Forecast Discussion for SEW NWS Office
000
FXUS66 KSEW 171039 CCA
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
310 AM PDT Fri Sep 17 2021

.SYNOPSIS...A strong frontal system will move through Western
Washington today with active weather expected throughout the
weekend. High rainfall amounts and windy conditions will be the
primary concerns. Conditions will quiet down for the early portion
of next week. Another weather system is expected mid week.

&&

.SHORT TERM /TODAY THROUGH SUNDAY/...Current satellite imagery shows
plenty of clouds pushing into the area ahead of the strong frontal
system expected for today. Matching radar showing echoes stretching
inland as far as Puget Sound...however only obs along the coast
showing precip at this time.

Models and ensembles remain in good agreement for the short term,
the front will slowly make its way through the area today, crossing
the Cascades into E WA late tonight/early Saturday. The associated
large upper trough will follow right behind keeping the area wet
with showers as well as introducing some instability for some
thunder potential. This trough ejects from the area by Sunday
afternoon, however enough moisture and instability linger to keep
conditions wet for the remainder of the weekend. High temps during
this period will cool...today being the warmest with highs in the
mid to upper 60s, Saturday on average 3-5 degrees cooler then low
60s area-wide on Sunday. For details on the impacts from this
system, read on to the elements listed below:

Winds: Southerly winds increasing throughout the day, peaking in the
late afternoon/early evening hours. The windiest spots still
expected to be the coast, San Juan Islands, Island County, as well
as Western Whatcom and Skagit Counties. These locations likely to
see sustained speeds of 25-35 mph with gusts 40-50 mph, particularly
adjacent to the waters. As such, inherited Wind Advisory still looks
on track and will remain unaltered. Winds inland will be
significantly less than directly near the waters, with sustained
speeds 15-25 mph and gusts 25-35 mph. It is worth noting that as
trees still have their leaves and being weak from the summer heat
and lack of moisture, some tree damage and power outages in the
region will be possible. Once the front passes tonight, winds will
gradually ease but breezy conditions are expected on Saturday.

Precipitation: Rain will continue to move inland as already
mentioned above. The coast and remainder of the Olympic Peninsula
will likely see rain throughout the morning while locations east of
the Sound may see some rain start up in the afternoon, but best
chances will hold off until late this afternoon/early this evening.
Rainfall intensity will initially be light, but is expected to pick
up as the front moves through. In general 1-2 inches is expected to
fall between Friday and Sunday with the majority falling on Friday
and early Saturday. As usual the coast and mountains will see higher
rainfall amounts with 2-4 inches and 4-6 inches respectively. With
soils being so dry and compact there will be a bit of a delay in
water being soaked up by the ground. As a result, ponding of water
is possible especially at the start of the heaviest rain. Urban
flooding may also be a concern if storm drains are not cleared out
or become blocked due to debris created from the winds. No river
flooding is expected, see hydrology section below for more details.
Snow levels today will not be much of an issue today, being well
above 10,000 feet. With frontal passage however, they are expected
to fall to around 6000 feet by Saturday, allowing for snowfall and
accumulation on the higher ridgetops within the CWA.

Thunderstorms: Dynamics do not really seem to line up well for any
thunder as the front moves through today...instead waiting for the
parent trough to start kicking up some instability. Thus, no
expectation for thunderstorms today, however a slight chance is
still expected for Saturday and even into Sunday as the trough
finally makes its way out of the area. Lightning, brief heavy rain,
small hail and gusty winds are all possible with any storms that do
develop.

All in all, a very active weekend made seemingly even more so given
the dry summer W WA is coming out of.  18

.LONG TERM /MONDAY THROUGH THURSDAY/...Generally dry conditions will
start off the week as an upper level ridge builds over the area
Monday and Tuesday. Deterministic models disagree on the amplitude
of the ridge, thus allowing ensembles to latch onto the prospect of
slight chance PoPs being possible for the northern portions of the
CWA. The ridge axis moves east of the Cascades midday Tuesday
allowing the feature to continue to be the main influence on W WA
weather at least into Tuesday evening. Some disagreement on how
quickly the next system enters, but looking for some chance PoPs
over the area for Wednesday as a weaker front pushes
through...dissipating as it moves eastward. Thursday looks to be a
return to dry conditions.  18

&&

.AVIATION...A vigorous frontal system will move through Western
Washington today and tonight. Cigs and vsby will drop as the rain
moves ashore this morning--especially along the coast, over the
Olympic Peninsula, and north of Seattle. For many areas there will
be a lull in the rain between the warm front and the cold front this
afternoon--but it will be windy in the warm sector. The windy
weather will occur in the normal windier spots for which there are
wind advisories. The cold front arrives this evening with poor vsby
and low cigs in the heavier rain. The mountains and foothills will
become obscured pretty quickly today in the warm front and
conditions will drop again later today with the cold front with
widespread MVFR and areas of IFR conditions and layers of merging
layers of frontal clouds. 19

KSEA...Occasional light rain today with breezy southerly wind. The
poorest vsby and lowest cigs will be this evening with the cold
front passage.

&&

.MARINE...Gale warnings are up for the typically windier areas like
the coastal and northern waters--including the Admiralty Inlet area
as a vigorous frontal system moves through the area today and
tonight. A trough will move ashore over the weekend with onshore
flow easing as high pressure builds into the area Monday and
Tuesday. A cold front will reach the area around daybreak Wednesday
but it will likely be much weaker than today`s system. 19


&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Wind Advisory until 8 PM PDT this evening for Admiralty Inlet
     Area-Central Coast-North Coast-San Juan County-Western
     Skagit County-Western Whatcom County.

PZ...Gale Warning until 11 PM PDT this evening for Admiralty Inlet-
     Coastal Waters From Cape Flattery To James Island 10 To 60
     Nm-Coastal Waters From Cape Flattery To James Island Out 10
     Nm-Coastal Waters From James Island To Point Grenville 10
     To 60 Nm-Coastal Waters From James Island To Point
     Grenville Out 10 Nm-Coastal Waters From Point Grenville To
     Cape Shoalwater 10 To 60 Nm-Coastal Waters From Point
     Grenville To Cape Shoalwater Out 10 Nm-East Entrance U.S.
     Waters Strait Of Juan De Fuca-Northern Inland Waters
     Including The San Juan Islands-West Entrance U.S. Waters
     Strait Of Juan De Fuca.

     Small Craft Advisory until 11 PM PDT this evening for Central
     U.S. Waters Strait Of Juan De Fuca-Puget Sound and Hood
     Canal.

&&

$$

www.weather.gov/seattle

NWS SEW Office Area Forecast Discussion