Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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499
FXUS66 KSEW 150001
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
401 PM PST Tue Nov 14 2017

.SYNOPSIS...A cold front will move slowly across Western
Washington tonight, bringing areas of heavy rain, heavy mountain
snow, and windy conditions. An upper trough will follow the front
later Wednesday through Thursday night, bringing showery weather.
Friday and early Saturday could be relatively dry, but a
transition to a wet and warmer pattern is likely during the
weekend. A dry reprieve looks possible next Monday, then more
heavy rain around next Tuesday.

&&

.SHORT TERM...A deep cold-core upper low continues to spin out
around 50N 136W, directing active southwest flow aloft into
Western Washington. This evening, a cold front will slowly
approach the coast, providing the focus and lift for a band of
heavy precip to gradually move across the Olympic Peninsula this
evening, then across the Puget Sound Region during the late
evening and overnight hours. S-SW 850 mb winds of 60-75 knots this
evening will really enhance rainfall rates along windward
slopes of the Olympics and North Cascades. Rainfall amounts over
the next 12 hours will exceed 3 inches over the windward Olympics
tonight, and approach 3 inches near Mount Baker. In addition, snow
levels will gradually drop tonight, dropping into the 3000-3500
foot range by daybreak. Above that level, all of this heavy rain
will come as a huge dump of snow. Have upgraded the Winter Weather
Advisory for the Olympics to a Warning, with over 2 feet of new
snow forecast at Hurricane Ridge.

As for wind, southeast gradients will peak over the next 6 hours
in advance of the cold front. Wind gusts near 50 mph are still
forecast for our southeast wind-prone locales such as the coast
and areas north of Everett. A Wind Advisory continues in those
places. These gusts would be significant on any day, but it
should be noted that these speeds are less than what was observed
yesterday. Outside of these areas, wind gusts over the Puget Sound
Region are forecast to be 35-40 mph tonight.

The front will slip southeast of the forecast area later
Wednesday. Wind and precip intensity will subside. Still,
abundant post-frontal showers will continue Wednesday through
Thursday night, owing to instability and cold air aloft under the
upper low/trough, and owing to continued onshore flow.

Flat upper ridging should develop on Friday and continue until at
least early Saturday. This will bring the best chance of dry
weather this week, though a little light rain still cannot be
ruled out under the warm advection zone around the periphery of
the ridge. At least PoPs will drop below 50 percent for a 12-hour
block around Friday.

.LONG TERM...The GFS and ECMWF have finally come into better
agreement with the weekend weather. They both now drop a slow-
moving front southward through Western Washington starting late
Saturday, though it should take until Sunday evening for the front
to move south-southeast of the area. Should be plenty of heavy
rainfall focused along the slow-moving front, similar to the
current situation. Windy conditions will again precede this front,
especially in southeast-wind prone locales.

Looks like a dry respite next Monday and Monday night, as a
sharper transient ridge passes across the Pac NW.

Yet another heavy rain and southeast-wind producer is in our
sights for next Tuesday.  Haner

&&

.AVIATION...A large upper level low and surface low centered
offshore will maintain strong and moist SW flow aloft over W WA
beyond 00Z Thursday. A very slow moving cold front associated
with the low was just W of the WA coast at 22Z and will continue
moving very slowly inland across W WA through Wednesday. The air
mass is moist and unstable over the coast and coastal waters and
somewhat unstable over the interior. The strong surface low will
continue to produce windy conditions throughout most of W WA
through tonight.

Due to the very slow moving nature of the front, lower CIGS to the
west have not yet spread much over the greater Puget Sound
region, so CIGS are still VFR SCT-BKN050-080 over much of the area.
CIGS have dropped to MVFR at PWT and SHN which have been more
exposed to rain showers moving N along the frontal boundary. The
front should continue creeping eastward but MVFR CIGS may hold off
over central Puget Sound until after 09Z. Gusty S surface winds
15 to 30 KT will continue over much of the area.

KSEA...The slow moving front has not made much progress eastward
so CIGS have stayed up in the BKN050-070 range. CIGS are now
expected to lower to MVFR after 06Z. Gusty southerly surface
winds 15-30 KT will continue. Kam

&&

.MARINE...A very slow moving cold front just offshore will
continue creeping toward the coast tonight, then move inland on
Wednesday. Gales are still expected over the coastal waters
offshore from Grays Harbor, but the SW to NE orientation of the
front has allowed winds to drop to SCA levels over the northern
coastal waters. Gales are still expected over the northern inland
waters, including the E entrance to the Strait, N inland waters,
and Admiralty Inlet. Winds in Admiralty inlet have not reached
gale today, but the stronger mesoscale models indicate gales are
quite possible tonight. High end SCA south winds are expected over
Puget Sound with E-SE SCA winds in the West and central strait.
Winds will gradually ease through Wednesday as the front slowly
crosses W WA.

The main offshore surface low will approach northern Vancouver
Island Wednesday, then move southeast over the WA coast by
Thursday evening. A weak ridge will cross the region on Friday.
Another strong frontal system will slowly approach the coast on
Saturday then move across the area Saturday night and Sunday. Kam

&&

.HYDROLOGY...Another enhancement of heavy rain is expected
tonight. 24-hour rainfall of 5 or more inches is expected over the
windward Olympics, and 2-3 inches over the North Cascades, for the
24 hours ending mid-day Wed. This will keep the Skokomish river
above flood stage, and could possibly boost it into the Moderate
flood category tonight. Will also be watching other rivers flowing
off the Olympics such as the Satsop and Dungeness. Earlier issued
a Flood Watch for Grays Harbor County to cover the Satsop. For
the North Cascades, will be keeping an eye on the Nooksack River.

Snow levels will fall by Wednesday morning into the 3000-3500 foot
range, so that will lessen the contribution of further runoff at
that time.

Landslide indices are also creeping up and could become marginally
conducive for landslides during tonight`s rain. Any particularly
intense burst of rainfall tonight could pose the first landslide
test of the rainy season. Haner

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Winter Storm Warning until 3 PM PST Wednesday for Cascades of
     Pierce and Lewis Counties-Cascades of Whatcom and Skagit
     Counties.

     Winter Weather Advisory until noon PST Wednesday for Cascades of
     Snohomish and King Counties.

     Wind Advisory until 4 AM PST Wednesday for Admiralty Inlet Area-
     San Juan County-Western Skagit County-Western Whatcom
     County.

     Flood Watch from late tonight through Wednesday afternoon for
     Central Coast-Lower Chehalis Valley Area.

     Wind Advisory until midnight PST tonight for Central Coast.

     Winter Storm Warning until 4 AM PST Wednesday for Olympics.

PZ...Small Craft Advisory for rough bar until 6 AM PST Thursday for
     Grays Harbor Bar.

     Gale Warning until 6 AM PST Wednesday for 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 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.

     Gale Warning until 10 AM PST Wednesday for Admiralty Inlet-East
     Entrance U.S. Waters Strait Of Juan De Fuca-Northern Inland
     Waters Including The San Juan Islands.

     Small Craft Advisory until noon PST Wednesday for Central U.S.
     Waters Strait Of Juan De Fuca-West Entrance U.S. Waters
     Strait Of Juan De Fuca.

     Small Craft Advisory until 6 PM PST Wednesday 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



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