Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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828
FXUS66 KSEW 210504 CCA
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
844 PM PST Mon Feb 20 2017

.SYNOPSIS...Light rain should be mostly over by midnight as a
frontal system moves northeast out of the area. Another system will
brush the south half of the Western Washington interior with some
light rain on Tuesday as it moves northeast across Oregon. An upper
level trough moving over the region Wednesday through Friday will
bring cool and showery weather. Sunny but cool weather is likely
over the weekend.

&&

.SHORT TERM...RADAR shows the areas of light rain over W WA starting
to diminish this evening. Based on the HRRR it should be mostly dry
by midnight, but moisture is still abundant in the lower air mass so
some spotty drizzle is still possible overnight. Snow over the
Cascades has become very light or ended so the Winter Weather
Advisory for the Cascades in Pierce and Lewis counties will be
allowed to expire at 10 PM.

Models still show the next system moving NE across Oregon on Tuesday
with light rain just brushing the southern half of the W WA
interior. The 18Z GFS and 00Z NAM bring rain up as far as Seattle
while the latest HRRR only gets as far N as Tacoma. The coast and
northern half of the interior should be dry on Tuesday.

Once the surface low from Tuesdays system moves over NE Oregon in
the afternoon, onshore flow will develop across W WA. This should
set up a convergence zone that could affect King and Snohomish
counties late Tuesday afternoon through Tuesday night. In this case
the convergence zone might start off over King county then move N
over Snohomish county overnight.

The longwave pattern over the NE Pacific is progressive, so the cold
upper level trough currently offshore will move slowly E until it is
over W WA Tuesday night. The trough will bring a pretty cool air
mass, enough to lower snow levels down to around 500 feet in spots
Wednesday morning through Thursday. With low snow levels the
scattered showers generated by the large trough could bring some
light snow showers to the lowlands Wednesday morning. Temperatures
should be warm enough Wednesday afternoon for just rain showers over
the lowlands. The showers will be spotty enough so that any
accumulations should be brief and insignificant. The upper level
trough and cold air mass will hang around on Thursday for a
continuing chance of spotty snow showers, mainly during the late
night and morning hours. Kam

.LONG TERM...Previous discussion from the 315 PM AFD...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...Southwesterly flow aloft will continue tonight and into
Wednesday before becoming more westerly late Wednesday afternoon.
Rain continues to dissipate...focused mainly along the coast and in
a convergence zone north of KPAE. Visibilities in showers may be
impacted...but otherwise are expected to improve area-wide. Cigs
will gradually increase through the night in most locations as
well...reaching MVFR conditions tonight before some locations lower
back down Tuesday morning. Another surface low will pass by well to
the south on Tuesday...with some rain possible over the SW portion
of the area Tuesday afternoon. Another Puget Sound Convergence Zone
is possible around Seattle and Everett Tuesday evening.

KSEA...Surface winds still mainly from the south through midnight
with winds turning more northerly overnight and remaining that way
into Tuesday afternoon...then another shift back to southerlies for
Tuesday evening. Wind speeds generally 5-10 kts. Vicinity showers
still possible this evening but radar trends remain decreasing. Cigs
improving to MVFR to very low end VFR tonight before falling again
Tuesday morning. Will be on the northern fringe of a large rain area
to the south on Tue during the late morning and early afternoon
hours. SMR/Haner

&&

.MARINE...Winds have fallen below SCA criteria...so have taken down
headlines for remainder of tonight period. A 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. This looks like it may bring a round of
SCA winds to the coastal waters...but may be borderline/low-end.
Will issue SCA for southern coastal waters for evening forecast
package and evaluate remaining waters for early morning issuance.
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/SMR

&&

.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 Tuesday morning through Tuesday afternoon
     for the coastal waters from Point Grenville to Cape Shoalwater
     out 60 NM.

&&

$$

www.weather.gov/seattle

You can see an illustrated version of this discussion at
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