Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA
FXUS66 KSEW 202317
Area Forecast Discussion
National Weather Service Seattle WA
315 PM PST Mon Feb 20 2017
.SYNOPSIS...Rain will taper this evening as a frontal system moves
northeast out of the area. Another system will brush the area
Tuesday as it moves by even further to the south. An upper trough
over the region Wednesday through Friday will bring cool and showery
weather. Sunny but cool weather is likely over the weekend.
.SHORT TERM...Light rain fell over most of the state this afternoon,
with snow in the mountains. Precipitation will continue into the
evening, then taper. Far northern areas have remained dry today.
Highs today have been 45-50. A winter weather advisory remains in
effect for the central and south Cascades. A total of 5 to 10 inches
will fall on the volcanoes, with 3 to 6 inches in the passes. The
snow level is around 2500 feet.
Another frontal system will move inland across northern California
and Oregon tonight and Tuesday. The north side of this system will
bring some precipitation to mainly the southeast corner of our
forecast area on Tuesday. Precipitation amounts will be relatively
light, and much of the area north and west of Seattle will have dry
weather all day. Highs Tuesday will be in the mid 40s.
An upper trough will move over the region from the west Tuesday
night, and then deepen over the area as it slowly moves east through
Wednesday night. It will bring generally light, scattered showers
and probably a weak Puget Sound convergence zone. The snow level
will fall to 1000 to 2000 ft, then to as low as 500 feet Thursday
morning. Several more inches of snow will fall in the mountains.
Showers over the lowlands could have snow at times, but little or no
accumulation is expected. Highs Wednesday and Thursday will be in
the low to mid 40s, with overnight lows in the low to mid 30s. Burke
.LONG TERM...An upper low is forecast by both the GFS and Euro
models to move south off the Washington coast on Friday. This will
likely spawn some showers, especially near the coast. The upper low
will continue to move south on Saturday, giving dry and mostly sunny
days on Saturday and Sunday. Despite the sun, heights will be low
and high temperatures probably won`t get much higher than 45. Burke
.HYDROLOGY...No flooding is expected during the next week. 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...Active southwest flow aloft is causing a surface low
to exit northeast through Eastern Washington this afternoon,
supporting onshore flow through this evening. Rain will ease and
dissipate early this evening, bringing improved visibilities. Yet
another surface low will pass by well to our south on Tue, with
rain clipping southwest Washington around mid-day Tue. Late in the
day on Tue, a Puget Sound Convergence Zone is possible around
Seattle and Everett.
KSEA...Expect a southerly component to surface winds until about
midnight, then winds will become more northerly again overnight
and persist through most of Tue afternoon. Yet another shift to
southerlies between 00z and 03z (4 and 7 pm) on Tue PM. Otherwise,
visibilities will improve through early this evening as rain
lightens up and dissipates. Will be on the northern fringe of a
large rain area to the south on Tue during the late morning and
early afternoon hours. Haner
.MARINE...A 998 mb low was moving NE through eastern Washington
this afternoon, bringing an increase in the westerly component of
gradients over the waters through early this evening. SCA strength
westerly are expected in the Strait. Northwesterlies have also
picked up in the Strait of Georgia, so hoisted a SCA for the
northern inland waters, targeted to waters north of the San
Yet another surface low center will cross the south Oregon coast
on Tue AM and track northeast to SE Washington by Tue PM. This
will bring another round of northerly gradients on Tue AM to
Western Washington, followed by westerly gradients later in the
day. Onshore flow will then continue Wed but weaken. A weak
surface low will develop near Cape Flattery on Wed night and then
slide southward through the coastal waters on Thu. Light to
moderate offshore flow is expected on Friday. Haner
WA...Winter Weather Advisory until 10 PM PST this evening for
Cascades of Pierce and Lewis Counties.
PZ...Small Craft Advisory until 9 PM PST this evening for Coastal
Waters From Cape Flattery To James Island 10 To 60 Nm-
Coastal Waters From Cape Flattery To James Island Out 10 Nm-
Northern Inland Waters Including The San Juan Islands-West
Entrance U.S. Waters Strait Of Juan De Fuca.
Small Craft Advisory until 3 AM PST Tuesday for Central U.S.
Waters Strait Of Juan De Fuca-East Entrance U.S. Waters
Strait Of Juan De Fuca.
You can see an illustrated version of this discussion at