FXUS66 KSEW 192314
Area Forecast Discussion
National Weather Service Seattle WA
315 PM PST Sun Feb 19 2017
.SYNOPSIS...Rain at times will continue through Monday evening, with
snow in the mountains. An upper trough will bring showers Tuesday,
and it will remain over the region through Friday for cool weather,
a few sun breaks, and a chance of showers each day. Clearing is
possible over the weekend.
.SHORT TERM...Moist southwest flow aloft over Western Washington
has plenty of showers embedded. Pops are high with moderate QPF.
Highs today were mainly in the mid 40s. Showers will be reaching the
Cascades somewhat more this evening, and a snow advisory remains in
There will be a lull in the precipitation later tonight and Monday
morning, then another system will arrive from the south Monday
afternoon. This system continues to weaken in the models though snow
amounts in the Cascades will probably reach advisory amounts,
especially in the south. Highs on Monday will be 45-50.
A poorly defined upper trough will be over Western Washington
Tuesday and Tuesday night for scattered showers. The south Cascades
may continue to receive snow. Parts of the forecast area should have
sun breaks on Tuesday. A Puget Sound convergence zone will probably
develop, for some enhancement of showers from around Everett into
the Cascades of Snohomish and King Counties. The snow level should
be around 2500 ft Tuesday, falling to 1500 ft Tuesday night. Highs
Tuesday will be in the mid 40s.
On Wednesday the upper trough sharpens a bit, perhaps enhancing
shower activity in the afternoon. More snow will fall in the
mountains, and the snow level will fall to around 1000 feet. Highs
will be in the lower to mid 40s. Some showers could contain snow,
but no accumulations in the lowlands are expected. Burke
.LONG TERM...The upper trough will dig offshore Thursday and Friday.
On Thursday the trough will be close enough for a decent chance of
showers, but by Friday it may be far enough away for the shower
chance to diminish. Highs will be in the lower to mid 40s Thursday,
and a few degrees warmer on Friday. Showers could still have snow at
times both days.
The upper trough is forecast to be well to the south on Saturday,
and under a cool dry northerly upper flow, the day is forecast to be
sunny. Highs will be in the 40s. The next system arrives from the
northwest in both the Euro and GFS models on Sunday. Burke
.HYDROLOGY...No flooding is expected during the next week. There
will be light rain at times with low snow levels. USGS landslide
guidance remains above the threshold at which landslides typically
occur. This means there will continue to be an elevated threat of
landslides for at least the next few days. Burke
.AVIATION...Moist SW flow aloft will continue over W WA tonight as
an upper level shortwave trough over W WA moves slowly NE over B.C.
overnight. Conditions are expected to be predominantly VFR BKN030-
070 but periods of MVFR CIGS BKN020-030 are likely due to shower
Moist SW flow aloft will continue on Monday. Another weak upper
level shortwave trough will move NE over the region Monday afternoon
allowing showers to continue. The air mass will be a little more
stable on Monday.
KSEA...Moist SW flow aloft will continue tonight with periods of
showers crossing the terminal. Conditions will be predominantly VFR
with CIGS BKN035-070 but short periods of MVFR CIGS BKN020-030 are
expected due to shower activity. Surface winds will be southerly 5-
15 KT through tonight. Kam
.MARINE...A 993 MB surface low near Cape Flattery at 2 PM will
move NE to the central Vancouver island coast early this evening
before dissipating. Small craft advisories are in effect for most of
the inland waters and the S flow was strong enough so that a gale
broke out over the East entrance and over the N inland waters N of
the San Juan Islands. Winds on the coast are just about all below 20
KT so the SCA will be replaced by a SCA for hazardous seas as
planned. Winds will ease this evening and all areas should be below
SCA levels by midnight.
A second surface low will approach the area Monday morning before
moving inland across NW Oregon Monday afternoon. The southern track
of the low will shift winds over W WA to northerly, but they are
expected to remain below SCA levels. A third low will follow a
similar track as the second low but will move inland over the
central Oregon coast Tuesday morning.
A surface ridge will build offshore from W WA Tuesday and Wednesday
with moderate NW flow developing over the coastal waters. SCA winds
are expected to develop over the coastal waters Tuesday morning and
the increasing onshore flow should bring SCA W winds through the
Strait Tuesday afternoon.
A weak surface low associated with an upper level trough will move
down over Vancouver Island and the WA coast on Wednesday. The
weakening pressure gradients across the area will allow winds to
WA...Winter Weather Advisory until 10 PM PST this evening for
Cascades of Pierce and Lewis Counties-Cascades of Snohomish
and King Counties-Cascades of Whatcom and Skagit Counties.
PZ...Small Craft Advisory until 4 PM PST this afternoon 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 hazardous seas until noon PST Monday
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 4 PM PST Monday for
Grays Harbor Bar.
Gale Warning until 7 PM PST this evening for East Entrance U.S.
Waters Strait Of Juan De Fuca-Northern Inland Waters
Including The San Juan Islands.
Small Craft Advisory from 7 PM this evening to midnight PST
tonight for Northern Inland Waters Including The San Juan
Small Craft Advisory until midnight PST tonight for Admiralty
Inlet-Puget Sound and Hood Canal.
You can see an illustrated version of this discussion at
NWS Seattle/Tacoma, WA (SEW) Office Area Forecast Discussion