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Area Forecast Discussion (AFD)

Issued by Seattle/Tacoma, WA (SEW)

FXUS66 KSEW 232113

Area Forecast Discussion
National Weather Service Seattle WA
213 PM PDT Fri Aug 23 2019

A trough of low pressure will slowly move across Central BC
through the weekend. This will bring a few showers and near to
below normal temperatures for Western Washington. The trough will
slide east and high pressure will build in behind it by Sunday and
into early next week. Low level offshore flow will develop by mid
week and produce well above normal temperatures and dry


Satellite and radar depict the steady eastward progress of the
showers across the northern tier of the forecast area this
afternoon. Rainfall totals were highest across the San Juans and
Whatcom County. Peak totals of about .10 to .20 inches were
observed in those areas. Measurable rain appears to have been
recorded as far south as line from Port Angeles to Marysville,
with dry and partly cloudy conditions south of there. A decrease
in shower activity is expected this afternoon with most areas dry
by sunset. Precipitable water values drop off as the atmosphere
briefly dries out this evening. However it is short lived as rain
showers should develop again off the coast late tonight and likely
reach the coast by sunrise on Saturday. There will be a low chance
for showers across the area on Saturday with the greatest chance
being over the Olympic Peninsula and coast earlier in the day.
This activity will not be significant but the showers and clouds
will help to keep temperatures below normal for many areas on
Saturday. The troughing will be slow to move east and leave the
area open for a few more showers Sunday across the far northern
areas closest to Canada. But otherwise it will remain cloudy with
highs lingering near normal. 

A change in the pattern will take place overnight Sunday and into
Monday. The trough of low pressure will finally exit well east of
the area. This will permit a ridge of mid level high pressure
which has been lingering well south of the area to begin building
north. Heights will rise quite a bit overnight Sunday and into
Monday and so Monday will be a much drier day with temperatures
bumping up about 5 degrees from Sunday to reach near or slightly
above normal. 

This warming trend will continue through midweek. Heights will
continue to climb and crest in the 585-588dm range. 850mb temps
will climb as well to around 18-20C by Tuesday evening with winds
becoming more favorable for supporting low level warmth. HIghs on
Tuesday are expected about 10 degrees warmer than Monday, so
should be well above normal with lots of sun and lower to mid 80s.
Wednesday looks to be the hottest day at present as 850mb temps
crest in the +20 to +23C range with low level offshore flow. This
suggests highs in the mid to upper 80s with readings near 90
possible from Seattle to the south for some late summer heat.
Current Heat Impact Levels indicate some impacts for vulnerable
populations but not hot enough for more widespread concerns at
this time. Record highs for Seattle on T-W-Th are 88,95,91 and so
those appear to be safe. 

Thursday looks set to be a cooler day, by at least a few degrees
if not more. heights fall and 850mb temps do as well. There is
some suggestion of some precip even in the area so the temperature
drop may be even more than current forecasts indicate. At present
have shows the greatest cooling on the coast and a few degrees of
cooling inland but will have to monitor trends in the extended.
Friday at this point looks like it may remain above normal across
the interior so have left forecast highs in the 80s as the details
on breakdown of the warmer pattern looks to be a bit in question.


.AVIATION...A weak system passing across the northern half of the 
state today has brought widespread mid and high level cloud cover to 
the terminals today, with a widespread area light to moderate 
rainfall from roughly Clallam Co into Skagit and Whatcom Counties. 
Most of the activity was confined to KCLM and KBLI, with some brief 
reductions in cigs and vsbys. The majority of the rain has moved 
east and dry conditions are expected for the remainder of today, 
with any cig and/or vsby reductions rapidly improving.

All other TAF sites have been VFR with ceilings generally above 10k 
ft under no vsby restrictions. This will continue to be the case 
thru the evening and part of the overnight. Low level marine stratus 
will develop along the coast and push inland some, likely reaching 
KHQM and KOLM but may spare the Sound terminals with perhaps 
coverage capping at FEW by daybreak. Clouds should then lift and 
scatter some into the afternoon. N/NE winds 10kts or less this 
afternoon will become light overnight and gradually veer to the S/SW 
through the day Saturday at again 10kts or less.

KSEA...VFR ceilings through the evening and much of the overnight. A 
few MVFR stratus may move into the vicinity of the airfield by 
daybreak Saturday. Winds N/NE 10kts or less into the overnight 
hours, veering S/SW by Saturday afternoon under 10kts.



.MARINE...Quasi-zonal to weak troughing pattern will hang in place 
in the mid and upper levels tonight and Saturday, while at the 
surface, onshore flow will continue as higher pressure remains 
located offshore and lower pressure remains inland. A weak shortwave 
trough embedded in the mid level flow is skirting this region today. 
With it has been a rather widespread area of light to moderate 
rainfall across mainly the Strait of Juan de Fuca and the Northern 
Inland Waters. Most of this activity is currently pushing east as of 
21z and will continue to clear the waters thru the remaining hours 
of the afternoon. Wind and waves have remained rather tranquil so 
far today. By late afternoon/early evening, gradients will 
strengthen enough to produce small craft advisory level winds down 
the central and eastern Strait. These winds will last thru much of 
the night and may bleed into the adjacent Northern Inland Waters 
where a small craft advisory has been issued with this forecast 
update to account for this.

Winds will ease across these waters Saturday morning /tho there 
could be an observation or two of small craft winds in the Strait 
between 12-18z/. Onshore flow will continue with little change in 
the synoptic pattern, although a weak front will cross the local 
area. In its wake, a stronger westerly push down the Strait is 
possible. Winds in latest guidance are capped at 30kts or below so 
will hold off on a Gale Watch but the evening and midnight shifts 
may need to issue if gradients continue to look strong. 

Onshore flow looks to continue through Monday before shifting to 
offshore as a thermal trough builds north from California. The 
offshore flow pattern may then last thru much of the week.



.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 from 4 PM this afternoon to 3 AM PDT 
     Saturday for Central U.S. Waters Strait Of Juan De Fuca-
     East Entrance U.S. Waters Strait Of Juan De Fuca.