Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 191107
AFDSEW

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

.SYNOPSIS...Light onshore flow caused by weak systems passing to the
north will provide some morning clouds and slightly cooler
temperatures through Sunday. High pressure builds Monday and Tuesday
with warmer temperatures and morning low clouds staying mostly
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 builds Friday with a return to dry
and warmer weather.

&&

.SHORT TERM...Fog/satellite imagery and obs shows areas of stratus
across much of interior Western Washington this morning. Most cloud
cover appears to be shifting from the north to south covering much
of greater Puget Sound, the southwest interior and the north
Washington coast. Some partial clearing is evident along the central
coast but moist onshore flow may result in some clouds forming in
place just about anywhere over the next few hours this morning.
Models agree that low level moisture will decrease with clouds
mixing out allowing mostly skies across to dominate most the
area during the afternoon. Highs will be slightly below average
today in the low 70s around Puget Sound.

Another weak system passes to the north on Sunday. Onshore flow will
increase again but weaker than this morning. Some morning low clouds
will develop around area again but with less coverage. Highs
should warm a few degrees with weaker onshore flow, closer to
average in the mid to 70s in warmer inland areas.

Virtually all models on building a temporary ridge over the region
Monday into Tuesday. The timing could not be better as the low level
flow will become light with low clouds likely remaining near the
coast Monday morning. A few patchy low clouds cannot be ruled out
inland but anything that does form would likely burn off quickly by
around the 9AM hour. Most the area should be mostly clear during the
eclipse viewing hours mid to late morning. Highs will warm, reaching
near 80 around Puget Sound.

.LONG TERM...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...Northwesterly flow aloft over Western Washington into
Sunday with a weak upper level ridge building offshore. At the
surface, high pressure remains offshore with lower pressure east
of the Cascades.

Wide variety of conditions early this morning over Western
Washington. Ceilings near 4000 feet over the central Puget Sound
area with ceilings as low as 2000 feet over the south Sound and
Southwest Interior with some weak low level convergence. Ceilings
1000-2000 feet along the North Coast. The remainder of the area
reporting scattered clouds. Little change in the conditions the
next few hours. Locations with ceilings will see the clouds
scatter out in the 15z-18z time frame. Just scattered clouds
across the area this afternoon.

KSEA...Ceilings near 4000 feet early this morning. Could see the
ceilings lower to near 2000 feet temporally until 16z. Scattered
clouds by 18z. Northeast wind 4 to 8 knots becoming north
northwesterly by late morning. Felton

&&

.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 beginning tonight through Tuesday night. A
weak front will approach the Coastal waters Wednesday. Felton

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
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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You can see an illustrated version of this discussion at
www.wrh.noaa.gov/sew/gafd/latest_webafd.html



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