Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 152255
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
355 PM PDT Sun Oct 15 2017

.SYNOPSIS...A weak upper level ridge over the area this afternoon
will shift southeast on Monday as wet frontal system approaches
the area from the northwest. The cold front will slide southeast
across the area Tuesday morning. A vigorous warm front will lift
back northward on Tuesday night then stall over or near the area
Wednesday and Wednesday night before moving southeast on
Thursday. This front will give periods of locally heavy rain and
windy conditions to the area. Cooler and showery weather will
follow on Friday. A wet and windy atmospheric river pattern is
possible again next weekend.

&&

.SHORT TERM...It is a really nice afternoon in the Pacific
Northwest with plenty of sunshine and temperatures around the 60
degree mark. While there will be some patchy fog over the southern
two thirds of the interior late tonight into Monday morning, the
nice weather will generally continue through Monday morning.
Conditions will gradually deteriorate from the north on Monday as
a strong and wet frontal system from northern Vancouver Island
southwestward into the Pacific moves southeast. The southeastern
half of the forecast area will likely eek out another nice day,
but rain will be developing from around Forks to the northern
interior near the Canadian Border by late afternoon. Highs on
Monday will again be in the upper 50s to mid 60s across the area.

Rain and breezy conditions ahead of the front will shift southeast
across the area late Monday night into Tuesday morning. A period
of moderate to locally heavy rain will accompany the front, but
the front will be moving. Across the northern half of the forecast
area, snow levels will fall to around 4500 feet behind the front.

The front will hang up near or just south of the area Tuesday
evening then will work back northward as a strong warm front later
Tuesday night into Wednesday. Moisture being transported eastward
from the tropical West Pacific into the front in strong westerly
flow aloft will give periods of heavy rain as the front stalls
over or near the area Wednesday through Thursday. 850 mb WSW
winds of 50-60 kt with the high moisture content of the air would
produce significant rainfall totals in the mountains. The 12Z
forecast models were consistent with each other in taking most of
the moisture into British Columbia midday Wednesday before
bringing the front back into the area Wednesday night or Thursday.
The 18Z GFS appears slower to move the front northward, then
hangs the front over the northern half of the area through midday
Thursday -increasing total rainfall amounts. The key message is
that there continues to be a considerable amount of uncertainty in
the details of how much rain will fall in different locations
across the area and what the specific impacts will be on area
rivers.

In the warm sector on Wednesday, snow levels will rise back to
nearly 9000 feet and temperatures will rise to around 60 despite
the rainfall. A Hydrologic Outlook was issued to address potential
flooding impacts from the heavy rain. See the hydrology section
below. Albrecht

.LONG TERM...Locally heavy rainfall may continue into Thursday as
the cold front shifts very slowly to the southeast across the
area. A cooler and showery pattern with snow levels dropping to
around 4000 feet or so is expected Thursday night through Friday
night.

The latest long term models continue to show the redevelopment of
an atmospheric river pattern over the area starting Saturday of
next weekend and continuing through Sunday. The 18Z GFS shows
impressive rainfall amounts on the west slopes of the Olympics
and in the North Cascades. The ECWMF has lower totals but shows an
impressive pattern that typically results in heavy rainfall. It
is too early for details at this point, but it is looking quite
wet and there is the potential for more significant river flooding
over portions of the area next weekend. Albrecht

&&

.AVIATION...Flat upper ridge over Washington this afternoon to shift
slowly east through Monday morning. Westerly flow aloft. A Pacific
frontal system is forecast to approach Western Washington from the
NW during the day Monday with increasing moisture aloft. At the
surface, light offshore flow to become onshore Monday in response to
falling pressure to the north with the approaching front. Areas of
low level moisture and IFR conditions along the coast and into the
SW interior and south Sound associated with the developing onshore
flow Monday morning. Air mass stable.

KSEA...VFR through tonight with some higher clouds well above 10K ft
passing over the terminal at times. Low stratus associated with the
developing onshore flow overnight will get close to the terminal
after 12z Monday. North wind around 5 kts to becoming light and
variable after 02z, then light southerly by 12z Monday. Buehner

&&

.MARINE...An approaching Pacific frontal system moving into British
Columbia tonight and Monday will drop pressures to the north with
onshore flow developing across Western Washington. Anticipate small
craft advisory winds to develop along the coast Monday. The front is
expected to sag southeast across the area Monday night and Tuesday.

The next stronger Pacific frontal system is set to arrive Wednesday
and anticipate gale force winds over the coastal waters. Solid small
craft advisory winds are likely over the inland waters. Forecasts
should be monitored closely as the precise track of the front could
bring stronger or weaker winds than forecast, especially over the
inland waters.

These Pacific frontal system will also help generate rising ocean
swells, building above 10 feet by Monday night for the coastal
waters, and likely close to 20 ft by late Thursday. Buehner

&&

.HYDROLOGY...An atmospheric river event is expected Tuesday night
through perhaps Thursday. On average, the focus of heaviest
precipitation is aimed toward the Olympics and North Cascades.
Forecast models started the more northward focus with last nights
runs, but the latest 18Z GFS solution is slightly farther to the
south. Rainfall amounts in the Olympics are expected to be 7 to 10
inches over a 36 hour period from late Tuesday night through
Thursday morning. The North Cascades will likely see 3-6 inches,
and the lowlands will likely see between 0.75 and 1.5 inches.

All area rivers will see rises. Since rivers are currently rather
low, the river most likely to flood will be the particularly
flood-prone Skokomish River in Mason County. A few other rivers
may approach flood stage. More widespread flooding is possible if
the atmospheric river hangs over the area without moving to the
north of the area on Wednesday afternoon. In the lowlands, a
period of heavy rainfall could result in some urban flooding due
to autumn leaf litter clogging drains.

Another period of heavy rainfall is increasingly likely next
weekend. With rivers starting at a higher level, the weekend event
could result in more widespread and significant flooding if it
occurs. Albrecht,

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory from 6 AM Monday to 5 PM PDT Tuesday 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-Coastal Waters From Point
     Grenville To Cape Shoalwater Out 10 Nm.

&&

$$

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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