Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 222322

Area Forecast Discussion
National Weather Service Seattle WA
322 PM PST Wed Nov 22 2017

.SYNOPSIS...A cold front will bring more rain to Western
Washington tonight through Thanksgiving. Scattered showers will
continue through Thursday night as an upper level trough follows
the front. A weak warm front will bring a little light rain Friday
night. The next system to bring widespread light rain should
arrive Saturday afternoon. This active weather pattern should
continue into early next week.


.SHORT TERM...RADAR shows areas of rain mainly over the mountains
and coast this afternoon. A weak cold front was over the
Washington offshore waters this afternoon, and was slowly
approaching the coast. The front is expected to strengthen tonight
as an upper level shortwave trough near 40N/135W at 22Z/2 PM and
embedded in the strong SW flow aloft, moves NE across the front.
Models have been consistent in indicating this will be a good rain
producer tonight and Thursday. However, the latest 12Z models,
have trended lower on how much rain will fall. In addition,
precipitable water values in the band just offshore were only
around 1.2 inches at 20Z/12 PM. The latest rainfall estimate for
the Olympics is for 1 to 3.5 inches of rain in the 24 hours ending
at 4 PM Thursday. The Cascades should get lower amounts, with 1
to 3 inches through 4 PM Thursday. These estimated amounts could
be a little high but with flooding already in progress from
Tuesdays rain, a conservative forecast is better.

The timing of the cold front should bring the heaviest rain to the
Olympics later this evening, then to the Cascades after midnight.
The front should be through the interior lowlands by midday
Thursday, but it will probably take the whole afternoon for the
rain to taper off over the Cascades. I will keep scattered showers
in the forecast behind the front Thursday afternoon, but there is
not a lot of convection showing up offshore behind the front. The
upper level shortwave trough to the W and trailing the cold front
could generate some showers through Thursday night as it crosses
the area.

A slow moving cold front sliding SE along the B.C. coast will
bring some light rain to the area on Friday, particularly the
northern part of W WA.

On Friday, another deep upper level longwave trough will develop
offshore along 140W, which will rev up the down stream SW flow
aloft over the PacNW. The models have been consistent the past
couple of days, showing an upper level shortwave trough embedded
in the flow moving NE across W WA late Saturday. This system
should produce widespread light rain. The past few GFS runs as
well as the latest 12z ECMWF have the rain moving N across W WA
Saturday afternoon. The latest 18Z GFS has sped that up a little
and now shows some light rain in the morning. QPF should be less
than an inch.

The warm air mass from Tuesday`s warm front has ended up breaking
several high temperature records for the date. Temperatures in the
60s were common and Puyallup and JBLM-McChord even broke 70. We`re
still looking at a record high minimum temperature for Sea-Tac. If
the temperature doesn`t cool below 56 degrees before midnight,
56 will be the warmest minimum temperature for Seattle in the
month of November going back through the entire climate record
back to the 1890`s. Kam

.LONG TERM...The GFS and ECMWF show the offshore longwave trough
weakening on Saturday then moving E across W WA Sunday night. The
GFS is a little faster than the ECMWF. This will keep the weather
active with rain at times. The GFS has stuck with bringing a warm
front E across the area Monday night, but the ECMWF, after showing
the same solution for the past 2 days now significantly weakens
the warm front offshore. The models diverge even more for Tuesday
and Wednesday. The GFS shows a dry period with a moderate
amplitude upper level ridge over the area Tuesday night and
Wednesday, but this will probably end up being a discarded
solution by next week. Kam


.AVIATION...Moist southwest flow aloft will continue tonight and
Thursday morning, then become more westerly Thursday afternoon as an
upper level trough passes. At the surface, southeasterly pressure
gradients will ease to moderate tonight, then become strong
southwesterly Thursday. Rain at times will continue in the stable
air mass.

Mostly VFR conditions continue, with patchy MVFR/IFR in rain.  There
are some breaks in the clouds. More clouds and rain are forecast
tonight, and this will probably bring widespread MVFR ceilings. Rain
will spread from the northwest tonight, and turn showery Thursday as
gusty southwest winds develop.

KSEA...MVFR tonight, improving to VFR late Thursday afternoon. South
winds 8-12 kt tonight, becoming southwest and increasing to 15g25 kt
Thursday afternoon. CHB


.MARINE...A front will arrive from the northwest late tonight. Low
end gale conditions are likely on the coast and over inland waters
from Admiralty Inlet north this evening. Small craft advisory winds
are likely elsewhere. Winds will ease somewhat late tonight, then
gradients will increase again Thursday. Most areas will have small
craft advisory strength winds, but local gales are possible,
especially after the front passes Thursday afternoon and winds
become gusty southwesterly. Southerly gradients will ease on Friday.

Another system will arrive around Saturday night. Strong southerly
gradients are possible over the entire area Sunday. Gales are
possible over most waters with small craft advisory conditions over
Puget Sound and Hood Canal. CHB


.HYDROLOGY...Just waiting for the next round of precip to kick in
and assess where all the rivers are at the time. Some rivers have
crested, some have fallen below flood stage, some are still rising.

Warnings and watches change as needed. For the current list, please

The next front will move through tonight and Thanksgiving Day, with
a forecast of generally 1 to 3.5 inches over the mountains. This is
slightly less than previously forecast as the models are all
trending down in amounts. River forecast updates will likely show
some lover crests with the new QPF numbers. The snow level will be
around 7500 feet as this system arrives, falling to 4000 feet late
Thursday as precipitation decreases.

With this second round of rainfall tonight and Thursday, there is a
possibility that flooding will become more widespread and/or
prolonged. There is at least a minor threat that any flood-prone
river in our area reaching flood stage.

In addition, there is a threat of landslides across the lowlands of
Western Washington. The USGS landslide guidance shows soil wetness
above the threshold of concern, and additional rain through Thursday
will drive those numbers higher and act as the trigger for
landslides. JBB/McDonnal


WA...Flood Watch through Thursday afternoon for Admiralty Inlet Area-
     Bellevue 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.

PZ...Small Craft Advisory for rough bar until 4 PM PST Thursday for
     Grays Harbor Bar.

     Small Craft Advisory from midnight tonight to 4 PM PST Thursday
     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.

     Gale Warning until midnight PST tonight 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-East Entrance U.S.
     Waters Strait Of Juan De Fuca-Northern Inland Waters
     Including The San Juan Islands.

     Small Craft Advisory until 4 PM PST Thursday for West Entrance
     U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory until midnight PST tonight for Central U.S.
     Waters Strait Of Juan De Fuca.



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