Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 142259
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
259 PM PST Sat Jan 14 2017

.SYNOPSIS...An upper ridge and light winds will keep a cold dry
air mass over Western Washington through Sunday. A front should
reach the area on Monday, with wet and warmer weather Tuesday and
Wednesday. An upper trough should bring cooler and showery weather
Thursday and Friday.

&&

.SHORT TERM...High pressure and offshore flow will continue
through the weekend for more cool and dry weather across Western
Washington. Low temperatures tonight will be in the teens to 20s
with highs on Sunday in the mid 30s to lower 40s. A weak system will
split offshore but will send mid/high clouds inland.

A pattern change early next week will bring wetter, warmer and
windier weather back to Western Washington. This change will take
place on Monday as a strong jet lines up over WA. There is a
timing discrepancy in the models, but most areas should see rain
as we move toward Monday evening. Strong, moist westerly flow will
continue on Tuesday and models are ramping up the precip totals in
the mountains. Heavy rainfall, with rising snow levels, may cause
river flooding. A Hydrologic Outlook has been issued. It will be
wet and windy in the lowlands over this period as well. The warmer
air mass will cause temps to spike into the lower 50s on Tuesday.
33

.LONG TERM...The wet and active period of weather will continue
into Wednesday with more heavy rain expected in the mountains.
However, the models have significant differences in the exact QPF
amounts. The GFS stands as the wettest solution to this
point...with a whopping 13 inches in the Olympics (Tue-Wed). A
blend of the mesoscale models was used for now but still shows up
to 9 inches in the Olympics...with 3-7 inches in the Cascades.
Heavy precip should come to an end Wednesday night as a cool upper
level trough moves inland.

The forecast through the end of the week will remain cool and
showery with a trough over the Pac NW. We may see a break in the
weather late next weekend as a weak ridge moves in. 33

&&

.AVIATION...The dry and stable air mass will remain in place for the
remainder of the weekend...keeping VFR conditions over Western
Washington. High clouds continue to work their way over the area and
are expected lower some tonight and into tomorrow. Low level flow is
still set up to be easterly...but gradients will continue to loosen
resulting in generally light winds. That said...the prospect for
stratus and fog may pop up again late tonight and into Sunday
morning for prone areas before cigs improve in the afternoon.

KSEA...Scattered high clouds this afternoon gradually increasing and
lowering overnight tonight. Low level winds to remain light and
easterly this afternoon and tonight...but as pressure gradients
loosen further winds will become variable by late Sunday morning.
SMR

&&

.MARINE...Surface high pressure east of the Cascades will result in
light offshore flow for the rest of today and into Sunday. Winds
expected to start to increase Sunday evening in advance of
approaching front expected to enter the waters Monday. Frontal
systems Tuesday and Wednesday will bring breezy southerly flow to
the area. SMR

&&

.HYDROLOGY...River flooding is not expected through Monday. A
pattern favorable for heavy rain and high freezing levels --
probably 8000 ft or a little higher -- will develop Monday night
or Tuesday and continue through Tuesday night or early Wednesday.
At this time it looks like two-day rain totals of 5 to 9 inches
are possible over the Olympic Mountains, with around 3 to 7 inch
totals possible over the Cascades. This could result in minor
flooding on the more flood-prone rivers.

Note: Snow is not a significant contributor to flood flows on
Western Washington rivers. Even when there is snow on the ground,
river flooding is almost exclusively caused by runoff from heavy
rain.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...None.
&&

$$

www.weather.gov/seattle

You can see an illustrated version of this discussion at
www.weather.gov/seattle/gafd/latest_webafd.html



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