Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 141540
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
840 AM PDT Sat Oct 14 2017

.SYNOPSIS...A little light rain is likely over the Olympic
Peninsula and near the Canadian Border today as a weak warm front
brushes the region. A flat upper level ridge will move over the
Pacific Northwest tonight through Monday, bringing a mainly dry
period to the area and allowing temperatures to warm to a little
above normal Sunday and Monday. A wet and windy weather pattern
will develop Tuesday with additional weather systems reaching the
area Wednesday through early next weekend.

&&

.SHORT TERM...IR satellite imagery this morning shows a weakening
warm front extending from around 50N 130W southeastward into the
Oregon offshore waters. This front will gradually dissipate later
today as it moves into the Washington coastal waters. A few
hundredths of an inch of light rain is likely along the west
slopes of the Olympics and along the northern portions of the
Washington coast. Areas further south on the coast and in the far
north interior near the Canadian border will just see a few
sprinkles, while the interior from about Everett southward will
only see mid and high clouds. Temperatures today will stay in the
50s with the clouds.

A flat upper ridge will build into the Pacific Northwest tonight
through Sunday night then shift east of the area on Monday. This
ridge, with warming at 850 mb to around 10C by Monday, will give
drying and warming to the region Sunday and Monday with high
temperatures getting into the upper 50s to mid 60s. There will be
enough lingering low level moisture that there will be areas of
late night and morning fog, especially in fog-prone valleys and in
the interior from about Seattle southward.

The short term forecast is in good shape today. No updates are
planned. Albrecht

.LONG TERM...From the previous long term discussion: A
progressive upper trough will move eastward across B.C. on Tue.
This will push a cold front through Western Washington. Tuesday`s
cold front will be accompanied by breezy conditions and a heavy
but short- lived shot of rain.

A break between storms is expected around Tue night.

An even stronger storm appears likely on Wed. Wednesday`s front
has a few classic hallmarks of a heavy rain producer, including SW
850 mb winds of 50+ kt on the equatorward side of the sharp
front, combined with precipitable water values of 1"-1.3". That
front will exit around Wed night, leaving a cool and showery upper
trough in its wake next Thu and Fri. Haner

&&

.AVIATION...Flat upper level high pressure over Western Washington
is expected to build a little tonight and Sunday. WNW flow aloft
becoming westerly overnight. A weak warm front moving into British
Columbia will spread mainly mid level clouds into the area today and
a little light rain across northern sections. Air mass stable.
Decreasing clouds tonight with patchy fog developing in fog prone
areas like the south Sound and Snohomish Co valleys.

KSEA...Mid level clouds to be prominent through this evening then
scattering out tonight into Sunday morning. Southerly wind 4 to 7
kts then easing to under 4 kts after 05z. Buehner

&&

.MARINE...A weak front will move east into British Columbia today.
Small craft advisory strength southerly winds to affect the northern
2/3 of the coastal waters, easing this evening. Higher pressure is
forecast to build onshore tonight into Monday with generally light
winds over the area.

A series of stronger fronts are expected to impact the coastal
waters region next week. The first system is anticipated to arrive
Monday night into Tuesday. The following stronger system is set to
arrive Wednesday. Exact details with these systems are not firm and
the resulting strength of winds are not set yet. With the higher
level of uncertainty, forecasts should be monitored closely next
week.

Of greater certainty is rising ocean swells well above 10 feet
reaching the outer coastal waters starting the middle of next week.
Buehner

&&

.HYDROLOGY...A moderate to strong front will move across the
region late Monday night into Tuesday. It appears that this front
will be accompanied by strong westerly flow aloft and will be
enough of a fast mover that rain in the interior lowlands will be
limited to around 0.5 inch. The mountains will likely see 1-3
inches. With most area rivers running at base flow, this front
will likely not produce flooding on area rivers.

Of increasing concern is a second stronger front and atmospheric
river event that will possibly start giving heavy rainfall to the
area starting Tuesday night and will hang up over the area
Wednesday through Wednesday night. Models have trended higher with
the snow levels (now expected to average over 8000 feet) and
rainfall amounts (5-7 inches in the Olympics, 2-5 inches in the
Cascades, and 1-2 inches in the interior Lowlands). Most of the
rainfall may occur in a short 6-12 hour burst on Wednesday per
latest model guidance from the GFS, ECMWF, and UWWRF. Rainfall of
this amount would drive most rivers to bankfull and would cause
flooding on the normally flood prone Skokomish River in Mason
County. Low spots in urban areas of the I-5 corridor could see
some street flooding as fall leaves clog area drains.

A period of strong onshore flow with showers and lower snow levels
is expected Thursday and Friday.

There are indications that a third front will bring another
period of heavy rainfall and possibly some minor flooding to area
rivers next weekend. Albrecht

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory until 5 PM PDT this afternoon 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.

&&

$$

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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