Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA
FXUS66 KSEW 191604
Area Forecast Discussion
National Weather Service Seattle WA
904 AM PDT Wed Oct 19 2016
.SYNOPSIS...Strong southwest flow aloft will drive an approaching
wet warm front across western Washington tonight. The associated
cold front will cross the area Thursday morning. Scattered post-
frontal showers will follow Thursday afternoon and night. Shower
activity will increase Friday as an upper level trough moves inland.
After a minor lull Saturday, a cold front will reach the area
Saturday night. Moist southwest flow aloft Sunday through early next
week will allow several weather systems to maintain the active
.SHORT TERM...An old occluded front along 130W this morning will
progress slowly toward the WA coast today, being pushed eastward by
the strong jetstream crossing the NE Pacific. This old front is not
very strong and the models have been having difficulty in timing the
spread of rain inland. The GFS and NAM remain the slowest while the
ECMWF and Canadian are fast. The forecast goes with the slower
GFS/NAM this morning, keeping the coast dry, then compromises
between the GFS/ECMWF in bringing the chance of rain over western
Puget Sound by late afternoon. Regardless, the initial rain will be
quite light. Clouds ahead of the approaching front will gradually
spread over W WA today with partly sunny areas this morning
filling in during the afternoon.
Warm advection will increase tonight as the remnants of the old
front move inland, with light rain increasing. The warm advection
tonight comes in two waves, first with the old front, then followed
by a stronger warm front associated with a surface low that moves NE
toward Vancouver island later tonight. The second warm front should
be through the area by sunrise Thursday. That will leave the cold
front to move slowly SE across the area Thursday morning, reaching
the Cascades by midday. The two warm fronts and the cold front
combined may bring 1 to 1.5 inches of rain to the coast and SW
interior, about a half inch to an inch over the rest of the interior
lowlands and 1-3+ inches to the Olympics, and 1-2 inches to the
north Cascades. The heavy rain over the Olympics will probably push
the Skokomish river back above flood stage on Thursday. The surface
low will also bring locally windy conditions to the coast and north
interior late tonight.
After the cold front goes through a post-frontal showery pattern
will take over along with onshore flow, persisting through Thursday
night. Scattered showers will predominate and a convergence zone
will probably form over Snohomish county. The overnight warm
advection will help push max temps Thursday up to around 60. The
cold front is weak and won`t completely overcome the effects of the
On Friday an upper level trough moving mainly through B.C. will
brush W WA. It looks like there is enough organization to the
incoming wave to almost call it a front. Regardless, shower activity
will increase. Kam
.LONG TERM...A large upper level low over the NE Pacific will
dominate the pattern Saturday through Tuesday. The GFS and ECMWF
still show a front moving through W WA later Saturday. However the
new 12Z GFS has significantly changed the details of this system, so
confidence is lower. Moist SW flow aloft continues the rest of the
weekend for continuing active weather. Both models eventually bring
the weakening low down over W WA early Tuesday. Confidence is low
in regarding the details but high on the continuing wet pattern. Kam
.AVIATION...Light westerly flow aloft over Western Washington this
morning will back to southwest and rise late today and tonight as
a fairly vigorous Pacific frontal system moves into the region.
The warm front will move into the forecast area tonight, followed
by the cold front on Thursday. Low level moisture over Western
Washington will dry around midday, and the air mass will become
moist all levels this evening.
There is patchy fog and low stratus around Western Washington this
morning, mainly in the southwest interior, giving local IFR
conditions. The low level moisture will dissipate during the next
few hours, and otherwise VFR conditions prevail with ceilings
mainly above 7000 ft. Conditions will deteriorate tonight,
becoming generally MVFR and locally IFR in rain.
KSEA...South to southeast wind 6-12 kt through tonight. KSEA is
unlikely to have fog or stratus again this morning. Rain should
begin this evening, with ceilings lowering to 012-020 overnight.
.MARINE...High pressure over the region this morning will move
east today. Hazardous seas will continue for 10 ft swell in the
outer coastal waters through this afternoon. A warm front will
bring gale force winds to the coastal waters this afternoon, and the
trailing cold front will arrive on Thursday. Gales should
continue over the coastal waters, with small craft advisory winds
in the inland waters north of Puget Sound. Gale force winds are
likely at the east entrance to the Strait of Juan de Fuca early
A weaker front will move through the waters Friday, followed by a
stronger frontal system late Saturday and Sunday. McDonnal
.HYDROLOGY...Another 1.75 TO 3.5 inches of rain on the Skokomish
River Basin during the 18-hour period ending at 11 AM Thursday
should be enough to drive the river near or slightly above flood
stage Thursday afternoon. Otherwise, apart from the Skokomish River,
flooding is not anticipated on area rivers the next 7 days.
WA...Flood Watch from Thursday morning through Thursday evening for
Hood Canal Area-Lower Chehalis Valley Area-Olympics.
PZ...Gale Warning from 2 PM this afternoon to 5 AM PDT Thursday for
Coastal Waters From Cape Flattery To James Island 10 To 60
Nm-Coastal Waters From James Island To Point Grenville 10
To 60 Nm-Coastal Waters From Point Grenville To Cape
Shoalwater 10 To 60 Nm.
Small Craft Advisory for hazardous seas until 2 PM PDT this
afternoon for Coastal Waters From Cape Flattery To James
Island 10 To 60 Nm-Coastal Waters From James Island To
Point Grenville 10 To 60 Nm-Coastal Waters From Point
Grenville To Cape Shoalwater 10 To 60 Nm.
Gale Warning from 8 PM this evening to 5 AM PDT Thursday for
Coastal Waters From Cape Flattery To James Island Out 10 Nm-
Coastal Waters From James Island To Point Grenville Out 10
Nm-Coastal Waters From Point Grenville To Cape Shoalwater
Out 10 Nm.
Small Craft Advisory from noon today to 1 AM PDT Thursday for
East Entrance U.S. Waters Strait Of Juan De Fuca.
Gale Warning from 1 AM to 5 AM PDT Thursday for East Entrance
U.S. Waters Strait Of Juan De Fuca.
Small Craft Advisory from 2 PM this afternoon to 2 PM PDT
Thursday for Northern Inland Waters Including The San Juan
Small Craft Advisory from 6 PM this evening to 5 AM PDT Thursday
for West Entrance U.S. Waters Strait Of Juan De Fuca.
Small Craft Advisory from 1 AM to 1 PM PDT Thursday for
An illustrated version of this forecast discussion can be seen at