High Wind in Washington, September 17 to 18, 2021

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

.UPDATE...Morning forecast is largely on track, with only minor
tweaks needed to the winds. A warm front moving through the area
will continue to bring more widespread rain to the region this
morning and afternoon, ahead of a cold front that is slated to
push through the region tonight and overnight. Latest observations
already indicate half an inch of rain has fallen for areas along
the coast and a quarter inch of rain has already fallen for areas
across the northern interior (primarily Whatcom Co) as of 9 AM.
The previous discussion follows below, highlighting the impacts
expected from this system through the weekend. Updates to the
aviation, marine, and hydro sections follow. 14

&&

.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. River
flooding is not expected at this time but sharp rises on areas
rivers are likely. 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 the area
today and tonight with southwest flow aloft. The air mass will be
stable today then become unstable near the coast tonight.
Widespread ceilings 4000-6000 feet this morning should lower to
1500-2500 feet this afternoon and evening with locally lower
ceilings. Visibilities will be reduced at times in rain. Southerly
winds will increase today and become gusty this afternoon and
evening.

KSEA...Ceilings 4000-5000 feet this morning could lift a bit for a
few hours this afternoon as a warm front shifts a little north
then should lower to 1500-2500 feet this evening as a cold front
moves through. Southerly winds 5-10 knots will rise to 12-18 knots
with gusts 25-30 knots late this afternoon and evening.

&&

.MARINE...A vigorous frontal system will bring gale force
southerly winds to most waters today. A trough will move trough
the waters over the weekend. Winds are generally expected to
remain below advisory strength although the Coastal Waters could
see some southerly small craft advisory strength winds on
Saturday. High pressure offshore will produce light onshore flow
Monday and Tuesday. A weak front could reach the area on
Wednesday.

&&

.HYDROLOGY...A vigorous frontal system will bring heavy rains to
the area through tonight. River flooding is not expected at this
time but sharp rises on areas rivers are likely. Due to the dry
soils there is some uncertainty as to how long it will take for the
water to absorb into the banks and how the rivers will respond to
the sudden influx. As a result, river conditions throughout the area
will be monitored carefully especially those more flood prone rivers
in the area, with a focus on the Skokomish River. Localized minor
urban flooding is also possible on Friday in areas of heavier rains.

&&

.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.

&&

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