Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 211043
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
330 AM PDT WED SEP 21 2016

.SYNOPSIS...An upper trough will gradually shift south of the
region the next couple days. The Western Washington lowlands will
have mostly sunny weather Wednesday and Thursday, with scattered
afternoon and evening showers over the mountains. A weak front
could bring some rain Friday, then a strong ridge will bring dry
and warmer weather Saturday through Monday.

&&

.SHORT TERM...Satellite fog product imagery shows some patchy fog in
the usual areas like the southwest interior and coast, but also
along parts of the I-5 corridor north to Bellingham. Otherwise, most
observation sites are reporting clear skies with cool temperatures
in the 40s to around 50, and even some upper 30s in cold pocket
locations like shelton. Any fog should dissipate quickly by mid
morning as temperatures begin to warm.

The upper trough over the Pacific Northwest has backed off the coast
with the closed 500mb low center moving into Oregon this afternoon.
Moisture is very limited over Western Washington, mainly surface
based which will make it hard to trigger showers. Some models like
the GFS20 shows some spotty light QPF over the mountains where there
will be some added lift along terrain. Will keep some low chance
pops in the mountains this afternoon, otherwise dry weather is
expected.

Thursday should be a dry day across the area including the
mountains. The closed upper low will move into the interior west and
moisture should remain to the south of our area. Upper heights will
rise briefly in the afternoon which should allow for a mostly sunny
day, expect for patchy morning fog once again.

The split in the upper level flow over the west will cause the next
front to weaken quickly as it reaches Western Washington. Most
models agree that rain will reach the coast late Thursday night or
Friday, then become spotty over the interior Friday afternoon. It is
possible the system could dissipate before it manages to make much
progress inland. Will keep the chance pops in place for the interior
with a higher likelihood of rain at least reaching the coast.

.LONG TERM...All the global models including the GFS/ECWMF/and
Canadian continue to show a strong ridge building north over the
region Friday night through the weekend. This will lift the flow
well to the north into B.C. keeping Washington dry. There will
likely be some areas of late night and morning fog to contend with,
but otherwise mostly sunny skies. Temperatures will warm
considerably, well into the 70s and possibly low 80s. Some models
show thermally induced low pressure along the Oregon coast possibly
reaching the Washington coast by Sunday which could induce some
offshore flow and give a further boost in temperatures. The current
extended forecast indicates mostly 70s to around 80 which seems
reasonable at this point.

Models show the ridge shifting inland on Tuesday with southwest flow
aloft over Washington. Some models show a weak system clipping the
area to the north but it does not appear significant enough to
mention precipitation yet. Most likely, it will induce onshore flow
and result in increased marine clouds and cooler temperatures.
Mercer

&&

.AVIATION...A weak upper level trough over Western Washington will
continue to dig southwest into the coastal waters of Oregon. Flow
aloft will remain light and northeasterly today, with northerly low
level flow. Showers overnight along the Cascades have diminished as
the air mass has dried and become more stable. Clear skies and light
winds have helped patchy IFR fog form in sheltered areas, especially
over the the South Puget Sound and Southwest Interior this morning.
Fog will burn off quickly later this morning. Showers will redevelop
over the Cascades this afternoon and evening as the upper level
trough begins to swing onshore into Oregon.

KSEA...VFR conditions this morning. Fog will likely form near the
terminal in the low lying and sheltered valleys to the east.
Northeasterly winds 4-6 KT will become Northerly 6-12 KT around
midday.

&&

.MARINE...Light northerly winds this morning will strengthen this
afternoon especially over the coastal waters, and become more
onshore this evening as higher pressure builds over the offshore
waters of Oregon and Washington. Mesoscale models continue to
highlight small craft advisory criteria winds over the central and
southern coastal zones late this afternoon and evening. Onshore flow
will develop further Thursday. Models show a front reaching the
coastal waters Friday with widespread small craft advisory southerly
winds becoming northwesterly late in the afternoon and evening.
Surface gradients and winds continue to look lighter through the
weekend as high pressure develops over the region.

&&

.SEW Watches/Warnings/Advisories...
WA...None.
PZ...small craft advisory central and southern coastal zones    late
this afternoon and evening.

&&

$$

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You can see an illustrated version of this discussion at
www.weather.gov/seattle/gafd/latest_webafd.html



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