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000
FXUS66 KSEW 060333
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
833 PM PDT Wed May 5 2021

.UPDATE...No changes needed to the forecast this evening. The
larger scale pattern this evening is characterized by a large
scale trough over the Pacific, with ridging slowly departing the
Pacific Northwest towards the Intermountain West. The Pacific
trough will continue to approach the area overnight and into
Thursday morning, with a sfc frontal system accompanying it.
Enough moisture and lift will be maintained for an organized band
of rain to approach the WA by mid-morning Thursday. Rain along
the front will have some trouble holding together as it pushes
east towards Puget Sound late morning into the afternoon.
Nevertheless, rain chances will increase and continue through the
afternoon hours. Scattered shower activity is likely behind the
front throughout the latter half of tomorrow, with instability
remaining too low for isolated thunderstorm activity.

Previous discussion can be referenced below, with an updated
marine and aviation section.

Kovacik

&&

.PREV DISCUSSION... /issued 259 PM PDT Wed May 5 2021/

.SYNOPSIS...Dry conditions will continue through tonight, before
a return to wet and unsettled conditions resume Thursday as a
frontal system will brings rain, breezy winds, and cooler
temperatures to western Washington. Showers will persist Friday
and into the weekend as an upper level trough traverses the Inland
Northwest and a weak system moves into the area. Drying trend
commences Sunday into early next week as upper level ridging
strengthens across the region.

&&

.SHORT TERM /TONIGHT THROUGH SATURDAY/...Satellite shows a
broken layer of high clouds moving over western Washington this
afternoon ahead of the next system offshore. Meanwhile, latest
observations show temperatures as of 2PM currently trending in the
low to mid 60s across the region. Given ample cloud cover, have
trended the temperature forecast down a couple of degrees this
afternoon and have highs topping out in mid to upper 60s for areas
in the vicinity of the Sound. A few spots could see 70 with offshore
flow, but this will likely be near the foothills of the Cascades.
Overall, expect the remainder of the evening to remain dry today,
with increasing cloud cover ahead of the system offshore.

A frontal system will approach the region and move inland on
Thursday, bringing a round of rain and breezy winds to western
Washington. Rainfall amounts look to be a couple hundredths to a
few tenths of an inch for the lowlands at this time. Southerly
winds will become breezy late Thursday morning and look to peak
across the region by late afternoon into early evening. Breeziest
areas look to remain along the coast and the north interior along
the frontal passage and along the Strait of Juan de Fuca with a
surge of post- frontal onshore flow. Highs are still expected to
be above normal and in the 60s on Thursday.

Rain will become more showery on Friday as an upper level trough
moves inland and becomes slightly negatively tilted. Cooler air
aloft will result in steepening lapse rates, increasing instability
across the area in the post-frontal environment, so have kept a
slight chance of thunder for areas across the interior late Friday
morning into Friday afternoon. Guidance is still on track
indicating that Friday will be the coolest day of the week-with
afternoon temperatures only topping out in the low to upper 50s.

The upper level trough will continue to move east-southeastward
into the Intermountain West on Saturday, while upper level ridging
across the eastern Pacific starts to build back into the Pacific
Northwest. Latest guidance still hinting at showers persisting
over the weekend as a weak system rides the periphery of the
ridge and moves into the area under northwesterly flow aloft. Rain
amounts look to remain rather light at this time, with
temperatures remaining cool- primarily in the 50s.

.LONG TERM /SUNDAY THROUGH WEDNESDAY/...Latest guidance still
hinting at light showers persisting into Sunday, however expect an
overall drying trend to commence late in the day as upper level
ridging across the eastern Pacific starts to strengthen and build
across the area. Ensemble guidance then shows a general drier
trend persisting through the first part of next week. Highs will
start to creep back up above normal early next week-back in the
mid to upper 60s. 14

&&

.AVIATION...Abundant cloud cover is prevailing over the region this
evening ahead of the approaching frontal system. Surface winds will
be fairly variable, transitioning to southwesterly in the early
morning hours as the front arrives onshore.

VFR conditions in place for all terminals for much of the next 24
hours. Some MVFR cigs are possible overnight and early Thursday
morning along the coast, particularly near KHQM. Surface winds will
begin to increase across the area out of the southwest with gusts
between 25-30 knots possible, particularly along the coast and
locations nearest the inland waters.

KSEA...VFR conditions through the period with increasing high clouds
this evening ahead of the next front. Prevailing wind out of the
north through this evening generally running 5-10 kts. Winds will
swing around to the south overnight, between 10-12Z, before becoming
more southwesterly. As the frontal system moves inland,
southwesterly winds will increase between 18-20Z. Gusts between 20
to 25 knots are possible through Thursday evening.

Kristell

&&

.MARINE...Expect flow to swing back to onshore flow late tonight
into Thursday morning and will increase to advisory levels for at
least the coastal waters and the preferred portions of the Strait
Thursday morning. Have stuck with this thinking even though latest
model runs coming in a little higher on wind speeds. A Small Craft
Advisory for most of the waters, excluding Puget Sound, will go into
effect at 12Z. The next shift will need to evaluate the possibility
for additional headlines tomorrow afternoon in the wake of the
front. Winds will remain elevated through Friday before gradually
decreasing. Strong nocturnal westerly pushes expected in the Strait
through the weekend. Another push of advisory level winds will be
possible offshore on Sunday with the passage of another system.

Offshore seas will increase Thursday afternoon, with westerly swells
building to 6 to 8 feet before gradually decreasing Friday through
the weekend. At least 2 to 4 foot wind waves will be possible across
portions of the inland waters Thursday and Friday. Another increase
in swells possible on Sunday with the passage of the next system.
Swells will remain below 10 feet.

18/Kristell

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory from 5 AM Thursday to 5 AM PDT Friday for
     Admiralty Inlet-Central U.S. Waters Strait Of Juan De Fuca-
     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.

&&

$$

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