Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 270449
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
849 PM PST Sun Feb 26 2017

.SYNOPSIS...Showers will diminish tonight as an upper level low
dissipates over Oregon this evening. An upper trough will move
across Washington Monday afternoon and evening. Cool northwesterly
flow aloft will be a good snow producer for the mountains
Wednesday through Friday. An upper level trough will bring showers
Saturday, which will taper Sunday.

&&

.SHORT TERM...Showers are winding down across Western WA as a
trough rotates east through central Oregon. Most of the remaining
showers will affect the central Cascades this evening then
diminish. Temperatures will drop into the upper 20 to lower 30s
tonight.

The air mass will remain cool and slightly unstable on Monday with
a longwave trough over the West. Showers will mainly impact the
mountains but there is a chance of a rain/snow mix in the
lowlands, especially in the interior where snow levels will be
near the surface. Snow accumulations are not expected.
Temperatures will remain below normal with highs near 40.

On Tuesday, the trough over the West will shift east toward the
Rockies. The air mass will moderate over Western WA as 500 mb
heights build and low level onshore flow increases. Models are
showing light precip over the area with warm air advection. The most
likely scenario is just light rain on the coast. Rain will
increase in coverage Tuesday night as a warm front moves inland.
The biggest impact will be heavier showers in the mountains. Snow
levels will remain low (near 2,000 feet) with several inches of
snow in the Cascades, including the passes. This pattern will
extend into Wednesday as onshore flow continues. 33

.LONG TERM...Previous discussion...Upper flow will turn more
westerly on Thursday and Friday and the air mass will get
noticeably more moist. This pattern will give copious snow to the
mountains. The Euro is not quite as wet as the GFS. Highs will be
45 to 50. On Saturday an upper trough will move through the
region, spawning showers. Models diverge after that. Burke

&&

.AVIATION...An upper trough over the Pacific NW is expected to
continue sliding southeastward through Monday. Cool unstable NW flow
aloft over Wa. A weak shortwave in the flow is forecast to move
through the area Monday and generate additional showers with mainly
MVFR conditions across Western Wa.

KSEA...Showers ending this evening as the upper trough continues
further southeast. Some partial clearing overnight yet occasional
MVFR cloud ceilings at times too. Northerly wind to ease overnight
and become light southerly by 18z Monday.

&&

.MARINE...Weak lower pressure is forecast to linger over Western
Washington through Monday. A weak Pacific frontal system is expected
to move ashore Tuesday evening with stronger onshore flow. The next
stronger frontal system is due to arrive Thursday and Thursday
night with another system to follow Friday.

&&

.HYDROLOGY...No flooding is expected during the next week.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Winter Weather Advisory until 10 PM PST this evening for
     Cascades of Pierce and Lewis Counties.

PZ...Small Craft Advisory for hazardous seas until 3 AM 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 until 5 AM PST Monday for Grays Harbor Bar.

&&

$$

www.weather.gov/seattle

You can see an illustrated version of this discussion at
www.weather.gov/seattle/gafd/latest_webafd.html


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