Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 182222

Area Forecast Discussion
National Weather Service Seattle WA
322 PM PDT Tue Oct 18 2016

.SYNOPSIS...An upper level trough will move east tonight allowing
showers to diminish by early evening. A weak upper level ridge will
bring a short dry period through Wednesday morning. A wet frontal
system will bring light rain to the area by late Wednesday afternoon
with rain increasing Wednesday night through 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. Moist southwest flow
aloft will result in a wet period Sunday through early next week.


.SHORT TERM...An upper level shortwave trough is moving across W WA
this afternoon, destabilizing the air mass enough for a couple
isolated thunderstorms. The showers should die off this evening as a
the trough departs and a weak upper level ridge moves inland.

The weak low amplitude upper level shortwave ridge moving over the
area tonight will bring a short lull in precipitation through
Wednesday morning. A 140 KT jet stream presently developing over the
NE Pacific will drive a frontal system across W WA late Wednesday
through Thursday. Models continue to show small differences in
timing the initial rain reaching the area on Wednesday. The current
forecast still follows the slightly slower GFS solution, but rain
should reach the coast by late Wednesday morning then spread far
enough E during the afternoon to reach Puget Sound late in
the day.

Strong low level warm advection fueled by the westerly jetstream
will bring moderate to heavy rain to W WA Wednesday night. It looks
like the coast and SW interior will get around 1.5 inches. The rest
of the interior may get between a half and one inch. The S and SE
Olympics could get 2 to 3 inches which would push the Skokomish
river back 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 off 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 Thursdays max temps up 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

Moist SW 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 B.C. brushes the area.

.LONG TERM...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 builds tonight, then starts to shift
eastward Wednesday afternoon ahead of the next trough. Light
southwest flow aloft, becoming moderate southerly flow Wednesday
afternoon. Air mass weakly unstable and moist in the lower/mid
levels. Showers, with isolated lightning strikes mainly south, will
decrease early this evening. The probability of TS or VCTS for any
terminal is less than 20 percent early this evening. Air mass
stabilizes after sunset this evening.

Mostly VFR cigs this evening with areas of MVFR associated with
showers this evening. Residual moisture should allow areas of MVFR
cigs and local IFR to develop again later tonight. Gradients look to
remain light with a slower shift the ridge Wednesday. Will stick
with the GFSMOS/LAMP and indicate a few hours IFR around 12-16z
Wednesday morning.

KSEA...South-southwest wind 5-10 kt becoming mainly 5 kt or less
later this evening. Mostly VFR with showers ending after dark and
partial scattering. This should allow fog and/or low stratus to form
later tonight/Wednesday morning. IFR cigs will remain in the
forecast, mainly for the 12-16z window. DTM


.MARINE...High pressure tonight into early Wednesday with hazardous
seas for swell near 10 feet outer coastal waters through the
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
possible ahead of this incoming front over the coastal waters
Wednesday night with small craft  north of Puget Sound.

A weaker front will move through the waters friday, followed by a
stronger frontal system late Saturday and Sunday. DTM


.HYDROLOGY...The Skokomish River fell below flood stage shortly
after midnight last night. The river is expected to remain below
flood stage through Wednesday. A wet frontal system will reach the
region by Wednesday night and continue through Thursday morning.
Moist S-SW flow aloft and strong warm advection with this system
will probably drop another 3 inches of rain over the Skokomish river
basin through Thursday afternoon. This should be enough to drive the
river above flood stage (17.0 feet) again on Thursday. A flood watch
has been issued.

Apart from the flood-prone Skokomish River, flooding is not
anticipated on other area rivers the next 7 days.


WA...Flood Watch for Mason county.

PZ...Small Craft Advisory for hazardous seas outer coastal waters
     through Wednesday.
     Small Craft Advisory for rough Grays Harbor bar through this
     Gale watch coastal waters Wednesday night.



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