NWS Seattle Forecast Discussion

discussion.md

Area Forecast Discussion

Issued by NWS Seattle/Tacoma, WA

000
FXUS66 KSEW 140310
AFDSEW

Area Forecast Discussion  
National Weather Service Seattle WA  
810 PM PDT Sun Sep 13 2020  

.SYNOPSIS...Smoke from wildfires will remain an issue into Monday
resulting in air quality impacts across the region. A frontal
system will bring some showers Monday night and Tuesday. An upper
low will slowly move onshore later in the week.

&&

.SHORT TERM /TONIGHT THROUGH WEDNESDAY/...Stubborn smoke continues
to hang on around most of Western Washington with unhealthy air
quality expected to linger into Monday. Surface gradients remain
light and this will do little to disperse anything across the
lowlands. A broad upper trough offshore will send a weakening
frontal system into the region Monday night into Tuesday, but models
are backing off considerably with precip chances. The trend now
appears that the upper trough offshore will largely stay there until
later in the week. Will hold off on making changes to the forecast
this evening, but overnight shift may need to scale back precip
chances Monday night and Tuesday.  27

Previous discussion...Smoke and air quality
continue to be the main issues across the region. Most areas
continue to report AQI values in the Very Unhealthy and Hazardous
categories. And visibility is mostly limited to less than 2 miles
in a combo of smoke and haze. The HRRR model does show total
smoke thinning out over western WA with stronger winds aloft (and
there`s less smoke offshore). However, mixing and ventilation
remain poor and there`s still plenty of smoke trapped near the
surface.

A frontal system will move inland on Monday (mainly Monday
afternoon or Monday night) bringing much needed rain to the
region. This, along with a stronger air flow pattern and deeper
mixing, will help clean things out (at least for areas west of
the Cascades). So improvements are on the way. Moist southwest
flow sets up over the Pac NW on Tuesday as a upper level low spins
offshore. This brings another period of rain to western WA which
will further help our air quality.

More smoke on Wednesday? We`re under south flow on Wednesday with
light offshore winds as the low continues to spin off the coast.
This flow pattern could potentially bring more wildfire smoke
(especially from OR and northern CA) into the region. But the
extent over western WA is unclear at this time. The smoke should
be mostly aloft. We will see better mixing through the period
along with additional showers so this should help to limit
impacts. 33

.LONG TERM /THURSDAY THROUGH SUNDAY/...Western WA will see
additional showers Thu-Sat as the upper low slowly wobbles
inland. The air mass is slightly unstable over the Cascades where
we may see a slight chance of thunderstorms (near the crest).
These showers will be wet, though, and critical fire weather
conditions are not expected. Onshore flow will prevail late in the
weekend as this system exits. This brings a drier start to next
week with temperatures near average. 33

&&

.AVIATION...Southwest flow aloft continues this evening and into
Monday. While PIREPs indicate that smoke layer is beginning to thin
from the top, widespread 1/2 to 2 sm visibility continues across
nearly all of the lowlands into the evening. With continued weak
onshore low-level flow expect lowering MVFR to local IFR ceilings
again tonight. An approaching front late Monday will bring
increasing mid level clouds and rain chances, but may provide enough
mixing to increase visibility late Monday afternoon.

KSEA...Visibility remains 1 to 1 1/2 SM this evening with light
surface winds. Expect little change tonight with restricted
visibility and lowering ceilings through early Monday. Increasing
mid clouds late Monday afternoon ahead of front, but increased
mixing may lead to better visibility.   Cullen

&&

.MARINE...Smoke will reduce visibility - especially over the inland
and inner coastal waters into Monday. A frontal system lifts north
through the waters on Monday into Tuesday. A surface low will
develop west of the waters Tuesday then drift towards the Oregon
coast Wednesday and Thursday.

Small craft advisory southeast winds may develop Monday night ahead
of the front over the coastal waters. Meanwhile, seas will build to
around 5 to 7 feet behind the front. Increasing onshore flow on
Thursday may bring small craft westerlies to the Central and Eastern
Strait of Juan de Fuca Thursday afternoon and evening. Cullen

&&

.HYDROLOGY...The daily hydrology discussion has ended until the
start of the next rainy season; it will only be updated as needed
until then.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...None.
&&

$$

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