Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 180940

Area Forecast Discussion
National Weather Service Seattle WA
240 AM PDT Wed Apr 18 2018

.SYNOPSIS...A weak upper trough will result in mostly cloudy skies
today with isolated showers, mainly over the mountains. An upper
ridge will bring dry mild weather Thursday into Friday. A frontal
system will move quickly across the area Friday night, followed by
showers and sun breaks on Saturday. A stronger upper ridge will
bring another stretch of dry mild weather Sunday through Tuesday.


.SHORT TERM...The small upper ridge that moved east across Western
Washington on Tuesday is shifting east of the Cascades early this
morning, and an upper low is digging southeast well offshore of
the Pacific Northwest. At the same time, the low level flow has
turned weakly offshore/northeasterly. Radar shows the last remnant
of the Puget Sound convergence zone that was quite active on
Tuesday, extending eastward from Whidbey Island into the northern
side of Snohomish County.

The offshore upper low will continue tracking southeast, moving
into the offshore waters of northern California late today and
then inland across central California tonight. A weak upper trough
extending northward from the low will move east across our
offshore waters today and inland tonight. This feature is really
rather insignificant, and showers associated with it will be
generally quite light and mainly limited to the mountains. It will
induce southerly flow aloft though, bringing some higher level
moisture into Western Washington and keeping skies mostly cloudy.
Highs today will be mainly in the 50s.

An upper ridge, tilting positively into the region from an upper
high well west of California, will move into Western Washington on
Thursday. As we have been advertising for several days now,
Thursday will be a day of mostly sunny weather with highs near to
slightly above normal. Temperatures should break the 60 degree
mark over the interior lowlands from roughly the Seattle area
southward. The ridge axis will shift east of the Cascades Thursday
night, leaving westerly flow aloft over Western Washington.

The next frontal system will approach the region from offshore on
Friday, with the leading edge of its precipitation spreading
inland Friday afternoon and evening. The front cold front and its
upper trough will move quickly across the forecast area Friday
night. McDonnal

.LONG TERM...Some showers and probably a Puget Sound convergence
zone will follow the cold front/upper trough on Saturday, but they
could decrease during the day as an upper ridge begins to build
upstream. Models have been consistent and in good agreement that
the upstream upper ridge will bring dry weather to Western
Washington Sunday and Monday, and it now looks quite likely that
the stretch of dry weather will continue through Tuesday as well.
What`s more, high temperatures should be well into the 60s over
much of the area Monday and Tuesday, with a few spots breaking
70 degrees. I know ... it sounds crazy. McDonnal


.AVIATION...Upper level trough over the southern Oregon offshore
waters will shift into northern California late today with light
southerly flow aloft over Western Washington. The air mass is
generally stable. High and mid level moisture will continue to
rotate around the trough across the region today, but the low level
moisture will dissipate through the day and most shower activity
will be confined to the mountain and coastal areas. MVFR ceilings
across the north interior are expected to dissipate this morning as
the remnants of a convergence erode from the south. VFR ceilings
will prevail across the area from this afternoon through tonight. At
the surface, high pressure will building into southern British
Columbia for developing low level offshore flow and north winds
through Puget Sound terminals.

KSEA...VFR ceilings with mid and high level moisture moving through.
Light and variable wind becoming northerly 6 to 11 knots from 18Z
through tonight.     27


.MARINE...Offshore surface flow this morning due to a 1013 mb
low moving southeast across the OR waters. Weak onshore flow is
expected to return this afternoon as high pressure builds over the
WA coastal waters. Onshore flow will then persist through Friday.
Look for a cold front to move across the area Friday night.



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