Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 141015
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
315 AM PDT Mon Aug 14 2017

.SYNOPSIS...Areas of clouds this morning will give way to mostly
to partly sunny skies in the afternoon. Temperatures will
generally be 1-4 degrees below normal today. Weak ridging building
in from the west combined with decreasing onshore flow will give a
warming trend to the area Tuesday through Thursday, with high
temperatures 3 to 5 degrees above normal by the end of the week.
Onshore flow will increase next weekend for cooler conditions and
an increase in marine cloudiness. Dry weather is expected through
next weekend.

&&

.SHORT TERM...Satellite imagery shows splotches of higher-based
stratus clouds across the region early this morning with clearing
in places. The air mass has a bit of an autumn feel in the areas
that have cleared; Arlington was 49 degrees at 2 AM and Olympia
was 50. Cloudy areas have stayed 5-10 degrees warmer. Expect
clouds across the area to decrease later this morning leaving
partly to mostly sunny skies in the afternoon. The exception will
be over the coastal waters and near the Pacific beaches were
marine stratus may increase somewhat as high pressure builds
offshore and subsidence increases to its east trapping moisture in
the marine layer.

The period Tuesday through Wednesday will see flat high pressure
building well offshore giving northwesterly flow aloft to our area
and 500-mb heights building to over 580 DAM by Wednesday afternoon.
Systems moving over the top of the ridge will be die before they
reach the area, and will likely result in just some increase in
mid and high clouds at times midweek. The building heights will
result in warming in the interior and a day-to-day decrease in
the amount of low stratus in the interior during the late night
and morning hours. High temperatures by Wednesday are expected to
rise into the mid and upper 70s in the interior lowlands from
south of Everett into the southern interior.

Changes to the forecast were minimal today. Enjoy the fine late
summer weather that is expected across the area!  Albrecht

.LONG TERM...The broad upper level ridge well offshore will shift
east across the area Thursday afternoon or Thursday night. Expect
a minimum of stratus over the interior zones on Thursday, and high
temperatures will rise into the mid 70s to lower 80s across the
interior. Friday will likely be a transition day as the upper
ridge shifts eastward and onshore flow increases. Friday may still
be rather warm in the interior before the onshore flow kicks in.
Onshore flow and marine air will give cooling over the weekend and
an increase in clouds.

Models show a good deal of disagreement on what kind of weather
the Pacific Northwest will see on the day of the total solar
eclipse (Monday, August 21). But it still is a week out. The GFS
for several runs generally shows clouds and rain moving into the
area on Monday. The ECMWF has shown the front and precipitation
with some solutions, but has trended drier over the past day.
Albrecht

&&

.AVIATION...An upper trough will slowly shift east of the region
today. Light northwest flow aloft with a stable air mass. Moist in
the lower levels this morning, otherwise becoming somewhat dry at
all levels. Mostly VFR cigs but localized MVFR stratus possible
early this morning although coverage looks to be less than was
expected. All areas VFR by 18Z Monday with scattering clouds. Patchy
brief IFR fog possible around the southwest interior 11-16z.

KSEA...NE wind 6-9 kt becoming variable to 5 kt after 12-13z. N wind
to 10 kt after 19z. Sct-Bkn clouds around 3-4K ft early this morning
could lower into an MVFR cloud deck for a couple hours between 12-
16z but trends suggest this is becoming less likely and will refrain
from indicating MVFR in the 12z TAF issuance. dtm

&&

.MARINE...Residual westerly flow with near small craft winds in the
central/eastern Strait will ease quickly early this morning. Another
weak diurnal push of winds could bring a few hours of low-end small
craft winds this evening to the central/eastern Strait.

A surface ridge will continue to rebuild over the WA offshore waters
will remain in place through the end of the week. The resulting W-NW
onshore flow over the waters will be generally light. However,
diurnal westerly winds can be expected to increase to near small
craft strength during some late afternoon/evening hours in the
central/east Strait. dtm

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory until 6 AM PDT early this morning 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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