Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 222153
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
300 PM PDT Sun Oct 22 2017

.SYNOPSIS...Scattered showers will end tonight. Flooding will
continue on a few rivers, but most are on the way down. Monday will
have a few showers in the mountains, and Tuesday will be dry. After
some light rain Wednesday, dry weather should return.

&&

.SHORT TERM...Radar shows scattered showers. These have had a few
lightning strikes and some gusty winds, but they are waning and
should be mostly done by sundown. Skies will clear somewhat
overnight and patchy fog will develop.

Monday and Tuesday now appear completely dry. Skies will become
completely clear Monday night and the resulting fog late at night
and Tuesday morning will be more extensive. Most of the fog will
evaporate on both days and there will be a slow warming trend. Highs
Monday will be 60-65, and they will be a few degrees higher Tuesday.

The weak system that will arrive from the northwest on Wednesday has
been looking even weaker in recent model runs. The forecast is now
for a chance of showers in the lowlands on Wednesday, with showers
likely in the mountains. There will be considerably more clouds on
Wednesday, so high temperatures will be 55-60. Burke

.LONG TERM...After the weak front Wednesday, an upper ridge will
build off the coast, then slowly move inland through the weekend.
This should give clear skies, dry weather, and highs mostly in the
60s Thursday through Sunday. Burke

&&

.HYDROLOGY...While some showers remain on radar...the threat for
heavy rain over the area has ended. Still...the effects of said rain
are still being felt on some of the area rivers. Flood warnings
remain in place...although the bulk of activity is trending
downward. For example...while a flood warning remains in effect for
the Skokomish River...current hydrographs indicate that waters have
fallen below flood stage. As such...that particular warning will be
allowed to expire upon the next FLS issuance. The Skykomish is in a
similar situation. The Snoqualmie remains in minor flood stage but
continues to trend downward...so that warning looks to remain in
place into tonight. The Cowlitz at Randle looks like it has crested
just over moderate flood stage and will start a downward trend soon.
While the Snohomish remains below flood levels at the time of this
writing...it is close and is still trending upward. That flood
warning will remain in place...but will merit watching in the
evening hours as the current trend does not really seem to fit well
with current hydrograph forecast.

The flood watch in place for Lewis and Thurston counties remains in
place due to a combination of the increased level of the Cowlitz and
the continuing presence of showers there. As activity...both flood
and showers...dwindles this evening...hard to imagine this watch not
being pulled down at some point in the night.

All in all...general activity continues a downward trend as the area
enters a phase where watches and warnings will slowly expire through
the night.  SMR

&&

.AVIATION...West flow aloft over Western Washington this afternoon
will veer to northwest tonight and continue Monday, an an upper
ridge builds offshore and moves slowly toward the Pacific
Northwest coast. The air mass will gradually dry, especially at
mid and upper levels, with low level onshore flow turning weakly
offshore tonight.

We still have scattered light showers with BKN035-055 across
Western Washington. Isolated, brief thunderstorms are possible for
the next couple hours at the coast and over the Olympics. Relatively
clear skies and light winds tonight, combined the moist ground,
should lead to patchy fog in the more fog-prone valleys late
tonight and Monday morning.

KSEA...Southwest wind 7-13 kt gusting 20 kt, easing this evening,
becoming southeast 4-8 kt tonight, then becoming northerly Monday
afternoon. VFR conditions with ceilings lifting above 6000 ft
later this evening. McDonnal

&&

.MARINE...Onshore flow this evening will turn weakly offshore
late tonight and continue through Tuesday, as a ridge builds over
the Pacific Northwest. Westerly swell 10 feet or higher will
continue over the coastal waters and at the west entrance Strait
of Juan de Fuca through Monday evening.

A weak cold front will move southeast across the area on
Wednesday, with small craft advisory winds over some marine
zones. Northerly offshore flow will follow the front Thursday and
Friday as a ridge builds over southern British Columbia. McDonnal

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Flood Watch through Monday morning for Cascades of Pierce and
     Lewis Counties-Southwest Interior.

PZ...Small Craft Advisory for hazardous seas until 11 PM PDT Monday
     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 Monday for
     Grays Harbor Bar.

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

&&

$$

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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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