Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 201530
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
830 AM PDT Wed Sep 20 2017

.SYNOPSIS...A large upper level low will maintain a chance of
showers and cooler temperatures over Western Washington through
Thursday. Drier north flow aloft from an upper level ridge will
bring more sunshine and a little warming Friday through Tuesday.

&&

.SHORT TERM...A large upper level low remains over the PacNW with
the main low center over Alberta. An upper level shortwave trough
rotating around the main low was over the S WA coast this morning,
and was dousing the central/south WA coast, the SW interior, and the
S Cascades with rain. Models slide the shortwave trough over the S
Cascades by 21Z, allowing the area of rain to break up. Otherwise
the rest of W WA will have scattered showers. NAM bufr soundings
show the lower air mass unstable up to a little above 700 MB.

As the shortwave trough slides E tonight, somewhat drier N flow
aloft from an approaching offshore upper level ridge will develop
over W WA. The lower air mass will still remain fairly moist on
Thursday, so not a lot of clearing is expected. The air mass should
be stabilizing for just a slight chance of showers.

Dry N flow aloft ahead of the approaching offshore upper level ridge
will strengthen on Friday for a dry day with more sunshine. High
temperatures will warm a bit but should still be a couple of degrees
below normal. Kam

.LONG TERM...Previous discussion from the 326 AM AFd...The medium
range solutions were suggesting that the tail end of frontal systems
passing to our north might bring a threat of light rain to the
northwest part of the CWA this weekend into Monday. The GFS was
starting to come more in line with the wetter ECMWF solution. For
now, decided to keep the inherited dry forecast but will need to
introduce rain if the 1200 UTC cycle continues with the idea of
wetter conditions. At any rate, expect the ridge to be a bit dirty
(mid and/or high clouds streaming across the area) through Monday.

Anticipate the upper ridge to rebound the end of the period for
dry weather with near normal temps.

&&

.AVIATION...There are areas of rain or showers from about
Hoquiam south and south of Seattle this morning--and there were some
showers up over the San Juans. An upper level trough is over the
region with light flow over Western Washington. Drier northerly flow
will develop slowly and northerly flow aloft will develop so that
will help drive the system over Oregon slowly away. There is some
clearing north of Seattle this morning and overall more clearing
from the north is likely through this evening before low clouds
redevelop overnight due to the light flow, stabilizing air, and low
level moisture.

KSEA...Winds should come around to light nw today and most of the
showers should stay south of the area now.

&&

.MARINE...A weather system is over the region but the blustery
pressure gradients are south of the WA/Oregon border--Western WA
has rather light gradients. Weak low pressure will remain over the
WA coastal waters through Thursday and then high pressure will build
Friday and Saturday.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory from 6 PM this evening to 2 AM PDT Thursday
     for Coastal Waters From Cape Flattery To James Island 10 To
     60 Nm-Coastal Waters From James Island To Point Grenville
     10 To 60 Nm-Coastal Waters From Point Grenville To Cape
     Shoalwater 10 To 60 Nm.

&&

$$

www.weather.gov/seattle

An illustrated version of this forecast discussion can be seen at
www.wrh.noaa.gov/sew/gafd/latest_webafd.html



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