Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 242137
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
238 PM PDT Mon Jul 24 2017

.SYNOPSIS...An upper level ridge will bring sunny and warmer
weather to Western Washington through Tuesday, except for some
isolated morning low clouds near the coast. An upper level trough
will approach the region Tuesday night producing onshore flow and
allowing more clouds and cooler weather to return Wednesday and
Thursday. Southwest flow aloft with weak onshore flow will produce
typically mild weather this weekend. A ridge may build northward
over the area early next week.

&&

.SHORT TERM...Visible satellite imagery shows clear skies over W WA
and the coastal waters this afternoon, except for a small patch of
stratus along the central coast and a little cirrus overhead. The
low amplitude upper level shortwave ridge out near 135W continues to
slowly approach the area. Northerly low level offshore flow has
developed over W WA ahead of the ridge and will continue through
Tuesday as the ridge traverses the region. Warming from the ridge
and weak low level offshore flow on Tuesday should push highs up
into the lower to mid 80s in the warmest spots. Models and forecasts
remain on course.

The ridge axis should be over the coast Tuesday afternoon then shift
over the ID Panhandle by 12Z/5 AM Wednesday morning. A weak upper
level trough will be right behind the ridge. This eastward
transition of the ridge and trailing trough are good indicators of a
marine push. Onshore flow will increase Tuesday evening, but the
weak to moderate pressure gradients indicated by the models suggest
only a shallow marine layer will reach Puget Sound Wednesday morning
before dissipating by midday. Stratus may only make it part way up
Puget Sound, just brushing Seattle and possibly not reaching
Everett. This may yield about 5 degrees of cooling in areas with
stratus but there may be only slight cooling elsewhere.

Models indicate slightly stronger onshore flow developing Wednesday
evening, possibly enough to cover the lowlands with a deeper marine
layer Thursday morning. The stratus is still expected to dissipate
Thursday afternoon. There should be enough cooling to knock highs
back to the 70s.

On Thursday, the upper level shortwave trough just offshore will
move NE across the area. This will be a weak dry trough with only a
minor cooling contribution. Another evening of slightly weaker
onshore flow will recharge the marine layer for Friday morning. Kam

.LONG TERM...Thursday nights onshore flow should maintain the
fairly deep marine layer Friday morning. It still looks like there
will be breakout Friday afternoon. Temperatures should be about the
same or slightly warmer. By this time a large upper level trough
will have formed over the NE Pacific, centered near 50N/145W. W WA
will be under SW flow aloft between the trough and the large SW U.S.
upper level ridge through the weekend. The GFS and ECMWF now agree
that an upper level low swinging around the large trough will move
NE toward the central B.C. coast Saturday night. The associated weak
front extending S from the low could bring a few showers to the
coast late Saturday night or Sunday morning. For now, the models
agree on the showers, but this could easily change over the next
several model runs.

The models have now postponed building the big SW U.S. ridge
northward to Monday. We`ll see how that evolves over the next bunch
of model runs, as well. Kam

&&

.AVIATION...A broad upper ridge will be over the area tonight and
Tuesday with west to northwest flow aloft. At the surface,
onshore flow will continue with high pressure offshore and lower
pressure inland. The air mass is dry and stable. There might be
some low clouds or patchy fog near the coast Tuesday morning but
this will probably be fairly limited.

KSEA...Clear. Northerly winds 5-10 knots, strongest this
afternoon and early evening. Schneider

&&

.MARINE...Varying degrees of onshore flow will continue through
Friday with high pressure offshore and lower pressure inland.
Small craft advisory strength northwest winds will continue over
the outer Coastal Waters through Tuesday night. There will also
be small craft advisory northwest winds for the inner Coastal
Waters the next couple of evenings.

The Central and Eastern Strait of Juan De Fuca should have small
craft advisory west winds each evening for the next several days.
Gales are possible each evening starting Tuesday. For now, the
best chance of gale force winds looks like Wednesday evening.

A weak front will move into the Offshore Waters on Saturday. This
will weaken the onshore flow and turn winds a bit more southerly,
especially over the Coastal Waters. Schneider

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory until 6 AM PDT Wednesday for Coastal Waters
     From Cape Flattery To James Island 10 To 60 Nm-Coastal
     Waters From James Island To Point Grenville 10 To 60 Nm-
     Coastal Waters From Point Grenville To Cape Shoalwater 10
     To 60 Nm.

     Small Craft Advisory from 6 PM this evening to midnight PDT
     tonight for Coastal Waters From Cape Flattery To James
     Island Out 10 Nm-Coastal Waters From James Island To Point
     Grenville Out 10 Nm-Coastal Waters From Point Grenville To
     Cape Shoalwater Out 10 Nm.

     Small Craft Advisory from 6 PM this evening to 3 AM PDT Tuesday
     for Central U.S. Waters Strait Of Juan De Fuca-East
     Entrance U.S. Waters Strait Of Juan De Fuca.

&&

$$

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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