Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 191117
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
330 AM PDT Thu Oct 19 2017

.SYNOPSIS...A wet frontal system will push south with a transition
from rain to showers today. Snow levels will slowly fall through
Friday with snow accumulation likely in the mountains and possibly
at higher passes. Another strong front will affect Western Washington
with another round of heavy rain and strong winds. Drier weather is
expected to develop early next week as a ridge of high pressure
strengthens.

&&

.SHORT TERM...Radar indicates the heaviest rainfall this morning is
along a nearly stalled frontal band from the south Washington coast
northeast through the I-5 metro corridor up to Seattle and eastward.
Rain has decreased to showers behind the front over the Olympic
Peninsula and north interior. Rain amounts the past 24 hours range
from 2 to 5 inches in the mountains, with locally higher amounts in
the Olympics. This has resulted in some minor river flooding. See
the hydrology section below for further details and the latest flood
bulletin for current river specific information.

Rain will taper to showers across parts of the area today. However,
another frontal waves still offshore will move into Western
Washington this afternoon. Amounts do not look particularly heavy
with this feature but could keep it rather damp across the area.
Winds have eased for the time being with breezy conditions through
Friday. A lull in shower activity is expected between systems
tonight through Friday night.

Models remain in fairly good agreement that the next strong system
will affect the area over the weekend. A negative tilt warm front
will spread rain into northwest Oregon late Friday night. The warm
front will lift north into Western Washington on Saturday with
increasing rain and breezy to locally windy conditions. The
southeast gradient will be tight but the direction is not favorable
for high wind in most spots. However, the coast and north interior
would be the spots to watch for the highest wind gusts. Elsewhere it
will be windy Saturday night and some advisory level wind gusts
possible. Statements to address specifics on the wind and rain this
weekend are likely to be issued today as new model data arrives.

.LONG TERM... Rainfall will gradually ease Sunday night and
Monday. River flooding could still be occurring on some rivers. Long
range models show a drying trend Monday and Tuesday. The ECWMF
differs from the GFS and brings some light rain through on
Wednesday. Kept low pops in the forecast. The overall pattern looks
to shift toward a more benign pattern with mostly dry weather and
mild temperatures. Mercer

&&

.AVIATION...A frontal system will continue to slowly sag
southeast through the area today with southwest flow aloft and low
level onshore flow behind the front. The air mass is moist and
stable. It will become unstable near the coast later today.

KSEA...Despite a deep moist air mass, low clouds have been
decreasing. Best guess is it will be mostly mid clouds this morning
then MVFR low clouds this afternoon as another batch of rain
develops. South wind around 10 knots today. Schneider

&&

.MARINE...A front will continue to slowly shift inland today.
Onshore flow will prevail behind the front into Friday. Small craft
advisory strength winds will gradually ease today although they
could linger in spots into tonight and Friday.

Another vigorous frontal system will move through the area Saturday
and Sunday. Onshore flow behind the front will ease and eventually
turn weakly offshore later Monday. 20-24 foot west swell will affect
the Coastal Waters later today through Friday morning before easing
below 20 feet later Friday. Schneider

&&

.HYDROLOGY...2 to 5 inches of rain fell over the north and central
Cascades the past 24 hours. Heaviest amounts fell overnight and the
south facing slopes of the Olympics had locally higher amounts. This
drove the Skokomish river in Mason county to just below flood stage
before starting to fall. The warning and watch for the Skokomish
river in Mason county will be ended early this morning.

Other rivers are still rising, mainly the Nooksack, Stillaguamish,
and Skagit. The Nooksack at Cedarville has the highest chance of
reaching flood stage this morning, but other rivers could come
close. Refer to the latest flood bulletin for current information.

The lowlands had 1 to 2 inches of rain with locally higher amounts.
Rain will start to decrease this morning, especially from Seattle
north. However, significant ponding of waters on roadways will be
likely during the morning commute. Clogged drainages may cause
water to pool in some intersections. Motorists should exercise
caution.

Rivers will recede later Thursday through Friday night as snow
levels drop to 3000-3500 feet and precipitation becomes more
showery.

Hydrologically significant precipitation appears increasingly
likely Saturday through Sunday across the southern half of the
forecast area (especially from a Stevens Pass-Everett-southern
Olympics line southward). Many rivers may reach flood stage if
precipitation develops as expected. At this time it appears that
any flooding that occurs will be minor.

Conditions will dry out starting later Sunday or Monday of next
week allowing all rivers to recede.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Flood Watches and warnings in effect for portions of the area.
     Refer to the latest flood bulletins for current information.

     High Surf Advisory from 11 AM Thursday to 6 AM PDT Friday for
     Central Coast-North Coast.

PZ...Small Craft Advisory for rough bar until 5 PM PDT Friday for
     Grays Harbor Bar.

     Small Craft Advisory until 6 AM PDT Friday 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 until noon PDT today for Admiralty Inlet-
     East Entrance U.S. Waters Strait Of Juan De Fuca-Northern
     Inland Waters Including The San Juan Islands.

     Small Craft Advisory until 9 AM PDT this morning for Puget Sound
     and Hood Canal-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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