Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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442
FXUS66 KSEW 281605
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
900 AM PDT THU JUL 28 2016

.SYNOPSIS...Upper level ridging and weak low level onshore flow
will continue over Western Washington through Friday resulting in
sunny days and above normal temperatures. An upper level trough
passing to the north of the area will bring lower temperatures over
the weekend. A stronger upper level trough could arrive around
Monday night.

&&

.SHORT TERM...marine stratus is already evaporating from the strait
and will also soon be gone from the southwest interior. High heights
and warm dry air aloft will boost temperatures this afternoon to
their highest levels so far this month, especially over the
interior. The warmest part of the region will be from Olympia south,
where 90-degree readings are possible.

Upper heights start to slowly fall on Friday, then an upper trough
will brush by to our north on Saturday. With this setup, low-level
onshore flow will become strong Friday evening in association with a
marine push. A little cooling trend will start on Friday, with
stronger cooling Saturday.

Otherwise, the only other short-term forecasting challenge is
figuring out the coverage of morning marine stratus. Friday
morning`s coverage should be similar to this morning. Deeper
penetration into the interior is expected Saturday morning
following Friday evening`s push, with marine stratus pushing
inland through the Chehalis Gap to the Seattle metro area shortly
after sunrise Saturday. Haner/Burke

.LONG TERM...Previous discussion...A little shortwave ridging is
expected Sunday, so removed the already-low PoPs from the forecast.
A progressive upper low is forecast to move eastward across the area
later Monday. Recent GFS runs have been catching up with the faster
ECMWF. In advance of this feature, cooling aloft, deepening
moisture, and difluent S-SW flow aloft could easily produce some
showers on Monday night, possibly as early as Monday afternoon over
the Olympic Peninsula. Have added low chance PoPs to the forecast as
a starting point, with room to increase PoPs if models remain
consistent with this solution. In fact, the latest model solutions
would even suggest the possibility of a few lightning strikes in
western Washington, but that will not be reflected yet in the
official forecast.

Beyond Monday night, cool northwest flow aloft will contribute to below
normal temperatures into the middle of next week. Haner

&&

.AVIATION...Light westerly flow aloft. The air mass is dry and
stable with shallow low level moisture over the coastal waters.
Some low level moisture with IFR cigs and visibilities will move
back into the Chehalis gap and through the Strait of Juan de Fuca
and the northern interior near the water 10Z-17Z Friday, but the
remainder of the region will remain clear with good visibilities.
Albrecht

KSEA...Clear skies with good visibilities. Light northeasterly
wind will become northwest 7-9 knots 19Z-04Z then will shift to
northeast and diminish after 04Z. Albrecht

&&

.MARINE...A ridge of high pressure over the offshore waters will
strengthen today through Saturday. Pressures east of the Cascades
will fall Friday night and Saturday. The combination of the
pressure changes will result in a gradually increasing northwest
flow over the coastal waters today through Saturday. Inflow
through the Strait of Juan de Fuca will be light to moderate
through midday Friday, then increase late Friday afternoon through
Saturday night.

Small craft advisories will come into effect today for the coastal
waters as winds and seas gradually increase. A small craft
advisory is also in effect for the central and eastern portions of
the Strait of Juan de Fuca tonight. Gale watches were issued for
the outer portions of the coastal waters from Point Grenville
northward to Cape Flattery for Friday night through Saturday and
for the central and eastern portions of the Strait of Juan de Fuca
for Friday night. Seas will also become increasingly high and
steep over the coastal waters as the long fetch of northwesterly
flow from off Vancouver Island into the coastal waters increases.
Albrecht

&&

.SEW Watches/Warnings/Advisories...
WA...None.
PZ...Small Craft Advisory until 5 PM PDT Friday 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 2 PM this afternoon to 5 PM PDT Friday
     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.

     Gale Watch from Friday afternoon through Saturday afternoon 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.

     Gale Watch from Friday afternoon through late Friday night for
     Central U.S. Waters Strait Of Juan De Fuca-East Entrance
     U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory from 5 PM this afternoon to 2 AM PDT Friday
     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.weather.gov/seattle/gafd/latest_webafd.html



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