857 
FXUS66 KSEW 161036
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
336 AM PDT Wed Sep 16 2020

.SYNOPSIS...Smoke and degraded air quality will remain the 
primary issue across Western Washington through much of the week,
with an Air Quality Alert remaining in effect into Thursday. A
broad upper level trough offshore will continue to influence the
region today, before moving inland late in the week and bringing 
more widespread shower chances to the area. 

&&

.SHORT TERM /TODAY THROUGH FRIDAY/...A broad upper level trough
across the northeastern Pacific will continue to influence the
region over the next several days. Radar early this morning shows
showers continue to progress northward along the coast and across
portions of the northern interior as of 300 AM. Many parts of 
Western Washington received another dose of light rain last 
night- with many places recording at least a hundredth of an inch.
The heaviest rainfall generally remained across the Olympic 
Peninsula and across portions of the Southwest Interior. Showers 
will continue along the coast through the early morning hours, 
before briefly tapering somewhat by mid morning.

The broad upper level trough will gradually move closer to the 
region today, allowing for flow aloft to transition more 
southerly. This will likely aid in smoke advection into the region 
and thus, do not expect much of an improvement in air quality 
today. Forecast soundings also suggest a prolonged inversion, 
likely keeping smoke trapped in the lower levels. Have continued 
to trend lower than guidance on temperatures- with highs across 
the region topping out in the 60s along the coast and low to mid 
70s across the interior. A more robust upper level shortwave will 
be ejected northward across the area this afternoon. Have opted to
keep a chance of thunderstorms along the coast for now. However, 
with additional smoke being advected into the region, will need 
continue to closely monitor conditions throughout the day. 
Nevertheless, the shortwave will bring another round showers to 
the coast and portions of the Olympic Peninsula today.

The upper level low will continue to rotate offshore before 
gradually approaching the coast and making its way inland late 
Thursday and into Friday. This will bring more widespread shower 
activity to the region, and given slightly more favorable 
dynamics- a chance for thunderstorms as well. Thunderstorm 
coverage will remain dependent on smoke, however. Little 
improvement in smoky conditions will act as more of a hindrance 
to any convection. And thus, this will continue to be monitored in
the coming days.

.LONG TERM /SATURDAY THROUGH TUESDAY/...Showers will grace the 
beginning of the long term as the aforementioned upper level low 
progresses across the Pacific Northwest. Guidance suggests that 
Sunday will be drier as a brief upper level ridge traverses the 
region. Ridging will remain short-lived, however, as by Monday 
flow aloft flattens, potentially allowing for a system to the 
north to brush past the area. Ensembles then hint at another round
of troughing moving into the region early in the week, keeping 
the pattern more unsettled. 14

&&

.AVIATION...Moist and unstable southerly flow aloft for Western
Washington today and tonight. At the surface, weak offshore flow
will switch to light onshore this afternoon and tonight. Smoke
will continue to blanket the region with visibilities generally
1-3SM and areas of MVFR ceilings. There will also be some patches
of IFR ceilings and visibilities less than 1SM in fog this 
morning.

KSEA...Smoke will continue with visibilities 1-3SM and ceilings
generally 2000-3000 feet. Light wind will become west to northwest
3-7 knots this afternoon. Schneider

&&

.MARINE...Light offshore flow will switch to light onshore this 
afternoon as weak high pressure moves over the coast. Weak onshore
flow will persist for the rest of the week with high pressure 
along the coast and lower pressure inland. Winds will be generally
sub- advisory strength through the period although the Strait 
could see a few hours of marginal small craft advisory strength 
west winds Thursday evening and then maybe again at times next 
weekend. Schneider

&&

.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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