Scientific Forecaster Discussion
fxus66 ksew 191606
Area forecast discussion
National Weather Service Seattle Washington
900 am PDT sun Aug 19 2018
Synopsis...a weak upper trough will bring a slight chance of
thunder to the central Cascades today and Monday. An upper ridge
will bring dry and warm weather Tuesday and Wednesday.
Temperatures will cool to near normal Thursday through Saturday as
onshore flow returns. Smoke will move into the area today and
continue through around Wednesday for reduced air quality.
Short term...the current forecast still appears on track through
at least Tuesday. The main concern is how much smoke will infiltrate
western Washington through Tuesday and affects on air quality. Smoke
remains mostly aloft over western Washington this morning, with with
good to moderate air quality reported at most stations. The weak
onshore flow near the coast is also bringing some stratus into the
southwest interior and western Strait which should push back to the
coast later this morning.
The weak trough skirting southeast across Oregon and SW Washington
may initially help deepen the mixing layer over the area today.
However, the flow pattern at the surface and aloft will be changing
later today after the trough starts to exit the area. The ywl-bli
gradient is near +7mb and probably close to inducing some Fraser
outflow which will introduce some low level smoke rather rapidly to
the north interior later today. If this does develop, the near
surface smoke will probably filter south into much of Puget Sound.
However, the other mechanism is the increasing smoke aloft which
should become even more noticeable this afternoon and evening. Smoke
plume models suggest this will settle into the lower levels tonight
into Monday and Tuesday with poor ventilation not allowing for any
significant improvement. Air quality will probably become unhealthy
first up north where Fraser outflow may introduce surface smoke,
then worsen across most of western Washington by Monday and Tuesday.
A partial marine push on Wednesday may improve some air quality near
the coast and possibly further inland areas later in the days.
In coordination with local air quality agencies, an air quality
alert remains in effect through Wednesday. Smoke is expected to
increase across the area today and especially Monday and Tuesday.
Air quality should get worse over the next day or two and there will
likely be reduced visibilities at times.
Models still show some weak convection over the Cascades of Pierce
and Lewis County including mount rainer. Will maintain the slight
chance of thunder later today for this area although smoke may
hinder surface heating enough to inhibit building cumulus. Mercer
Long term...previous discussion...an upper ridge over the area will
begin to weaken on Wednesday. At the same time, low level offshore
flow will turn onshore and increase during the day. The coast should
be 10+ degrees cooler but the interior will see only minor cooling.
Low clouds should also return to the coast.
A marine push Wednesday night will bring low clouds inland and
cool highs back to near normal all areas on Thursday. An upper
trough will also move over the area on Thursday. It looks largely
dry although an isolated shower cannot be ruled out for now. This
pattern change should also clear most of the smoke out of the area
and improve air quality.
A broad upper trough will remain over the area Friday and
Saturday with low level onshore flow. It look largely dry with
some night and morning clouds and temperatures near normal.
Aviation...a weak upper level trough will cross western Washington today
with a slight chance of thunderstorms near the Cascade crest, where
heating and instability is the greatest. Elsewhere, dry conditions
are expected. Low level marine clouds will mainly affect the coast
this morning and will burn off this afternoon. Wildfire smoke will
continue to impact the region as northerly flow brings in smoke from
b.C. May see 4-6sm visibility as smoke mixes down to the surface.
Light winds and stagnant conditions may lead to a decrease in
visibility overnight due to smoke. 33
Ksea...wildfire smoke will continue to increase across western Washington
today with northerly flow. May see 4-6sm visibility in fu with
possible ceiling 4000-6000 ft. Surface winds will be north/northwest to 10 kt.
Marine...northerly flow will prevail today due to high pressure
over b.C. And lower pressure over Oregon. This flow pattern will
bring wildfire smoke into western Washington. The flow will remain
northerly or offshore through Tuesday. Strong onshore flow will
develop on Wednesday with gales possible through the Strait of Juan
de Fuca. Onshore flow will continue through the end of the week. 33
Fire weather...a critical few days of fire weather coming up for
western Washington, but some of the effects will be mitigated by
thick smoke, of all things.
First, the air mass will get warmer and drier as northeasterly
offshore wind flushes the marine layer out to sea. Offshore flow
will peak on Monday morning in a Fraser-outflow like display. (In
the winter, this is a cold wind. In the Summer, it's a warm and dry
wind.) As a result, a critical combination of wind and low relative humidity is
expected over the north Cascades, in the Bellingham area, and across
the northwest olympic peninsula on Monday. The issuance of a red
flag warning for parts of northwest Washington may eventually be
needed. On Tuesday as the gradient becomes more easterly across the
Cascades, so may need to watch for east wind downwind of the Cascade
gaps and passes.
Second, isolated lightning is possible late this afternoon and early
this evening in the vicinity of Mount Rainier. Moisture and
instability are marginal for thunderstorm development, so any storms
will be short-lived and produce a very small number of lightning
strikes, if any at all.
Third, the mid-level Haines index will be near a 6 through Tuesday
night, indicating a dry and unstable air mass between roughly 4,500
ft and 10,000 ft msl (850-700 mb). This typically Perks up fires
over the mountains, but the influx of thick smoke from British
Columbia will provide significant shading, help stabilize the lower
atmosphere, and significantly mitigate this effect.
Finally, visibility will deteriorate today through Monday morning as
smoke arrives from British Columbia. Surface visibility may lower to
2 miles or less in places. This could affect the ability to use air
Ops to help with initial attack, and it may hamper the detection of
new fires. Haner