Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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920
FXUS66 KSEW 142232
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
331 PM PDT Tue Aug 14 2018

.SYNOPSIS...An upper ridge will produce dry and warm weather
through Wednesday. Given this flow pattern it will also be smoky at
times. Onshore flow will gradually develop beginning Wednesday and
continue into the weekend as a weak upper trough approaches and
eventually moves through. This will cool highs back to near normal
by Thursday and Friday. High pressure aloft will rebuild over the
area on Saturday before another upper trough moves into the region.

&&

.SHORT TERM...Onshore flow was unfortunately not strong enough to
help erode lower level smoke and haze this afternoon. Although
isolated pockets of improvement may occur into the evening
hours...expect the majority of the haze to linger into the overnight
hours. This will allow for poor air quality conditions to continue
for the remainder of the day for most locations. Given these
trends...will go ahead and re-issue SPS for smoke/poor air quality
through tonight. Please refer to SPSSEW for complete details on
expected impacts.

Recent water vapor imagery reveals a synoptic pattern characterized
by a weak positively tilted trough across the eastern Pacific and a
ridge building northward from the Great Basin vicinity...all while
strongest belt of westerly flow remains confined to higher latitudes
across Canada. Closer to the sfc...a weak thermal trough oriented N-
S is still evident over the inland waters/lowlands of western
Washington. Associated pressure gradients will allow for weak
onshore flow this afternoon and into Wednesday morning...where
marine stratus are expected to make it to the coastline and perhaps
just slightly inland. This weak marine push still does not appear
strong enough to completely erode haze from wildfire smoke tomorrow.
Therefore...similar impacts will remain likely thru much of the
day...tho slightly stronger pressure gradients may allow for some
improvement into the afternoon.

Overnight Wednesday...stronger marine push is expected as the
aforementioned upper trough creeps closer to the coast. Should see
better coverage of marine stratus along coastal zones and into the
southwest interior by Thursday morning. Stratus should erode by the
afternoon and the trough should move onshore later in the day. This
should help clear out much of the smoke. Lack of appreciable deep
moisture should keep conditions dry during the passage of the trough
Thursday and Friday...with high temps dropping back down the upper
60s-upper 70s. Kovacik


.LONG TERM...Upper level trough axis should push east of the local
area late Friday night into Saturday. Weak ridging will take its
place through the day on Saturday allowing for a rather pleasant day
and temps beginning to warm back up a few degrees. The effects of
the ridge will be short-lived however as better consensus amongst
the 12z operational GFS and ECMWF show the development of a weak rex
block along the BC and Washington coastline by early Sunday. This
would place the CWA under better influence of the trough portion of
the block, which may impact the expected warming trend to some
degree. Will opt to not get too carried away and await later runs
before determining impacts /if any/ to the forecast. At this point
in time it should be noted that no precip is expected thru the
extended. Trough should linger thru the remainder of the extended.
Kovacik

&&

.AVIATION...Upper level high pressure will remain over the area
through early Wednesday. Weak upper level trough will move across
western Washington later Wednesday into Thursday. Light northerly
flow at the surface tonight. Flow aloft will shift more to the
southwest Wednesday which should help improve/clear out smoke and
haze. Visibility restrictions will likely linger until then though.

KSEA...Light north to northwest winds. Smoke and haze will continue
to reduce visibility down to 3-5SM through Wednesday morning. As
flow aloft shifts to the southwest, some improvement in visibility
is expected during the day Wednesday. CEO

&&

.MARINE...Light flow becoming onshore this evening. Surface haze
and smoke will linger into Wednesday morning. Stronger onshore flow
will develop later Wednesday and continue through the end of the
week as a weak upper level system moves across the area. Small craft
advisory west winds are likely each night for the central and
eastern Strait of Juan de Fuca through Saturday. Winds may be
pushing gale strength Thursday night. There will also be the
possibility of small craft advisory northwest winds at times for the
outer coastal waters. CEO

&&

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

&&

$$

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