Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 182217

Area Forecast Discussion
National Weather Service Seattle WA
317 PM PDT Sun Mar 18 2018

.SYNOPSIS...An offshore upper low will bring spotty showers to
the mountains and coast through Monday, mainly during the
afternoons and evenings. The interior lowlands should have dry
weather. A weak ridge will be over the region Tuesday for 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...The upper low over our offshore waters has continued
to move slowly southwest and weaken today, and this afternoon it
is near 47N/128W at around 5440 meters. Spotty showers have
continued over the coastal waters and along the Washington coast,
and a few showers have popped up over the Olympics this afternoon,
but so far there has been little development over the Western
Washington interior lowlands or the Cascades. Light southerly flow
continues aloft, and the air mass is slightly unstable with
moisture in the lower 7000 ft. Showers should mostly dissipate
after sunset as the air mass stabilizes.

The upper low will weaken further tonight and 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 a partly to mostly sunny afternoon is

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 the forecast area.

A more significant change in the weather will start to develop
Wednesday, as a frontal system moves toward Western Washington
from the southwest. This system should eventually bring widespread
precipitation to the forecast area; however model solutions are
still not consistent with this feature, and the 18Z GFS is holding
off its arrival till late Wednesday afternoon or evening. McDonnal

.LONG TERM...The frontal system that moves into Western Washington
from the southwest Wednesday or Wednesday night will keep rainy
weather across the area on Thursday. As mentioned above and in
previous discussions, the models are not in strong agreement with
the details of this system, and that applies to Thursday as well.
One change we`ve seen in the models is that they`re now showing a
bit more surface development, and breezy to windy conditions are
likely on Thursday.

The upper trough following the frontal system should bring cool
showery weather to the area Friday through Saturday. The snow
level could fall to around 1000 to 1500 ft, so showery
precipitation could fall as snow over the higher lowland areas
though accumulations are really unlikely. The upper trough will
probably start moving off to the east on Sunday, for decreasing
showers and more sun breaks. McDonnal


.AVIATION...An upper level low off the WA coast will continue
drifting SW. The low will be off the northern OR coast by Monday
morning. Light sly flow aloft will become light ely overnight.
Expect areas of generally MVFR CIGs along with patchy fog (VSBYs
generally in the 1-3SM range) to dvlp overnight.

KSEA...VFR conditions are expected; although, there will probably
be occasional MVFR CIGs (near 2K ft) between 1000 UTC (3 AM PDT,
Monday) and 1800 UTC (11 AM PDT). Winds will generally be light
and variable.


Light onshore or westerly flow will continue through Tuesday. The
flow will become nearly flat on Wednesday. Expect a frontal
system to impact the waters on Thursday for the possibility of
gale force winds over parts of the area. The weather models were
not in agreement on the track and strength of Thursday`s surface
Low; therefore, the forecast reflects a compromise between the
model solutions.


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



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