Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA
FXUS66 KSEW 180331
Area Forecast Discussion
National Weather Service Seattle WA
830 PM PDT Mon Oct 17 2016
.SYNOPSIS...Showery weather will continue tonight and Tuesday as an
upper level trough passes. An upper level ridge will bring a brief
lull in the rain Tuesday night through Wednesday morning. A Pacific
frontal system will bring rain Wednesday afternoon through Thursday
morning followed by scattered showers. Another upper level trough
will increase shower activity Friday.
.SHORT TERM...A band of showers over southwest Washington is moving
northeast and it will reach the cities of Puget Sound later this
evening. There is also a persistent shower in the Cascades along the
Skagit-Snohomish county line. This had some lightning earlier though
not for the last hour or so. Showers will diminish somewhat Tuesday.
Skies will be mostly cloudy and highs will be mostly 55-60.
Shower activity should diminish Tuesday night through Wednesday
morning as an upper level shortwave ridge crosses the region. The
leading edge of the next frontal system will spread light rain to
the coast around Wednesday afternoon. Rain will move inland
Wednesday night with the cold front following Thursday. Southeast
pressure gradients become fairly tight as the warm front moves
inland Wednesday night with possible wind advisory level southeast
winds over the coast and north interior. Snow levels will spike up
to 8000 feet across the area so the mountains will probably get a
good shot of rain. Scattered showers follow the front Thursday.
Highs will be 55-60 again on Wednesday, then several degrees warmer
on Thursday. Burke/Kam
Concerning tidal overflow on Tuesday: Astronomical high tides will
be about the same on Tuesday as they were on Monday, but atmospheric
pressures are expected to be about 10 mb higher on Tuesday than
today. Therefore, while tidal levels will come close to the tidal
overflow value, they are not expected to exceed it. Albrecht
.LONG TERM...Previous discussion...GFS, ECMWF, and Canadian models
all agree that moist SW flow aloft will prevail Friday through
Monday. However the models still have differences with strength and
timing of the individual features. The models agree that a weak
front will move through W WA late Saturday. Behind the front Sunday
and Monday it looks like just strong moist SW flow aloft probably
with embedded small weak shortwave troughs. POPs were increased to
show likely levels for the coast and high chance POPs for the
.AVIATION...A surface and upper level trough extending from the
Gulf of Alaska southeastward into the Oregon coastal waters will
lift northeastward across the area tonight and early Tuesday. Then
a ridge of high pressure will build along the coast late Tuesday
into Tuesday night. The air mass is cold, moist, and unstable
aloft. The air mass will slowly stabilize and dry starting later
Conditions are generally VFR around the area, except in heavier
showers along the coast where they are occasionally MVFR.
Radars, observations, and lightning detection data show isolated
thunderstorms at 745 PM PDT in the Cascades from northeast of
KAWO to the Cascade crest and spiraling an area of mid level spin
over the coastal waters. The isolated thunderstorms in the
interior will weaken this evening. The thunderstorms over the
coastal waters will spin onto the Washington coast at times
overnight but the threat of thunderstorms at any particular
terminal location will be low enough to keep out of the TAFs.
Ceilings are expected to drop to 2000-2500 feet by about 12Z then
scatter out midday Tuesday. Albrecht
KSEA...South wind 6-10 kt. An area of showers will move into the
terminal area by 04Z and last for a few hours, lowering ceilings
at times to 3500 feet. Thunderstorm activity is expected to stay
away from the terminal and approaches. Ceilings will lower to the
2000-3000 foot range 12z-19z period Tuesday morning, then scatter
out in the afternoon. Albrecht
.MARINE...A weak surface trough of low pressure over the coastal
waters will lift northeast later tonight into Tuesday morning.
Then a ridge of high pressure will build along the coast late
Tuesday afternoon into Tuesday night. Small craft advisory winds
in the Puget Sound have ended, so the advisory will be cancelled.
Small craft advisory conditions continue over the coastal waters
and on the Grays Harbor Bar due to hazardous seas and rough bar
conditions. Seas there will subside toward morning.
A moderate to strong frontal system associated with a low moving
from the offshore waters into northern Vancouver Island late
night will move onshore late Wednesday night or Thursday morning.
Expect south to southeast winds to rise to 20 to 30 knots over the
coastal waters Wednesday afternoon to rise to gale force Wednesday
night. Gales are also becoming increasingly likely with this front
Wednesday night or early Thursday at the Entrances to the Strait
of Juan de Fuca and in the Northern Inland Waters. A Gale Watch
will likely need to be issued with the next forecast package late
A weaker front will move through the waters late Thursday night or
.HYDROLOGY...The Skokomish River will continue to fall slowly
tonight and should fall below flood stage sometime late tonight
or early Tuesday morning. Rainfall amounts through Tuesday will
slow but not stop its fall. A moist Pacific frontal system will
arrive over Western Washington Wednesday night through Thursday
morning. This system could bring three inches of rain to the
Skokomish river basin in 24 hours from 00Z Thursday to 00Z Friday,
which is probably enough to send it above flood stage again.
Otherwise, flooding is unlikely on area rivers the next 7 days.
PZ...Small Craft Advisory for hazardous seas until 6 AM PDT Tuesday
for Coastal Waters From Cape Flattery To James Island 10 To
60 Nm-Coastal Waters From Cape Flattery To James Island Out
10 Nm-Coastal Waters From James Island To Point Grenville
10 To 60 Nm-Coastal Waters From James Island To Point
Grenville Out 10 Nm-Coastal Waters From Point Grenville To
Cape Shoalwater 10 To 60 Nm-Coastal Waters From Point
Grenville To Cape Shoalwater Out 10 Nm.
Small Craft Advisory for rough bar until 9 AM PDT Tuesday for
Grays Harbor Bar.
You can see an illustrated version of this discussion at