Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 190432

Area Forecast Discussion
National Weather Service Seattle WA
930 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...Radar is still picking up a few isolated showers
around the area but activity will dissipate shortly with loss of
diurnal heating. An upper low off the Wa/Oregon coasts will shift
westward and dissipate on Monday. The region will stay between
systems through Tuesday. Expect mostly dry weather, except for a few
isolated diurnal driven showers, mainly over the mountains.

A more significant change in the pattern develops around Wednesday.
Models have not been very consistent on the timing and strength of a
developing trough and frontal system. The GFS is deeper and more
negative tilt while the ECMWF is further north with the low. Both
solutions bring rain or showers into the region Wednesday. Most of
the uncertainty is in regards to surface low development and track.
The pattern favors some surface low development but the latest
GFS/ECMWF do not show much development. No updates planned this
evening. Mercer

.LONG TERM...Previous discussion...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...Upper level low pressure will shift westward and
dissipate Monday. Light southerly flow aloft will become light
easterly overnight. Latest satellite imagery shows some areas of mid
cloud developing. This could inhibit much MVFR stratus from forming
Monday morning. Mostly VFR skies by late Monday morning.

KSEA...Mid clouds. A slight risk of MVFR from 12-16z. 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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