Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 211045
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
330 AM PDT Mon Aug 21 2017

.SYNOPSIS...High pressure will build over the Pacific Northwest today
and Tuesday, then shift eastward. After some areas of morning low
clouds and fog, mostly sunny skies and warmer temperatures are
expected. A weak trough will approach the region Wednesday then
quickly move across the area Wednesday night and Thursday. Cooler
temperatures will prevail and there will a chance some showers,
especially coast and mountains. Another ridge builds Friday through
the weekend with a return to dry weather and above average
temperatures.

&&

.SHORT TERM...High clouds that passed through the area yesterday
have pretty much cleared out as high pressure aloft build over the
Pacific Northwest. The ridge will maintain dry weather the next
couple days with light northerly flow. Clear skies this morning has
allowed for radiational cooling with areas of fog and low clouds
forming in place. Satellite fog product imagery shows this low level
moisture expanding rapidly along the coast and reaching down the
Strait and through the Chehalis gap into parts of the southwest
interior. While tops on these clouds are likely quite low, probably
less than 1-2k ft, it does appear that the low clouds could deepen
and penetrate further into greater Puget Sound than was expected.
Low clouds or fog could reach most of the interior waters and
surrounding coastlines except perhaps not reaching the north
interior and possibly the east lowlands and foothills. The good news
is that the shallow cloud tops should burn off rapidly, hopefully by
9 to 10 AM just before the eclipse peaks. If clouds do hang in and
expand over the next few hours, the best chance of viewing the
eclipse will probably be east of Lake Washington and locations with
some elevation. Hopefully the shallow nature of the fog and stratus
will allow it to burn off quickly this morning. Once the shallow
moisture mixes, sunny skies will prevail the rest of the day. Highs
will warm into the low 80s today and again Tuesday with no rain
expected.

The pattern begins to change by Wednesday as an upper trough
approaches the region. Heights will fall and the flow will become
onshore with cooler high temperatures. Models seem to hold back on
the deeper moisture until the main trough axis shifts through early
Thursday. This will be the highest chance for measurable rain
around Western Washington, although models agree that amounts will
be unimpressive. Orographic enhancement should squeeze out a bit
more rainfall along west facing slopes, perhaps a tenth of an inch
in spots.

.LONG TERM...High temperatures will struggle to reach above 70 on
Thursday with the chance of showers ending by evening. Long range
models still agree on building another ridge on Friday through the
weekend. The latest GFS is not as amplified with the ridge compared
to past runs. However, the ECMWF still builds 500 mb heights above
5800M by Saturday and Sunday. Highs could reach the low to mid 80s
if this pans out. Mercer

&&

.AVIATION...Building upper level ridge just off the coast early this
morning will move onshore by late today. NW flow aloft to back to
westerly later today. At the surface, lower pressure east of the
Cascades with higher pressure off the coast for low level onshore
flow into Western Washington. Air mass dry except for areas of a
shallow surface-based marine layer mainly along the coast and in the
Strait this morning. Cirrus overhead early this morning to also thin
out by midday today as the upper ridge axis moves ashore.

The thin cirrus has helped impede the low stratus formation
overnight. But as the cirrus thins out through 16z, should see some
stratus form mainly along the coast, the Strait and north sound. The
inland stratus should be thin and short-lived - generally dissipated
by 17z.

KSEA...Northerly wind 4-8 kt to ease a bit by sunrise, but pick up
in the 5-10 kt range again before 18z. Not much of a chance of
stratus at the terminal around 13z-16z, but may be out over the
sound. Otherwise VFR conditions through this evening. Buehner

&&

.MARINE...Higher pressure off the coast with lower pressure east of
the Cascades will maintain varying levels of low level onshore flow
into Western Washington through late this week.

Small craft advisory west winds are likely in the central and
eastern Strait of Juan de Fuca each evening through early morning,
although gale westerlies are possible Wednesday night as a weak
upper level trough approaches British Columbia and tracks inland
Thursday. Buehner

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory through 5 AM PDT Tuesday for Central U.S.
     Waters Strait Of Juan De Fuca-East Entrance U.S. Waters Strait
     Of Juan De Fuca.

&&

$$

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