FXUS66 KSEW 050333

Area Forecast Discussion
National Weather Service Seattle WA
833 PM PDT Sat Apr 4 2020

.UPDATE...Dry conditions across most of Western Washington this
evening with light offshore flow. Light rain is sliding northward
across portions of southern WA with an upper level low offshore,
however not much in the way of precipitation is expected with dry
air over the area. Slight chance of a showers across the Southwest
Interior and South Sound into tonight. Dry conditions for Sunday.
Winds light northerly.


.SYNOPSIS...Upper level low offshore will move south tonight and
Sunday. A weak upper level trough will move over the area from the
north Sunday. Upper level ridge to build offshore Monday with the
ridging moving inland in the middle of next week. Another weather
system will try to move through the ridge next Friday or Saturday.


.SHORT TERM /TONIGHT THROUGH TUESDAY/...Satellite imagery shows
mostly cloudy skies over most of Western Washington this
afternoon. Doppler radar not picking up any shower activity at
23z/4 pm with the leading edge of the band of showers moving up
from the south near Portland. Temperatures were in the mid 40s to
lower 50s.

Upper level low well offshore will continue to dig south tonight.
This will make the flow aloft over Western Washington more
southernly pushing the shower activity over the Portland area
north into the Southwest Interior. This feature is splitting
slightly as it moves north so expect the shower activity to die
off before it reaches the Central Puget Sound. Mostly cloudy
conditions will continue overnight. Lows in the 30s and lower

Upper level low offshore continuing to move south Sunday with the
southerly flow aloft shutting off then being replaced by a drier
northwesterly flow aloft as an upper level trough moves into the
area. Air mass slightly unstable with the trough but with the
limited moisture will confine the chance of showers mostly to the
higher terrain. Some mid and high level moisture will keep skies
partly sunny/mostly cloudy. Highs a little warmer, in the lower to
mid 50s.

Upper level trough hanging over the area Sunday evening before
kicking off to the east early Monday morning. This will keep the
threat of showers in the Cascades intact Sunday night. For the
remainder of the area cloud cover decreasing overnight as the
trough moves east. Lows Monday morning in the 30s with the cooler
locations near freezing.

Northwesterly flow aloft continuing on Monday as an upper level
ridge slowly starts to build over the eastern Pacific. Low level
onshore flow will bring some morning clouds to the area with
mostly sunny skies in the afternoon. Highs in the 50s with lower
60s possible over the Southwest Interior.

Upper level ridge continuing to slowly build offshore Monday
night into Tuesday with dry northwesterly flow aloft. Surface flow
remaining onshore combined with the ridge being relatively weak
will keep high temperatures mostly in the 50s but Tuesday will be
a dry day.

agreement Wednesday and Thursday with the upper level ridge
nosing into the area Wednesday night and Thursday. Thursday looks
to be the warmer day of the two sunny days with warmer locations
getting into the mid 60s. The forecast high of 63 on Thursday
would be the warmest day in Seattle since October 24th. Most of
the high Wednesday and Thursday will be in the 50s and lower 60s.

Model consistency breaks down Friday and Saturday with the ECMWF
keeping the ridge in place Friday followed by a very weak system
Saturday. The GFS breaks the ridge down sooner with rain
spreading into the area Friday with a trough trailing the system
for Saturday. ECMWF ensemble solutions for Friday have about 20
percent of the members indicating light precipitation, a little
more wet then the operational solution. The GFS has a little over
half the ensemble members indicating precipitation, drier than the
operational run would indicate. End result will have chance pops
in the forecast Friday. A few more of the ensemble members are
wet for Saturday in the ECMWF but the precipitation is light.
Given the inconsistency in the models will stay with chance pops
for Saturday. Felton


.AVIATION...Southerly flow aloft with an upper level low offshore
tonight. Mainly dry conditions expected with light rain chances
mainly along the WA/OR border tonight. VFR cigs into Sunday.
Northerly winds 5 to 10 knots.

KSEA...VFR cigs. North winds to 10 knots tonight. JD


.MARINE...An area of low pressure will slide south offshore of
OR/CA into Sunday. Northerly flow across Western Washington. The
flow will turn onshore on Monday as high pressure builds over the
NE Pacific. Small Craft Advisory, or possibly gale winds, through
the Strait of Juan de Fuca Monday evening. Light onshore flow
Tuesday and Wednesday across the area. JD


.HYDROLOGY...River flooding not expected in the next 7 days.





NWS SEW Office Area Forecast Discussion

Discussion Script by Ken True at Saratoga Weather - WFO Script by SE Lincoln Weather