Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

Current Version | Previous Version | Text Only | Print | Product List | Glossary Off
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

000
FXUS66 KSEW 190545
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
900 PM PST Sat Nov 18 2017

.SYNOPSIS...A vigorous front will bring rain and windy conditions
to Western Washington Sunday and Sunday night. Weak high pressure
over the area on Monday will provide a brief lull in the active
weather. The next in a series of fronts arrives Tuesday and
Wednesday with rain and windy conditions. Wet weather will persist
through through the end of next week.

&&

.SHORT TERM...Light warm advection rainfall has reached the north
coast and extreme north interior this evening. Radar returns further
offshore are not very impressive and IR imagery showing brighter
clouds tops lifting northward. This matches up well with most models
holding off on the bulk of rainfall until later Sunday morning.
Warmer air aloft will infiltrate the area causing snow levels to rise
as precipitation arrives. The north Cascades may stay cool enough
for some decent snow accumulations but some rain will probably mix
in at times. Will plan on dropping the winter storm watch for the
Cascades of Pierce/Lewis counties and just go with an advisory for
the north Cascades above 5000 feet for up to 10 inches Saturday
afternoon and night. Will need to adjust the timing on this.

The new 00z models are little changed from earlier runs and shows
south/southeasterly gradients increasing later tonight and Sunday
ahead of a strong front. A meso low will also develop northeast of
the Olympics which should induce some stronger wind gusts around the
Admiralty Inlet/Whidbey Island area. Southerly winds of 20 to 35 mph
with gusts 50 to 55 mph still appear reasonable for the north
interior and central/north coast. Wind advisories remain in effect
for these locations Sunday. There is some questions as to whether
the stronger winds could extend a bit further south into parts of
Everett or other areas around Puget Sound. The latest UW WRF4km only
shows spotty gusts close to 40 mph around the Sound. This is still
sub-advisory but it will still be a windy day across all of Western
Washington.

Models are also fairly progressive with the front with heaviest rain
falling Sunday afternoon through Sunday evening, then tapering
overnight. Heaviest rainfall will occur along the south slopes of
the Olympics where strong orographics will enhance rainfall. Even
then, amounts may struggle to reach much above 2 to 3 inches. The
Skokomish river will still be at risk of minor flooding and the flood
watch remains in effect. Refer to the latest flood watch statement
and hydro section below for details.

The system sags south on Monday with a brief lull in wind and rain
over the area. Another warm front arrives Monday night and Tuesday
with another round of rain and locally windy conditions. This system
will bring warmer air into the region and high temperatures could be
well above average by Tuesday and Wednesday. Mercer


.LONG TERM...From previous discussion...The strong warm front will
be over the region Tuesday. The heavier rain shifts to the B.C.
coast Wednesday and Wednesday night before shifting south over the
area Thursday and breaking up somewhat. It will be mild, rainy, and
breezy and the rivers will be running high, with the Skokomish River
in Mason county flooding. There is a chance of flooding on the other
rivers, but the fact that this pineapple express is more southerly
probably means the best orographics are limited to the Olympics. But
it will still make a mess of our early season snowpack which is a
shame for the skiers. Seattle ought to be up around 60 for a high
Wednesday.

&&

.AVIATION...A flat upper ridge aloft will exit to the east
tonight. A strong cold front will spread down the British
Columbia coastline tonight and move southward through Western
Washington later Sunday. The front will be preceded by heavy rain
and strong southerly surface wind. Moderate westerly flow aloft
this evening will become strong on Sunday while strong south to
southwest winds develop between 1500 and 6000 ft MSL midday Sunday.
The air mass is stable and will become increasingly moist late
tonight and Sunday. Expect mainly VFR conditions across the area
today with conditions lowering to IFR in precipitation and low
ceilings from the northwest on Sunday. Albrecht

KSEA...Only FEW-SCT035 with moisture above 120 through most of
tonight. CIGS around 035 will rapidly develop about 12Z late
tonight with rain developing later Sunday morning. CIGS and VSBYS
will lower Sunday afternoon to 008-012 into the evening hours in
occasionally moderate rain. South wind at the surface 6-10 kt
through Sun morning will increase to 15G25KT about 18Z. Winds at
025 will increase to 19060kt 20Z-02Z Sunday with peak winds
between 040 and 060 of 19070kt. Albrecht

&&

.MARINE...A strong cold front extending WSW from the Queen
Charlotte Sound into the Pacific will slowly drag southeast down the
British Columbia coastline tonight and then push southeastward
through the Washington waters Sunday afternoon through the early
evening hours. In advance of the front, winds will rise. The wind
increase will progress from north to south through tonight with
gales expected over most waters ahead of the front. Winds will
rapidly ease and shift out of the northwest behind the front later
Sunday afternoon and evening.

High pressure will bring light winds to the waters on Monday. A
strong warm front will push northward through the waters on Tuesday.
A strong low pressure center developing well offshore will push
northeast into Haida Gwaii on Wednesday. Haner/Albrecht

&&

.HYDROLOGY...The Skokomish river in Mason county will likely rise to
near or over flood stage and could stay up around flood stage for
several days.

For all other rivers, the new 00z models do not show especially
heavy precip over the Cascades mid week. The flow will be mostly
southerly through the week which is not usually not conducive for
the heavier rain amounts needed to cause river flooding in most
Cascade basins. Regardless, the system still bares watching with a
warm air mass in place and a long fetch of available moisture.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Winter weather advisory Sunday morning through late Sunday night
     for the Cascades of Whatcom and Lewis counties above 5000 feet.

     Flood Watch from Sunday morning through Monday morning for Hood
     Canal Area-Lower Chehalis Valley Area-Olympics.

     Wind Advisory from 6 AM to 3 PM PST Sunday for Admiralty Inlet
     Area-San Juan County-Western Skagit County-Western Whatcom
     County.

     Wind Advisory from 6 AM to 5 PM PST Sunday for Central Coast.

     Wind Advisory from 4 AM to 3 PM PST Sunday for North Coast.

PZ...Small Craft Advisory for rough bar from 6 AM Sunday to noon PST
     Monday for Grays Harbor Bar.

     Gale Warning from 3 AM to 7 PM PST Sunday for Coastal Waters
     From Point Grenville To Cape Shoalwater Out 10 Nm.

     Gale Warning from midnight tonight to 7 PM PST Sunday for
     Coastal Waters From Point Grenville To Cape Shoalwater 10
     To 60 Nm.

     Gale Warning from 3 AM to 3 PM PST Sunday for Admiralty Inlet-
     Coastal Waters From James Island To Point Grenville Out 10
     Nm-East Entrance U.S. Waters Strait Of Juan De Fuca-
     Northern Inland Waters Including The San Juan Islands.

     Gale Warning until 3 PM PST Sunday 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.

     Gale Warning from midnight tonight to 3 PM PST Sunday for West
     Entrance U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory from midnight tonight to 6 PM PST Sunday
     for Central U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory until 6 AM PST Sunday for Puget Sound and
     Hood Canal.

     Gale Warning from 6 AM to 3 PM PST Sunday 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



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