Scientific Forecaster Discussion
fxus66 ksew 282135
Area forecast discussion
National Weather Service Seattle Washington
230 PM PDT Wed Jun 28 2017
Synopsis...a surface ridge offshore will maintain onshore flow into
early next week. This will produce typical early Summer weather with
mild temperatures, morning low clouds, and afternoon sunshine. An
upper level ridge will weaken the onshore flow and produce a minor
warming trend Thursday and Friday. Stronger onshore flow will bring
more cloud cover this weekend, and possibly light rain to the coast.
Short term...satellite imagery shows that while the northern third
of western Washington has cleared out nicely, the rest of the region
has been slow to clear. There has been clearing from the edges, so
some hours of mostly clear conditions are likely for the Seattle
Metro area this evening, even if the southwest interior remains
mostly cloudy. For tonight have a partly cloudy forecast for most
areas, except mostly cloudy on the coast.
The upper level ridge offshore will slowly move east tonight and
Thursday. The 500 mb heights will rise over the area with the ridge
axis just offshore; heights will be in the low to mid 580s. Low
level onshore flow will weaken, resulting in less morning cloud
cover for the interior Thursday morning. With less cloud cover and
the warming temperatures aloft, highs on Thursday will be a few
degrees warmer with mid and upper 60s on the coast and widespread
70s over the interior.
The upper level ridge will move through the area Thursday night into
Friday morning and will be just east of the area Friday afternoon.
Onshore pressure gradients don't begin to increase again until
Friday afternoon, giving a mostly sunny day across most of the area
on Friday. Friday will be the warmest day in the forecast period
with mostly lower 80s from Seattle south. North of Seattle 70s
will be common with highs near 70 on the coast.
Heights fall on Saturday as an upper trough moves through. Highs
will fall into the 70s, 60s coast, but the GFS and in fact all
models are stubbornly dry with this weak upper trough. Have kept a
mostly sunny forecast going on Saturday but there could be at least
morning clouds. The surface gradient is forecast to be strongly
onshore Friday night into Saturday which would imply a fair amount
of cloudiness. Burke
Long term...the upper pattern Sunday through Tuesday has a weak
trough over the area. At the surface the flow is moderate to strong
onshore. There will be morning clouds and possibly afternoon sun
each day, with highs in the low to mid 70s. Have limited any nonzero
pops to the coast. Wednesday looks a bit warmer as heights rise and
onshore surface flow weakens. Burke
Aviation...upper level high pressure offshore will slowly make its
way eastward today with flow aloft mainly from the northwest through
Thursday. Low level flow will remain onshore. Low level stratus are
hanging on longer than initially anticipated...but current satellite
trends show that burn off is starting to kick in and would expect
skies to start to clear as mid-afternoon approaches. Thus...cigs
likely to remain MVFR for a little bit longer before scattering out
after 22z-ish. Clearing skies will prevail for the remainder of the
afternoon and into the overnight hours before another low stratus
deck can be expected to roll over the area early Thursday morning.
Ksea...above discussion applies. Winds southwesterly 6-10 kts
turning northerly by this evening before becoming light and
variable overnight. Smr
Marine...higher pres offshore with lower pres east of the Cascades
will maintain onshore flow of varying strength through the week.
Most recent model data shows Small Craft Advisory winds over the Strait by late this
afternoon...so posted the advisory with morning package and that
still looks good. Small Craft Advisory winds may return for the Strait Thu
evening/night as well. Strengthening onshore flow Friday afternoon
and evening still looks like it may result in gales for portions of
the Strait...but will leave for next shift to determine whether or
not to Post a watch. Winds over remaining waters look to be fairly
uneventful in the near term...although models hint that winds over
the coastal waters may approach Small Craft Advisory thresholds Friday evening.
Pz...Small Craft Advisory for central and east Strait late this
afternoon and tonight.