Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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610
FXUS66 KSEW 180520
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
900 PM PST Fri Nov 17 2017

.SYNOPSIS...Weak high pressure will be over the area tonight and
Saturday. A strong front will reach the area Sunday--heavy rain
is likely in the Olympics, and windy weather is possible for some
areas. Periods of wet weather with rather mild temperatures are
likely next week--with windy conditions possible at times.

&&

.SHORT TERM...Current radar loop shows some spotty light showers
around. Models show warm air advection tonight and Saturday ahead of
the next system. This will keep some light rain around but should
lift north into B.C. by Saturday morning. Winds will pick up over
the coastal waters Saturday, and windy conditions are expected
across the entire area Sunday. Models differ on the strength of the
cold front approaching Sunday. The GFS20 shows much more surface
development along the front but other models such as the 12Z ECMWF
are not as intense. Wind advisory criteria winds are likely over the
north interior and probably the coast. Puget Sound may not be as
windy but it will depend on where or if a meso low forms in the lee
of the Olympics.

Strong flow will produce heaviest rain amounts over the south slopes
of the Olympics. See the hydro discussion below and refer to the
latest flood statement. The lowlands could get up to an inch but
very little rain will fall where rain shadowing occurs in the lee of
the Olympics. Rain should ease off by Monday which is reflected in
the current forecast. Mercer

.LONG TERM...From previous discussion...The front that moves through
Western Washington Sunday afternoon will be over Northern California
and Oregon later Sunday night and Monday morning. The GFS shows a
dry day for Western Washington on Monday. Rain moves back into
Western Washington Tuesday afternoon and evening. The heavier rain
shifts to the B.C. coast and Vancouver Island for Wednesday and
Wednesday night then back down through the area Thursday. It will
become unusually mild--the 700mb temp will rise to around 0c and
850mb temps will peak around +9c. That does not bode well for the
ski areas by the middle of next week. The good news is that the
models do not have especially heavy rain, but that might change
again and bears watching.

&&

.AVIATION...A flat upper ridge centered over the Washington
coastal waters will shift into Eastern Washington Saturday
afternoon and evening. A strong and wet cold front will sag
southeast through British Columbia on Saturday and will enter the
area from the northwest Saturday evening. Moderate westerly flow
aloft. The air mass is moist and somewhat unstable, except
somewhat drier over the interior lowlands to the lee of the
Olympics and terrain of Vancouver Island. The air mass will become
increasingly stable later tonight into Saturday.

Scattered convective showers are affecting the Washington coastal
waters and coastal zones and the interior lowlands from about KSHN
and KOLM southward. The showers will gradually decrease in both
coverage and intensity overnight. Conditions are generally VFR,
with MVFR ceilings in showers. The low level air mass is moist
enough that MVFR ceilings with bases 010-020 will become more
widespread after 06Z with diurnal cooling. The lower levels are
expected to dry somewhat Saturday morning as the upper level ridge
shifts overhead then to the east of the area. Albrecht

KSEA...South wind 9-13 kt this evening will ease to 7-10 kt late
tonight and Saturday morning then rise to 9-14 kt with gusts 18-22
kt midday Saturday. Good VFR this evening with cigs near 030
developing about 06Z and continuing through early Saturday
morning. There is a 30-40 percent chance that CIGS will remain
better than indicated in the current TAF as westerly flow aloft
interacts with the Olympics. Albrecht

&&

.MARINE...A strong cold front extending WSW from Haida Gwaii into
the Pacific will slowly drag southeast down the British Columbia
coastline tonight through Sunday, then will move southeast through
the waters Sunday evening. Winds will gradually increase from the
south and southeast ahead of the front late tonight into Saturday
night with gales expected over most of the waters Saturday night.
Winds will rapidly ease behind the front Sunday night.

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. Albrecht

&&

.HYDROLOGY...Heavy rain will fall in the Olympics on Sunday driving
the Skokomish river in Mason county to near or above flood stage. A
flood watch remains in effect for Mason County. Other rivers should
not experience flooding with the Sunday system.

Mild wet weather is likely from about Tuesday afternoon through
about Thursday of next week. The Skokomish river will again be at
risk of flooding. Other rivers need to be watched but the threat
of flooding is low.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Flood Watch from Sunday morning through Monday morning for Hood
     Canal Area-Lower Chehalis Valley Area-Olympics.

PZ...Small Craft Advisory from 3 AM to 3 PM PST Saturday 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.

     Gale Warning from 3 PM Saturday to 3 PM PST Sunday for Coastal
     Waters From Point Grenville To Cape Shoalwater 10 To 60 Nm.

     Gale Warning from 3 PM Saturday to noon 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-Coastal Waters From James Island To Point Grenville
     Out 10 Nm.

     Small Craft Advisory from 9 AM to 6 PM PST Saturday for Coastal
     Waters From Point Grenville To Cape Shoalwater Out 10 Nm.

     Gale Warning from 6 PM Saturday to 3 PM PST Sunday for Admiralty
     Inlet-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 from 3 AM to 6 PM PST Saturday for East
     Entrance U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory from midnight tonight to 6 PM PST Saturday
     for Northern Inland Waters Including The San Juan Islands.

     Small Craft Advisory from 6 AM to 6 PM PST Saturday for
     Admiralty Inlet-West Entrance U.S. Waters Strait Of Juan De
     Fuca.

     Gale Warning from 6 PM Saturday to noon PST Sunday for West
     Entrance U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory from 9 AM to 3 PM PST Saturday for Puget
     Sound and Hood Canal.


&&

$$

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



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