Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 210356

Area Forecast Discussion
National Weather Service Seattle WA
900 PM PDT Thu Oct 20 2016

.SYNOPSIS...A weak upper level ridge tonight will give a brief lull
in precipitation. A convergence zone over North Puget Sound will
dissipate. A weak warm front will bring some light rain to the area
on Friday. After another minor lull Saturday, another cold front
will reach the area late Saturday and Saturday night. A large upper
level low offshore will maintain moist southwest flow aloft.


.SHORT TERM...A convergence zone shows up nicely on radar but it has
shown signs of diminishment the last few hours. Still, a spotter
between Bothell and Mill Creek reported 2.03 inches of rain since 9
PM last night--impressive for a lowland site. The convergence zone
will probably dissipate by midnight. It`s dry elsewhere.

A weak weather system will move onto the coast Friday and spread
rain inland Friday afternoon and evening. Rainfall amounts with this
system will be measured in tenths of an inch rather than whole
inches. After it dissipates Friday night, Saturday will be mostly
dry. Highs both Friday and Saturday will be 55-60.

A weak front will arrive around Saturday night and Sunday will have
some rain, mainly over the mountains. Sunday will be slightly
warmer than Saturday. Burke

.LONG TERM...Previous discussion...The above mentioned upper level
longwave low will remain the main feature Monday through most of
next week. Moist SW flow aloft around the low will send a series of
weak shortwaves across W WA through the period. The models have not
been all that consistent with the smaller scale features so
confidence in the details is low. Confidence is still good that this
will be a fairly wet active period of weather. Kam


.AVIATION...Southwest flow aloft will prevail through Friday
night. A weak cold front will through the coastal waters on Friday
and weaken as it moves inland late Friday afternoon and Friday
evening. The air mass will be moist and stable.

While some drier air aloft is filtering into Western Washington
this evening, convergent onshore flow continues to produce
showers in a Puget Sound Convergence Zone over far northern
portions of King County and in Snohomish County. This convergence
zone will gradually fade away by 10Z. Partial clearing and
radiational cooling this evening combined with a wet ground will
result in areas of stratus and patchy fog forming overnight. Some
of the stratus and fog is already forming early this evening from
Whidbey Island to Port Townsend in the northern interior and
around Olympia. By 10Z-12Z expect areas of IFR conditions in the
interior lowlands in low stratus and fog, and some LIFR conditions
can be expected. Low clouds will decrease about 18Z as mid and
high level moisture spreads inland ahead of the next frontal
system. Areas of -RA will spread inland during the afternoon
hours. Albrecht

KSEA...Partial clearing this evening combined with radiational
cooling, light pressure gradients, and a very wet ground will
result in LIFR in low stratus developing at the terminal around
12Z. The evening 03Z TAF was updated to account for this. Stratus
will likely scatter out midday as mid and high level moisture
associated with the next in a series of fronts moves in during the
afternoon. Light rain from a OVC050 deck is expected by late
afternoon. Winds will S to SE at 4-7 kt. Albrecht


.MARINE...A series of fronts will move through the area over the
next several days.

The first front in this series -a relatively weak front- will
bring small craft advisory southeast to south winds to the coastal
waters and to the western portions of the Strait of Juan de Fuca
starting early Friday morning. Winds are expected to remain below
20 knots in the remainder of the waters as the front weakens while
moving inland. While winds behind the front over the coastal
waters and in the western Strait will weaken, expect a westerly
swell of 10 to 13 ft Friday night to require Small Craft
Advisories to continue.

A stronger front will bring small craft advisory winds to most
waters later Saturday or Sunday. While gales are possible for the
coastal waters with this system, winds this strong are not
currently indicated in the models or forecast.

Another fairly vigorous frontal system will affect the waters
Sunday night and Monday. Yet another system will move through the
waters on Tuesday. Schneider/Albrecht


.HYDROLOGY...Heavy rain caused minor flooding on a few rivers. The
Snoqualmie at the falls, the Skokomish, and the Skykomish all
crested above flood stage earlier today and are now falling. The
lower reaches of the Snoqualmie and Snohomish may still reach flood
stage. The Skagit River near Concrete has been rising and it will
probably crest right around flood stage in a few hours. All rivers
will recede Friday. Burke


WA...Flood Watch until 6 AM PDT Friday for Skagit, Snohomish, and
     King counties.

PZ...Small Craft Advisory from 6 AM to 3 PM PDT Friday 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-West Entrance U.S. Waters
     Strait Of Juan De Fuca.



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