Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 200308
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
815 PM PDT Thu Oct 19 2017

.SYNOPSIS...Showers will continue tonight and Friday. The snow level
will fall to 4000 feet with light snow accumulations in the
mountains.
A strong warm front will move over Western Washington Saturday with
periods of heavy rain into Saturday night--some flooding is possible.
Drier weather is forecast to develop early next week as high
pressure builds over the region.

&&

.SHORT TERM...Showers continue this evening--but there will be
a little break between systems later Friday. The next strong weather
system will affect the area over the weekend. A warm front will
spread rain into northwest Oregon late Friday night. The warm front
will move north into Western Washington on Saturday with increasing
rain and breezy conditions. The southeast gradient will be tight but
the direction is not favorable for strong winds, but it will be
blustery. Heavy rain is likely over much of Western Washington
Saturday night with a high snow level around 8000 feet. The rain
will taper off Sunday, becoming showery Sunday night. Conditions
will be mild Saturday and Sunday under the influence of the warm
front--after highs in the lower to mid 50s on Friday, highs will
rise into the mid 50s and lower 60s over the weekend.

.LONG TERM...Monday will be dry in most places, with just a chance
of showers in the mountains, and highs in the lower 60s. A strong
ridge is forecast to build over the region Tuesday, with heights
near 590dm. If this were summer, temperatures might reach 90
degrees. As it is October, highs will be in the 60s under sunny
skies on Tuesday. A weak system will move through the area on
Wednesday, followed by cooler northwest flow aloft Thursday. Burke

&&

.AVIATION...An upper trough will be over the area tonight and
Friday with westerly flow aloft. At the surface, low level onshore
flow will strengthen tonight and Friday. The air mass will remain
moist and unstable, especially at the coast.

There will be a mix of low and mid clouds tonight and Friday. In
general, ceilings will be lower in and around showers. Some overall
improvement is likely later Friday with ceilings becoming mostly
VFR.

KSEA...Ceilings should continue to bounce between MVFR low clouds
and VFR mid clouds tonight and Friday morning. The MVFR low clouds
are expected to mostly go away Friday afternoon. South wind 7-13
knots. Schneider

&&

.MARINE...A surface trough will move inland from the Coastal
Waters this evening. Onshore flow will develop behind the trough and
strengthen through Friday. Small craft advisory winds will develop
over most of the coastal and inland waters zones on Friday; the
strongest winds will likely be the westerlies in the Central and
Eastern Strait of Juan de Fuca, and gales there are not out of the
question Friday afternoon.

Another fairly strong frontal system will move through the area
Saturday and Sunday, with widespread small craft advisory winds.
At this time, we are forecasting winds to remain below gale force.
Onshore flow behind the front will ease and eventually turn weakly
offshore later Monday and continue Tuesday.

West swells of 19 to 23 feet over the Coastal Waters will gradually
subside later tonight and Friday. Schneider

&&

.HYDROLOGY...Models continue to agree that a heavy rain event will
occur Saturday afternoon through Sunday afternoon. The heaviest rain
will fall Saturday night. Most models show the focus of rainfall
will be in Cascades from about Stevens Pass southward, with less in
the North Cascades and Olympics. However, past experience with these
heavy rain events shows that models can easily misjudge where the
rainfall eventually falls. The European model does show heavy rain
in the North Cascades. The current river model run is based on the
QPF from a blend of mesoscale models that does not include the
European model.

The river model shows minor flooding on most rivers flowing off the
Cascades from the Skykomish River southward, with a few rivers
touching moderate stage. This flooding will occur mainly on Sunday
from heavy rain Saturday night. Due to uncertainty in where the
heaviest rain will fall, will issue a flood watch for all of Western
Washington, 5 PM Saturday through 5 PM Sunday, this afternoon. Burke

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Flood Watch from Saturday afternoon through Sunday afternoon
for      Admiralty Inlet Area-Bellevue and Vicinity-Bremerton and
  Vicinity-Cascades of Pierce and Lewis Counties-Cascades of
Snohomish and King Counties-Cascades of Whatcom and Skagit
Counties-Central Coast-East Puget Sound Lowlands-Eastern      Strait
of Juan de Fuca-Everett and Vicinity-Hood Canal Area-     Lower
Chehalis Valley Area-North Coast-Olympics-Seattle and      Vicinity-
Southwest Interior-Tacoma Area-Western Skagit      County-Western
Strait of Juan De Fuca-Western Whatcom      County.

     High Surf Advisory until 6 AM PDT Friday for Central Coast-North
     Coast.

PZ...Small Craft Advisory for rough bar until 6 PM PDT Friday for
     Grays Harbor Bar.

     Small Craft Advisory until 6 AM 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-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 hazardous seas until midnight PDT
     Friday night 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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