Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA
FXUS66 KSEW 210431
Area Forecast Discussion
National Weather Service Seattle WA
931 PM PDT WED JUL 20 2016
.SYNOPSIS...An offshore upper low will move toward Western
Washington Thursday and inland Thursday night. The area will have
partly to mostly sunny skies, but there is a chance of showers
mainly over the mountains during the afternoon and evening hours.
Thunderstorms are possible over the Cascades Thursday. A weak
upper ridge should bring sunny weather Saturday through Wednesday.
.SHORT TERM...Southwest flow aloft will prevail tonight with an
upper trough offshore. Weak low level onshore flow should allow
low clouds to reform at the coast tonight and push partially
inland Thursday morning but much of the interior will generally
The upper trough will shift closer to the area on Thursday with
the air becoming a bit more unstable. Moisture will be limited but
a few showers or thunderstorms are possible over the Cascades late
in the day.
The upper trough will drag across the area Thursday night and move
into Eastern Washington Friday morning. There will be a chance of
showers pretty much everywhere as the upper trough moves through.
The shower threat should end later Friday as drier Westerly flow
There will also be a significant increase in onshore flow Friday
with and behind the upper trough. Highs Thursday will be similar
to today - a few degrees above normal - and will then cool around
10 degrees Friday to a few degrees below normal with much more
Saturday should be a typical summer day as an upper ridge builds
in the from the west. Morning clouds should give way to afternoon
sunshine and highs will rebound to near normal. Schneider
.LONG TERM...Previous discussion...A nondescript pattern typical
of late July will prevail Sunday through Tuesday with light
westerly flow aloft, weak onshore flow at low levels, and 500 mb
heights around 5820 meters. Aside from morning late night and
morning marine layer clouds mainly at the coast, the days will be
generally sunny and the nights mostly clear. Temperatures will be
near to slightly above normal, with highs in the mid 60s to lower
70s at the coast and in the 70s to lower 80s inland. Slightly
warmer weather is likely Wednesday as onshore flow begins to
weaken or turn offshore. McDonnal
.AVIATION...An upper level low off the Pacific Northwest coast will
shift inland across Washington Thursday night. Light southwest flow
aloft becoming westerly Thursday night as the low passes. Low level
onshore flow will strengthen Thursday afternoon and evening. The air
mass will be stable and mostly dry except somewhat moist in the
lower levels. Increasing moisture at all levels Thursday afternoon
and night and becoming slightly unstable with scattered showers.
There will be a slight chance of thunderstorms over the
Cascades...mainly Thursday evening.
Mainly VFR scattered clouds over interior Western Washington through
Thursday. However, low MVFR is likely to develop along the Coast
affecting the KHQM terminal by midnight through Thursday morning.
Stratus cigs around 1-2K ft may also surge down the Strait late
tonight and partially into the western part of the southwest
interior. KCLM and KPAE could get a few hours of low marginal cigs
KSEA...Mostly clear skies at the terminal through Thursday
morning. Mid-level moisture and instability will arrive from the
south on Thursday afternoon and evening, with a brief shower
possible from mid-level cumulus clouds. More substantial marine
stratus holds off until pre-dawn hours on Friday morning.
.MARINE...Surface high pressure over the coastal waters and lower
pressure inland will give onshore flow. Lower pressure will
develop east of the Cascades Thursday with stronger westerly
onshore flow by evening. Guidance peaks UIL-BLI pressure gradient
around +3.4 to +3.8 mb on Thursday evening, which is a good
indicator of gales. So have hoisted a Gale Warning for the central
and eastern Strait for Thursday night.
Varying degrees of west to northwest onshore flow are expected
Friday through early next week. Haner
PZ...Gale Warning from 5 PM Thursday to 5 AM PDT Friday for Central
U.S. Waters Strait Of Juan De Fuca-East Entrance U.S.
Waters Strait Of Juan De Fuca.
You can see an illustrated version of this discussion at