Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 191550

Area Forecast Discussion
National Weather Service Seattle WA
850 AM PDT Mon Mar 19 2018

.SYNOPSIS...Today will be partly to mostly sunny with isolated
showers in the mountains. A weak ridge over the region Tuesday
will bring a mostly sunny day. Rain will return Wednesday and
Thursday as a system moves through, with cool showery weather
Friday into the weekend.


.SHORT TERM...An upper level low will spin offshore today then
weaken and merge with a broader cutoff low well off the CA coast.
Across Western Washington, skies will be partly sunny with patchy
morning fog in the south sound and in the San Juan Islands.
Models are showing isolated showers over the mountains this
afternoon and evening, otherwise expect dry conditions over the
lowlands. High temperatures will be close to normal with
temperatures in the low to mid 50s.

The pattern remain rather benign on Tuesday with weak onshore
flow. Isolated showers are expected again, mainly in the Olympics
and north Cascades.

Wet weather will return on Wednesday as a deep upper low forms
over the NE Pacific, setting up moist S/SW flow over WA. A cold
front will shift inland Wednesday night. Model consensus looks
good so far. 33

.LONG TERM...Previous discussion...Models have recently backed
off on the strength of the surface gradient over the region as a
surface trough moves toward the coast on Thursday. Now it appears
conditions will be no more than breezy. Rain will continue at
times on Thursday.

The upper trough following the frontal system will bring cool
showery weather to the area Friday through Saturday. The snow
level will fall to around 1000 ft, so precipitation could fall as
snow over the higher lowland areas. Despite being past the
equinox on Wednesday, this is a wintry pattern. Highs will be
mostly in the 40s, and snow will fall in the mountains. The
upper trough will move off to the east on Sunday, for decreasing
showers and more sun breaks. Burke


.AVIATION...An upper level low over the Oregon offshore waters is
dropping slowly southward while a ridge of high pressure aloft
along the Canadian border moves into the area. Light NE flow aloft
today will become northerly tonight and northwesterly on Tuesday.
Light pressure gradients will prevail at lower levels. The air
mass is somewhat moist. The stable air mass this morning will
become weakly unstable over the mountains late this afternoon and
evening resulting in the development of isolated light showers
there. The air mass will stabilize again overnight tonight.

Patches of MVFR and IFR conditions are seen across the area this
morning with fog and low stratus concentrated over the San Juan
Islands and in the southwest interior. Patches of stratus are
producing ceilings at some of the central Puget Sound terminals at
times this morning as well. Stratus and fog will lift by about
18Z. Expect scattered CU this afternoon and evening with isolated
showers confined to the mountains during the late afternoon and
evening hours.

Conditions later tonight and Tuesday morning will be similar to
this morning with mainly VFR conditions and patches of stratus and
fog. Albrecht

KSEA...A patch of stratus has developed around KBFI and may nose
into KSEA for an hour or so this morning before 18Z, but with a
light southeast wind expect the stratus to likely remain north of
the terminal. VFR conditions are expected this afternoon through
Tuesday morning. Light winds will become north at less than 8 kt
this afternoon and evening. Albrecht


.MARINE...A ridge of high pressure extends from the offshore
waters into the coastal waters while another ridge sits over the
Cascades. expect light northerly flow over the coastal waters and
light to moderate afternoon and evening sea breezes near the
coastline and in the inland waters through Tuesday.

A developing frontal system will slowly move into the coastal
waters Wednesday then inland later Wednesday night or early
Thursday. Expect small craft advisory conditions do develop over
most of the waters early Thursday. Some forecast guidance suggests
that gales could occur over the coastal waters and from Puget
Sound northward into the Northern Inland Waters. Model guidance
has shown quite a bit of variability in the strength of the flow
for Thursday morning so confidence in the wind speed forecast is

Weaker frontal systems will affect the waters Friday through the
weekend. Albrecht


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



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