Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 202141

Area Forecast Discussion
National Weather Service Seattle WA
241 PM PDT Sun Sep 20 2020

.SYNOPSIS...Weak high pressure is over the area. A weak weather
system will dissipate as it moves over Western Washington Monday
afternoon and evening. A vigorous front will arrive by daybreak
Wednesday. Additional weather systems could keep the weather wet at
times into next weekend.


.SHORT TERM /TONIGHT THROUGH WEDNESDAY/...There ought to be enough
clearing for patchy fog later tonight into Monday morning. A weak
weather system will arrive Monday afternoon and evening for a chance
of showers. A chance of rain will arrive to the coast Tuesday as a
warm front brushes the area. Rain will increase on the coast Tuesday
night spreading inland around daybreak Wednesday as a vigorous front
arrives. The coastal and northern waters will probably see the first
southeasterly gale of the season by daybreak Wed. It will be wet and
the hydro section of the AFD has started up again--see below. 19

.LONG TERM /THURSDAY THROUGH SUNDAY/...Additional periods of
rain are possible into next weekend but the forecast is not on very
solid ground. It will probably be cool and showery Thursday and
Friday, and then a warm front might give us some rain Saturday--but
most of that moisture is likely to shift north to the B.C. coast and
the Alaskan panhandle as ridging begins. The forecast for Sunday
does not account for the ridge the global models build. Well, there
is plenty of time to sort out the details and with strong westerlies
crossing the Pacific any little extra buckle changes everything five
six and seven days out so the forecast is simply a blend of all the
guidance. 19


.AVIATION...Westerly flow aloft will transition more southwesterly
tonight as an upper trough approaches the area. VFR cigs for most
this afternoon with MVFR in spots with mid cumulus clouds. VFR cigs
this evening with a few high clouds. IFR stratus possible early
Monday morning, mainly for the Southwest Interior and into the
central Puget Sound. Patchy fog possible across the South Sound
Monday morning. VFR cigs by late Monday morning through the
afternoon. Light surface winds under 5 knots.

KSEA...VFR this afternoon with MVFR briefly at times. VFR into
tonight. IFR stratus possible late tonight into early Monday
morning, however, clouds may mainly be south and west of the site.
Any clouds will clear quickly tomorrow morning for VFR cigs. Surface
winds light southwesterly will shift northerly this evening. JD


.MARINE...Light onshore for through Monday. Increasing winds by
Tuesday afternoon as a strong cold front approaches the area.
Widespread advisory level winds across the area. Gales will be
possible at least for the coastal waters, eastern Strait of Juan de
Fuca, and the northern inland waters. Advisory level winds will be
possible across the waters late week and through the weekend as a
series of systems move through the area. JD


.HYDROLOGY...The mountains should see 2-4 inches of rain with
the first decent frontal system of the season Wednesday. That is the
sort of rainfall that can drive the flood prone Skokomish river in
Mason county up around flood stage--but there is a lot of capacity
to absorb moisture early in the season. It is likely that this first
round of moisture restores the rivers to reasonable flows but it
will be worth watching the Skokomish--especially if Thursday and
Friday are especially wet too. 19



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