITH SMALL CRAFT ADVISORY WINDS DEVELOPING OVER MOST
OF THE WATERS WEDNESDAY NIGHT OR THURSDAY. WINDS EASING THURSDAY
NIGHT. THE ONSHORE FLOW PATTERN WILL RETURN FRIDAY INTO THE WEEKEND
WITH SMALL CRAFT ADVISORY WESTERLIES IN THE CENTRAL AND EASTERN
STRAIT LATE IN THE DAY. FELTON

&&

$$

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




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