007
FXUS66 KSEW 160940
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
240 AM PDT Tue Jul 16 2024

.SYNOPSIS...High pressure will continue to remain over the region
for the majority of the week. Onshore flow will continue to allow
the marine layer to move inland each night, with slight day-to-day
variations. An upper-level shortwave impulse will bring a chance for
elevated convection over the Cascades late tonight into Wednesday.
Upper level ridging will remain in place over the region through at
least this weekend as a longwave trough slowly begins to approach
from offshore into early next week.

&&

.SHORT TERM /TODAY THROUGH THURSDAY/...Latest satellite imagery
shows the marine stratus layer slowly creeping eastward, south of
the Olympic mountains into the lower Chehalis Valley and through
the Strait of Juan de Fuca and over Whidbey Island. Patchy fog is
being observed along the Pacific Coast and the Strait of Juan de
Fuca. Stratus and patchy fog will continue to push inland this
morning, but weaker pressure gradients compared to yesterday, it
will not be able to make it as far inland and skies should remain
mostly clear south of Everett, north of Skagit County, and north
of Tacoma. Low temperatures this morning should reach the low to
mid 50s.

With the warm low-level air mass advecting northward into the region
and with less marine stratus this morning, temperatures should warm
quickly across the region, with high temperatures 8 to 12 degrees
warmer compared to yesterday, reaching the mid 80s to near 90, with
low to mid 90s possible through the Southwest Interior and Cascade
valleys. Mid to upper level cloud cover will increase tonight as a
shortwave trough approaches the area from the southwest. This will
result in limited cooling overnight, with overnight lows in the low
to mid 60s for most of the region. As a result, Tuesday`s HeatRisk
values are Orange/Moderate for most of the region, with pockets of
Major/Red for the Cascade Valleys and the Duwamish/Green River
valley in the Seattle metro.

Wednesday will start cloudy as the shortwave trough passes over
the region, but skies will clear throughout the day. With the
shortwave trough directly overhead, this will be the day with the
best chance for elevated convection over the Cascades. There will
be some moisture to work with with PWATs from around 0.75 to 1 in
(around or slightly positive anomaly), however, it will be mostly
contained in the upper levels with a deep and dry boundary layer,
with little to no precipitation making it to the surface. The
marine layer will help maintain a low-level inversion over the
Olympics, though there may be a brief period in the late afternoon
when the inversion dissipates in the higher elevation locations
and the thermodynamic profile could be realized. Confidence and
probability of occurrence is much lower than over the Cascades.
The most likely scenario for temperatures is that cloud cover and
substantial onshore flow should keep high temperatures slightly
cooler than Tuesday, in the low to mid 80s, in the 70s along the
coast. This corresponds to Moderate/Orange HeatRisk through the
interior north of the Tacoma metro to the Canadian border. If
skies clear early, there`s a chance that temperatures could reach
the low to mid 90s through the interior and the Cascade valleys.
Probabilities for seeing a high temperature of 90 or greater reach
up to 30% across the south and eastern sides of the Seattle metro
area and into the eastern King County lowlands.

Thursday looks to be cooler with continued onshore flow behind
the shortwave trough, pushing the marine layer into the interior
Thursday morning.

.LONG TERM /FRIDAY THROUGH MONDAY/...The pattern look to remain
warm and dry with the ridge axis slowly moving eastward with highs
through the weekend remaining in the mid 70s to upper 80s and lows
in the low to mid 50s. Saturday looks to be the warmest day in the
long term with the ridge momentarily amplifying as it retrogrades
back towards the west coast. Deterministic and ensemble models
are showing a large upper level trough creeping in from the
northwest early next week, which could allow temperatures to drop
back into the 70s and lows in the upper 40s to low 50s into the
beginning of next week, though it depends if the trough will
actually able to push the ridge is actually able to sweep the
ridge away that quickly.

LH

&&

.AVIATION...Upper level ridge over Western Washington with
southwesterly flow aloft. Flow aloft becoming southerly later this
afternoon through tonight with upper level trough moving into
Western Oregon. Onshore flow in the lower levels this morning
becoming light northerly this afternoon and tonight. Increasing
low level onshore flow Wednesday.

Stratus along the coast, down the Strait of Juan de Fuca to near
Whidbey Island and inland over the Lower Chehalis Valley at 2
am/09z. Stratus with the weakening onshore flow will not make it
much further east this morning. The layer is shallow with tops
near 1000 feet. With the stratus, LIFR cigs with local visibility
2 sm or less in fog until 16z. Ceilings lifting to IFR 16z-18z
with the stratus clearing out 18z-20z. For the remainder of the
area clear skies.

Stratus reforming along the coast 03z-06z moving inland after 07z
tonight but staying west of Puget Sound. As upper level trough
moves into the northern portion of Western Oregon late tonight
increasing middle level clouds over the area. There is a chance
for a colorful sunrise Wednesday morning with the increasing alto
cumulus clouds.

KSEA...Clear skies until around 09z Wednesday. Increasing middle
level clouds through the morning hours Wednesday. Northerly winds
4 to 8 knots switching to southerly 09z-12z Wednesday. Felton

&&

.MARINE...Light onshore gradients this morning becoming northerly
this afternoon. Onshore gradients increasing later Wednesday
morning into Wednesday afternoon and evening. High pressure over
the waters Thursday through the weekend with varying degrees of
onshore flow.

Small craft advisory winds in the Central and Eastern Strait of
Juan de Fuca Wednesday evening into early Thursday morning and
possibly again Thursday evening. Felton

&&

.FIRE WEATHER...High pressure will persist over the Pacific
Northwest, maintaining warm and dry conditions. Low-level onshore
flow across the region will keep marine air present, limiting
minimum relative humidity values to 30-35%, with localized pockets
as low as 25%. Good relative humidity recovery is expected each
night.

The primary period of concern is Tuesday night into Wednesday as
an upper-level disturbance moves across the region, providing
ample instability for thunderstorm development over the Cascades.
With an unstable atmosphere, as well as lightning risk with very low
chances of wetting rains, a Fire Weather Watch remains in effect
for zones 658 and 659.

It is worth noting that with this pattern and very receptive
fuels, all it takes is a single rogue lightning strike from a
thunderstorm to ignite these fuels.

LH/Mazurkiewicz

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Fire Weather Watch from this evening through Wednesday evening
     for West Slopes of the Central Cascades Generally above
     1500 Feet-West Slopes of the North Cascades Generally above
     1500 Feet.

PZ...None.
&&

$$