566
FXUS66 KSEW 170322
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
822 PM PDT Tue Jul 16 2024

.UPDATE...Clear skies for western Washington this evening with
continued warm temperatures. Mid to high clouds are slowly sliding
northward across the Oregon/Washington border ahead of an upper
trough which will move through on Wednesday, serving as the
catalyst for convective development over the mountains in the
afternoon. Clouds will increase into Wednesday morning, perhaps
associated with a few weak showers, mainly over the mountains and
coastal waters. Otherwise, warm overnight temperatures are
expected with lows in the upper 50s to mid 60s. Only adjustment
is to include haze/smoke into the North Cascades due to southerly
flow aloft from the Pioneer Fire tonight.

&&

.SYNOPSIS...An upper level disturbance will pass through
Wednesday with a slight chance of scattered thunderstorms along
the Cascades, Olympics and coastal waters. There is increased
fire danger for the Cascades and Olympics during the day
Wednesday due to the thunderstorms. Remainder of the week stays
warm and dry, with the potential for cooler temperatures early
next week with an upper level low passing by to the northwest of
Washington.

&&

.SHORT TERM /TONIGHT THROUGH SATURDAY/...Upper level pattern
continues to show upper level ridging, with the axis just to the
east now over western Montana, and southwest flow aloft over much
of the state. At the surface a thermal trough continues to build
up along the coast of the state. Satellite remains relatively
clear over the western part of Washington, with a few mid-level
cumulus clouds over the north Cascades, and a cloud deck of
altocumulus over the coastal waters. Temperatures have already
soared into the mid and upper 80s in south Puget Sound this
afternoon (with a few 90s in the Southwest Interior). Lows tonight
will be noticeably warmer than this morning, with more widespread
low 60s and a few upper 50s (warmest temperatures east of Puget
Sound over to the Cascades, and the Olympics). Heat risk remains
predominately orange (moderate) across the region (although a
couple isolated areas may see red/major heat risk).

The pattern changes slightly for Wednesday as a small upper level
trough moves up into the state from Oregon/Pacific. Cloud cover
will increase as the trough approaches early Wednesday morning. An
unstable airmass in place (via the thermal trough) will help bring
ingredients in place (CAPE of 800-1600 J/kg in the Cascades &
Olympics, limited PWAT with 1 inch, and up to 100 m2/s2 of 0-3 km
SRH) for a slight chance of elevated scattered thunderstorms. The
best chance of seeing a storm will be over the coastal waters
early Wednesday morning, and the Cascade/Olympic foothills
Wednesday afternoon and evening. No severe weather is expected,
but given the drier nature of the storms, there is an increase in
fire danger due to the potential for several cloud to ground
lightning strikes (see the fire discussion below for further
details).

The high temperatures for Wednesday will depend on how
much marine air/stratus makes its way into the sound during the
morning. The model spread in high temperatures remains relatively
large. If the cloud cover rolls in Wednesday morning (most likely
scenario), highs will range from the low to mid 80s. If there is
no cloud cover in the morning, there is a 25% chance (per NBM)
that areas south of Puget Sound may reach low 90s. Heat risk for
Wednesday will be a mix of minor (yellow) and moderate (orange),
but may increase as temperature confidence changes (based on cloud
cover Wednesday morning).

Thursday and Friday will remain dry as weak ridging builds back
into the region Thursday. A few models have a weak trough passing
on Friday, but at this time it appears to pass through dry. High
temperatures remain in the low to mid 80s for inland areas, with
highs in the 60s along the coast. Light southwest winds Thursday
should help make the air feel cooler to persons outside. Lows will
bottom out in the mid and upper 50s both nights as well.

.LONG TERM /SUNDAY THROUGH WEDNESDAY/...The pattern is expected to
remain warm and dry through the weekend. Saturday appears to be
the warmest day with the ridge from Friday builds over eastern
Washington (widespread upper 80s and low 90s). The forecast cools
down slightly going into next week as the ridge moves eastward,
and a low passing to the northwest returns stronger onshore flow.
There are more ensemble/deterministic models in agreement with
this pattern change, so high temperatures as a result will drop
into the low 80s Monday, and potentially the upper 70s Tuesday.
The threat of precipitation is very low in this period.

HPR

&&

.AVIATION...Southerly flow aloft with an upper ridge across the
Pacific Northwest. An upper trough will slide northward across the
area into Wednesday. Clear skies this evening with lingering
stratus along the coast. LIFR/IFR stratus will spread inland into
portions of the Southwest Interior tonight, and generally remain
west of Puget Sound. Otherwise, mid to high clouds will also
increase from the south to north into Wednesday morning.
Thunderstorms are expected to develop late Wednesday morning
through the evening across the Cascades and potentially the
Olympics. Convection is expected to remain confined to the
mountains during this period. Light N/NW winds this evening will
become light, variable into Wednesday morning before increasing
more W/SW on Wednesday.

KSEA...Clear skies this evening. Increasing VFR mid to high clouds
later tonight into Wednesday morning. Thunderstorms are expected
to develop and remain over the Cascades Wednesday afternoon. N/NE
winds into tonight will become more variable Wednesday morning,
before increasing W/SW during the afternoon. JD

&&

.MARINE...Light northerly onshore gradients strengthening Wednesday
into Wednesday evening. High pressure will remain over area waters
Thursday through the weekend with varying degrees of onshore flow. A
slight chance of thunderstorms early Wednesday morning over the
coastal waters may result in locally gusty winds.

Otherwise, strongest winds continue to be expected over the Central
and Eastern Strait of Juan de Fuca Wednesday afternoon into early
Thursday morning - with Small craft advisory westerlies. Seas 4 to 6
feet will linger generally over the coastal waters through the
forecast period.

&&

.FIRE WEATHER...The overall pattern continues to be driven by very
persistent high pressure over the Pacific Northwest with warm and
dry conditions. This has resulted in elevated overall fire weather
conditions over the region.  Low-level onshore flow across the
region has maintained the marine layer helping limiting minimum
relative humidities and supporting good overnight relative humidity
recoveries.

The primary issue of concern remains the potential for thunderstorm
development Wednesday as an upper-level disturbance moves across the
region. This will enhance instability and provide for thunderstorm
development over the Cascades and Olympic Mountains - resulting in
critical fire weather conditions.

High resolution ensembles, like the HREF have shown uncalibrated 6
hour probabilities of lightning as high as 65% along and near the
Cascade Crest Wednesday afternoon and again Wednesday evening.
Similarly, the HREF has produced 6 hour probabilities of lightning
as high as 40% over the core of the Olympic mountains during the
afternoon and again over the evening hours. Given the overall
unstable conditions, fuel conditions and lightning risk with very
low chances of wetting rains a Red Flag Warning has been issued for
zones 658, 659, 652 and 661.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Red Flag Warning from 8 AM to 11 PM PDT Wednesday for East
     Portion of the Olympic Mountains-West Portion of the
     Olympic Mountains-West Slopes of the Central Cascades
     Generally above 1500 Feet-West Slopes of the North Cascades
     Generally above 1500 Feet.

PZ...Small Craft Advisory from noon Wednesday to 5 AM PDT Thursday
     for Central U.S. Waters Strait Of Juan De Fuca-East
     Entrance U.S. Waters Strait Of Juan De Fuca.

&&

$$