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Area Forecast Discussion (AFD)

Issued by Seattle/Tacoma, WA (SEW)

FXUS66 KSEW 250217

Area Forecast Discussion
National Weather Service Seattle WA
715 PM PDT Sat Aug 24 2019

.SYNOPSIS...Lingering moisture coupled with northwesterly flow aloft 
will keep mid to high level clouds over Western Washington into 
Sunday morning. A ridge will build north into the area late Sunday 
and continue through Wednesday kicking off a late summer warming 
trend bringing daytime highs into the mid to upper 80s and allowing 
dry conditions to persist through much of the week. A troughing 
pattern returns late in the week bringing a chance for showers.


.SHORT TERM /TONIGHT THROUGH TUESDAY/...Current radar is fairly 
clear and satellite imagery some clearing over southern portions of 
the CWA. Clouds continue to linger over approx the northern two 
thirds of the area but trends show some slow scattering. Low level 
onshore flow might prove to limit this moisture from 
the exiting system coupled with marine air will allow for stratus 
development overnight and into Sunday morning. Skies are expected to 
clear Sunday afternoon and thru the remainder of the day as upper 
level ridging moves into the area. Monday sees the influence of the 
ridge strengthen over the area. Low level flow will still be onshore 
as large area of sfc high pressure remains offshore so could have 
some stratus near the coast but otherwise sunny skies. Low level 
flow then turns offshore on Tuesday as a thermal trough builds north 
from California. With upper level ridge influence, this will make 
for noticeably warmer temps, into the lower to mid 80s for most.


Discussion...Upper level ridge will remain in place to begin the 
extended, as will low level offshore flow. Wednesday will likely be 
the warmest day of the week with highs at least mid to upper 80s 
across the interior.

By Thursday, the ridge will begin to dampen, with axis shifting
east, opening the region up to more of a SW flow aloft regime.
A few weak shortwave disturbances appear to be embedded in the
larger scale flow, which may help enhance at least some mid and 
high level cloud cover, even maybe a shower or two across parts of
the Pacific Northwest. This may keep temps a few degrees cooler 
if this were to verify, however, mid level heights still remain 
fairly high, so regardless it will continue to be warm with highs 
in the 80s for many.

By Friday, a system well offshore the Washington Coast will
amplify. This will help reestablish the ridge to some degree 
across the Pacific Northwest, despite some weak perturbations 
becoming trapped underneath. By the end of the forecast period on 
Saturday, larger scale trough looks to return to the region, with
which some showers may begin to spread into the area along
a sfc cold frontal boundary.



.AVIATION...Northwesterly flow aloft as an upper level trough 
departs the Pacific Northwest. Onshore flow at the surface. VFR cigs 
in place throughout W WA this evening and will persist into early 
tonight before marine stratus push brings cigs down to MVFR late 
tonight/early Sunday morning. Cigs expected to lift back into VFR 
late morning/early afternoon as upper level ridging begins to exert 
influence over the area.

KSEA...VFR cigs with mid and high level clouds this evening before 
MVFR cigs with stratus developing at or around 12Z. North to 
northeast winds this evening and tonight before becoming 
southwesterly after 12Z and into Sunday afternoon. Wind speeds 5-10 
knots. SMR


.MARINE...Onshore flow continues tonight and into Monday. Small 
Craft Advisory winds currently being observed in portions of the 
Central and Eastern Strait...and as such will leave inherited 
headlines in place with evening forecast package. As this onshore 
flow persists into Sunday...there is the potential for another round 
of SCA winds for Sunday evening and Sunday night. Offshore flow is 
expected Tuesday and will persist through much of next week. JD/SMR


.HYDROLOGY...The daily hydrology discussion has ended until the
start of the next rainy season; it will only be updated as needed
until then.


PZ...Small Craft Advisory until 5 AM PDT Sunday for Central U.S. 
     Waters Strait Of Juan De Fuca-East Entrance U.S. Waters 
     Strait Of Juan De Fuca.