Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 200320

Area Forecast Discussion
National Weather Service Seattle WA
820 PM PDT Mon Mar 19 2018

.SYNOPSIS...Weak high pressure is over the area. Moist southwest
flow will develop on Wednesday. Rain will increase Wednesday night
as a cold front moves ashore. A deep upper level low will keep the
air mass cool and unstable Thursday through Saturday. A ridge will
bring drier weather on Sunday.


.SHORT TERM...No updates this evening, quiet weather with just a
few patches of clouds here and there. Winds are light so we might
see some patchy fog or low clouds develop overnight. Lows will drop
into the 30s.

The pattern will remain rather benign on Tuesday with weak
onshore flow. Models show some moisture over B.C. which may clip
the area. Isolated showers are expected again, mainly in the
Olympics and north Cascades.

Wet weather will return Wednesday and Thursday as a deep upper
level low forms over the NE Pacific, setting up moist S/SW flow
over WA. A cold front will shift inland Wednesday night, the focus
for steadier precip. A trough axis will swing through on Thursday
keeping the air mass cool and unsettled.

.LONG TERM...The air mass will remain cool and unstable Friday and
Saturday as a cold upper level low crosses the region. Snow levels
will be low with a chance of snow showers on the higher hills.
Significant accumulations are not expected since daytime temps will
be in the mid to upper 40s but lows will be in the 30s so a skiff on
the grass at night could be possible. A weak upper level ridge will
bring drier weather on Sunday. Temperatures will be a few degrees
warmer although still trend below normal. Rain will return on Monday
as the next Pacific frontal system arrives.


.AVIATION...There is an area of mid level clouds over the North
Cascades and a few smaller patches of clouds elsewhere--but much of
Western Washington is clear this evening. Weak high pressure is over
the area. The air will be fairly stable through Tuesday, but a
couple of showers over the mountains cannot be ruled out again
Tuesday afternoon. The air is probably not dry enough to rule out
some patches of morning fog or low clouds--mainly over southwest
Washington and the river valleys.

KSEA...The mid level deck has pulled away from the area, so it looks
like we will see mostly clear skies through tonight. Perhaps the mid
level clouds will show up again later on Tuesday as a weather system
over British Columbia slowly sags south toward the area.


.MARINE...A ridge of high pressure will remain over the waters
through Tuesday night giving light winds and weak afternoon/evening
sea breezes.

A frontal system will move slowly east through the coastal waters on
Wednesday and inland later Wednesday night or early Thursday. Some
small craft advisory level winds will likely develop Thursday or
Thursday night. Weaker frontal systems will affect the waters Friday
through the weekend.


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



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