Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 182310
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
300 PM PST Sat Feb 18 2017

.SYNOPSIS...Scattered showers will taper tonight. Two frontal
systems in quick succession will keep rain at times over the area
Sunday and Monday. An upper trough will move over the region next
week for cooler weather with a chance of showers each day.

&&

.SHORT TERM...Radar shows a blob of light rain moving northeast out
of the area, with scattered showers replacing it. Solid cloud cover
over western Washington has kept temperatures in the 40s today.
There will be minimal clearing tonight, even as the showers taper,
and overnight lows will be 35-40.

Another weak system will arrive from the southwest Sunday. This will
be a high pop but low QPF event over the lowlands. However, the
mountains could get up to an inch of moisture, most of which will be
in the form of snow. During the period from 18Z Sunday to 06Z
Monday, 6 to 11 inches of snow could fall in the Cascades, with the
highest amounts on the volcanoes.

Rain will taper Sunday night, then yet another system will arrive
Monday afternoon or evening. Earlier model runs had the Monday
system fairly moist, but later runs have been showing less QPF.
Temperatures will max out around 50 both Sunday and Monday, which is
about normal.

A filling surface low will move northeast across the area Monday
night and Tuesday, and there will be some drying and maybe even a
little clearing on Tuesday. Onshore flow Tuesday could cause a Puget
Sound convergence zone to form Tuesday afternoon and evening. Highs
on Tuesday will be a few degrees lower than Monday, in the 40s. Burke

.LONG TERM...An upper trough will move over the area Wednesday,
followed by drier northwest flow aloft on Thursday. Another upper
trough will dig offshore on Friday and Saturday. The upshot is
cooler weather and a chance of showers. 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 foot
level. Burke

&&

.HYDROLOGY...All rivers are below flood stage and falling. Rain
during the next three days should not present a threat of flooding,
with total amounts of roughly 1 to 3 inches in the mountains and the
snow level in the 2000 to 3000 ft range.

USGS landslide guidance remains above the threshold at which
landslides typically occur. This means there will continue to be
an elevated threat of landslides through at least early next week.

&&

.AVIATION...An upper level shortwave trough is moving NE across W WA
this afternoon. The axis was over Puget Sound at 23Z which is
slightly faster than expected. The rain band is moving N and
extended from Seattle up to Bellingham at 23Z. Behind the trough,
scattered showers were moving inland from the coast. The lower air
mass remains quite moist and widespread MVFR/IFR conditions are
still present. Some lifting of CIGS and visibility improvement can
be expected through 10Z but not enough to return the area to VFR
conditions.

A cold front W of the North and Central Oregon coast at 23Z will
move NE across W WA 12Z-21Z with areas of rain while the upper level
trough and surface low track just offshore toward central Vancouver
island. The air mass will remain moist on Sunday with mainly
MVFR/IFR conditions continuing.

KSEA...The rain band from the upper level trough has moved just N of
KSEA at 23Z. For the rest of the evening there will only be
scattered light showers. The lower air mass will remain moist enough
so that MVFR conditions are expected to continue. Surface winds will
be southerly 4-8 KT. Kam

&&

.MARINE...A cold front offshore W of the Oregon coast will move NE
tonight then move N across W WA Sunday morning. The associated 992
mb surface low will move NE across the coastal waters on Sunday,
eventually reaching central Vancouver Island Sunday evening before
dissipating. SCA winds are expected around the low center over parts
of the coastal waters late tonight and Sunday, as well as over the
central and east Strait. There is a chance that SCA S winds will
develop over Puget Sound Sunday afternoon, but this is not as
certain. There could also be a brief westerly surge through the
strait behind the low Sunday evening but it may not be very strong.

Another low will approach the area from the SW on Monday.
Uncertainty is high since the GFS, ECMWF, and NAM all have
significantly different solutions. The forecast is closest to the
12Z/18Z GFS which sends a 991 MB low NE across the S WA coast Monday
afternoon which could generate SCA winds over parts of the coastal
waters and possibly Puget Sound. The 12Z ECMWF takes a weaker low NE
across NW Oregon with little impact for W WA. The 18Z NAM takes an
almost insignificant low NE across Grays Harbor, again with little
impact to WA waters.

After the Monday low, a large surface ridge will develop offshore,
putting W WA under NW flow Tuesday and Wednesday. Kam

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small craft advisory James Island to Cape Shoalwater out 60 NM.
     Small craft advisory central and east strait.

&&

$$

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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