Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA
FXUS66 KSEW 240415
Area Forecast Discussion
National Weather Service Seattle WA
915 PM PDT Sun Apr 23 2017
.SYNOPSIS...Showery weather will continue for much of the
upcoming week as a series of weather systems move through. There
should be a dry spell Monday night and the first part of Tuesday
as high pressure aloft crosses the area. Another dry period is
likely Friday and Saturday.
.SHORT TERM...Showers will taper over the north half of the area
tonight as an upper short waves exits. A frontal system moving
onshore to the south will keep rain likely over the south part of
the area tonight into Monday morning. A general shower threat
will continue all areas on Monday with a weak upper low over the
area. The highest chances will be over the south part of the
forecast area closest to the upper low.
The upper low will move inland Monday evening and an upper ridge
will move over the area later Monday night. Showers should taper
and eventually come to an end Monday night. The weak upper ridge
will be over the area most of Tuesday for a generally dry day. An
approaching front should spread rain to the coast in the afternoon
but the interior looks like it will remain dry until late in the
afternoon or early in the evening.
The aforementioned relatively weak front will move through the
area Tuesday evening for a little rain. Another wetter system fast
on its heels will reach the area late Tuesday night. Wednesday
morning is looking rather soggy, especially south part of the
area. This front will be east of the Cascades by afternoon but
showers will linger. A Puget Sound Convergence Zone also looks
likely in the afternoon. Schneider
.LONG TERM...Previous discussion...Models are in general
agreement Wednesday night and Thursday in showing onshore flow and
showery conditions continuing. On Thursday night and Friday it
appears that showers will diminish as the onshore flow relaxes
with the long-wave offshore ridge pushing eastward a bit.
The ECMWF, GFS and a majority of their ensemble members generally
agree that the period Friday night and Saturday stands the highest
probability of being dry with temperatures finally get to, or
perhaps rising a bit above, the normal value for the tail end of
April. But inconsistencies start arising Saturday night as the
long wave ridge offshore temporarily retrogrades. The ECMWF and
GFS both show a short wave trough driving southeastward into the
area on the front side of the ridge by Sunday. Handing of this
short wave trough and others that may or may not follow is
becoming increasingly inconsistent in all model solutions.
.AVIATION...An upper level low will move from the Olympic
Peninsula late tonight to eastern Oregon on Monday evening.
Meanwhile, a surface low will move by to our south on Monday.
Moderate southwest flow aloft will become light and variable on
Monday, then become light northerly on Monday evening. The air
mass will be moist and somewhat unstable through Monday morning,
then the air mass will become a bit more stable with a shallowing
of the moist layer late Monday. With a surface low moving by to
our south, widespread precip on Monday will mainly be confined
along and south of a line from UIL-SEA-SMP.
KSEA...Showers will diminish over the next few hours as large-
scale lift turns to subsidence. The air mass will be richly moist
through tonight, so mere thinning of cloud layers is expected.
Northerly surface flow for much of Monday will lead to some drying
and clearing. A return to S-SW surface wind on Monday evening will
spell a quick return to a richly moist low-level environment,
along with a return of clouds at or below 020. Haner
.MARINE...A 1001 mb surface low will move eastward onto the north
Oregon coast on Monday morning, then move east of the Cascades on
Monday afternoon. A weak surface ridge will drift across the
waters early Tuesday. On Tuesday night, an occluded front will
cross the waters. Following Tuesday night`s front, strong onshore
flow will develop on Wednesday and Thursday, with westerly gales
possible in the central and eastern Strait of Juan de Fuca. Haner
You can see an illustrated version of this discussion at