Weather Service Report


613 
FXUS66 KSEW 300451
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
951 PM PDT Thu Sep 29 2016

.SYNOPSIS...A low pressure system will continue to spin offshore
through Saturday. Expect the best chance of precipitation on
Saturday. The low pressure system offshore will move inland on
Sunday for a drying trend. Daytime temperatures will be below
normal through this weekend.

&&

.SHORT TERM...
A low pressure system will remain offshore through Saturday. The
present upper level low well west of the northern Vancouver Island
will continue drifting south overnight through Friday while
gradually weakening. Another, but stronger, upper level low
dropping southwest off the British Columbia coast will eventually
take its place well off the WA coast Saturday morning. This Low
will then continue drifting south. Sunday morning, the Low is
anticipated to be off the far northern CA coast. The system will
finally move inland Sunday afternoon. This track reflected the
latest GFS solution. The 1200 UTC Thursday ECMWF solution was
farther north (brought it inland over southwest OR) than the GFS.

Concerning the forecast details: Stratus and/or fog will probably
be a little more widespread Friday morning compared to this
morning due to continued onshore flow. Also, stratus was already
developing over the areas where the dense high clouds have
vacated. 

It looks like it will remain dry over much of the CWA through
Friday. The precip threat will slowly increase Friday night
through Saturday morning, with relatively wet weather anticipated
Saturday afternoon. The Short Range Ensemble Forecast (SREF)
continued to indicate the possibility of thunderstorms over the
coastal waters, and parts of the coast and southwest interior on
Saturday. Will need to keep an eye on this. There has been
isolated lightning strikes over the offshore waters with the
current weakening upper level Low. Therefore, cannot rule out
isolated thunderstorms over parts of the CWA on Saturday with a
stronger Low offshore.

The threat of showers should become confined to mainly the
southern part of the CWA on Sunday.

Daytime temperatures during this period will be below normal.

.LONG TERM from the previous discussion...
In the wake of the upper lows, moist northwesterly flow aloft
will develop. Models are in rough agreement that weather systems
will arrive around Tuesday and again around Thursday. The Thursday
system looks stronger. Have kept chance pops most of the Monday to
Thursday period, but bumped them up somewhat Tuesday and Thursday.
Temperatures will be near normal, with highs in the 60s and lows
in the 50s. Burke

&&

.AVIATION...Southerly flow aloft will prevail tonight and Friday
with an upper trough offshore. At the surface, weak onshore flow
will continue with a ridge along the coast and lower pressure
inland. The air mass is stable with high level moisture. Low
clouds are expected to form at the coast tonight and then over much
of the interior for a few hours Friday morning before burning off
late morning.

KSEA...High clouds tonight. Low clouds will likely affect the
terminal for a few hours Friday morning. Northerly wind 3-6 knots
will become light after midnight then southerly 4-8 knots Friday.
Schneider

&&

.MARINE...Onshore flow will prevail tonight with high pressure
offshore and lower pressure inland. Marginal small craft advisory
west winds in the Central and Eastern Strait of Juan De Fuca will
ease after midnight tonight.

A weak frontal system will spiral offshore on Friday. A second
weak low will track south down 130 west on Saturday then shift
toward southern Oregon on Sunday. A stronger system will arrive on
Monday. Schneider

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory in effect until midnight tonight for
 the central and eastern Strait Of Juan De Fuca.

&&

$$

www.weather.gov/seattle

An illustrated version of this forecast discussion can be seen at
www.weather.gov/seattle/gafd/latest_webafd.html

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Western Regional Climate Center, wrcc@dri.edu