Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 251648
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
948 AM PDT SAT APR 25 2015

.SYNOPSIS...AN UPPER LOW OVER THE AREA WILL MOVE INTO EASTERN
WASHINGTON LATE TODAY OR EARLY THIS EVENING. HIGH PRESSURE ALOFT
WILL PREVAIL TONIGHT THROUGH MONDAY. HOWEVER...A WARM FRONT WILL
BRING SOME RAIN TO THE AREA ON SUNDAY. LOOK FOR TEMPERATURES TO
CLIMB WELL ABOVE NORMAL ON MONDAY.

&&

.SHORT TERM...
THERE WAS A STATIONARY BATCH OF SHOWERS...LOCALLY HEAVY...OVER
WESTERN SKAGIT AND SNOHOMISH COUNTIES. THE SHOWERS WERE OCCURRING
IN THE VICINITY OF THE UPPER LOW. KATX RADAR ESTIMATED LOCALIZED
RAINFALL AMOUNTS OF UP TO 0.90 INCH IN THE THREE HOUR PERIOD
ENDING ABOUT 9 AM PDT....HOWEVER THE OBSERVED AMOUNTS WERE MUCH
LESS. PER THE DUAL POL HYDRO CLASS DATA...IT LOOKS LIKE THE RADAR
PRECIP ESTIMATES WERE BEING CONTAMINATED BY HAIL. THIS BATCH OF
SHOWERS /IF IT HOLDS TOGETHER/ SHOULD DRIFT EAST-SOUTHEAST WITH
THE UPPER LOW.

THE AIR MASS WAS DESTABILIZING OVER THE CWA AT THIS TIME...THANKS
TO THE PRESENCE OF THE UPPER LOW AND SURFACE HEATING. THEREFORE
ANTICIPATE SCATTERED SHOWERS TO BREAK OUT OVER THE CWA WITHIN THE
NEXT 2 TO 3 HOURS. THE CURRENT FORECAST HAS A MENTION OF TSTMS FOR
THE CASCADES BUT CANNOT COMPLETELY RULE OUT AN ISOLATED LIGHTNING
STRIKE ELSEWHERE...ESPECIALLY IN THE VICINITY OF THE UPPER LOW. ANY
TSTMS THAT DO DEVELOP OVER THE LOWLANDS WILL CONTAIN A LARGE
AMOUNT OF SMALL HAIL.

LOOK FOR THE UPPER LOW TO MOVE INTO EASTERN WASHINGTON LATE TODAY
OR EARLY THIS EVENING. AN UPPER RIDGE WILL BUILD INTO THE AREA
FROM THE WEST IN ITS WAKE FOR DRIER CONDITIONS.

A WARM FRONT WILL CLIP WESTERN WA ON SUNDAY FOR A LITTLE LIGHT
RAIN...MAINLY OVER THE OLYMPIC PENINSULA. THERE IS A CHANCE OF RAIN
IN THE INTERIOR AS THE FRONT MOVES NORTH INTO B.C. BUT PRECIPITATION
WILL REMAIN LIGHT.

MONDAY WILL BE WARMER AND DRIER WITH UPPER LEVEL RIDGING OVER THE
PAC NW. WHAT IS LEFT OF THE WARM FRONT MAY CLIP THE COAST SUNDAY
MORNING...OTHERWISE EXPECT DRY WEATHER AS WE HEAD INTO THE
AFTERNOON. THE WARMER AIR MASS WILL BOOST TEMPS WITH HIGHS IN THE
LOW TO MID 70S IN THE INTERIOR. 33

.LONG TERM FROM THE PREV DISCUSSION...
THE UPPER LEVEL RIDGE WILL SHIFT FARTHER INLAND TOWARD IDAHO ON
TUESDAY WHILE A TROUGH DIGS OFFSHORE. MODELS SHOW A COLD FRONT
PASSING THROUGH WESTERN WA FOR A CHANCE OF SHOWERS. IT IS LOOKING
WETTER AND COOLER OVER THIS PERIOD WITH THE GFS AND ECMWF NOW
ALIGNED.

A MEAN LONG WAVE TROUGH WILL PERSIST OVER THE PAC NW THROUGH THE
LONG TERM PERIOD. THERE IS A CHANCE OF SHOWERS THROUGH THE
FORECAST WITH TEMPERATURES CLOSE TO NORMAL. NO MAJOR CHANGES WERE
MADE FROM THE PREVIOUS FORECAST. 33

&&

.AVIATION...AN UPPER LEVEL LOW PRODUCING A CLUSTER OF SHOWERS OVER
NORTHWEST WASHINGTON THIS MORNING WILL DRIFT SOUTHEAST INTO EASTERN
WASHINGTON BY TONIGHT. LIGHT FLOW ALOFT WILL BECOME NORTHERLY THIS
AFTERNOON. AIR MASS MOIST AND SOMEWHAT UNSTABLE.

MULTIPLE CLOUD LAYERS TODAY BETWEEN 4000 AND 8000 FEET WITH CEILINGS
AS LOW AS 2000 FEET IN SCATTERED SHOWERS POPPING UP AROUND THE AREA
THROUGH THIS AFTERNOON. THE SHOWER ACTIVITY WILL DECREASE THIS
EVENING.

KSEA...SOUTHERLY WINDS 4 TO 8 KNOTS BECOMING NORTHWESTERLY IN THE
LATE AFTERNOON OR EVENING HOURS. CEILINGS BETWEEN 4000 AND 8000 FEET
WITH A CHANCE OF BRIEF CEILINGS NEAR 2000 FEET WITH ISOLATED
SHOWERS. THE CLUSTER OF SHOWERS ASSOCIATED WITH THE UPPER LEVEL LOW
WILL LIKELY PASS NORTH OF SEATAC. JSMITH

&&

.MARINE...WEAK HIGH PRESSURE WILL BUILD OVER THE WATERS TODAY AND
REMAIN OVER THE WATERS TONIGHT GIVING LIGHTER WINDS TO THE AREA.
THERE IS A CHANCE OF SCA LEVEL WINDS IN THE CENTRAL AND EASTERN
STRAIT LATER THIS EVENING...BUT I HAVE HELD OFF ISSUING AN ADVISORY
FOR NOW SINCE THE WINDS ARE MARGINAL. A WEAKENING FRONT WILL MOVE
THROUGH THE AREA SUNDAY AFTERNOON AND EVENING. THIS SHOULD BRING
SMALL CRAFT ADVISORY WINDS TO THE COASTAL WATERS...AND POSSIBLY
PARTS OF THE NORTHERN INTERIOR WATERS.

A SECOND FRONTAL SYSTEM WILL REACH THE AREA TUESDAY. MODELS ARE
CONTINUING TO HAVE TROUBLE SETTLING ON THE TIMING AND STRENGTH OF
THIS SYSTEM SO CONFIDENCE REMAINS LOW FOR THE WIND FORECAST ON
TUESDAY. HIGH PRESSURE WILL BUILD IN BEHIND THE FRONT ON WEDNESDAY.
JSMITH/FELTON

&&

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

&&

$$

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