Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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040
FXUS66 KSEW 181700
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
900 AM PST Sat Feb 18 2017

.SYNOPSIS...A weak upper trough will bring some rain to Western
Washington today. 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...Light rain developed this morning and is currently
widespread. Have already updated the forecasts and grids with higher
pops today. Highs today will be mostly 45-50, which is 5-10 degrees
lower than yesterday`s highs.

There will be a break in the rain tonight, then another weak system
will arrive from the southwest Sunday. 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. In fact rainfall amounts in the
lowlands will not be particularly heavy with any of these systems,
but pops are pretty high. Temperatures will max out around 50 both
Sunday and Monday, which is about normal.

Precipitation in the mountains will be higher. The snow level will
be around 3000 feet so most of the precipitation will be in the form
of snow. Snow advisory amounts are possible at times, especially
Monday and Monday night. Burke

.LONG TERM...Previous discussion...Tuesday should be somewhat breezy
and showery in the wake of the Monday and Monday night frontal
system. An upper trough will move over the area Wednesday, for
cooler weather and a chance of showers. Drier northwest flow aloft
will follow the trough on Thursday. A deeper upper low will probably
dig over the region from the north on Friday; the GFS has been
consistent with this solution, while the ECMWF has been less so. At
any rate, Friday should be a cool and showery day, with the snow
level probably falling to around 1000 ft. McDonnal

&&

.AVIATION...An upper level shortwave trough along the WA coast at
16Z will move slowly NE across W WA through 06Z. Steady light rain
has spread northward ahead of the trough and has lowered conditions
to IFR over the S Puget Sound region and the SW interior, with MVFR
conditions over the rest of the Puget Sound region. The continuing
rain will eventually lower conditions over the N interior to MVFR
CIGS after 20Z. The trough axis will move across KSEA around 00Z and
across KBLI around 06Z with rain ending and CIGS lifting to VFR.

On Sunday, an offshore upper level shortwave trough weak surface low
moving NE toward central Vancouver Island will pass just W of the WA
coast on Sunday. The associated cold front will move NE across W WA
Sunday morning for areas of rain and probably another round of
MVFR/IFR conditions.

KSEA...An upper level trough moving across W WA today will maintain
light rain over the terminal through 00Z. CIGS around 040 are
expected to lower to BKN020 around 18Z and continue through 00Z.
CIGS down to around 010 are not out of the question. The rain should
end and CIGS increase to BKN025-040 after 00Z and behind the trough.
Northerly surface winds 4-7 KT will shift to SW as the trough axis
moves through around 00Z. Kam

&&

.MARINE...A weak surface ridge will remain over W WA today which
will keep winds generally weak. A cold front offshore approaching
130W will approach the area tonight. A weak surface low will develop
along the portion of the front W of the southern Oregon coast. The
low and front will move NE tonight with the low buzzing by the WA
coast on Sunday while generating SCA winds. The front will move NE
across W WA Sunday morning. The low will eventually reach central
Vancouver Island early Sunday evening before dissipating.

The 12Z GFS and 00Z ECMWF agree that another low will develop
offshore on Monday then approach the region Monday afternoon. The
track of this low should take it NE across the south WA coast early
Monday evening. This low looks a little stronger but the track may
keep winds below SCA levels for much of the coast. There is still
uncertainty regarding this low as the 12Z NAM no longer shows the
low developing. A large surface ridge is expected to develop
offshore Tuesday and Wednesday for a shift to moderate NW flow. Kam


&&

.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 2500 to 4000 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.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...None.

&&

$$

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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