Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 201544
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
900 AM PDT Sun May 20 2018

.SYNOPSIS...A weak upper level trough offshore this morning will
move into the area this afternoon and evening keeping a chance of
showers in the forecast with the best chances in the mountains. The
trough will dissipate over Western Washington early Monday
morning. An upper level ridge will begin to build over the area
later Monday and remain over the area for most of the week. The
flow aloft will become southerly in the middle of the week
bringing some moisture up from the south with a chance of showers
again for the mountains Wednesday.

&&

.SHORT TERM...Satellite imagery shows cloudy skies over Western
Washington this morning. Doppler radar still shows a few light
showers. Temps at 8am are in the 50s.

Upper level trough just offshore early this morning will slowly
move over Western Washington today. Most of the energy associated
with the trough moving inland well to the south of the area. The
air mass is moist over the region with multiple cloud layers up to
8000 feet. While there is little in the way of dynamics with the
trough with the deep marine layer just a little lift could result
in a light shower so will keep the chance pops in the forecast.
Onshore surface gradients weakening during the day but with the
deep marine layer in place even the strong late May sun will have
a hard time burning completely through the cloud cover. Some
sunbreaks in the afternoon for the interior which will help high
temperatures get into the 60s. Highs along the coast and near the
Strait of Juan de Fuca will be around 60.

What is left of the trough dissipating over the area tonight. With
the trough axis right over Western Washington still a chance for
a mountain shower in the evening hours. Onshore flow increasing in
the evening hours but KOTH-KSEA gradient peaking out near plus 3
mb and the KHQM-KSEA peaking below 2 mb so expect the marine
layer to be thinner overnight into Monday morning. Lows will be in
the mid 40s to mid 50s.

Upper level ridge building into Western Washington on Monday with
the surface gradients going northwesterly in the afternoon. With
the thinner marine layer and the lack of reinforcing onshore flow
expect plenty of sunshine in the afternoon after morning clouds.
Highs on Monday with more sunshine will be warmer, in the 60s to
lower 70s.

Upper level ridge remaining over Western Washington Monday night
into Tuesday with northwesterly surface gradients continuing into
Tuesday afternoon. 500 mb heights only in the mid 570 dms but
temperatures in the lower levels warming nicely with model 850 mb
temperatures in the plus 14 to 16C range Tuesday afternoon. Marine
layer will be confined to the coast late Monday night into
Tuesday morning. With more sunshine and warming temperatures aloft
highs on Tuesday in the 70s for most locations in the interior
and mid to upper 60s along the coast and near the water in the
north.

.LONG TERM...Extended models in good agreement on Wednesday with
an upper level low off the California coast moving east and the
flow aloft over Western Washington becoming southerly. This will
push some mid/high level moisture over the area. Air mass over the
mountains will be slightly unstable during the peak daytime
heating hours. This combined with the mid level moisture will
result in a chance of showers over the Cascades on Wednesday.
Highs in the 70s will once again be common with some lower 80s
over the interior from Seattle southward.

Surface gradients go onshore late in the day Wednesday with the
onshore flow weakening Thursday. Flow aloft becoming more
southwesterly eliminating the chance for mountain showers. Weak
marine push Wednesday night into Thursday morning with high
temperatures on Thursday 4 to 8 degrees cooler versus Wednesday.

Some small differences begin to show up in the models for Friday
and Saturday with the GFS moving the upper low northward faster
than the ECMWF solution. The further north GFS solution would
increase the chances for showers mainly over the mountains on
Saturday. South southwesterly flow aloft will keep the air mass
aloft warm over the region. Low level flow remaining onshore which
will keep highs in the mid 60s to mid 70s both days. Felton

&&

.AVIATION...An upper level trough axis lies over the offshore
waters this morning. The air mass is moist and generally stable with
light southwest flow aloft. The air mass will gradually dry today
with the flow aloft veering to northwesterly this evening as the
trough axis moves onshore. Cigs generally MVFR area-wide with a few
exceptions...OLM and BFI are actually seeing VFR conditions while
PWT and PAE are reporting IFR. Conditions expected to improve to
mainly VFR from mid-afternoon into the evening hours. Some MVFR in
stratus is expected to redevelop overnight as continued low level
onshore flow brings some low level marine cloudiness into the
interior of Western Washington.

KSEA...MVFR ceilings are expected to persist through the morning as
residual moisture from shower activity remains over the region. Some
improvement to VFR ceilings expected after about 21Z. MVFR ceilings
may redevelop after 07Z tonight as stratus pushes into the interior
from the coast. Southwest winds 5 to 10 knots through the morning
will make a gradual shift toward the northwest by around 00Z.  27/SMR

.MARINE...High pressure offshore and lower pressure east of the
Cascades will result in generally weak onshore flow the next
several days. Small craft advisory westerlies are likely in the
central and eastern Strait of Juan de Fuca during the diurnal peak
late afternoon through early morning hours most days.  As has been
the pattern...inherited headline covers this expectation for tonight
and into Monday morning. Thus...no need for any alterations
to the forecast with the late morning forecast package. McDonnal/SMR

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory from 3 PM this afternoon to 3 AM PDT Monday
     for Central U.S. Waters Strait Of Juan De Fuca-East
     Entrance U.S. Waters Strait Of Juan De Fuca.

&&

$$

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