Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 181019

Area Forecast Discussion
National Weather Service Seattle WA
319 AM PDT Sat Sep 18 2021

.SYNOPSIS...Active weather to persist today and Sunday with
additional  rainfall, upper elevation mountain snow, breezy
conditions and thunderstorms. Drier and warmer conditions expected
on Monday and Tuesday before another system affects the region
midweek. Drier weather for the end of the week.


.SHORT TERM /TODAY THROUGH TUESDAY/...Moderate rain continues
across  Western Washington at this hour as the cold front begins
to move inland at this hour. As of 100 AM local time, up to 1" of
rain has fallen at locations in an around Puget Sound, with
higher totals in the 1 to 3" range in portions of the Cascades,
Olympics and along the outer coast. Several observation sites set
record rainfall totals yesterday, including Bellingham (1.45") and
Quillayute (3.05"). Expect persistent moderate rain to continue
through mid-morning, before tapering off from west to east as the
cold front clears the region. An additional 0.5 to 1" of rain is
possible in most locations, with isolated totals higher than 1" in
the mountains through this morning.

*Thunderstorms: The upper level trough responsible for this
 weekend`s weather will push over the Pacific Northwest later
 today, cooling mid-level temperatures down to around -25 degrees
 C by this afternoon. This, coupled with the potential for some
 sun breaks behind the front later this morning will yield
 sufficient instability to support thunderstorms this afternoon.
 The HREF has been consistent in indicating the highest
 probabilities for thunder occuring between 18 and 00Z today, with
 CAPE values peaking around 500 J/KG in portions of Western
 Washington. The one limiting factor with this setup is that the
 best bulk shear will be east of the area when conditions are most
 favorable for convection. Nonetheless, roughly 25 to 30 kts of
 effective bulk shear will be enough to support more robust
 updrafts.The greatest threats from any thunderstorm this
 afternoon will be dangerous lightning, heavy rain (rainfall rates
 up to 1"/hr at times possible), gusty winds, and small hail with
 the strongest updrafts.

*Additional Precipitation: Outside of the additional rainfall
 this morning mentioned above, thunderstorms will yield isolated
 higher amounts in portions of the region today, as rainfall rates
 will vary from storm to storm. In general, an additional 0.5 to
 1" of rain could be expected this afternoon through Sunday
 morning with this activity. Snow levels are expected in the 6000
 to 7000 foot range today through Monday, with impactful snow
 amounts expected. Anyone recreating outdoors in high elevations
 should stay tuned to the forecast and be prepared for snowfall
 accumulation at elevation today, especially at Mt. Rainier
 National Park.

*Wind: The heaviest winds associated with this system passed last
 night, with numerous instances of power outages and tree/limb
 damage across the region. Breezy south to southwesterly winds
 will persist through the day, particularly in any breaks in
 precipitation where strong winds aloft could mix to the surface.
 Occasional wind gusts to 30 mph will be possible through this
 afternoon. Stronger wind gusts possible under any thunderstorms.

Active weather will persist into Sunday with the passage of another
upper level shortwave trough late Saturday night/early Sunday
morning. Additional chances for thunderstorms are expected during
the day once again as adequate instability will be in place once
again with sunbreaks. HREF guidance currently indicating the
potential for a convergence zone Sunday evening. Heavy rain,
thunder, and small hail would be the main threats associated with
the development of a convergence zone.

Shortwave ridging will begin to build into the Northwest Monday and
Tuesday bringing a return of drier and slightly warmer weather.
Tuesday would be the warmest day, with temperatures likely reaching
the low 70s in parts of Western Washington.

.LONG TERM /WEDNESDAY THROUGH FRIDAY/...Dry and quiet weather will be
short-lived across the region as model guidance is in good agreement
in another system moving across the region on Wednesday, bringing
another round of rain and breezy conditions to Western Washington.
Drier conditions look to resume Thursday through the end of the



.AVIATION...The front over Western Washington early this morning
will move inland today and the rain will turn to showers by
daybreak. The flow aloft is southwest and the air is moist and
becoming unstable with a chance of a few thunderstorms. An upper
trough is just offshore and the trough axis will move ashore around
daybreak Sunday. Areas of MVFR cigs with the shower activity today
and the mountains will be mostly obscured.

KSEA...Showery with a southerly breeze today and occasional MVFR
cigs this morning.



.MARINE...A front will move inland today. The associated 985mb low
pressure center is near Haida Gwaii and the trough of low pressure
extending south just offshore will gradually weaken over the
weekend. High pressure will build over the area Monday and Tuesday.
A weak front could reach the area Wednesday.



.HYDROLOGY...Additional rainfall expected today and Sunday, with
the heaviest rainfall coming from thunderstorms. 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 will be
possible under any thunderstorms, as rainfall rates could approach
1"/hr at times.


PZ...Small Craft Advisory until 5 PM PDT this afternoon for 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.

     Small Craft Advisory until 9 AM PDT this morning for Central
     U.S. Waters Strait Of Juan De Fuca-East Entrance U.S.
     Waters Strait Of Juan De Fuca.

     Small Craft Advisory until 11 AM PDT this morning for Northern
     Inland Waters Including The San Juan Islands.

     Small Craft Advisory until 3 PM PDT this afternoon for Puget
     Sound and Hood Canal-West Entrance U.S. Waters Strait Of
     Juan De Fuca.


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