Heat in Washington, July 4, 2023

Area Forecast Discussion
National Weather Service Seattle WA
905 AM PDT Tue Jul 4 2023

.SYNOPSIS...Dry and warm conditions continue across Western
Washington as high pressure remains over the region, maintaining
heightened fire weather concerns. Warming temperatures continue,
peaking Wednesday, before the high breaks down. This will bring
temperatures down closer to seasonal normals. Some afternoon and
evening showers are possible over the Cascade crest late in the week.

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.SHORT TERM /TODAY THROUGH THURSDAY/...A quick look at 24 hour
trends and the morning sounding out of KUIL shows that today's
forecast is on track. Some modest warming at 850 millibars suggests
high temps about 5 or 6 degrees warmer than yesterday for areas
away from the water. A layer of smoke remains aloft from Canadian fires,
but appears to be thin enough as to not have a significant effect on
high temperatures. A shallow marine layer is snaking its away
northward along the Oregon coast signaling cooler temps for the WA
coast tomorrow, but thermally induced low pressure will remain
across interior areas where high temps are likely to reach their
zenith on Wednesday. Previous discussion follows with updates to
marine and aviation portions.  27

As has been the case the past couple of mornings, much of W WA
seeing clear skies and temps generally in the 50s. Like 24 hours
ago, both Everett and Sea-Tac are the outliers with temps as of 2 AM
PDT still in the mid 60s.

High temps today expect to jump up 5-7 degrees on average today when
compared to temperatures yesterday as the upper level ridge nudges
eastward today. This will be the peak for locations along the coast
as temps there will get into the mid 70s to around 80. While the
interior will get toasty as well, ranging in the mid 80s to around
90, the highest temperatures are yet to come as an additional 1-3
degrees of warming are expected for Wednesday...bringing highs into
the upper 80s to lower 90s. In contrast, the coast and other water-
adjacent areas will seem temps fall a bit into the 70s. A
retrograding trough from Canada will push the ridge's influence
westward, allowing for the start of a cooling trend on Thursday.
Highs Thursday will drop on average 3-6 degrees areawide, but this
will be more noticeable along the coast where temps will dip back
down into the mid 60s to around 70. Inland, temps will remain above
average mainly in the 80s although the south Sound and SW interior
will still see temps in the upper 80s.

As has been consistent, even though temperatures will be above
seasonal normals for the short term, Heat Risk concerns remain
Moderate /orange/ for both today and Wednesday before dropping off
significantly Thursday. Given the holiday, it is still important to
remember it is still hot. Hydrate frequently with water and keep
outdoor exertion to a minimum if at all possible. If not, remember
to take frequent breaks so as not to run the risk of heat-related
illnesses.

Lastly, while a more in-depth discussion of Fire Weather concerns
will be addressed in that portion of this discussion, it is
noteworthy to mention that north to northwesterly flow may allow for
smoke from Canadian wildfires to drift into portions of W WA. This
smoke is expected to remain aloft, thus not giving any rise to air
quality concerns, but it may result in a little bit of haze in the
afternoon hours.  18

.LONG TERM /FRIDAY THROUGH MONDAY/...An upper level low over SW BC
Friday will not only help to facilitate increased onshore flow and
thus further cool temperatures into the mid to upper 70s
inland/lower to mid 60s along the coast, but also start to
reintroduce PoPs back into the area. Unfortunately, this does not
look to be much to get excited about, mainly some drizzle along the
coast or some afternoon and evening showers along the Cascade crest.
The remainder of the CWA will remain dry. Temps will reach their
nadir Saturday before warming begins on Sunday as the upper level
low ejects to its native Canada and W WA starts to fall once again
under the influence of the Pacific ridge. While high temps will
remain in the mid to upper 60s along the coast, interior locations
will see highs rise once again into the mid 70s to lower 80s.

&&

.AVIATION...Upper level ridging will continue to promote north to
northwesterly flow aloft today. Winds at the surface are
primarily north to northwesterly as well, except near the building
thermal trough along the coast - where winds have turned lightly
offshore. Winds will increase to 8-12 kt again by this afternoon
and will ease back down to 6 kt or less by tonight. Conditions
will remain VFR today. Satellite does show smoke over the region
this morning, though expect the majority of the smoke to remain
aloft - with no impacts expected at the area terminals.

KSEA...VFR today with north winds increasing to 8-12 kt, gusting
to 10-15 kt, primarily between 21-06z. 14

&&

.MARINE...A thermally induced surface trough will persist along
the Washington coast today, weakening the onshore flow into Wednesday.
North to northwesterly winds will persist for the majority of the
area waters, except for areas along the immediate coast (in the
vicinity of the trough) where flow has started to turn lightly
offshore. Winds will increase across the Coastal Waters tonight,
the strongest of which look to persist over the outer Coastal
Waters. A Small Craft Advisory remains in effect starting tonight
as a result. Stronger onshore flow will return Wednesday night
and then again Thursday night, with marine pushes likely bringing
a strong push of westerly winds down the Strait of Juan de Fuca -
where additional headlines will be likely.

Seas across the coastal waters look to hover at about 5-7 feet
into Wednesday, before building to 6-8 feet on Thursday. Seas
then look to subside down to 3-5 feet over the weekend. 14

&&

.FIRE WEATHER...Elevated fire weather conditions expected today and
Wednesday as hot, dry and unstable conditions remain in place over
much of W WA. The inherited Red Flag Warning for Fire Weather Zones
655 /SW interior and Chehalis River Valley/ and 659 /West Side of
the Central Cascades/ will remain in place as minimum RH values
there look to fall into critical thresholds. Additionally, will be
monitoring Fire Wx Zones 658 and 657 throughout this period as well,
given that RH values there are borderline, but generally expected to
remain above criteria. While RH values remain above criteria
elsewhere, given the holiday it is certainly worth mentioning that
the persistent hot and dry conditions have cured finer fuels and
made a good dent in the moisture in some of the larger fuels...thus
making the risk for fire starts high, especially when you factor in
burning embers from holiday fireworks.

It is worth mentioning that the prospect of smoke from Canadian
wildfires could actually prove to be a bit of a benefit, taking a
slight edge off the heating or slightly impairing instability.
Current forecast is hesitant in allowing those factors too much
purchase, however, it may be enough of a limiting factor to restrict
Red Flag conditions to those already under the Warning, possibly
making any additional expansion difficult.

Once the ridge breaks down Thursday, RH values climb both pretty
quickly and substantially with no additional headlines needed for
the remainder of the week, the possible exception being if
convection over the weekend results in the formation of isolated
thunderstorms over the Cascades.  18

&&

.CLIMATE...Forecast high for Seattle of 87 degrees on the 4th not
a record ( record 92 in 2015 ) but it would be the 7th warmest 4th
of July in 79 years of records. 80 degree plus days on July 4th
not too common with 13 ( 17% ) in the 78 years of records at
Seattle-Tacoma airport. Felton

&&

.HYDROLOGY...The daily hydrology discussion has ended until the
start of the next rainy season; it will only be updated as needed.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Red Flag Warning from 7 AM this morning to 7 PM PDT Wednesday
     for Black Hills and Southwest Interior Lowlands-West Slopes
     of the Central Cascades Generally above 1500 Feet.

PZ...Small Craft Advisory from 5 PM this afternoon to 5 AM PDT
     Thursday for Coastal Waters From Cape Flattery To James
     Island 10 To 60 Nm-Coastal Waters From James Island To
     Point Grenville 10 To 60 Nm-Coastal Waters From Point
     Grenville To Cape Shoalwater 10 To 60 Nm.
&&