Scientific Forecaster Discussion
fxus66 ksew 282223
Area forecast discussion
National Weather Service Seattle Washington
320 PM PDT sun may 28 2017
Synopsis...an upper ridge will shift inland through Memorial Day.
Onshore flow will increase Monday and Tuesday and coastal marine
layer low clouds will push farther inland each morning. An upper
trough and southerly flow aloft will bring an increasing chance of
showers and a few thunderstorms by Tuesday night. Weak weather
disturbances will keep clouds and a chance of showers in the forecast
through the end of next week.
Short term...the coastal marine low clouds and patchy fog (pic1)
will push in a bit farther Monday morning than they did this morning.
After areas of morning low clouds and patchy fog, most of western Washington
will still be mostly sunny--just a bit cooler. The coast wont see
much change, still cloudy and chilly along the ocean beaches. An
upper shortwave trough will reach the area Tuesday afternoon and
evening and that will further deepen the marine layer and touch off
some showers (pic2) and that Marks the end of the dry streak and
finishes off the warm spell.
Long term...the rest of the week will probably see western Washington in
a regime of deep marine layer clouds that break out in the afternoon
and evening. There is a chance or slight chance of showers each day
but the forecast is fairly meaningless and the differences in timing
of weak shortwaves makes it impossible to say which days have the
best chance of being decent and which will be gloomier. One way it
often turns out is with some areas of morning drizzle and then late
afternoon sunbreaks. The pscz area is obviously a likely place to
see drippy weather this time of year with weak shortwaves and
middling heights. In the GFS, upper level heights do recover a bit
around the end of the week, so perhaps Friday and Saturday will be
decent days before an upper trough moves into the northeast Pacific
early the following week. And then there is the European model (ecmwf) which is cool
and showery as an upper low moves over the area by next Saturday. A
period of sunny weather with above normal temps like we have seen
this weekend is not at all likely for some time based on the global
models I see today.
Aviation...an upper level ridge of high pressure will slowly shift
eastward over the northern Great Basin on Monday (pic10) (pic11).
Light onshore flow will gradually increase through Monday, then
become strong onshore Tuesday. Meanwhile, areas of IFR stratus and
fog near the coast and in the Strait will push further inland late
tonight and Monday morning, likely reaching the ksea terminal and
possible the kbfi and kpae terminals in addition to khqm, kclm, and
kolm (pic12). This should again mostly burn off by midday, except
perhaps continuing at khqm through the day. It will become slightly
unstable this evening and again Monday afternoon/evening with
towering cumulus over the Cascades. Thunderstorms are not expected
to affect any terminals through Monday afternoon.
Ksea...VFR with light northerly winds through this evening.
Northwest winds 5 to 9 kt, then light south wind to 5k after 10z.
IFR stratus still appears likely between 11-17z Monday morning, then
quickly burning off by midday.
Marine...high pressure offshore along with lowering pressure east
of the Cascades will result in strengthening onshore flow through
Tuesday. This will lead to the potential for gale force winds over
parts of the Strait of Juan de Fuca tonight (pic13), and late Monday
(pic14) and Tuesday afternoon and evening. Gale force winds appear
likely tonight with a Gale Warning in effect but there is some
uncertainty in the timing. It could happen as early as 7 PM PDT
tonight or as late as midnight Monday. Small craft winds will
prevail in the the Strait through Tuesday, possibly reaching gale or
near gale force during the evening and overnight hours. Small craft
winds are also possible in waters adjacent to the East Entrance to
the Strait as well as the West Entrance.
A strong marine push on Tuesday could potentially bring more
widespread small craft winds to other interior waters including
Puget Sound. Winds may pick up suddenly as the marine push develops.
There is also a threat of thunderstorms Tuesday. Forecasts should be
Pz...small craft advisories are in effect for the Strait of Juan de
Fuca, northern inland waters, and Admiralty Inlet.
Gale Warning in effect from this evening through late tonight for
the central and eastern Strait of Juan de Fuca beginning at 6 PM.
Gale watch central and eastern Strait Monday night.