Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

Home | Current Version | Previous Version | Text Only | Print | Product List | Glossary On
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50
000
FXUS66 KSEW 232327
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
327 PM PST Thu Nov 23 2017

.SYNOPSIS...An upper level trough crossing Western Washington will
maintain scattered showers tonight. A cold front will stall over
Vancouver Island Friday and Friday night, which will produce a
little light rain over mainly the north part of Western
Washington. Another frontal system is expected to bring more rain
later Saturday into Sunday morning. A large upper level trough
will cross the area Sunday night and Monday with scattered
showers. This active weather pattern should continue into mid
next week.

&&

.SHORT TERM...RADAR and satellite imagery show this mornings cold
front moving over the Cascades, so the current episode of heavy
rain is essentially over. Visible imagery shows open cellular
cumulus just offshore, which will move inland tonight for
scattered showers. Today`s rain combined with the heavy rain this
past Tuesday have pushed many rivers up above flood stage.

Also on track is the cold front extending W from the central B.C.
coast, and moving S toward Vancouver Island. After tonight`s
scattered showers this front should be close enough as it becomes
quasi- stationary over Vancouver Island to bring some light rain
to the area. The best chance of rain will be over the N part of W
WA, especially the coast and mountains. The rain will probably
reach a little farther S over W WA on Friday, then recede back
north a little Friday night as the flow aloft over the region
becomes more southwesterly. This rain will not be hydrologically
significant so it won`t impact the current flooding.

A deepening large upper level trough well offshore will shift the
upper level flow over W WA back to a moist SW flow aloft. This
will allow a system embedded in the strong SW flow aloft to bring
widespread rain to the area Saturday afternoon through Sunday
morning. The GFS and ECMWF have been a little inconsistent lately
on the details, so how much rain falls and during what periods is
still a little uncertain. This period of rain could be a little
heavier but with the big break between wet systems it is unlikely
to cause renewed flooding, although there is always a chance that
the Skokomish river will flood.

Snow levels will lower to 3500 to 4000 feet tonight through
Saturday, but precipitation over the Cascade passes will be light
enough for snowfall to remain below advisory thresholds. Snow
levels will rise back to 5000-7000 feet Saturday night and Sunday
morning as that next system from the SW arrives. Kam

.LONG TERM...The GFS and ECMWF both agree that the large offshore
upper level trough will move E across W WA Sunday afternoon
through Monday. This will be a cool showery period. The models
still show a warm front reaching the area Monday night, but have
been inconsistent with this feature for the past several model
runs. So at this point the Tuesday-Thursday period still looks
like an active weather period, but confidence is low regarding how
it will evolve. Kam

&&

.AVIATION...A trough will shift east tonight with a break in the
steady rain across Western Washington through Friday. The next front
and associated trough will stall off Vancouver Island Friday.
Moderate to strong southwest flow aloft. Air mass stable and
generally moist, with some drying in the mid/upper levels tonight.

KSEA...Rain shadow off the Olympics will keep most residual shower
activity to the north and south of the KSEA/KBFI terminals. The
lower air mass will remain somewhat moist and may settle into MVFR
cigs after midnight as the air mass stabilizes. However, stratus is
not expected to fill in and become widespread and periods of VFR can
be expected into Friday morning. Southwest wind 10-12 kt w/ gusts 22
kt easing late this evening. dtm

&&

.MARINE...Onshore flow will increase this evening then begin to east
later tonight and Friday morning. Marginal small craft winds are
expected for most waters, especially through midnight. A lull in
winds will develop later tonight and Friday morning.

The next front will stall off Vancouver Island on Friday. Gradients
will gradually rise over the coastal waters with small craft
conditions possible by afternoon through Saturday.

A stronger frontal system will lift north outside the WA coastal
waters late Saturday afternoon through Saturday night. The front
should eventually swing through the coastal waters and weaken. Gale
force S/SE winds are possible over the coastal waters, entrances to
the Strait, and north inland waters. There is significant
uncertainty on how this system develops and tracks. Forecasts should
be monitored for changes. dtm

&&

.HYDROLOGY...Hydrologically significant rainfall appears to have
ended, though light rain continues to fall, mainly in the mountains.
Rivers that are flooding now are responding to rainfall over the
past three days, which was copious in the mountains. The North
Cascades received 4-9 inches, the Central and South Cascades got
3-6 inches, and the Olympic peninsula got 4-10 inches. Upstream
reaches of rivers are receding now, though some downstream reaches
have a way to go until they crest. A cooling trend is forecast--the
snow level was about 6000 feet this morning and will fall to around
4000 feet this afternoon.

Here is a summary of river flooding, from north to south:

The Nooksack River at North Cedarville fell below the moderate flood
stage this morning, and will fall below minor flood stage tonight.
The Nooksack at Ferndale is still rising and is forecast to have
minor flooding on Friday.

The Skagit River is experiencing major flooding. At Concrete, it
will remain a couple of feet above the major flood stage for a few
more hours this evening, then slowly recede through Friday. At Mount
Vernon the river is forecast to go above the major flood stage
tonight, then recede below flood stage Saturday.

Flooding has ended on the main fork of the Stillaguamish River
and the flood warning has ended.

The Skykomish River near Gold Bar rose above moderate flood stage
this afternoon and will fall below moderate flood stage this
evening. The Snoqualmie River near Snoqualmie Falls and near
Carnation will experience minor flooding tonight and Friday. The
Tolt River crested below flood stage twice today. Further
downstream, moderate flooding is occurring or forecast on the
Snohomish River.

In the southern part of the state, the White River near R Street in
Auburn and the Nisqually River near National are having minor
flooding. The Cowlitz River at Randle will just touch the moderate
flood stage this evening. In the Olympics, the Satsop and Skokomish
Rivers will continue to have minor flooding.

A flood watch remains in effect through tonight, though the threat
of further flooding has receded.

For the current list of flood watches and warnings, please check:
www.wrh.noaa.gov/sew/get.php?pil=RVA&sid=SEW

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.

Looking further out, the flood threat should decrease Friday through
early next week. More fronts will move through Western Washington,
but rainfall will not be as heavy and snow levels will be in the
3000 to 5000 foot range. Burke

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Flood watch in effect for Lewis, Grays Harbor, Jefferson,
     Clallam, Pierce, King, Snohomish, and Whatcom counties.

PZ...Small Craft Advisory for rough bar until 10 AM PST Friday for
     Grays Harbor Bar.

     Small Craft Advisory until 4 AM PST 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-Puget Sound and Hood Canal-West
     Entrance U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory until 10 PM PST this evening for Admiralty
     Inlet-Central U.S. Waters Strait Of Juan De Fuca-East
     Entrance U.S. Waters Strait Of Juan De Fuca-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


USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.