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Forecast Discussion for Seattle/Tacoma, WA

FXUS66 KSEW 210926

Area Forecast Discussion
National Weather Service Seattle WA
226 AM PDT Fri Jul 21 2017

A strong upper level low pressure will track into the central BC
coast today into Saturday. This will bring showers to the north
coast areas but remaining dry elsewhere. A ridge of high pressure
will gradually build back north into the area late Saturday and
push the rain out of the area. This will bring return of dry and
warm weather to the area with near normal temperatures most of the
coming week. No significant rain chances expected through the
coming week.


Showers produced mainly spotty and light rainfall amounts on
Thursday across Western Washington. Most areas that saw any rain
were less than a tenth of an inch, with Seattle itself not seeing
any rain at all. Some spots in the foothills, coast, and in the
North Cascades, received more rainfall, with up to a half inch
falling in higher elevation spots in Whatcom County. The system
responsible for the showers has lifted well northeast of the area.
Overnight a quick moving shortwave ridge has lifted through the
region. A more substantial mid level low pressure center is now
pushing into the north central BC coast with showers showing up
off the coast at this time. The cloud shield is moving quickly
east with this system. However this system will not be much impact
for the area. Heights today will actually rise across the area and
showers will be confined to the north coastal areas. Otherwise it
will be mostly cloudy with temperatures remaining near normal.
Heights will remain high through Saturday and limit rain to the
north coast areas still, as the main mid level low pressure system
stays well north of the area. Some moist low level advection on
Saturday morning will push surface dewpoints to near 60 degrees
across the southwest, and may help to push showers a little
farther south on the coast then they were on Friday. But the
system will have little impact outside of the coast and most areas
Saturday will actually be warmer as a mid level ridge builds
north. This will push heights over 584 dm from the Sound
southward and allow for low 80s to return. As the low over the
north central BC coast weakens quickly and lifts inland Saturday
night, any showers will end rapidly and a warm night will be
expected with surface moisture remaining high. Sunday will bring
little change in the weather as heights fall a little but no
major features to impact the region, so expect another warmer than
normal day.

Key Short Term Messages:
The weak system tonight and Saturday will bring rain chances to
the north coast and interior north of Everett. Any rain would be
quite light except for spots on the north coast.


The coming week will start off with warmer than normal and quiet
weather conditions. Low level onshore flow will persist and
heights will remain around 580+ DM for the area. This will bring
interior temps into the 80s again. An upper low is set to push
into BC on Wednesday which will bring a pronounced fall in mid
level heights as well as strengthen the onshore flow. This combo
will drop temps to below normal on Wednesday. The GFS model
continues to show spotty shower activity on the southern edge of
the BC low on Wednesday but other models are dry. Will not carry
rain chances at this time. Temps should moderate some late week.
If there are no rain showers Wednesday, then the dry streak should
have no problem reaching past 40 consecutive days.

Key Long Term Messages:
Dry and warm weather will persist through the coming week. There
may be some showers around on Wednesday but the chances are low.
The dry streak will likely continue.



.AVIATION...Westerly flow aloft over Western Washington early this
morning will continue through tonight, as a flat upper ridge
builds over the Pacific Northwest. Light onshore flow will
continue at the surface. The air mass will remain somewhat moist,
mainly below 6000 ft.

With the exception of a BKN-SCT035 cloud layer from around KPAE
northward, skies were mainly clear below 12,000 ft. Some clouds
should form in the lower moist layer during the next few hours
for areas of MVFR ceilings, probably with bases 012-020. With the
exception of the north coast -- where lower conditions are likely
to continue through tonight -- the low clouds that do form should
lift and clear during the late morning, followed by VFR
conditions through tonight.

KSEA...Light wind becoming south to southwest 3-8 kt this morning,
then veering to northerly this evening. A MVFR cloud layer with
bases around 015-020 will probably affect the terminal, at least
intermittently, 12Z-18Z. However, confidence that this will occur
has decreased during the past few hours. McDonnal


.MARINE...Light onshore flow will prevail across Western
Washington today, as a small ridge sits along the coast. A weak
front will move into the offshore waters today and dissipate
offshore Saturday. Marginal small craft advisory southerlies are
possible tonight over the northwest part of the coastal waters.

Stronger onshore flow will develop Sunday and continue into
Tuesday, with high pressure offshore and lower pressure inland.
Small craft advisory northwest winds are likely at times over the
coastal waters, with small craft advisory westerlies in the
Central and Eastern Strait of Juan de Fuca. McDonnal




You can see an illustrated version of this discussion at

NWS Seattle/Tacoma, WA (SEW) Office Area Forecast Discussion

Discussion Script by Ken True at Saratoga Weather - WFO Script by SE Lincoln Weather