Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 282207
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
308 PM PDT SUN AUG 28 2016

.SYNOPSIS...A weak frontal system just offshore will continue
to spread clouds across Western Washington tonight, with some light
sprinkles over the coast and north interior. The frontal system will
stall just offshore Monday and Tuesday allowing more sunshine to
reach the interior and temperatures to warm a little. The front will
finally move across the area Tuesday night and Wednesday with areas
of light rain. A large upper level trough over the region will bring
mostly cloudy skies and a chance of showers to the area late this
week.

&&

.SHORT TERM...Radar has been lit up this afternoon as weak warm
advection from the weak frontal system offshore has been sending
light showers or sprinkles inland over W WA. The showers have been
affecting mainly the coast, Olympics, and north interior, but some
light showers have made it down over Seattle. The warm advection
will gradually weaken this evening with the light showers
diminishing overnight.

The large upper level low W of the central B.C. coast will dig
southward off the PacNW coast tonight and Monday. This will change
the flow aloft over the region from westerly to southwesterly, which
is more parallel with the weak frontal system just offshore.
This will cause the frontal system to stall just offshore through
Tuesday afternoon, but it will still be close enough to keep a
slight chance of showers for the coast. Over the interior the
increasing SW flow aloft will be drier and warmer, so skies over the
interior should be mostly sunny on Monday with temperatures up a few
degrees.

W WA will remain dry on Tuesday with the front remaining offshore,
except for the slight chance of showers on the coast. The GFS and
NAM are showing SW onshore flow developing Monday evening with the
OTH-SEA gradient getting up to 4 MB. This should spread stratus over
most of the interior by Tuesday morning. GFS time-height sections
indicate the stratus will burn off by early afternoon for at least
partly sunny skies.

Models now bring the offshore stalled front slowly inland Tuesday
night and Wednesday. The kicker appears to be an upper level
shortwave trough that is rotating around the S part of the large
upper level low farther offshore. The models have been trending
slower with the front the past couple of days and it is possible
that the front could be delayed even further. Precip generation
should be helped by the SW onshore flow redeveloping Tuesday evening
which will keep the lower air mass relatively moist. Although this
system is weak and slow moving, it looks like it will bring the
first measurable widespread precip to the area since August 7th. Kam

.LONG TERM...The large upper level low offshore will move slowly
inland across the region Thursday through Friday. It looks like
about three upper level shortwave troughs rotating around the main
low will move across W WA. This should be a mostly cloudy and
showery period with temperatures a little cooler than normal. The
model solutions have been inconsistent for this period so confidence
is only moderate. Confidence is lower for Saturday and Sunday. For
now I will maintain the broad brush chance of showers forecast. Kam

&&

.AVIATION...An upper level trough will dig off the Pacific
Northwest coast tonight and Monday. Light southwest flow aloft.
stable and somewhat moist at all levels. A few high based sprinkles
are possible through early this evening mainly north of KSEA/KBFI.
Gradual drying at levels tonight except residual low level moisture
at the coast and near the surface over the southwest interior.

BKN-OVC cigs around 8-10k ft with scattered clouds around 3k ft.
Sprinkles or spotty light rain possible through early this evening
mostly north of KSEA/KBFI. VFR conditions should generally prevail.
Clouds will decrease across the entire area this evening with just
some scattered residual clouds into Monday morning. Patchy low
clouds and fog could produce a few hours of IFR conditions at KHQM
and KOLM early Monday morning.

KSEA...northwest wind 4-6 kt becoming northerly by early evening,
then light after sunset this evening. Clouds decrease this evening
with status unlikely to develop tonight. VFR conditions likely
through Monday. dtm

&&

.MARINE...Light surface flow over the waters tonight. A weak front
will reach the coastal waters and stall Monday through Monday night.
Winds will likely remain just below small craft strength over the
coastal waters, then possible reach small craft strength Tuesday
before the front reaches the coast Tuesday evening. The front will
weaken and push through the inland waters Tuesday night with winds
likely to stay below 20 kt. Another front may reach the area late
Thursday. This system could be strong enough for another round of
small craft advisory winds over the coastal waters. dtm

&&

.SEW Watches/Warnings/Advisories...
WA...None.

PZ...None.

&&

$$

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An illustrated version of this forecast discussion can be seen at
www.weather.gov/seattle/gafd/latest_webafd.html



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