Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 170324
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
830 PM PDT Mon Oct 16 2017

.SYNOPSIS...A front will move through the area Tuesday morning
giving rain and breezy conditions to the area. A strong front will
move into the area Wednesday with a period of heavy rain and windy
conditions. Showers are expected Thursday and Friday. A strong warm
front will arrive for the weekend.

&&

.SHORT TERM...A front will move through Western Washington Tuesday
morning. Showers taper off quickly behind the front and with the
exception of a brief PSCZ most of Western Washington will be dry by
evening. The dry weather gives way to a wetter and stronger front on
Wednesday, with heavy rain in the mountains due to strong winds
aloft and a rather moist plume of air into the region. The heaviest
rain will be in the Olympics and North Cascades Wednesday afternoon
and evening. The rain gives way to showers on Thursday.

.LONG TERM...Cool upper level troughing and onshore flow will give
showers to the area with snow in the mountains as low as about
3500 feet at times Thursday night through Friday night. The weekend
will be the next period with potentially impactful weather as
another strong jet and atmospheric river aim into the area. Rainfall
amounts and winds with this system could be the same or a bit higher
than Wednesday`s system depending on how things evolve. Early next
week appears that a drying and warming trend will take place as
riding strengthens over the western US. Albrecht

&&

.AVIATION...A vigorous frontal system will move through the area
tonight and Tuesday with westerly flow aloft. The air mass is
moist and stable. Low level moisture will increase tonight as the
front moves onshore and rain develops.

KSEA...Moisture will increase tonight with mid level clouds this
evening lowering to MVFR low clouds early Tuesday. Low clouds
should break up late Tuesday behind the front. South wind 5-10
knots will increase to 15-25 knots Tuesday. Schneider

&&

.MARINE...A vigorous frontal system will move through the area
tonight and Tuesday. Small craft advisory winds are expected all
waters. Gales are possible Coastal Waters and entrances to the
Strait late tonight but the forecast is 20-30 knots for now. A few
hours of gale force west winds are likely in the Central and
Eastern Strait of Juan De Fuca behind the front Tuesday.

Another strong frontal system will move through the area Wednesday
with gales likely most waters. Onshore flow will prevail Thursday
and Friday. A third strong front will reach the area Saturday for
another possible round of gales.

West swell 20-23 feet are possible over the Coastal Waters
around Thursday night. Schneider

&&

.HYDROLOGY...A period of heavy rain with high snow levels is
expected late tonight and Tuesday morning. Dry antecedent
conditions and current low river flows mean that flooding will not
be an issue with this front.

A stronger atmospheric river pattern is expected to affect the
region Wednesday and Wednesday night. Models have trended a bit
more progressive with this frontal system with flow a bit more
southwesterly aloft. This reduces the expected rainfall amounts in
the mountains somewhat and reduces the threat of river flooding
for most rivers. The typically flood-prone Skokomish River in
Mason County would still have about a 25 percent chance of
flooding in this pattern. The main issue may be for the lowlands
as 1-3 inch rainfall totals combine with windy conditions to cause
autumn leaves to collect in drains and give the potential for
some urban flooding. Will keep the hydrologic outlook statement
going due to the continued threat of urban flooding and do not
anticipate the need to issue any flood watches at this time due to
the low probability of any river flooding.

Rivers will recede late Thursday into Friday in response to
lighter and showery precipitation and a lowering of snow level to
as low as 3500 feet.

The weather pattern over the weekend looks like another floody
atmospheric river pattern for Western Washington. The 12Z ECMWF
is rather aggressive with precipitation amounts as compared to the
GFS. We will need to keep a close eye on the weekend system as the
pattern evolves. Albrecht

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Gale Watch from Wednesday morning through Wednesday 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 until 5 PM PDT Tuesday for Admiralty Inlet-
     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-Puget Sound and Hood
     Canal-West Entrance U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory for rough bar from 11 AM to 11 PM PDT
     Tuesday for Grays Harbor Bar.

     Gale Watch from Tuesday morning through Tuesday afternoon for
     Central U.S. Waters Strait Of Juan De Fuca-East Entrance
     U.S. Waters Strait Of Juan De Fuca.

     Gale Watch from Wednesday morning through Wednesday afternoon
     for East Entrance U.S. Waters Strait Of Juan De Fuca-
     Northern Inland Waters Including The San Juan Islands.

     Small Craft Advisory until 8 AM PDT Tuesday for East Entrance
     U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory until 11 PM PDT Tuesday for Northern Inland
     Waters Including The San Juan Islands.

&&

$$

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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