Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 191603
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
903 AM PDT Thu Sep 19 2019

.SYNOPSIS...A weak upper ridge will build inland today. A weak
system will slide down the back of the ridge tonight and Friday for
a slight chance of showers. Better chances for rain are expected
Saturday night and Sunday as frontal system arrives.

&&

.SHORT TERM /TODAY THROUGH SATURDAY/...Low level stratus and patchy
fog has formed across portions of the western WA lowlands this
morning with visibility down to a mile or less in spots. The fog
is shallow and we should see improving conditions late this
morning. Otherwise, expect a mix of clouds today but dry
conditions. 33

Previous discussion...A weak upper ridge will be the primary
weather feature of the day as it builds over W WA...keeping
conditions dry and partly to mostly sunny. Models remain
persistent in showing an extremely weak feature hot on its heels
though...dipping down into the area tonight and into Friday...but
impressive is not one of the first thousand words to describe this
feature. The ECMWF plays things a little wetter with it...but
ultimately the best moisture remains just along the border of the
PQR/SEW CWAs. Ultimately...POPs with this feature will remain
pretty low with the most widespread impact simply being increased
cloud cover. Another round of ridging builds in for Friday night
and into Saturday but both flattens and moves eastward rather
quickly with the ridge axis east of the Cascades by Saturday
afternoon. Models remain in good agreement regarding next frontal
system expected to reach the coast Saturday night.

Afternoon high temperatures will remain fairly static throughout the
short term...ranging in the mid to upper 60s for lowland
locations. SMR

.LONG TERM /SUNDAY THROUGH WEDNESDAY/...Previous discussion...The
aforementioned frontal system will bring widespread rain to the
CWA for the bulk of Sunday before it moves east of the Cascades
Sunday evening and showers taper off Sunday night. An upper level
ridge does follow closely behind but like the ridges mentioned in
the short term...does not hang around for any prolonged amount of
time...making a hasty exit Monday evening. The system behind that
ridge is a point of contention for the models...with the GFS
fizzling the system out pretty quickly while the ECMWF also
weakens the system as it passes through but slowly enough so that
precip with the system would still be a very real possibility.
Opted to take an average of these two solutions...leaning maybe a
bit more toward the drier GFS. THe gulf in model solutions widens
for Wednesday as the GFS has a so-weak-as- to-nearly-be-zonal-flow
ridge over W WA while the ECMWF keeps a troughier pattern
overhead. Having leaned toward the GFS for Tuesday...opted to
follow suit for Wednesday as well. SMR

&&

.AVIATION...Northerly flow aloft will persist today as an upper
level ridge over the offshore waters builds into the area and an
upper level trough continues to dig into the Great Basin. Low
level flow will remain weakly onshore. Some moisture in the low
levels has allowed for fog development across the area during the
overnight and early morning hours. Current observations indicate
a mixed bag of ceilings across the region, with areas either VFR
or IFR/LIFR in areas with fog. Expect areas of IFR/LIFR to dissipate
by late this morning and giving way to VFR conditions this afternoon.
High clouds will increase this afternoon ahead of a weather system
moving into British Columbia.

KSEA...Satellite imagery has indicated fog has made it into the
terminal this morning, yielding IFR/LIFR ceilings at times.
Expect fog to dissipate by 18-19Z, yielding VFR conditions for
the afternoon hours and into the evening. Surface winds will
remain light and variable becoming S-SW 7 knots or less from mid
morning through the afternoon. 27/14

&&

.MARINE...Relatively light onshore flow will prevail through
Friday with high pressure offshore and lower pressure inland.
Onshore flow will be enough for low-end small craft advisory
westerlies in the central and east strait this evening. A frontal
system will then approach from the west on Saturday and then move
through the area Saturday night into Sunday. 27/14

&&

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

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory from 5 PM this afternoon to 2 AM PDT Friday
     for Central U.S. Waters Strait Of Juan De Fuca-East
     Entrance U.S. Waters Strait Of Juan De Fuca.

&&

$$

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