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Area Forecast Discussion (AFD)

Issued by Seattle/Tacoma, WA (SEW)

FXUS66 KSEW 242208

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

.SYNOPSIS...Weak perturbations in the NW flow aloft will continue
to traverse the area into Sunday morning, allowing for some mid
and high cloud cover. A ridge will build north into the area late
Sunday and continue through Wednesday, with a late summer warming
trend likely with highs in the mid to upper 80s. This will
make for warm and dry conditions through much of the week before 
troughing pattern returns late in the week/early weekend with a
chance for showers.


.SHORT TERM /TONIGHT THROUGH TUESDAY/...The upper level pattern
this afternoon features a broad swath of NW flow aloft, extending
roughly from The Gulf of Alaska into the Pacific Northwest and 
Northern Rockies. This flow regime then gives way to a more split
flow pattern downstream across the Plains, with ridging across 
the Midwest & Great Lakes and weak troughing across the South. At 
the sfc, A weak low in the Haida Gwaii vicinity has essentially 
dampened, with trailing cold front that attempted to push inland 
across Washington state more of a diffuse boundary at this point. 
East of the Rockies, a very expansive area of high pressure
/center ~1028mb/ encompasses much of the eastern US. 

Earlier today, a fast moving shortwave trough darted across
Washington and as of 21z is located in central Montana. This wave
had enhanced some mid and high level moisture that has also
largely carried east. In addition, some light shower activity made
it to the coast and into the Lower Chehalis Valley and Olympic
Peninsula. Appreciable lack of dynamics both at the sfc and aloft
was responsible for the demise of most this activity into the
afternoon. Some shower activity is currently noted across Skagit
and Whatcom Counties. There is some enhanced shortwave activity
across far southern BC very near the Canada/US border that is
likely the source of sufficient lift for this activity.

Going forward to later this afternoon and evening, a better
onshore push of westerly low level wind is expected in the wake of
the fizzling frontal boundary. The strongest winds will slide down
the Strait of Juan de Fuca into the northern Puget Sound 
vicinity. These winds could potentially converge with more S/SW in
the central/southern Sound. Although not strong, this may allow 
for a convergence zone to develop with a few light showers as some
low level moisture hangs in place. Slight chance POPs remain in 
this general area into King and Snohomish counties for a few hours
this evening.

Some cloud cover is likely Sunday morning with a reinforcing
shortwave trough glancing the northern border of the area during
the morning hours. Heights will drop briefly but best moisture
remains up in Canada. Heights will then begin to rise as a ridge
across the Pacific starts to build N/NE. Skies are expected to
clear thru the day as this happens. Much of the same for Monday
with ridge influence a bit better than Sunday. 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.


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 
for most with MVFR along the northern tier of the area. Expect VFR 
through the afternoon into tonight with mid and high level clouds 
around. MVFR late tonight (09z-12z) with marine stratus push. Winds 
generally southerly becoming northerly for some this evening. Winds 
5-10 knots.

KSEA...VFR cigs with mid and high level clouds. MVFR cigs with 
stratus tonight into Sunday morning. Winds generally southerly 
becoming more north-northeasterly tonight. Winds 5-10 knots. JD


.MARINE...Onshore flow continues through the weekend. A weak front 
will move across the area today with another westerly push down the 
Strait this evening into tonight. Small Craft Advisory winds for the 
central and eastern portions of the Strait of Juan de Fuca later 
this afternoon through tonight. Onshore flow into Monday with 
offshore flow Tuesday and through much of next week. JD


.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.