Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 202240
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
340 PM PDT Tue Oct 20 2020

.SYNOPSIS...A weak weather system will move through the area
tonight into Wednesday, bringing cooler temperatures and another
round of light showers to portions of the area. Thursday will
remain dry, before a stronger system traverses the region on
Friday, bringing with it another round of lowland rain, mountain
snow, and cooler temperatures into the weekend.

&&

.SHORT TERM /TONIGHT THROUGH FRIDAY/...Primarily dry across
western Washington this afternoon, except for a few spotty showers
across Snohomish county. Elsewhere, satellite imagery showing
cumulus scattered across much of the area this afternoon. A weak
system will move through the region tonight into Wednesday,
bringing a round of light showers to portions of the area. Showers
will not be widespread, however hi-res guidance continues to
indicate the development of a convergence zone this evening
across portions of southern Snohomish and northern King counties.
Snow levels will fall to near 3000 feet overnight, so cannot rule
out a little snow at the Cascade passes, though do not expect this
to impact travel significantly. Showers will linger into Wednesday
and high temperatures will begin to cool a few degrees, topping
out in the low 50s across the area.

Thursday continues to look dry as Western Washington resides in
northwest flow aloft between an upper level ridge across the
northeastern Pacific and a trough exiting to the east. Expect to
see some sun on Thursday, with temperatures remaining in the low
50s.

The next stronger and colder system will traverse the region on
Friday, bringing with it widespread lowland rain and mountain
snow. Snow levels will drop and remain below pass levels, so will
likely see some accumulations across the passes. Ensemble
guidance continues to hint at fairly high probability of 6 inches
for the passes, however details still remain uncertain and will
become more refined in the coming days. Temperatures will continue
to cool, with highs only making it in the upper 40s across much
of the lowland areas.

.LONG TERM /SATURDAY THROUGH TUESDAY/...Cool start to the long
term in the wake of the aforementioned system. Showers will linger
across the mountains into early Saturday morning, but expect most
of the region to dry out relatively quickly during the day.
Temperatures will remain cool-topping out in the mid to upper 40s
and overnight lows will be in the low to mid 30s across the
region, potentially yielding the first widespread frost across
the area. Dry and cool weather will persist into Monday as western
Washington remains under northwest flow aloft and high pressure
starts to build across the Pacific. 14

&&

.AVIATION...Northwest flow aloft through Wednesday. Mainly VFR this
afternoon with MVFR in spots across the central Puget Sound. Showers
may develop with a convergence zone this evening near Snohomish/King
Counties for MVFR at times. Some showers into Wednesday as a system
moves through. VFR cigs expected to transition to MVFR for most into
Wednesday morning. IFR possible in spots. Winds mainly light under 5
knots through this evening with south winds increasing to 8 to 12
knots by Wednesday morning. JD

KSEA...VFR cigs. MVFR may briefly be possible this evening if any
convergence zone develops. MVFR expected late tonight into Wednesday
with a few showers. Winds light westerly may become light northern
(under 5 knots) this evening with southerly winds increasing
Wednesday morning. JD

&&

.MARINE...Onshore flow will increase tonight as a front moves
through. Small Craft Advisory winds for the coast, Strait of Juan de
Fuca, and Northern Inland waters through tonight. Strong northwest
flow on Wednesday along the coast for high end SCA winds. Flow will
be lighter Wednesday night and transition offshore Thursday. A
system will move through on Friday with for gales possible along the
coast. Offshore flow through the weekend. JD

&&

.HYDROLOGY...River flooding not expected over the next 7 days.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory from 11 PM this evening to 7 PM PDT
     Wednesday 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 from 9 PM this evening to 11 AM PDT
     Wednesday for Central U.S. Waters Strait Of Juan De Fuca-
     East Entrance U.S. Waters Strait Of Juan De Fuca-West
     Entrance U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory from midnight tonight to 11 AM PDT
     Wednesday for Northern Inland Waters Including The San Juan
     Islands.

&&

$$

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