Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 221604
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
904 AM PDT Mon Jul 22 2019

.UPDATE...A longwave trough extends roughly from the Gulf of
Alaska into the northern Pacific and towards the western US coast
this morning, with water vapor loops revealing numerous shorter
wave perturbations rotating about. Several of these can be seen
offshore the Washington Coast this morning in the vicinity of
130W. One of these continues to pull north into Vancouver Is, with
another trailing behind. This second, trailing wave will also pull
north before reaching the Washington Coast today, likely doing
nothing more than passing some mid and high clouds our way. A
frontal system is associated with the first wave, and that will
push thru western Wa later today and overnight.

Satellite loops this morning indicate benign weather in
place across western Washington this morning, though there is
noticeably more cloud cover. Starting with the Cascades, mid
level clouds were observed streaming northward, with a touch of
vertical extent noted. This indicates an unstable environment is
in place although shower and thunderstorm development appears to
be more favorable east of the Cascade crest. Elsewhere, some
marine stratus had developed across the SW interior and near Hood
Canal. This is expected to dissipate this morning with daytime
heating. A more expansive, deeper marine layer was seen across
the outer coastal waters and out to sea, along with cloud cover
associated with the cold front.

A mostly sunny and dry conditions can be expected today for most,
with best cloud cover across the Cascades, and stratus
redeveloping along the coast later in the day. Given the return
to onshore flow, temperatures will still be warm. but noticeable
cooler than the weekend, with upper 70s to near 80 the rule. Have
only made minor tweaks to the sky coverage for today with the rest
of the forecast on track.

See previous discussion below with updated marine and aviation
sections.

Kovacik

&&

.PREV DISCUSSION... /issued 230 AM PDT Mon Jul 22 2019/

.SYNOPSIS...Dry weather will continue into Monday with
temperatures slightly above normal. A cold front and upper level
trough will bring showers and a chance of thunderstorms in the
Cascades on Tuesday. Strong onshore flow will prevail on Wednesday
then weaken on Thursday. Mild weather will continue into the
weekend.

&&

.SHORT TERM /TODAY THROUGH WEDNESDAY/...Upper level ridging will
continue to weaken today as an upper trough over the Northeast
Pacific gradually shifts closer to the region. Low level onshore
flow has increased over the past 24 hours. This has brought some
shallow marine air inland. This combined with lowering heights will
knock around 10 degrees off of high temperatures over the interior
compared to yesterday. The main area of marine stratus is still
offshore early this morning, but should spread onto the immediate
coastline later this morning. For the remainder of the area, it will
be another sunny day ahead. Onshore flow increases tonight and
Tuesday as the upper trough offshore edges closer. This should
initiate a solid marine push tonight with stratus reaching most of
the lowlands by daybreak Tuesday. Temperatures will cool further on
Tuesday with the stubborn marine layer likely only yielding some
partial afternoon clearing over the interior lowlands. Increasingly
moist and unstable southerly flow aloft ahead of trough will produce
a slight chance of thunderstorms over the Cascades Tuesday afternoon
and evening. The upper trough will quickly exit to the east on
Wednesday with a flat upper ridge moving over the area. Any shower
threat should end by Wednesday morning. Highs will be near normal on
Wednesday.

.LONG TERM /THURSDAY THROUGH SUNDAY/...Low amplitude ridging will
produce dry conditions and slightly above average temperatures
Thursday into Friday with persistent low level onshore flow. The
models are advertising a weak trough passing mainly to our north
on Saturday. Apart from a chance of showers in the North Cascades,
it looks to do little more than deepen the marine layer and cool
temperatures a bit. Ridging weakly rebuilds for the later half of
the weekend for perhaps a slight warmup and continued dry weather.

27

&&

.AVIATION...Onshore flow will continue to increase today. A patch of
stratus made it in to Bremerton this morning, and there are a few
patches of low clouds elsewhere. There is also a solid deck of
marine stratus just off the coast which was not supposed to get to
the coast til evening--but there is a chance that will make it to
the coast for a few hours later this morning, then perhaps pull back
before coming ashore again late this afternoon or evening. The
marine clouds will be more extensive Tuesday morning. Also, there is
southerly flow aloft so there will be some mid and high level clouds
at times over the Cascades. The air is unstable over the Cascades.

KSEA...A patch of marine stratus made it to Bremerton this morning,
so it is a good bet that the stratus will make it to Seattle Tuesday
morning.

&&

.MARINE...Onshore flow will increase today and then continue for the
remainder of the week with high pressure offshore and lower pressure
inland. The westerly push in the Strait looks like it will be
stronger Tuesday evening--the UW wrfgfs shows gales in the Central
and Eastern Strait of Juan de Fuca Tuesday evening.

&&

.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...Small Craft Advisory from 5 PM this afternoon to 11 PM PDT this
     evening for San Juan County.

PZ...Gale Watch from Tuesday afternoon through late Tuesday night for
     Central U.S. Waters Strait Of Juan De Fuca-East Entrance
     U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory from noon today to 5 PM PDT Tuesday for
     East Entrance U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory from 5 PM this afternoon to 11 PM PDT this
     evening for Admiralty Inlet-Northern Inland Waters
     Including The San Juan Islands.

     Small Craft Advisory until 5 PM PDT Tuesday for Central U.S.
     Waters Strait Of Juan De Fuca.

&&

$$

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