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afdsew

Area forecast discussion
National Weather Service Seattle Washington
749 PM PDT Tue Sep 17 2019

Update /tonight/...there have been a few thunderstorms over the
interior this evening...further inland than earlier expected. The
forecast has been updated to expand the thunder threat into Puget
Sound. The thunderstorms have been quite isolated however and
even that threat should end after this evening over the land as
the air mass becomes a bit more stable. Otherwise, the current
forecast is on track and no updates are planned this evening. The
previous short and long term discussions are below. Schneider

&&

Synopsis...a cool upper level low will maintain showers and a
slight chance of thunderstorms through Wednesday. A weak upper
level ridge will build into British Columbia Thursday with the
ridge remaining over the region into Saturday. A weak system will
try and move through the ridge Friday. Another frontal system will
arrive Sunday.

&&

Short term /tonight through Friday/...showers will continue to
rotate through western Washington tonight from an upper level low over the
offshore waters. No lightning strikes yet, but there's plenty of
shower activity streaming inland in S/SW flow. The highest
instability, and best likelihood of seeing thunderstorms, will be
near the coast (closest to the cold core).

Expect scattered showers on Wednesday as the upper level low
remains overhead. The air mass is marginal for thunderstorms with
an Li near 0. Temperatures will again trend below normal with
highs mainly in the low to mid 60s.

We'll see a break in the weather Wednesday night and Thursday as
the low exits and high pressure nudges into the pac northwest from the
west. The low level flow remains onshore so we'll still see some
clouds over the area. Upper level ridging remains in place on
Friday but the models are showing some light precip clipping
western Washington in northerly flow. 33

Long term /Saturday through Tuesday/...Saturday is the driest
day of the upcoming weekend as the upper level ridge crosses the
region. Our next frontal system will then bring more rain Saturday
night into Sunday. This is another wet frontal system with
widespread wetting rains expected.

Dry, northerly flow will develop on Monday giving US a break in
the weather. But it's unclear how long this dry stretch will last
as the models start running out of phase. Therefore, will continue
to use a forecast blend through the rest of the extended forecast.

&&

Aviation...an upper trough will be over the area tonight and
Wednesday with southwest flow aloft tonight becoming northerly on
Wednesday. At the surface, a weak low over the coastal waters will
shift south tonight with onshore weak onshore flow developing on
Wednesday. The air mass is moist and unstable.

There have been a few thunderstorms this evening but this threat
should end in the next few hours as the air mass become a bit more
stable. There will be a slight chance of thunder again Wednesday
afternoon as daytime heating destabilizes the air mass.

Low clouds have scattered in spots this evening but these should
fill back in overnight as winds die down and the air mass
stabilizes. Low clouds Wednesday morning should lift and scatter
in places later Wednesday.

Ksea...mostly mid clouds this evening. Low clouds should return
Wednesday morning then lift by early afternoon. South wind 4-8
knots will become light tonight and turn northerly on Wednesday.
Schneider

&&

Marine...winds will ease tonight in the wake of a front.
Relatively weak onshore flow will prevail Wednesday through Friday
with high pressure offshore and lower pressure inland. A front
will approach from the west on Saturday and then move inland
Saturday night and Sunday. Schneider

&&

Hydrology...the daily hydrology discussion has ended until the
start of the next rainy season; it will only be updated as needed
until then.

&&

Sew watches/warnings/advisories...
Washington...none.
Pz...none.

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