Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 191525

Area Forecast Discussion
National Weather Service Seattle WA
825 AM PDT Sat Aug 19 2017

.SYNOPSIS...Light onshore flow caused by weak systems passing to the
north will provide some late night and morning clouds and
slightly cooler temperatures through Sunday. High pressure
offshore will shift eastward and across the area Monday and
Tuesday giving warmer temperatures and dry conditions; morning
low clouds will be confined to the coast. A low pressure trough
will bring a chance of showers late Wednesday and Thursday, along
with cooler high temperatures. High pressure will build over the
area Friday through next weekend for a return to dry and warmer


.SHORT TERM...Morning satellite imagery shows areas of stratus in
the interior and on the north coast. These clouds will gradually
erode midday resulting in mostly sunny skies this afternoon.
Rather deep marine air is in place over Western Washington this
morning, so high temperatures today will be held to near or
slightly below normal for this time of year today.

Latest short term models show the next weak system upstream weaker
and a bit farther north than what was shown on earlier solutions.
With this change, forecasts were updated to cut back a bit on
cloud cover for Sunday and to remove any chance of precipitation
from the forecast for the northwestern portion of the Olympic
Peninsula. There will still be some scattered mid level clouds on
Sunday, but they will have little impact on weather conditions.
High temperatures on Sunday will likely be a degree or two higher
than today, still around normal for late August.

A ridge of high pressure will build offshore Sunday night then
will move east across the region Monday and Monday night then to
Western Montana by Tuesday afternoon. With the incoming ridge, sky
cover for the solar eclipse on Monday morning is expected to be
minimal, with any low clouds confined to the immediate Pacific
coastline. Temperatures on Monday will be near 80 from Seattle
southward, held down a couple of degrees by the decrease in
insolation from the morning eclipse. Albrecht

.LONG TERM...From the previous long term discussion: High
pressure will prevail into Tuesday with dry weather above average
temperatures. The pattern will begin to break down by Wednesday as
models indicate an upper trough will approach the region. 500 mb
heights will fall and onshore flow will start to increase,
resulting in cooling high temperatures. There are some timing
differences between the ECMWF and GFS so the arrival of showers is
a bit uncertain. The GFS would bring showers to the coast and
possibly some north interior areas by late Wednesday or Wednesday
night. Most the showers shown by the ECMWF would occur on
Thursday. Models are in pretty good agreement that precipitation
will be rather light and spotty but mostly spots have a good
chance of measuring. The coast and mountains will almost certainly
get measurable precipitation. Thursday should be the coolest day
as the main trough axis shifts across. Highs will top out in the
low 70s, and possible a bit cooler depending on the depth of the
trough. By Friday, the air mass should begin to dry out with
temperatures warming into the weekended. Mercer


.AVIATION...Northwest flow aloft over Western Washington this
morning will continue through tonight, as an upper ridge remains
over the offshore waters. At the surface, onshore flow will
continue with high pressure offshore and lower pressure east of
the Cascades. The air mass is somewhat moist below 7000 ft this
morning; it will gradually dry today, with limited low level
moisture tonight.

Moisture in the lower part of the atmosphere is giving SCT-
BKN020 BKN050-060 over much of Western Washington at 8 am, with
local stratus OVC010-015 mainly in the south Puget Sound area and
southwest interior. These clouds will gradually become scattered
during the next few hours. Marine layer stratus tonight and Sunday
morning should be more limited.

KSEA...Northeast wind 6-12 kt becoming northwest late morning,
then northeast 4-10 kt after sunset. The clouds below 7000 ft
should become scattered by late morning. A stratus ceiling looks
unlikely tonight or Sunday morning. McDonnal


.MARINE...A typical August pattern will prevail through the
middle of next week. Onshore flow of varying strength will
persist due to high pressure offshore and lower pressure east of
the Cascades. Small craft advisory westerlies are likely in the
central and eastern Strait of Juan de Fuca in the evening and
early morning hours tonight through Tuesday night. A weak front
will approach the coastal waters Wednesday. McDonnal


PZ...Small Craft Advisory from 6 PM this evening to 5 AM PDT Sunday
     for Central U.S. Waters Strait Of Juan De Fuca-East
     Entrance U.S. Waters Strait Of Juan De Fuca.



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