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Area forecast discussion
National Weather Service Seattle Washington
346 am PDT Wed Jun 28 2017

Synopsis...a surface ridge offshore will maintain onshore flow
across western Washington into early next week. This will produce
typical early Summer weather with mild temperatures and 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 a bit more cloud cover this
weekend. A weak front could bring a few showers to mainly the coast
on Sunday.


Short term...satellite imagery shows stratus along the coast
through the lower Chehalis valley and into the central Puget Sound
area at 3 am/10z (pic0). In other locations over the interior the
stratus is just beginning to form. Temperatures at 3 am were in the
upper 40s and 50s.

Little change in the weather today versus Tuesday with low level
onshore flow continuing over the area with weak trofiness aloft
early in the day (pic1). Onshore gradients are almost identical to
yesterday morning. The stratus has been a little slower to
form/spread inland this morning but with the satellite imagery
showing the areas that were clear starting to fill in at 3 am
expect most of the lowland to have stratus by 15z. Model cross
sections show the marine layer depth pretty similar to yesterday.
End result is a persistence forecast for today with the stratus
burning back to the coast midday. High will be similar to tuesdays
readings with mid 60s along the coast and mid 60s to mid 70s

Upper level ridge offshore moving eastward later today and
tonight (pic2). 500 mb heights rising over the area with the ridge
axis just offshore at 12z Thursday (pic3). Low level onshore flow
weakens 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 70s common over the interior. Lows tonight
will mostly be in the 50s.

Upper level ridge moving through the area Thursday night into
Friday morning with 500 mb heights peaking in the lower to mid
580 dms (pic4). Onshore pressure gradients don't begin to increase
again until Friday afternoon making for a mostly sunny day across
most of the area on Friday. Friday will be the warmest day in the
forecast period with some lower 80s from Seattle south. North of
Seattle 70s will be common with highs near 70 on the coast.

Long term...extended models in good agreement for the weekend
(pic5)(pic6) with zonal flow aloft and low level onshore flow on
Saturday for morning clouds and some afternoon sunshine over the
interior. A weak trough approaches the coast on Sunday for a slight
chance of showers in the morning (pic7). Low level onshore flow
remaining intact through Monday with a weak trough remaining over
the area for a continuation of the morning clouds with some
afternoon sunshine scenario for the interior. Some differences show
up in the models for the 4th with the GFS beginning to build a ridge
(pic8) while the European model (ecmwf) keeps the weak trough in the area. Both
solutions are dry. Current forecast is closer to the less cloud
cover/ a few degrees warmer GFS solution. Will stay with that idea
for the morning forecast package. Felton


Aviation...a weak upper level trough will continue to shift east
and high pressure offshore will slowly nudge toward the area. The
low level flow will remain onshore (pic9). Areas of coastal marine
stratus will spread back inland around daybreak and will burn off by

Ksea...morning low clouds should set up around daybreak and then
burn off late in the morning.


Marine...higher pres offshore with lower pres east of the Cascades
will maintain onshore flow of varying strength through the week
(pic9)(pic10). The flow will become strong Friday afternoon and
evening for the possibility of gale force winds over parts of the
Strait of Juan de Fuca.


Sew watches/warnings/advisories...


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