Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 181525

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

.SYNOPSIS...Upper level low pressure over the Pacific Northwest
will bring spotty light showers and sun breaks 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...A 5420 meter upper low is over the Washington
offshore waters this morning, centered near 48N/127W; this is the
feature that formed over Vancouver Island on Saturday and drifted
south overnight. Light southerly flow aloft continues over Western
Washington to the east of the low. The air mass is slightly
unstable, moist below about 7000 ft, and the upper low is
producing some showers mainly over the coastal waters and along
the coast.

The upper low will weaken a bit as it moves slowly southwest
today. Daytime heating will destabilize the air mass a bit more,
which should result in some showers developing over the mountains.
The threat of showers over the Western Washington interior
lowlands looks rather slight, and precipitation that does occur
will be mostly light. Showers should mostly dissipate after sunset
as the air mass stabilizes.

The upper low will weaken further on Monday as it tracks slowly
southwest over the Oregon offshore waters. The air mass will be
less moist and less unstable Monday, and the threat of showers
should be limited to the mountains and mainly during the afternoon
and evening. There could be considerable cloudiness during the
morning, but at least over the interior lowlands it should give
way to some afternoon sunshine.

A few days ago models showed a weak ridge building over the
Pacific Northwest Monday and Tuesday, then just some ridging on
Tuesday, and now it looks more like it will amount to just the
absence of a trough. Tuesday still looks like a dry day though,
and it should be mostly sunny across most all of the forecast
area. McDonnal

.LONG TERM...Here is the long term section from the early morning
forecast discussion -- 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...A surface and upper level low over the Washington
coastal waters will drift very slowly southwestward into the
Oregon offshore waters by Monday. Light flow aloft, mainly out of
the southeast today and variable on Monday. The air mass is
somewhat moist and weakly unstable.

Conditions are on the lower end of VFR most areas this morning
with areas of MVFR and local IFR conditions in lower ceilings
occurring from around KBVS into the San Juan Islands. Area radars
show showers rotating around the low to the west of the area with
showers most concentrated over the coastal waters and on the
immediate coastline from about 20N KHQM southward. Only isolated
light showers are seen over the higher terrain east of KOLM.
Expect shower activity to increase somewhat during the late
afternoon and evening hours over the higher terrain. Isolated
light showers are possible elsewhere, but the chances will be low
enough (generally less than 30 percent) to only mention a VCSH at
the inland terminals KPAE southward. After 18Z, VFR conditions
are expected at all of the terminals, though brief MVFR ceilings
are possible around the light shower activity.

Showers will diminish after sunset and as the low moves southwest
of the region. Patches of MVFR are possible during the morning
hours on Monday after some radiational cooling overnight, and
lower IFR stratus with some 1-2SM BR is possible in the KOLM area
early Monday morning. Albrecht

KSEA...Lower end VFR conditions with ceilings around 035 will
improve to SCT035 BKN050 after about 18Z. A few showers may get
close to the terminal as they wrap around the offshore low
pressure system, but it is rather unlikely that any showers will
actually reach the terminal area. Will throw in a VCSH group for
the period 20Z-02Z to cover this. Expect S-SW winds 5-9 kt this
afternoon and light SE winds tonight and Monday morning. Albrecht


.MARINE...A weak surface low over the outer portions of the
Washington coastal waters will drift slowly southwestward into the
Oregon offshore waters on Monday. Winds will be generally light
offshore this morning with the weak low nearby, and only weak
onshore flow is expected this afternoon and evening with weak
daytime heating in the interior.

Onshore flow is expected to 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. Albrecht/CHB


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




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