Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 201604
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
900 AM PDT FRI MAY 20 2016

.SYNOPSIS...The air mass will dry somewhat today, with morning
clouds giving way to some sunshine across the western Washington
lowlands. Scattered showers should develop over the mountains. An
upper level trough will remain over the region this weekend and
most of next week for typical May weather with scattered showers,
some sunny periods, and highs in the upper 50s and 60s.

&&

.SHORT TERM...An upper level low is over the Pacific Northwest,
California and Nevada. A weather disturbance over Idaho should
develop today, moving north at first to the panhandle, and then
shifting westward across the northern portion of Eastern Washington
through Saturday morning. This disturbance could bring an increase
in showers over Western Washington on Saturday. For Saturday night
and Sunday the main center of the low will have migrated from
Northern California today to northwest Montana--while another weaker
lobe of the low forms over southwest British Columbia. That may
keep showers going at times.

.LONG TERM...The models do not agree very well for the extended
forecast period Monday through Thursday, especially with regard to
the details. It looks likely that an upper trough will remain
over the region. Most of the shower activity should be over the
mountains, and the western Washington should be dry most of the
time with some sunny periods. Highs will be mainly in the 60s. All
in all, it will be quite typical weather for late May. McDonnal

&&

.AVIATION...A large upper level low will remain over the
PacNW through Saturday. The low center will remain down over N CA
and S Oregon with an upper level trough extending N over WA. Flow
aloft over W WA will be light E-SE.  The air mass over W WA is
somewhat moist and unstable at lower levels, with a low level
inversion capping the low level cumulus clouds. CIGS are MVFR around
BKN-OVC020 from about KPAE southward with VFR CIGS around BKN050
over the N interior. The low level inversion will lift slowly
through 20Z, allowing CIGS to gradually improve. The inversion
should mix out around 20Z with SCT layers after 20Z.

A weak upper level shortwave trough rotating around the large low
will spread moisture westward across the Cascades 06z-12z with light
rain possibly reaching the Puget Sound lowlands after 12z.

The upper level low will weaken Saturday night and Sunday as the
center drifts up over the northern Rockies, but W WA will still
remain under the broad low.

KSEA...The somewhat moist and unstable air mass will allow BKN020-
030 cumulus clouds to continue over the terminal through 20z. A low
level inversion is capping the cumulus development. CIGS will
gradually lift through the morning. Once the low level inversion
mixes out around 20z the cumulus layer will become FEW-SCT030-045
with SCT higher clouds above. Surface winds will remain light and
variable through 17z-18z with NW winds 4-8 kt after 18z. Kam

&&

.MARINE...A weak surface trough along the WA coast will gradually
fill this afternoon. Pressure gradients across the area are weak,
with light winds expected today. The coastal trough will fill this
afternoon and evening while another weak surface trough forms over
the N interior this evening. This change will allow weak W to SW
onshore flow to develop overnight. The trough will deepen into a
weak low over Vancouver Island by Saturday evening with W to SW
onshore flow continuing over W WA. The onshore flow could become
strong enough through the Strait of Juan de Fuca for SCA winds 15-25
kt to develop in the central and eastern strait. Higher pressure
offshore will continue through early next week allowing varying
degrees of onshore flow to continue. Kam

&&

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

&&

$$

www.weather.gov/seattle

An illustrated version of this forecast discussion can be seen at
www.weather.gov/seattle/gafd/latest_webafd.html


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