Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 170413
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
900 PM PDT Wed Aug 16 2017

.SYNOPSIS...An upper level ridge moving over the Pacific Northwest
coupled with low level onshore flow will maintain seasonable summer
weather with areas of morning clouds into Friday. A weak upper level
trough is expected to bring some cooling and possibly some drizzle
late Friday into Saturday morning. Another upper level ridge will
rebuild over the region Sunday through Tuesday for a bit warmer
conditions.

&&

.SHORT TERM...Westerly flow aloft with embedded weak shortwaves will
continue through the rest of this week. One shortwave that moved
into British Columbia this morning brought some middle and high
clouds into Western Washington during the day today along with a few
sprinkles. Those clouds should continue inland tonight. The system
has induced stronger low level onshore flow and marine clouds will
spread inland overnight. Prog cross-sections show the tops to be
near 850 mb/5000 ft by Thursday morning and thick enough to perhaps
squeeze out a little drizzle.

The next positive tilt shortwave was crossing around 160W early this
afternoon. Progs concur that this shortwave will build some ridging
aloft ahead of it moving over the Pacific NW Thursday. So anticipate
the marine cloud deck to thin out during the afternoon as a result.

The following shortwave is forecast to track into BC on Friday with
heights aloft over Western Washington sagging into Saturday. Will
retain a small threat of showers closer to the Canadian border and
in the mountains with some orographic flow later Friday into
Saturday morning. Renewed stronger low level onshore flow should
result in a blanket of marine clouds across much of Western
Washington Saturday morning. Anticipate daytime temperatures to be a
bit cooler - close to or just under average Saturday.

.LONG TERM...What`s left of former Typhoon Banyon was located near
37N/160E and becoming extratropical. The mid-range progs continue to
build ridging downstream from this system over the Pacific NW Sunday
into Monday for a return to less morning marine clouds and more warm
sunshine early next week. The Seattle-North Bend OR surface pressure
gradient turns offshore by Monday morning. Again, there are 5 days
to go before the eclipse...but at this point any morning marine
clouds may be limited to just the coastal region and any inland
should be short-lived.

Heading into the Tuesday and Wednesday, the mid-range guidance
offers a developing upper level trough off the coast with a growing
threat of showers. The Euro is faster and stronger while the GFS and
Canadian are slower and weaker with this trend. For now have
introduced a threat of showers and some cooler temperatures on
Wednesday. Buehner

&&

.AVIATION...Northwesterly flow aloft will become westerly on
Thursday. Low level onshore flow continuing through Thursday.
Stratus along the coast this evening will spread inland over the
remainder of the terminals early Thursday morning. In general
ceilings in the 1000 to 2000 foot range Thursday morning with
ceilings improving late in the morning. The clouds will scatter out
over the interior in the early afternoon hours with the breakout
along the coast a little later in the afternoon.

KSEA...Stratus arriving at the terminal around 12z with ceilings
1000-1500 feet. Ceilings improving mid to late morning with the
stratus scattering out 19z-20z. Variable wind less than 6 knots
becoming southerly overnight. Winds becoming northwesterly Thursday
afternoon. Felton

&&

.MARINE...Onshore flow of varying strength will continue through
early next week due to persistent high pressure offshore with lower
pressure east of the Cascades. The onshore flow will be strong
enough for brief gale force westerlies over the central and eastern
Strait of Juan de Fuca late tonight. There is a possibility of gale
force winds over the same area Friday evening. Felton

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Gale Warning until 2 AM PDT Thursday for Central U.S. Waters
     Strait Of Juan De Fuca-East Entrance U.S. Waters Strait Of
     Juan De Fuca.

&&

$$

www.weather.gov/seattle

An illustrated version of this forecast discussion can be seen at
www.wrh.noaa.gov/sew/gafd/latest_webafd.html


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