Brier Weather Station

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 Latitude N 47° 46' 49"    Longitude W 122° 16' 48"    Elevation 341 ft

000
FXUS66 KSEW 180421
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
921 PM PDT Fri Aug 17 2018

.SYNOPSIS...High pressure aloft will prevail through Saturday. A
weak low pressure system will move over the region on Sunday but
will have little impact on our weather. This system will move
east of the region by Monday. Expect smoke to return to the area
from wildfires in Canada early next week, possibly causing the air
quality to become unhealthy again.

&&

.SHORT TERM...
High pressure aloft will prevail over the region through Saturday.
A weak low pressure system will move over the area on Sunday but
will have little impact. This system will move east on Monday,
allowing an upper level ridge to move back over the region.
Weakening onshore flow will lead to warmer daytime temperatures
during this period (Saturday thru Monday).

The air mass may become unstable enough over the Cascades Monday
afternoon for a slight risk of thunderstorms. If any thunderstorms
do develop, they will be high-based and not produce much (if any)
rainfall.

Meanwhile, deep northerly flow is anticipated to return early next
week. This will cause smoke from wildfires raging in British
Columbia to drift back into the CWA, possibly causing the air
quality to degrade into the unhealthy range again. If the smoke
becomes as dense as this past Tuesday and Wednesday, highs on
Monday, in particular, will end up being lower than forecast.

.LONG TERM...
Offshore and northerly flow will continue Tuesday. Thermally
induced low pressure will be over Western Washington. Aloft there
is likely to be a weak upper low over Oregon. This pattern will
allow smoke to continue at low levels, with continuing air quality
concerns. High temperatures will be warmer than normal, with lots
of 80s and perhaps some low 90s inland. The heat could be enough
to trigger a few thunderstorms in the Cascades and adjacent areas.
The coast could also be quite warmer than normal. If there is a
lot of smoke, temperatures will be lower than what guidance gives,
but it should be noted that MOS and other guidance currently show
upper 80s and low 90s over much of the area.

Models show a marine push Tuesday night, with low level onshore
flow for the rest of next week. This should return temperatures to
near normal and alleviate most air quality concerns. A weak
system around Thursday has introduced low chance pops to the
forecast then. Burke

&&

.AVIATION...A weak upper level ridge will prevail over the region
overnight for wly flow aloft. Low level onshore flow will weaken.
Expect areas of IFR CIGs/MVFR VSBYs (mainly over the coast)
overnight.

KSEA...VFR. There will probably be some stratus west of the
airport, over the Puget Sound waters between 1200 (5 AM PDT) and
1600 UTC (9 AM PDT). Light and variable winds will become
northerly 5-10 knots early Saturday afternoon.

&&

.MARINE...
A 1030 mb high well offshore with lower pressure east of the
Cascades will keep the flow onshore or westerly through Saturday.
The flow will become northerly Saturday night and persist into
early next week due to higher pressure over British Columbia with
lower pressure over Oregon.

&&

.FIRE WEATHER FROM THE PREV DISCUSSION...
Have some concerns for critical fire weather from Sunday through
Wednesday, but there are also mitigating factors.

First, mid-level Haines 6 conditions are forecast from Sunday
through Tuesday. This indicates dry and unstable conditions in the
850-700 mb layer, or roughly 4,500-10,000 feet msl. This typically
leads to active burning conditions over the mountains. However,
possibly thick smoke is expected to spread over the area from
British Columbia on Sunday and Monday, persisting through Tuesday.
Thick smoke has a shading and therefore stabilizing effect on the
lower atmosphere, which would heavily mitigate the Mid-Level
Haines 6 conditions.

In addition, a few thunderstorms with lightning are possible from
Monday through Wednesday, mainly over the Cascades. However, if
southeast steering flow develop, some of the storms could venture
across the Southwest Interior or into the Southern Puget Sound
lowlands. Still much uncertainty with the pattern at this point,
which would affect coverage and location of lightning. Haner

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory until 2 AM PDT Saturday for Central U.S.
     Waters Strait Of Juan De Fuca.

     Small Craft Advisory until 6 AM PDT Saturday for Coastal Waters
     From Cape Flattery To James Island 10 To 60 Nm-Coastal
     Waters From James Island To Point Grenville 10 To 60 Nm.

&&

$$

www.weather.gov/seattle

NWS SEW Office Area Forecast Discussion

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