Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

Home | Current Version | Previous Version | Text Only | Print | Product List | Glossary On
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50
000
FXUS66 KSEW 192234 CCA
AFDSEW

AREA FORECAST DISCUSSION...CORRECTED
NATIONAL WEATHER SERVICE SEATTLE WA
334 PM PDT SUN APR 19 2015

CORRECTED MINOR TYPO IN THE SYNOPSIS.

.SYNOPSIS...HIGH PRESSURE ALOFT WILL PREVAIL OVER THE PACIFIC
NORTHWEST THROUGH MONDAY. THE HIGH WILL MOVE EAST OF THE REGION ON
TUESDAY AND THIS WILL ALLOW A LOW PRESSURE SYSTEM TO APPROACH FROM
THE PACIFIC. LOOK FOR WHAT IS LEFT OF THE COLD FRONT TO MOVE ACROSS
THE AREA TUESDAY AFTERNOON. THE LOW PRESSURE SYSTEM ALOFT WILL MOVE
ACROSS THE REGION ON WEDNESDAY.

&&

.SHORT TERM...
A GRADUALLY WEAKENING UPPER RIDGE OVER THE PACIFIC NORTHWEST WILL
MOVE OVER THE INTERMOUNTAIN WEST ON TUE. THIS WILL ALLOW AN UPPER
TROF OVER THE NORTHEAST PACIFIC TO APPROACH THE REGION. THIS...IN
TURN...WILL SERVE TO INITIATE A MARINE PUSH. IT LOOKS LIKE THE
PUSH WILL BEGIN MON AFTERNOON BUT WILL STRENGTHEN ON TUE.
CONSEQUENTLY...ANTICIPATE COOLER AND CLOUDIER CONDITIONS ON TUE.
WHAT IS LEFT OF A COLD FRONT WILL MOVE ACROSS THE CWA TUE
AFTERNOON FOR A THREAT OF SHOWERS. IT LOOKS LIKE THE RISK OF TSTMS
WILL STAY EAST OF THE CASCADES...SO THE POSSIBILITY WAS REMOVED
FROM THE FORECAST.

CONCERNING MAX TEMPS FOR MON AND TUE...CONFIDENCE IN THE FORECAST
FOR MON WAS NOT VERY HIGH. THE WEAKENING UPPER RIDGE COMBINED WITH
INCREASED LOW LEVEL ONSHORE FLOW WOULD SUPPORT TEMPS BEING CLOSER
TO THE ECMWF MOS GUIDANCE. SINCE THE ECMWF TENDS TO HANDLE SPLIT
FLOW REGIMES BETTER...DECIDED TO LEAN TOWARD THIS MODEL. WITH THAT
SAID...TUE SHOULD BE NOTICEABLY COOLER ACROSS MUCH OF THE CWA.
THE NAM AND GFS APPEARED TOO WARM...ESPECIALLY WHEN FACTORING IN
THE STRENGTH OF THE ONSHORE PRESSURE GRADIENT. THUS...LEANED
TOWARD THE ECMWF GUIDANCE. ALTHOUGH...THERE IS A CHANCE THAT THE
FORECAST MAX TEMPS FOR TUE COULD BE 3-5 DEGREES TOO WARM.

THE ECMWF INDICATED THAT THE TROF IN THE NORTHERN BRANCH WILL
APPROACH THE AREA TUE NIGHT AND THEN MOVE ACROSS THE CWA ON WED.
CONDITIONS SHOULD BE COOL AND A BIT ON THE CLOUDY SIDE. THERE WILL
ALSO BE A THREAT OF SHOWERS. MAX TEMPS ACROSS MOST LOWLAND LOCALES
ON WED SHOULD ONLY BE IN THE MID TO UPPER 50S.

.LONG TERM...
THE EXTENDED PERIOD STILL APPEARED COOL AND UNSETTLED...HOWEVER
CONFIDENCE IN THE DETAILS WERE NOT HIGH DUE TO LACK OF CONSISTENCY
AND AGREEMENT IN THE MEDIUM RANGE SOLUTIONS. ANTICIPATE NEAR OR
SLIGHTLY BELOW NORMAL DAYTIME TEMPS DURING THIS PERIOD.

&&

.AVIATION...AN UPPER LEVEL RIDGE IS OVER THE PACNW. FAIR SKIES WITH
A DRY STABLE AIR MASS WILL REMAIN IN PLACE. THE FLOW ALOFT IS NW
WITH JUST A LITTLE CIRRUS.

KSEA...A LITTLE CIRRUS...NORTHERLY BREEZE.

&&

.MARINE...THERMALLY INDUCED LOW PRESSURE OVER CALIF EXTENDS NORTH
ALONG THE COAST. ONSHORE FLOW SHOULD DEVELOP MONDAY EVENING AND THEN
INCREASE TUESDAY AS A FRONT REACHES THE AREA. ONSHORE FLOW WILL
CONTINUE THROUGH THE END OF THE WEEK.

&&

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

&&

$$

WWW.WEATHER.GOV/SEATTLE

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




USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.