Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA
FXUS66 KSEW 190341
Area Forecast Discussion
National Weather Service Seattle WA
845 PM PDT Tue Oct 18 2016
.SYNOPSIS...A weak upper level ridge will bring a short dry period
through Wednesday morning. A frontal system will bring light rain by
late Wednesday afternoon, with rain increasing Wednesday night and
Thursday morning. A few showers are expected Thursday afternoon
behind the cold front. An upper level trough will bring some showers
Friday. A weak front will spread rain across the area late Saturday.
.SHORT TERM...Scattered showers are evident on radar. They have
diminished in intensity if not coverage this evening. As a weak
upper ridge builds over the area tonight, the showers should
continue to diminish and should be gone altogether by morning.
Rain from an approaching frontal system will begin on the coast by
Wednesday afternoon and spread inland Wednesday evening. Moderate to
heavy rain is likely Wednesday night. The coast and southwest
interior could receive 1.5 inches. The rest of the interior may get
between a half and one inch. The southern Olympics could get 2 to 3
inches which would likely push the Skokomish river above flood stage.
The associated weak cold front should reach the coast Thursday
morning around 13Z/6 AM per the GFS. The cold front will push slowly
inland so that rain should be tapering over the lowlands by midday.
The lower air mass will probably be weakly unstable behind the cold
front so there will be a few showers around and possibly some
convergence over central Puget Sound. The warm advection Wednesday
night should push Thursday`s max temps into the lower 60s over parts
of the interior. Even though a cold front is moving across the area
during the day, the cold advection will be pretty weak.
Moist southwest flow aloft will prevail Thursday night and Friday.
The models do not show any organized troughs impacting the area so
just scattered showers will continue in the weakly unstable
post-frontal air mass Thursday night. On Friday showers should
increase as the south end of a trough moving inland over British
Columbia brushes the area. Kam/Burke
.LONG TERM...Previous discussion...after a brief break between
systems Friday night through Saturday morning, another weak cold
front may bring some light rain to the area late Saturday. The ECMWF
shows the same feature but is a little faster than the GFS. POPs
were a compromise between the two models. By this time, a large
upper level trough will have formed offshore along roughly 130W.
Moist SW flow aloft gets cranked up behind the initial system on
late Saturday, so that the Sunday through Tuesday look pretty wet.
Models agree that the pattern will be active and wet, but chaos
still reigns when arguing over details. Kam
.AVIATION...High pressure aloft will build tonight, then start to
shift east Wednesday afternoon ahead of the next trough. Light
southwest flow aloft will become moderate southerly flow Wednesday
afternoon. The air mass is weakly unstable and moist in the
lower/mid levels. Scattered showers, mainly over Snohomish county,
will decrease early this evening. The air mass will stabilize late
Mostly VFR cigs this evening with a few areas of MVFR associated
with showers this evening. Residual moisture should allow areas of
MVFR cigs and local IFR to develop later tonight, especially since
the gradients should remain light. Will stick with the GFSMOS/LAMP
and indicate a few hours IFR around 12-16z Wednesday morning.
KSEA...South-southwest wind 5-10 kt. Mostly VFR with isolated
showers ending by midnight and partial scattering. This should allow
fog and/or low stratus to form Wednesday morning. IFR cigs will
remain in the forecast, mainly for the 12-16z window. DTM/13
.MARINE...High pressure will remain over the area tonight into early
Wednesday. Hazardous seas are likely for swell near 10 feet in the
outer coastal waters through the Wed afternoon. A warm front will
bring a burst of Small craft winds to the coastal waters Wednesday.
The 1005 mb low moving into Vancouver Island will send a cold front
through the waters Thursday. Gales are likely ahead of this incoming
front over the coastal waters Wednesday night with small craft
advisories north of Puget Sound.
A weaker front will move through the waters friday, followed by a
stronger frontal system late Saturday and Sunday. DTM/13
PZ...Small Craft Advisory for hazardous seas outer coastal waters
Small Craft Advisory for rough Grays Harbor bar through this
Small Craft Advisory east entrance Wednesday.
Gale warning coastal waters Wednesday night.
You can see an illustrated version of this discussion at