Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 181534

Area Forecast Discussion
National Weather Service Seattle WA
834 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...A cutoff low offshore Northern California will
spread high clouds across Western Washington today in southerly
flow aloft. A few showers will linger, mainly over the mountains.
Most of the lowland will be dry. Highs will be a couple degrees
below normal.

Isolated showers will end this evening as high pressure aloft
builds into the area. Thursday will be mostly sunny and a few
degrees warmer with an upper ridge overhead.

The ridge will shift inland on Friday. Areas of rain ahead of the
main frontal band will spread into the western part of the area
in the afternoon as a front nears. The GFS brings the leading edge
of rain to Seattle by late afternoon but other models are
slightly slower and those solutions are preferred for now.

The front will move onshore Friday night with a few hours of
rain in the middle of the night. It looks like a pretty quick
shot of rain with showers behind the front quickly becoming mostly
confined to the mountains Saturday morning. Schneider

.LONG TERM...Previous discussion...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.


.AVIATION...An upper level trough extending from the central
Oregon coast northward into the Washington offshore Waters will
give light south flow aloft today. Flow aloft will become
northwesterly late tonight into Thursday as an upper level ridge
moves into the coastal waters then will become westerly during the
day Thursday as the ridge moves eastward into Western Washington.
The air mass is stable. At the surface, a ridge of high pressure
over southeastern British Columbia will give light northeasterly
flow today. Flow will become more north to northwesterly tonight
into Thursday as another ridge builds offshore.

The air mass is mainly dry at lower levels with most of the region
seeing BKN-OVC100. The exception is along the north slopes of the
Olympics and the Kitsap Peninsula to the Hood Canal area where
northeasterly flow is giving MVFR and areas of IFR conditions in
low stratus and in the fog-prone valleys of the south interior
where there is some fog. The low level moisture is expected to
dissipate midday leaving just the mid and high level moisture
above about 10000 feet. Albrecht

KSEA...Good VFR conditions with ceilings 10000 ft and above. Light
winds will become northerly 10 kt this afternoon and evening then
light northeasterly later tonight into Thursday morning. Albrecht


.MARINE...A ridge of high pressure over Southeastern British
Columbia will give light north to northeast flow this morning.
Flow will back to more north or northwesterly tonight as ridging
strengthens along about 130W.

Thursday afternoon and evening will see increasing onshore flow as
pressures fall east of the Cascades and high pressure ridging
remains over the offshore waters. Small craft advisory westerly
winds are likely late Thursday afternoon and Thursday night
in the eastern two thirds of the Strait of Juan de Fuca. The
onshore flow will persist on Friday as a cold front approaches the
coastal waters.

A moderate cold front will move through the waters Friday night
and will be followed by strong onshore flow late Friday night into
Saturday as high pressure rebuilds offshore. Albrecht




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