Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 081156
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
330 AM PST Fri Dec 8 2017

.SYNOPSIS...A strong upper ridge will maintain dry and stagnant
conditions across Western Washington through the end of next week.
Inversions will keep temperatures cooler in the lowlands and warmer
temperatures in most mountain locations. Fog and Freezing fog will
expand in coverage and duration today and Saturday. High clouds
could increase this weekend as a frontal system passes by well to
the north with less fog coverage Sunday into next weekend. Air
quality will likely worsen over at least the next couple days.

&&

.SHORT TERM...High pressure and subsidence over the region has
suppressed the inversion across the lowlands. While the overall air
mass is dry, any residual surface moisture is resulting in fairly
widespread fog including along the I-5 corridor through Western WA.
Radiational cooling is very effective at the moment with
temperatures down to the mid and upper 20 across most areas away
from the water and warmer metro areas. Freezing fog will be the main
concern this morning as it will affect much of the area by causing
slippery roadways. Black ice can catch drivers off guard, so
motorists should allow extra space between vehicles. The flow is
much lighter than previous morning so fog could linger into the
afternoon, especially around the southwest interior/south Sound.
Other spots could have patchy fog into the afternoon as well.

A weak system will move over the ridge Saturday and Sunday. The main
effect will be high clouds traversing the area at times. This will
cut down the amount of radiational cooling and fog coverage at night
but also probably only worsen the temperatures inversion. Air
quality has also worsened somewhat overnight, with many stations now
in the moderate category for sensitive groups and even some reaching
unhealthy. Agencies will be coordinating the morning regarding the
air quality forecast, so check back for any new statements that
could be issued. Expect somewhat cooler daytime highs in the
mid/upper 40s and possibly hazy.

.LONG TERM...Differences have developed between the GFS and ECWMF
regarding the strength of the ridge next week. The new GFS now more
closely mirrors the ECMWF in tracking a weak upper low just off the
coast around late Monday. This could induce some onshore flow or it
may not have any surface reflection in the form of a front or
stronger flow to break up inversions. Stuck with the inherited
forecast and kept dry weather in the forecast for the next 7 day.
There are some hints by models that a gradual weakening of the ridge
could allow for a pattern change beyond day 7, beyond the forecast
period.  Mercer

&&

.AVIATION...A strong upper ridge axis will sit over Central
Washington through tonight. Light to moderate southwest flow
aloft. The air mass will be dry with persistent low-level
temperature inversions. This will lead to stubborn persistent fog
in places over the lowlands. The coast and mountains should remain
clear.

KSEA...Fog surrounded the terminal on all sides overnight, and
shallow fog is now encroaching on the airfield. Seems that the
thicker fog area that was over the South Sound last evening has
shifted up to Sea- Tac. With such a stable air mass, will be tough
to dissipate the fog now that it has formed. Just amended TAF to
show fog until noon, but confidence in this timing is low. Haner

&&

.MARINE...Easterly gradients between a surface ridge over the
Rockies and lower pressure offshore will maintain low level
offshore flow through this weekend. Small craft advisory strength
winds are likely at times near the West Entrance to the Strait of
Juan De Fuca. Haner

&&

.HYDROLOGY...River flooding is not expected during the next week.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory until 3 AM PST Saturday for Coastal Waters
     From Cape Flattery To James Island Out 10 Nm-West Entrance
     U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory from 3 PM this afternoon to 3 AM PST
     Saturday for Coastal Waters From Cape Flattery To James
     Island 10 To 60 Nm-Coastal Waters From James Island To
     Point Grenville 10 To 60 Nm-Coastal Waters From Point
     Grenville To Cape Shoalwater 10 To 60 Nm.

&&

$$

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