Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 221018
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
320 AM PDT Sat Jul 22 2017

.SYNOPSIS...An upper level trough and weak warm front will brush
Western Washington today bringing clouds to much of the area and a
chance of light rain to the coast and north part. Dry conditions and
diminishing cloud cover are expected late tonight and Sunday.
Onshore flow will return for dry and mild weather through most of
next week.

&&

.SHORT TERM...An upper level low off the coast of northern BC will
gradually push eastward today and as it does so...associated
moisture with the system may allow for some precip over portions of
the area. Precip currently looks to be confined to mostly the coast
and the northwestern fourth of the Olympic Peninsula...but Whatcom
and the northern half of Skagit counties may see a chance for precip
as well. For the remaining points south and east...including the
Seattle metro area...lawns look to remain parched for now. With no
precip recorded at Sea-Tac Friday...2017 has entered into the top 10
when it comes to the number of consecutive days without measurable
precip...entering into a three way tie with stretches in 1945 and
1979. Another dry day today would climb us further up the ladder to
the number 9 spot...tying with a stretch in 2013 that ran from June
28th to August 1st.

The outlook for the short term should see the current dry streak
claw its way further up the charts. By the time the aforementioned
low makes its way onto the Canadian coast...any moisture associated
with it moves northward out of the area leaving dry zonal flow in its
wake for Sunday. Upper level ridging building over the Pacific will
help to ensure continued dry conditions into the start of the new
week.

High temps today and Sunday look to be within a few degrees of one
another...keeping the coast in the upper 60s to right around 70
while the interior lowlands will find themselves in the upper 70s to
right around 80. W WA will start to feel the impact of the Pacific
ridge starting Monday as temps climb up a few degrees as the onshore
flow weakens somewhat.  SMR

.LONG TERM...A split in the flow Tuesday will allow for upper level
ridging to the the main weather feature for W WA while an upper
level low spins up off the CA coast...allowing for the prospect for
precip to climb into OR. Current models look to be in agreement that
nothing associated with this low will make it as far north as
WA...but given its proximity this may be a feature to keep an eye on
in future runs. The upper ridge pushes east Wed with an upper trough
expected over the area for Thu. Models in agreement that this
troughiness will linger into the weekend...but not much in the way
of precip expected for the area. This level of activity for the
second half of the upcoming week will allow for temps to fall a few
degrees allowing for more mild summer weather over W WA.  SMR

&&

.AVIATION...Westerly flow aloft early this morning will continue
through tonight, as a flat upper ridge builds over the Pacific
Northwest. A weak front moving into British Columbia early this
morning will dissipate during the day. At the surface, high
pressure will build off the coast today for a return to onshore
flow across Western Washington. The air mass will be somewhat
moist at mid and high levels today, then become moist at low
levels tonight.

Clouds associated with the weak front to the north are keeping
skies BKN-OVC with bases mostly 7000-9,000 ft. The cloud bases
should lower to 5000-8000 ft today. Moist low level onshore flow
tonight should eventually bring widespread MVFR conditions late
tonight with ceilings mainly 1100-2000 ft.

KSEA...The ceiling will probably be 6000-7000 ft most of
today, then lower to around 1300-1600 ft late tonight in moist
onshore flow. Southerly wind 4-8 kt should gradually veer to
southwest this morning, northwest this afternoon and evening, and
northerly tonight. McDonnal

&&

.MARINE...A weak front moving into British Columbia this morning
will dissipate today. Onshore flow will develop across Western
Washington this afternoon as high pressure builds offshore and
lower pressure continues east of the Cascades. Small craft
advisory westerlies are likely in the Central and Eastern Strait
of Juan de Fuca tonight.

Onshore flow will continue Sunday and Monday, with small craft
advisory northwest winds at times over the coastal waters. Onshore
flow should strengthen further on Tuesday, and westerly gales are
possible in the Strait of Juan de Fuca. McDonnal

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory until 5 AM PDT Saturday for Coastal Waters
     From Cape Flattery To James Island 10 To 60 Nm-Coastal
     Waters From Cape Flattery To James Island Out 10 Nm-Coastal
     Waters From James Island To Point Grenville 10 To 60 Nm-
     Coastal Waters From James Island To Point Grenville Out 10
     Nm.

&&

$$

www.weather.gov/seattle

You can see an illustrated version of this discussion at
www.wrh.noaa.gov/sew/gafd/latest_webafd.html



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