Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 210354
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
900 PM PDT Thu Jul 20 2017

.SYNOPSIS...Tonight and Friday will be dry. A weak disturbance will
move by to the north of the area Friday night and Saturday, giving a
chance of light rain to northern sections. Sunday and Monday will be
dry, sunny, and warmer than normal. There will be an increase in
night and morning clouds and temperatures will cool to near normal
starting about Tuesday.

&&

.SHORT TERM...Showers have ended. There are still plenty of clouds
over the about the northern half of the area, but the southern half
has cleared. 500 mb heights will rise tonight and Friday as a weak
shortwave upper ridge moves across the area. Conditions aloft will
dry and warm in response to this weak ridge. High temperatures on
Friday will be well into the 70s in most areas, with some low 80s in
the southwest interior.

A weak disturbance in the upper flow will move by, mostly to the
north, Friday night and Saturday. A little rain is likely on the
north coast, but this system appears weaker than the one just past,
so northern areas will only have a chance or slight chance of
showers, and southern areas will remain dry. 500mb heights are
in the mid 580s, which definitely works against any rain falling.
Highs Saturday will be in the low to mid 80s from about Seattle
south, 70s elsewhere.

Despite continued onshore flow on Sunday, high 500-mb heights and
warm conditions aloft will give temperatures in the mid 70s to
lower 80s in the interior as well as dry conditions. Burke/Albrecht

.LONG TERM...Previous discussion...Monday will be much like Sunday
with 500 mb heights remaining in the lower 580 DAM range and low
level flow remaining onshore. Then Tuesday onward we will see
lowering heights aloft as a mid 560 DAM upper low passes through
southern British Columbia and onshore flow increasing somewhat. The
forecast from Tuesday onward will feature more extensive night and
morning clouds and temperatures around or slightly below normal for
late July. A few model solutions hint at some isolated showers mid
week as the upper trough passes by to the north, but we are entering
our driest time of year and most solutions are dry. Albrecht

&&

.AVIATION...Westerly flow aloft over Western Washington this
evening will continue tonight and Friday, as a flat upper ridge
builds over the Pacific Northwest. Light onshore flow will
continue at the surface. The air mass will remain somewhat moist,
mainly below 6000 ft.

Cloud cover across Western Washington is SCT-BKN050 this evening,
and the trend will probably be for that cloud deck to scatter
during the next few hours. Low level moisture and weak onshore
flow should lead to areas of MVFR ceilings to develop late
tonight and Friday morning, followed by clearing during the late
morning and afternoon.

KSEA...Southwest wind 6-12 kt veering to northwest 4-8 kt during
the next few hours, then becoming variable 3-7 kt after midnight.
A MVFR cloud layer with bases around 015-020 is likely to form
late tonight/early Friday morning, with clouds lifting and
clearing for VFR conditions and ceilings above 12,000 ft by
afternoon. McDonnal

&&

.MARINE...A surface ridge will be along the coast tonight and
Friday with lower pressure inland for light onshore flow. It looks
now like the westerlies in the Strait of Juan de Fuca will remain
20 kt or less, and the small craft advisory will be ended.

A front will move into the offshore waters Friday and dissipate
offshore on Saturday. Marginal small craft advisory southerlies
are possible over the northern part of the coastal waters Friday
afternoon and Friday night. This will be followed by onshore flow
Sunday through Tuesday with high pressure offshore and lower
pressure inland. McDonnal

&&

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

$$

www.weather.gov/seattle

You can see an illustrated version of this discussion at
www.wrh.noaa.gov/sew/gafd/latest_webafd.html



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