Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA
FXUS66 KSEW 191714
Area Forecast Discussion
National Weather Service Seattle WA
915 AM PST Sun Feb 19 2017
.SYNOPSIS...Two frontal systems will move through Western Washington
in quick succession today and Monday giving rain at times, snow in
the mountains, and locally breezy conditions. An upper trough will
bring showers on Tuesday, and it will remain over the region the
rest of the week for cool weather, a few sun breaks, and a chance of
showers each day.
.SHORT TERM...Moist southwest flow aloft over Western Washington
has plenty of showers embedded. Pops are high with moderate QPF. The
upshot for today is cloudy skies with rain at times and highs in the
mid 40s. There have been a few lightning strikes off the Oregon
coast, so there is a chance of thunderstorms on the coast today.
The snow level is below 3000 feet and snow is falling in the passes
and at ski areas. QPF appears to be high enough for advisory amounts
of snow in the Cascades, with the bulk falling 18Z-06Z. Have issued
a snow advisory.
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 be flirting with warning amounts,
especially in the south.
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. Burke
.LONG TERM...Previous discussion...The upper trough will deepen over
the Pacific Northwest on Wednesday for a cooler day with a chance of
showers, especially for the coast and mountains. The trough will
move east Wednesday night, and northwest flow aloft will develop
Thursday. It looks like another trough will develop over the region
on Friday, but model solutions really begin to separate. The upshot
is cooler weather with a chance of showers each day and some sun
breaks. Highs will be in the lower 40s, with lows near freezing.
Some showers could occasionally have some snow, with the snow level
bouncing around the 500-1000 ft level. Saturday looks like a dry
weather day with north to northeast flow aloft. McDonnal
.HYDROLOGY...No flooding is expected during the next week. Mountain
precipitation during the next 48 hours should be 1 to 2.5 inches
water equivalent, with the snow level ranging from 2500 to 3500 ft.
The lowlands should receive 0.5 to 1.5 inches of rain. That is
insufficient to present a flood threat. Less precipitation is
expected Tuesday through Saturday, and rivers should generally be
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.
.AVIATION...An upper level shortwave trough over W WA at 17Z will
move slowly N over B.C. this afternoon and tonight. Moist SW flow
aloft will continue over W WA today. The air mass is expected to
become weakly unstable this afternoon which should enhance shower
activity. There is a small chance that isolated thunderstorms will
develop 20Z-02Z but the chance is not nearly high enough to include
in the TAFs. 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 should be a bit more
stable on Monday.
KSEA...An upper level shortwave trough will be over the area today
maintaining the moist air mass with showers. 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 7-15KT with gusts to 22 KT. Kam
.MARINE...A 992 MB surface low over the WA coastal waters at 8 AM
will move NE today, reaching the central B.C. coast this evening,
before dissipating. This low will generate SCA winds over the
coastal waters and all the inland waters except for the West
entrance. The SCA winds over the inland waters will continue into
this evening. The SCA winds over the coastal waters will end late
this afternoon, however a 10-12 foot swell train will reach the
coast late this afternoon and continue through Monday afternoon. A
SCA for hazardous seas will be in effect tonight as will a SCA for
rough Grays Harbor bar conditions.
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 NW flow over the coastal waters. A weak surface low associated
with an upper level trough will move down over Vancouver Island on
WA...Winter Weather Advisory for the Cascades.
PZ...Small Craft Advisory until 4 PM PST this afternoon for 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 from 4 PM this afternoon
to 4 PM 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 from 4 PM this afternoon to 4
PM PST Monday for Grays Harbor Bar.
Small Craft Advisory from 6 AM this morning to 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.
Small Craft Advisory from 4 AM early this morning to 7 PM PST
this evening for Central U.S. Waters Strait Of Juan De Fuca-
East Entrance U.S. Waters Strait Of Juan De Fuca.
Small Craft Advisory until to midnight PST tonight for
Puget Sound and Hood Canal-Admiralty Inlet-North Inland Waters.
You can see an illustrated version of this discussion at