Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 141056
AFDSEW

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

.SYNOPSIS...A weak warm front will bring a chance of light rain
to the Olympic Peninsula and areas near the Canadian border this
afternoon and evening. Otherwise, a flat upper level ridge will
move over the Pacific Northwest through Monday, bringing a mainly
dry period to the area and allowing temperatures to warm to near
normal Sunday and Monday. A wet and windy weather pattern will
develop Tuesday with additional weather systems reaching the area
through next Friday.

&&

.SHORT TERM...A flat upper ridge will spread into Western
Washington today. Rising heights and warming air aloft will push
across the area, amounting to a weak warm front. This will mainly
increase mid-high clouds, but there is a chance of light rain this
afternoon over the Olympic Peninsula and near the Canadian
border.

The flat upper ridge axis will pass across Western Washington on
Sunday and Monday, leading to dry and mostly sunny weather. 850
mb temps will reach +10C on Monday, leading to warmer surface
temps. Lingering low- level moisture under stable conditions will
favor early morning fog development along the I-5 corridor and in
the Puget Sound lowlands on Sunday morning and especially Monday
morning.

.LONG TERM...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...High pressure aloft will weaken today as a weak front
brushes the northwest portion of Wrn Washington. Light southwesterly
flow aloft becoming westerly tonight and Sunday between fronts. Air
mass mostly dry and stable with predominately VFR skies early this
morning - except for localized IFR fog in the most prone low lying
of the southwest interior and west side of Puget Sound due to
residual surface moisture. Mid level moisture will increase by
mid morning with any fog dissipating. Light spotty rain should
only brush the north coast and Olympics. Clouds scattering across
most the area during the evening.

KSEA...South wind 4-7 kt. Isolated fog possible in the vicinity of
the KSEA terminal, mainly until 14-15Z but is not expected at the
terminal. Will monitor for any fog development but it looks to stay
confined to south of SEA/BFI. dtm

&&

.MARINE...A weak front will move into the coastal waters this
morning and mostly dissipate near the coast by late in the day.
Small craft southerly winds will affect the northern 2/3 of the
coastal waters, easing late and becoming southwesterly.

High pressure will temporarily build over the area tonight through
Monday with lighter flow over the area. A series of stronger fronts
will begin to affect the waters by Monday night or Tuesday. However,
there is a high degree of uncertainty on the exact track of these
systems. Winds over the waters could be stronger or lighter than
indicated and forecasts should be monitored closely next week. dtm

&&

.HYDROLOGY...A moderate to strong front will move across the
region late Monday night into Tuesday. This front will give many
places in the lowlands 1 to 2 inches of rain and may produce even
more in the mountains. With most area rivers running at base
flow, this front will likely not produce flooding on area rivers
-with exception to the normally flood-prone Skokomish. With fall
leaves clogging area drains, low spots in urban areas may see some
street flooding with Tuesday`s front.

A stronger front will move across the area Wednesday or Thursday.
The model timing is generally faster than previous solutions, but
the ECMWF operational run is slower than the faster but slightly
wetter GFS. Both model solutions show a period of rather heavy
precipitation with snow levels between 5000 and 6500 feet.
Heavier model solutions would possibly result in minor flooding on
normally flood-prone rivers, such as the Skokomish River. Haner

&&

.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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