Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 200533 RRA
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
915 PM PST Sun Nov 19 2017

.SYNOPSIS...A vigorous front will move slowly through Western
Washington late tonight and early Monday morning. Winds will ease
Monday with some lingering light rain gradually ending. A
strong warm front will arrive Tuesday with more rain and breezy
winds. The middle of the week will be mild, rainy, and perhaps
windy at times.

&&

.SHORT TERM...WV imagery and MSAS surface analysis show a closed 998
mb low off the central Washington coast this evening. Buoy obs match
up exactly with the pressure center. Gradients have turned more
southeasterly and winds are gradually easing even though the low
that formed along the front is still not onshore. Some models hinted
at this development but other models missed it and would have pushed
the front through quicker. Wind advisories will probably be
allowed to expire later this evening, although some gusts are still
close to 45 mph in a couple spots around Admiralty Inlet. These
winds should ease shortly.

One big mystery is the lack of warming given the strong southerly
flow over 50 kt that was expected to penetrate the interior. It
appears the strong flow made it over lowland areas as a few spots up
north hit high wind or close to it. Models were terribly
inconsistent with the amount of warm air expected to reach the
Cascades today and tonight, which should have caused the snow
levels to rise. The problem now is the closed low off the coast
moving so slowly and possibly cutting off some of the warm
advection. It is pretty clear the north Cascades will get a lot of
snow by Monday morning with nearly a foot already at Mount Baker.
The passes are still surprisingly cold and models are too fast in
warming the air mass. However, I still think there is a good chance
that snow levels will finally rise above 5000 to 6000 feet across
the Cascades of Lewis/Pierce counties with a change over to rain in
the passes still expected but later than was forecast. Went with a
winter weather advisory, except up north where 1-2 feet are possible
by morning. Pass travel could be very difficult the next several
hours will heavy snow at times before the transition to rain
hopefully occurs not long after midnight. A few inches have already
fallen at Snoqualmie and Stevens pass.

00Z models show more precipitation lingering into Monday, but still
tapering off. There could be a convergence zone in the afternoon in
the typical north Seattle-Everett corridor. Then a brief break or
lull in rain is possible from afternoon to the evening hours. The
next warm front will bring rain back to the area late Monday night
through Tuesday. This system should bring much milder air into the
area with highs possibly close to 60 Tuesday and Wednesday. Models
have trended somewhat lower with rain amounts in the Cascades. The
flow aloft is not typically conducive for rain amounts that cause
river flooding. However, a lot of uncertainty still exists and the
river forecasts should be followed closely this week. See the
latest flood statements and hydro section below for further
details. Mercer

.LONG TERM...From previous discussion...After a pretty decent round
of rain Tuesday and Tuesday night--with some heavy rain in the
mountains, the heavy precip will be north of the area on Wednesday.
There will still be rain at times over Western Washington--but this
lull on Wed will help a great deal with the rivers. Then on Thursday
the rainfall picks up again--but precip amounts in the models are
actually pretty modest. Still, the RFC forecasts show pretty high
rivers midweek, so a hydrologic outlook was issued, but it merely
highlights the uncertainty in the forecast at this time.

&&

.AVIATION...A cold front is gradually moving southeast across the
area this evening, but a wave moving northeast along the frontal
band will slow the front somewhat through late tonight. At the
upper levels, a developing upper low offshore will move eastward
along the Canadian border on Monday. Strong southwest flow aloft
tonight will ease and become moderate to light westerly on Monday.


Rain will ease to showers from the northwest late tonight into
Monday morning and will be followed by mountain showers and a
Puget Sound Convergence Zone on Monday. The air mass will be moist
tonight with some drying along the coast and in the lowlands away
from the convergence zone during the day on Monday. Conditions
will gradually degrade to mainly lower end MVFR tonight and Monday
morning as the winds aloft ease, then will become mainly VFR with
MVFR limited to showers and the convergence zone on Monday.
Conditions will dry further Monday evening as weak high pressure
moves across the area. Albrecht

KSEA...CIGS near 040 early this evening will generally deteriorate
to BKN015 later tonight into Monday morning as pressure gradients
across the area weaken with the incoming front and low pressure
trough. Winds 19050kt at 020 and 21060kt at 050 will ease rapidly
around 08Z as the front weakens over the area. Convergence zone
activity is expected to hang up north of the terminal (near or
just north of KBFI) during the day Monday giving light opposing
winds around the terminal area. Ceilings will slowly lift and
become mainly VFR Monday afternoon or evening. Albrecht

&&

.MARINE...A cold front will move southeast through the
Washington waters this evening. The front is being preceded by
southerly gales. Winds will ease quickly following the frontal
passage. The front has slowed its forward progress somewhat as a
wave of low pressure moves northeast along the frontal band. This
has prompted the continuation of the gale warnings for some of the
inland and southern coastal zones until midnight tonight.

Seas will slowly subside late tonight into Monday. A small craft
advisory for hazardous seas will continue in effect over the
coastal waters into Monday.

A high pressure center will pass across the waters late Monday or
Monday night. A strong warm front will push northward through the
waters on Tuesday, preceded by strong easterly flow with gales
possible near the West Entrance. The warm front will be followed
by southerly flow on Wednesday. Strong low pressure over the
northeast Pacific will maintain southerly flow on Thu and Fri.
Albrecht/Haner

&&

.HYDROLOGY...Around a couple inches of rain fell along the south
slopes of the Olympics the past 12 hours. Additional rain tonight
will probably not be significant. The Skokomish river has started
rising sharply but it is questionable if the amount of rain will be
enough to cause minor flood stage. The flood watch remains in effect
and with so much uncertainty, will monitor trends closely the next
few hours before deciding on whether or not a warning is needed. If
it does reach flood late tonight/Monday morning, it probably will
not remain above flood stage as long as previously thought.

Rainfall will be heaviest Tuesday and Wednesday with the next warm
frontal system. A lull is expected on Wednesday, and then it picks
up a bit Thursday but is not heavy. We will have to watch
the rivers with the very high snow level around mid-week, but the
Skokomish River in Mason county is the only one that is an obvious
threat --- if not tonight and Monday, then certainly on Tuesday.
A Hydrologic Outlook was issued today for all counties except
Mason, Island, and San Juan -- highlighting the uncertainty of the
forecasts next week but the unusually high freezing level and
periods of rain in the mountains.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Winter Weather Advisory until 3 AM PST Monday for Cascades of
     Pierce and Lewis Counties-Cascades of Snohomish and King
     Counties.

     Winter Storm Warning until 10 AM PST Monday for Cascades of
     Whatcom and Skagit Counties.

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

     Wind Advisory until 10 PM PST this evening for Admiralty Inlet
     Area-San Juan County-Western Whatcom County.

     Wind Advisory until midnight PST tonight for Central Coast.

PZ...Small Craft Advisory for rough bar until 3 PM PST Monday for
     Grays Harbor Bar.

     Gale Warning until midnight PST tonight for Admiralty Inlet-
     Coastal Waters From Point Grenville To Cape Shoalwater 10
     To 60 Nm-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 for hazardous seas from midnight tonight to
     3 PM PST Monday for 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.

     Small Craft Advisory for hazardous seas until 3 PM PST Monday
     for Coastal Waters From James Island To Point Grenville 10
     To 60 Nm.

     Small Craft Advisory for hazardous seas until 3 PM PST Monday for
     Coastal Waters From Cape Flattery To James Island 10 To 60 Nm-
     Coastal Waters From Cape Flattery To James Island Out 10 Nm.

     Small Craft Advisory for hazardous seas until 9 AM PST Monday
     for West Entrance U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory until 9 AM PST Monday for Central U.S.
     Waters Strait Of Juan De Fuca.

&&

$$

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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