River Flood Warning 61, NWS Seattle, WA

Area Forecast Discussion
National Weather Service Seattle WA
900 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 indicates rainfall increasing in coverage and
intensity across Wrn Washington this evening. Satellite imagery
shows the tap of moist tropical air continues with another surge of
moisture expected to bring more heavy mountain rain tonight. The
leading edge of the cold front is probably near the north coast but
not moving inland much if at all. The Forks sounding shows temps
around 1K feet up around 12C while the Sand Point profiler indicates
20C at that level. The warmer air is clearly still in place over
interior Wrn Washington. Record highs were reached at several
location including Seattle at 68F, breaking the old record of 58F in
1959. That's some unusually warm air but can see why looking at the
source region way out just east of Hawaii. The flow is very strong
from the low levels all the way up on the 00z sounding.
Southwesterlies are about 40 kt down to 1k ft and 60-70 kt at a few
thousand feet up. As the next wave arrives, this should hold the
front up until Thursday morning. Very warm moist air and strong flow
at all levels spells heavy orographic rainfall overnight. The 00z
4km WRF came in surprisingly low on QPF, with up to 2-2.5 inches
only along favored southwest slopes, especially the Olympics. Models
trended toward somewhat less QPF tonight than had been expected.
Looking at satellite, soundings/profiler, and flow, my guess is that
rain amounts could be quite a bit heavier than the WRF. Forecast QPF
was only lowered slightly which was probably a good call. See the
Hydro discussion below and flood bulletins for further details.

Rain will be slow to taper off. Models disagree on how quickly the
front will finally push through. The new 00z NAM brings another wave
up during the day Thursday. Other models are not as wet in the
afternoon. But some adjustment to raise pops for the afternoon hours
might be needed with the next forecast update.

Friday and Friday night appear to show a lull in rainfall. Another
strong wave with possible surface low development will approach on
Saturday. There are differences on timing, track, and strength but
additional rain looks certain. Windy conditions could develop with
the lower later on Saturday depending on the track and depth. Will
need to keep an eye on this, there is a great deal of uncertainty at
this point. Mercer

.LONG TERM...Previous discussion...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, SW flow will continue over Western WA tonight
and Thursday for rain across the area. Expect mainly MVFR ceilings
with patchy fog possible. A trough will push inland Thursday
afternoon for strong onshore flow and showers with ceilings lifting.
Showers will mainly affect the mountains Thursday night with strong
westerly flow aloft. 33

KSEA...MVFR tonight, improving to VFR late Thursday afternoon. South
winds 8-12 kt tonight, becoming southwest and increasing to 15g25 kt
Thursday afternoon. 33

&&

.MARINE...Gales over the waters will ease by midnight but will
still see Small Craft Advisory winds over most zones. Strong onshore
flow will continue through Thursday as a trough moves inland. The
next front will stall over southern B.C. on Friday then dissipate.
Another vigorous 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. 33

&&

.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
check: www.wrh.noaa.gov/sew/get.php?pil=RVA&sid=SEW

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

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Flood Watch in effect for Lewis Grays Harbor Jefferson Clallam
     Pierce King Snohomish Whatcom.
     Other flood statements in effect.

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

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

     Small Craft Advisory from midnight tonight to 4 PM PST Thursday
     for East Entrance U.S. Waters Strait Of Juan De Fuca-
     Northern Inland Waters Including The San Juan Islands.

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

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

     Small Craft Advisory until 4 AM PST Friday for Puget Sound and
     Hood Canal.

&&

www.weather.gov/seattle

You can see an illustrated version of this discussion at
www.wrh.noaa.gov/sew/gafd/latest_webafd.html