Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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344
FXUS66 KSEW 280319
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
820 PM PDT Tue Jun 27 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 mostly clear skies across W WA
this evening except for low marine stratus that has pushed inland
along the central WA coast. Onshore pressure gradients are around
1.5 MB. This will help recharge the already somewhat moist marine
layer over the interior tonight, so Wednesday morning should start
off with our familiar low stratus clouds covering most of the
interior lowlands. Burn-off should occur around midday again
followed by mostly sunny skies Wednesday afternoon.

An upper level ridge offshore along 135W this evening will move
steadily eastward, with the axis reaching the offshore waters on
Thursday then moving slowly across WA on Friday. The ridge will
weaken the onshore flow for less morning cloud cover Thursday and
especially on Friday. More sunshine coupled with warming from the
ridge will bring a minor warming trend. Inland high temperatures
should be in the 70s on Thursday and in the mid 70s to lower 80s on
Friday. Coastal highs will be cooler, 60s on Thursday with some
lower 70s around on Friday. Kam

.LONG TERM...The upper level ridge will exit over the ID panhandle
Friday night leaving the way clear for a weak front to approach the
region this weekend. Onshore flow will increase again Friday night
which will produce more stratus and a deeper marine layer Saturday
morning. High temperatures will drop back into the 70s. The weak
front approaching the coast will slide inland on Sunday. There is a
slight chance for some spotty showers along the coast and maybe the
north interior. The marine layer will be pretty deep but this could
allow some intermittent sunbreaks to occur in the afternoon.

The onshore flow pattern will continue early next week with a
surface ridge offshore and moderate W flow aloft over the region. So
Independence Day will have the familiar morning low clouds with some
afternoon clearing and mild temperatures. Kam

&&

.AVIATION...Weak upper level trof over the region will cont moving
slowly E, with upper level ridge following in its wake. Contd NW
flow aloft. The low level flow will remain onshore. Areas of coastal
marine stratus should spread back inland overnight but Wednesday
should see a bit less of the stratus and it should burn off a bit
quicker.

KSEA...Morning low clouds from about 12-18z are a pretty good bet
again Wednesday morning--although OTH-SEA is only +2mb and yesterday
at this time it was +4mb, so I may trim it back a bit and it could
be that a four hour tempo group might be enough to cover it on Wed.

&&

.MARINE...Higher pres offshore with lower pres E of the Cascades
will maintain onshore flow of varying strength through this weekend.
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...
WA...None.
PZ...Small Craft Advisories central/eastern Strait of Juan de Fuca.

&&

$$

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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