Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 210546
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
946 PM PST Tue Nov 20 2018

.SYNOPSIS...The first in a series of weather systems will move
across the area Wednesday bringing light rain to the area. A
stronger system is expected Thanksgiving Day with widespread
lowland rain and mountain snow likely. Rain and mountain snow will
continue Friday with a brief break Saturday. Unsettled weather
returns Sunday through much of next week.

&&

.SHORT TERM...
There are no major changes to the following forecast thinking:
Ridging will move off to the east as the first in a series of
shortwaves approaches the coast tonight. This initial system will
be weak with western Washington left in the middle as main energy
splits and goes north into BC and south into Oregon/California.
Rain will overspread the area early Wednesday and continue
throughout the day with light accumulations. Snow levels will
generally be in the 5000 foot range with accumulations limited to
the highest terrain. This will limit travel impacts outside of
rain-slick roads.

No noticeable break between systems as next shortwave moves into
the region Wednesday night into Thursday/Thanksgiving Day. Models
continue to be in fairly good agreement with a slightly negative
tilt and stronger dynamics. More widespread heavier rain expected
with snow levels dropping down to 3500-4000 feet during the day
Thursday. Several inches of snow are possible down near pass
levels which may cause some travel headaches. Next shortwave
rotates through Thursday night into Friday with additional
precipitation. As this system moves through will likely see some
breezy southerly winds. Cooler mid-level air will wrap into the
system and further drop snow levels down to around 3000 feet. This
will drop several more inches of snow down to pass levels which
additional minor travel impacts. A snow advisory may be needed.
Those with travel plans Thursday and Friday should keep an eye on
the forecast and check on road conditions before departing. CEO

.LONG TERM FROM THE PREV DISCUSSION...
System will depart Friday night with a brief period of ridging
across Western Washington. A few showers may linger in the
mountains Saturday but most areas will see a break in
precipitation. Have cut back precipitation chances in the interior
to match current model trends.

Break is short-lived as next system moves across Saturday night
into Sunday. This one looks fairly weak and fast-moving so
scattered showers are a good bet with light precipitation amounts.
Models continue to advertise a wet and unsettled next week with
several chances for additional widespread precipitation. CEO

&&

.AVIATION...
No major changes to the following forecast thinking: An upper
level trough will push inland tonight and Wednesday for increasing
clouds and rain. The flow aloft is S/SW. Rain will reach the
coast by 12z with rain in the interior by 15-18z. Expect cigs in
the 3000-5000 ft range with showers in the vicinity through the
afternoon. 33

KSEA...VFR tonight. Rain spreading inland by 15-18z Wed. S/SE
winds at the surface to 10 kt. 33

&&

.MARINE...A stationary front extending from a 1001 mb low
was well offshore this evening. Expect the low to strengthen to
996 mb overnight as it moves east. The weak cold front will then
move across the area Wednesday afternoon and evening. This front
will likely dissipate before reaching the Cascades. A strong
occluded front will impact the area on Thursday for gale force
winds over a larger area.

&&

.HYDROLOGY...No flooding is indicated at this time for the next
seven days; however, there are some possibilities on the horizon. A
stormy period this Wednesday through Friday could put as much 2 to 4
inches of precipitation over the Olympics with less over the
Cascades. This could push the Skokomish River close to or exceed
flood stage. That is not the likely scenarios at this time because
the precipitation looks to be to spread out over the time period.
Lowering snow levels during the period, if low enough, could reduce
storm runoff into the streams as well.

The GFS is showing a much stronger storm mid-week next week that has
the possibility of flood-producing precipitation amounts over the
Olympics and Cascades. jbb

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Gale Watch from Thursday morning through Thursday afternoon 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 6 AM PST Wednesday for Coastal Waters
     From Cape Flattery To James Island 10 To 60 Nm-Coastal
     Waters From James Island To Point Grenville 10 To 60 Nm-
     Coastal Waters From Point Grenville To Cape Shoalwater 10
     To 60 Nm-East Entrance U.S. Waters Strait Of Juan De Fuca.

     Gale Warning from 6 AM to 4 PM PST Wednesday for Coastal Waters
     From Cape Flattery To James Island 10 To 60 Nm-Coastal
     Waters From James Island To Point Grenville 10 To 60 Nm-
     Coastal Waters From Point Grenville To Cape Shoalwater 10
     To 60 Nm.

     Small Craft Advisory until 4 PM PST Wednesday for Coastal Waters
     From Cape Flattery To James Island Out 10 Nm-Coastal Waters
     From James Island To Point Grenville Out 10 Nm-Coastal
     Waters From Point Grenville To Cape Shoalwater Out 10 Nm-
     West Entrance U.S. Waters Strait Of Juan De Fuca.

     Gale Warning from 6 AM to 6 PM PST Wednesday for East Entrance
     U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory until 10 PM PST Wednesday for Northern
     Inland Waters Including The San Juan Islands.

     Small Craft Advisory from 6 AM to 6 PM PST Wednesday for
     Admiralty Inlet-Central U.S. Waters Strait Of Juan De Fuca.

&&

$$

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