Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 132235
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
335 PM PDT Fri Oct 13 2017

.SYNOPSIS...An upper level trough east of the area this afternoon
will pull away this evening. A weakening frontal system will
dissipate as it moves into the coastal waters on Saturday. Then
high pressure will build into the region Saturday night through
Sunday night before moving off to the east on Monday. A series of
increasingly strong fronts will begin to affect the region with
cool, wet, and windy condition starting late Monday night or
Tuesday.

&&

.SHORT TERM...Skies continue to clear and showers continue to
dissipate as the upper trough that brought inclement weather to
the region last night and this morning moves well off to the east.
Isolated shower activity will increasingly become tied to the
Cascades this evening then will come to an end by midnight
tonight. With the clear and mostly calm conditions tonight, expect
it to get rather cool with lows in the mid 30s to lower 40s across
the region.

The combination of good mid-October radiational cooling, warming
aloft, and wet ground will result in the formation of some fog in
area river valleys overnight through midday Saturday. The fog
formation will be held in check somewhat as mid and high level
moisture increases from the west with the approach of a
dissipating warm front. The front will mainly give clouds to the
northern two-thirds of the region, with spotty light rain being
limited to the far north and the Olympic Peninsula on Saturday.

Once the front dissipates, expect high pressure over the region to
give dry and calm conditions Saturday night through Sunday night.
Sunday appears that it will be the nicest day of the next week or
so with highs getting into the upper 50s to mid 60s and most
areas seeing some sunshine.

A stronger front will be approaching the area from the west Monday
into Monday night. High pressure moving off to the east will
induce some offshore flow, so Monday appears that it will be dry
and mild with increasing mid and high level moisture. Rainfall
Monday, if it occurs, should be limited to the northern portion of
the Olympic Peninsula. Albrecht

.LONG TERM...A moderately strong front will sweep east across the
area late Monday night through Tuesday and will be followed by
another stronger front Wednesday or Thursday. Operational GFS and
ECMWF solutions have generally sped up with the second front. The
second front will produce a period of heavy rainfall, especially
in the mountains, and windy conditions. Snow levels will bounce
between 4000 feet between frontal systems to 6500 feet in the warm
sector. While the fronts, especially the second one, appear that
they will be rather strong, models remain less than ideally
consistent with the timing of the fronts and duration of heavy
precipitation. Confidence should increase as we get closer to the
potential event. Albrecht

&&

.AVIATION...Upper level troughing has moved well east of the area
and will continue to pull away tonight. Scattered light showers
are increasingly becoming limited to the Cascades and the Eastern
Puget Sound Lowlands near the Cascades. Shower activity is
expected to come to an end by midnight tonight.

Conditions this afternoon are mostly VFR, with MVFR ceilings
limited to showers. Expected VFR conditions later this afternoon
and tonight as onshore flow relaxes.

Clouds will increase over the Olympic Peninsula late tonight or
early Saturday morning as a weakening frontal system approaches
the area. But the general lack of cloud cover for most of the
night in the interior will result in good radiational cooling.
The combination of the radiational cooling, wet ground, and
warming temperatures aloft will result in the formation of some
fog late tonight with IFR or LIFR conditions through midday
Saturday in more fog-prone river valleys. At this time, fog is not
expected to be an issue at most of the area terminals, with the
exception of KOLM. Albrecht

KSEA...Mostly VFR conditions with SCT-BKN035 becoming SCT020 after
midnight. Residual showers should be completely out of the picture
by the 00z TAF issuance. Northerly wind 6 to 9 kt becoming more
NE this evening then SEly around 12Z. Albrecht


&&

.MARINE...Northwesterly winds and steep northwesterly waves along
the coast are easing and will continue to do so this evening.
Winds will shift to south and increase early Saturday morning as a
weakening frontal system moves into the coastal waters then
dissipates before moving inland. At this time, it appears that
small craft advisory conditions on Saturday will be limited to the
coastal waters from Point Grenville northward and out to about 30
nm from the coastline as flow aloft interacts with the Olympic
Mountains.

High pressure will build into the waters from the south Saturday
night through Sunday night giving a period of lighter winds.

A moderately strong frontal system will sweep eastward across the
waters late Monday night and Tuesday and will be followed by an
even stronger front on Wednesday or Thursday. Widespread small
craft advisory conditions can be expected with the front as it
passes Tuesday morning and local gales are possible. More
widespread gales and possible heavy seas are expected with the
second front. Albrecht

&&

.HYDROLOGY...Any precipitation that occurs across the area on
Saturday with a dying front will be quite light and will not
impact area rivers.

A moderate to strong front will move across the region late
Monday night into Tuesday. This front will give many places in the
lowlands 1 to 2 inches of rain and may produce even more in the
mountains. With area most rivers running at base flow, this front
will likely not produce flooding on area rivers -with exception to
the normally flood-prone Skokomish. With fall leaves clogging
area drains, low spots in urban areas may see some street flooding
with Tuesday`s front.

A stronger front will move across the area Wednesday or Thursday.
The model timing is generally faster than previous solutions, but
the ECMWF operational run is slower than the faster but slightly
wetter GFS. Both model solutions show a period of rather heavy
precipitation with snow levels between 5000 and 6500 feet.
Heavier model solutions would possibly result in minor flooding on
normally flood-prone rivers. Albrecht

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory from 6 AM to 5 PM PDT Saturday 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.

&&

$$

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


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