Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 232222
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
330 PM PDT Sun Oct 23 2017

.SYNOPSIS...High pressure building over the region will provide dry
weather tonight and Tuesday. Residual moisture will produce some
morning fog or low clouds late tonight and Tuesday morning. A weak
front will move through the area Wednesday with some light rain. Dry
weather will return Thursday and should prevail through the weekend.
Areas of fog will develop late in the week.

&&

.SHORT TERM...High pressure will build north over the region through
Tuesday providing dry and mild weather. There had been some fog last
night, but high clouds moved in late and seem to have ended most of
the fog. Tonight and Tuesday will be clearer, so there will probably
be more fog around the area late tonight and Tuesday morning. High
temperatures will be above average, mainly in the mid 60s Tuesday.

A decaying front will move southeast across the area on Wednesday.
Recent model runs have trended toward a weaker system with marginal
moisture and lift. Rain in the lowlands should be spotty and quite
light. Even the mountains will probably get less than a tenth of an
inch. However likely pops seem about right in the Cascades. Highs
Wednesday will be 55-60.

Chance pops will linger in and near the Cascades into Wednesday
evening, then skies will clear again. Heights will rise in dry
northerly flow. Highs Thursday will be in the low 60s. Burke

.LONG TERM...Skies will remain mostly clear through the weekend as
strong upper ridge moves slowly through the region. Some clouds are
possible Sunday and/or Monday as the tail of a weak upper trough
moves by far to the northeast. The weather will remain dry, however.
Have introduced fog each day Friday through Sunday. The fog could
get fairly extensive at night, and last into the afternoon in some
places. Generally temperatures will be slightly above average, in
the low 60s, except in places where the fog lingers, where it will
stay cool all day. Burke

&&

.HYDROLOGY...River flooding is unlikely for the next seven days.

&&

.AVIATION...A strong upper ridge axis will be located over Western
Washington through Tuesday morning, then it will start to flatten
and weaken. The air mass will be dry and stable, except for a
near-surface moist layer. Moderate northwest flow aloft today and
this evening will become light westerly late tonight and Tuesday.
A clear evening is expected, setting the stage for fog
development late this evening and overnight for the more typical
suspects...OLM, PWT, CLS, SHN, AWO, eventually HQM, and possibly
more. On Tuesday, light surface wind and low-level inversions will
make fog burnoff over the lowlands difficult with patches of fog
and low clouds lasting into the afternoon.

KSEA...Mid-level cloud layers will thin out and then dissipate
this evening, leaving clear skies for most of tonight. This will
lead to some fog development in valleys SE of the airfield late
tonight, but expecting fog to avoid the terminal on Tue morning.
Northerly surface flow could push some Lake Washington steam
stratus clouds into the vicinity of the terminal around sunrise
Tuesday, just going FEW005-007 in the TAF right now. Light
northerly surface wind is expected through Tue evening. Haner

&&

.MARINE...Strong high pressure centered near Western WAshington
will support weak northerly offshore flow through Tuesday. A weak
cold front will move southeast across the area on Wednesday,
preceded by moderate onshore flow in the morning, then followed
by SCA-strength northwesterlies over the outer coastal waters on
Wed PM. A thermal pressure trough will build up west of the
Cascades on Thursday and Friday, bringing northerly offshore flow.
Gradients and winds will weaken on Saturday. Haner

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory for hazardous seas until 11 PM PDT this
     evening for Coastal Waters From Cape Flattery To James
     Island 10 To 60 Nm-Coastal Waters From Cape Flattery To
     James Island Out 10 Nm-Coastal Waters From James Island To
     Point Grenville 10 To 60 Nm-Coastal Waters From James
     Island To Point Grenville Out 10 Nm-Coastal Waters From
     Point Grenville To Cape Shoalwater 10 To 60 Nm-Coastal
     Waters From Point Grenville To Cape Shoalwater Out 10 Nm.

     Small Craft Advisory for rough bar until 11 PM PDT this evening
     for Grays Harbor Bar.

     Small Craft Advisory for hazardous seas until 5 PM PDT this
     afternoon for West Entrance U.S. Waters Strait Of Juan De
     Fuca.

&&

$$

www.weather.gov/seattle

You can see an illustrated version of this discussion at
www.wrh.noaa.gov/sew/gafd/latest_webafd.html


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