Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 241014
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
315 AM PDT Sun Sep 24 2017

.SYNOPSIS...High pressure aloft will produce a dry mild day
today with clouds gradually increasing in advance of an incoming
weakening weather system. This system is expected to bring a chance
of rain tonight and Monday. Higher pressure aloft is expected to
build off the coast Tuesday and move onshore mid week for a period
of sunshine and above average late September temperatures.

&&

.SHORT TERM...Current satellite shows clouds associated with
incoming shortwave just starting to nick the far northwestern tip
of the Olympic peninsula at the time of this writing. While forecast
might be a touch aggressive with these clouds...will let it ride as
the trend in the forecast looks to be ahead of the satellite trend
by a couple of hours or so. Starting to see some fog development
around HQM this morning and left inherited forecast alone regarding
fog so looks to be shaping up that the more prone areas may see a
bit of patchy fog this morning.

Generally models have remained consistent for the short term as a
shortwave system embedded in the ridge will enter the area this
afternoon bringing an increase in clouds. Temperatures should still
end up being a degree or two warmer than yesterday in spite of this.
Precip associated with this looks to hold off until Monday...and
that is where there will be a bit of a dip in area temperatures.
Precip coverage continues to look scattered on the GFS and both
models show the feature weakening throughout the day. This would
lead to thinking that the best chances will be on the coast while
interior locations may only see slight chance POPs. Tuesday
continues to look like the transitional day...starting off cloudy
with clouds finally dissipating by late afternoon/early evening. The
most notable thing for Tuesday is that the warming trend will
return...with afternoon high temps jumping up 5 degrees on average
over the area when compared to those of Monday.  SMR

.LONG TERM...The ridge remains in control over W WA Wed and
Thu...allowing for dry and warm conditions to persist. Temperatures
will push into the mid 70s Wed...and a shift to offshore flow thanks
to a developing thermal pressure trough will push the temps Thu into
the upper 70s and possibly a few locations seeing 80. While
climatologically this is uncommon...it would seem this summer is not
done with the area just yet.

The ridge shifts eastward Friday and an incoming upper level trough
will serve to cool things down after the brief summer flashback.
However...models agree that any precip associated with this system
really breaks down by the time it arrives allowing for slight chance
POPs at best.  SMR

&&

.AVIATION...An upper level ridge will bring dry and stable weather
to the region today. The flow aloft is northerly. Patchy low
clouds or fog may form early this morning, mainly along the coast
and in the valleys. Otherwise, VFR conditions expected with mid
and high clouds. A weak warm front will spread rain to the coast
late tonight into Monday. 33

KSEA...Patchy low clouds possible this morning for brief MVFR
conditions. Otherwise, VFR conditions expected. Light N winds
switching to light S early this morning. 33

&&

.MARINE...High pressure over the NE Pacific will maintain light
onshore flow today. A warm front will clip the area on Monday
with possible Small Craft Advisory winds over the Coastal Waters.
The flow will turn offshore by midweek as a thermal trough forms
along the coast. 33

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...None.
&&

$$

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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