Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 260446
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
846 PM PST Sat Feb 25 2017

.SYNOPSIS...An upper trough will move into the region on Sunday
morning. A cold showery pattern will bring lowland snow, with one
to three inches forecast above an elevation of about 300 feet.
Most snow will melt Sunday afternoon. A chance of showers will
continue through much of next week, with snow levels rising after
Tuesday.

&&

.SHORT TERM...A cool upper level low will shift south from B.C.
tonight then track down the WA coast on Sunday. The air mass will
be cool enough to bring snow levels down near the surface...with
snow showers possible in the lowlands late this evening through
Sunday morning. The showers will be convective and hit-or-miss but
cannot rule out light accumulations up to 1 inch across the area.
Heavier showers could put 2-3 inches on the higher hills. Areas
near sea level may only see a rain/snow mix with little
accumulation if any. All in all, it`s a tricky forecast with
borderline conditions. A Winter Weather Advisory will remain in
effect for much of the area through 10 AM PST Sunday. Showers will
taper off Sunday afternoon as this system shifts farther south.
We`ll also see temps rising into the lower to mid 40s. So any snow
that does fall should not stick around for too long.

A broad upper level trough will remain over the West on Monday
with a cool and unstable air mass still in place over Western WA.
There is a chance of rain/snow showers as snow levels will remain
low. However, precip accumulations do look light. Temperatures
will continue to trend a few degrees below normal.

The trough over the West Coast will shift toward the Rockies by
Tuesday and the air mass will slowly moderate over Western WA. A
warm front will bring light precip to the area during the day,
with rain increasing during the evening as the front moves inland.
Snow levels will remain low in the Cascades with a few inches of
snow expected in the passes. 33

.LONG TERM...Previous discussion...Temperatures will be closer to
normal Wednesday through Friday. On Wednesday and Thursday, zonal
flow will bring rain to the coast and snow to the mountains, with
the Seattle metro area in something of a rain shadow. On Friday a
front is forecast to move through the area, with rain likely
everywhere. Saturday will have post frontal showers with lower
temperatures. The GFS and Euro are in broad agreement about this
progression. Burke

&&

.AVIATION...Upper trough digging SE down the British Columbia coast
to off the Pacific NW coast Sunday. Northwest flow aloft becoming
southwest early Sunday then becoming light southeast later Sunday.
Unstable air mass associated with the trough with increasing
moisture for areas of showers. As the trough continues southeast
Sunday afternoon and evening, the low level flow will turn more
northerly and precipitation easing and some clearing from the north.

Lower mainly MVFR ceilings and vsbys with the showers. Associated
cool air mass with precipitation changing to snow showers most
locations Sunday morning. Terminals on higher terrain such as KPAE
have the better chance of accumulating snow - 1-3 inches. Showers
should change back to rain by midday Sunday as temperatures warm.

KSEA...MVFR ceilings through the night. Showers likely to arrive
after 06z with best threat of snow from about 11z to 19z. The winter
weather advisory remains in effect for 1-3 inches of slushy snow as
temps to remain just above freezing. Decreasing showers Sunday
afternoon with improving ceilings and vsbys. Southerly winds 4-7 kt
turning southeasterly after 06z and then northerly around 21z
Sunday. Buehner

&&

.MARINE...A 1011 mb low near the central British Columbia coast
is forecast to deepen a bit to 1004 mb near the mouth of the
Columbia River by late Sunday afternoon. Low level flow turning
offshore overnight tonight. Lower pressure should remain over the
region into Monday.

A Pacific frontal system is forecast to move ashore Tuesday night
with stronger low level onshore flow. The next stronger frontal
system is due to arrive Thursday.

&&

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

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Winter Weather Advisory until 10 AM PST Sunday for Admiralty
     Inlet Area-Bellevue and Vicinity-Bremerton and Vicinity-
     East Puget Sound Lowlands-Everett and Vicinity-Hood Canal
     Area-San Juan County-Seattle and Vicinity-Southwest
     Interior-Tacoma Area-Western Skagit County-Western Whatcom
     County.

     Winter Weather Advisory until 10 PM PST Sunday for Cascades of
     Pierce and Lewis Counties-Cascades of Snohomish and King
     Counties-Olympics.

PZ...Small Craft Advisory until 7 PM PST Sunday 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 9 PM PST this evening for Central
     U.S. Waters Strait Of Juan De Fuca-East Entrance U.S.
     Waters Strait Of Juan De Fuca.

&&

$$

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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