Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 142225
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
325 PM PDT Sat Oct 14 2017

.SYNOPSIS...A weak warm front is dissipating as it moves inland
this afternoon. Clouds will decrease from the south tonight
through Sunday as a flat upper level ridge builds into the Pacific
Northwest. Expect a mainly dry period across the area Sunday
and Monday. A wet and windy weather pattern will develop late
Monday night or Tuesday and will continue through next weekend.

&&

.SHORT TERM...Clouds are rather extensive across much of the area
this afternoon as a weak warm front moves onshore and dissipates.
A few light sprinkles have been seen in the San Juan Islands and
along the northern Washington coast, but overall it has remained
dry. Cloud cover is expected to gradually erode from the south
tonight into Sunday as a flat upper level ridge builds into the
area.

With the flat upper ridge building into the area, there will be
warming aloft with 850 mb temperatures rising to over 10C. This
should result in maximum temperatures at the surface rising into
the lower to mid 60s Sunday and Monday. With any clearing
tonight, expect developing temperature inversions to result in
the formation of some fog, especially in the south interior and in
fog-prone river valleys. Areas over far northern portions of
Western Washington will see little -if any- fog as mid and high
level moisture lingers, inhibiting radiational cooling.

Enjoy Sunday and Monday as weather conditions will deteriorate
from the northwest later Monday into Monday night. A frontal
system will sweep southeastward across the area on Tuesday
bringing breezy and wet conditions. The progressive nature of the
front will keep mountain rainfall amounts from becoming excessive,
while westerly flow interacting with the coastal terrain will keep
rainfall amounts in the interior lowlands light to moderate.
Albrecht

.LONG TERM...Tuesday`s front will move just south of the forecast
area Tuesday afternoon or evening resulting in a very short break
between systems. However, models are showing the duration of the
break decreasing as the jet stream across the Pacific increases in
strength.

Operational models and their ensembles continue to show the
development of a 18-24 hour atmospheric river event late Tuesday
night through Wednesday night or early Thursday. Models have
waffled somewhat on what part of the forecast area gets hit
hardest, but the basic story is that several inches of rainfall
will fall on the west and southwest facing Olympics (the WRFGFS
shows a small area over 10 inches in 48 hours) and portions of the
Cascades will see 5-7 inches as well. The lowlands may see up to
a couple inches of rain with Wednesdays system. Snow levels in
the mountains will briefly fall to 4500 feet or so Tuesday
evening, then rise back to over 7000 feet Wednesday through late
Wednesday night or Thursday before falling again to around 4000
feet. Typically these events give windy conditions to the Puget
Sound lowlands as strong flow aloft interacts with the terrain and
develops a mesolow northeast of the Olympics.

A cooler and showery pattern with onshore flow is expected later
Thursday into Friday.

While models differ on the details, they all hint (especially the
operational ECMWF) at the arrival of another atmospheric river
event next weekend. Albrecht

&&

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

KSEA...Mid level clouds through this evening then scattering out
tonight into Sunday morning. Southerly wind 4 to 7 kts easing
variable 4 kts or less after 02z. Buehner

&&

.MARINE...A weakening warm front moved east into British Columbia
today and will dissipate this evening. Higher pressure is forecast
to build onshore tonight into Sunday then shift east 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 with likely small craft advisory winds for
the coast and northern interior waters. The following stronger
system is set to arrive Wednesday and anticipate gale force winds
along the coast. Forecasts should be monitored closely as more
details about these systems next week become available.

These Pacific frontal system will also help generate rising ocean
swells, building well above 10 feet for the outer coastal waters
by 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. Successive runs of the various
models have trended higher with snow levels and rainfall amounts.
Total rainfall amounts in excess of 7 inches are likely in the
Olympics, and portions of the Cascades may see 5-7 inches. The UW
WRFGFS shows 1-2 inches in the interior lowlands falling on
Wednesday, mainly from Everett southward. Autumn leaves blowing
off the trees and clogging drains could result in some urban
flooding.

Most of the rainfall on Wednesday may occur in a short 6-12 hour
burst per latest model guidance. Rainfall of this intensity
would drive most rivers to bankfull and would cause some flooding
on area rivers. The river most likely to flood during the upcoming
week is the normally flood-prone Skokomish River in Mason County.
Stay tuned to latest forecasts.

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

There are indications that a third front will bring another
period of heavy rainfall to the area next weekend resulting in a
renewed flood threat. 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 Out 10 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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