Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 180952 AAA

Area Forecast Discussion
National Weather Service Seattle WA
300 AM PDT Sun Mar 18 2018

Updated aviation section.

.SYNOPSIS...Upper level low pressure over the Pacific Northwest will
bring scattered light showers to Western Washington through Monday.
A weak upper ridge will be over the region Tuesday for mostly sunny
weather. Rainy weather will return Wednesday and Thursday as a
system moves through, with cool showery weather Friday and Saturday.


.SHORT TERM...An upper level low centered along the Washington
coast will slowly move south and weaken through Monday. By Monday
evening it should be little more than an open trough in Oregon. At
the surface the flow is light onshore. This is a good pattern for
clouds and scattered showers, which is indeed the forecast through
Monday. Highs today and Monday will be in the low to mid 50s, with
overnight lows 30-40.

The upper low will probably be far enough away on Tuesday to allow a
weak upper ridge to build over Western Washington. This will give a
mostly sunny and slightly warmer day. Highs Tuesday are forecast to
be in the mid to upper 50s. Burke

.LONG TERM...Southerly flow will start to bring moisture into the
region Wednesday. Precipitation will begin in the mountains, then
become more widespread during the day. The forecast has likely pops
now, but rain is looking more certain. A deep surface low is
forecast by models to move north just off the coast Wednesday night.
By Thursday there will be a strong southerly gradient over the area.
The forecast has lots of WINDY and BREEZY wording Thursday. The
gradient currently does not appear strong enough for high wind,
though some wind advisories might be needed.

After the surface low moves away, an upper trough will move toward
Washington Friday, and move through the area Saturday. This appears
cold with heights in the low 530s. The snow level could fall to as
low as 1000 feet by Friday. More vigorous showers could bring
local snow to the lowlands. Burke


.AVIATION...Light southeast flow aloft this morning will become
easterly tonight as an upper level low over the Washington coastal
waters shifts into the Oregon offshore waters. The air mass is moist
and somewhat unstable. Near the surface...weak gradients are
producing light and variable winds. MVFR ceilings at 09Z are mostly
near the coast in the vicinity of the upper level low. With the
moist air mass in place and light surface winds, expecting more
widespread MVFR ceilings to develop by sunrise. There will be some
improvement this afternoon, possibly to low end VFR. Isolated to
scattered showers will continue.

KSEA...Ceilings slow to materialize, but presence of the upper low
near the coast and light surface winds should help develop some MVFR
ceilings this morning that will persist into midday before lifting
to VFR. May see a few showers mostly 20Z-02Z as daytime heating adds
a little more instability to the atmosphere. S-SW winds 5 to 10
knots.  27


.MARINE...Winds will be generally light today, with only weak
onshore flow in the afternoon and evening. The onshore flow will
increase Monday afternoon and evening. Small craft advisory
westerlies are likely in the strait Monday evening. Onshore flow may
also be strong enough for advisories Tuesday evening.

A weather system will affect the area Wednesday as a low center
moves north along the coast. Winds will turn approach small craft
advisory criteria at times. After the low moves north, strong
southerly flow Thursday could bring gales. CHB


.HYDROLOGY...River flooding is not expected in the next 7 days.



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