Heat in Washington, June 26 to 30, 2021

Area Forecast Discussion
National Weather Service Seattle WA
834 AM PDT Sat Jun 26 2021

.SYNOPSIS...A strong upper level high across the Pacific Northwest
will bring record high temperatures to Western Washington this
weekend. Temperatures are expected to peak Sunday and Monday,
before onshore flow increases and marine air brings somewhat of a
cool down to the interior on Tuesday. Even after the heat peaks,
temperatures look to remain above normal through the next week.

&&

.SHORT TERM /TODAY THROUGH MONDAY/...The forecast is largely on
track still. With SeaTac achieving 73 degrees as of 730 AM, the
temperature has already gotten to the average high temperature for
this day in June. Some model guidance has suggested slightly
warmer temperatures for coastal regions for Sunday and Monday, but
will evaluate further for this afternoons forecast update.

The rest of the previous AFD follows below:

A strong 597 dam upper level high will center itself across
southern British Columbia this weekend and will be responsible for
driving temperatures into the 90s and triple digits for many
areas across Western Washington. Most recent observations early
this morning show overnight temperatures are already trending
about 5 to 10 degrees warmer than they were yesterday at this
time. Overall, expect temperatures to remain rather warm overnight
and to only fall into the 60s across much of the area. SeaTac has
a chance to break, or at least tie, its daily record for warmest
overnight low today-which was 67 degrees back in 2006.

With the unusually warm air mass in place over the Pacific Northwest
and British Columbia through Monday, new records will likely be
set for both afternoon highs and for unusually warm overnight low
temps. Saturday will likely be the day where areas set new record
highs for the month of June, while Sunday and Monday will likely
be the days where areas set record all-time high temps across the
region.

As for temperatures, coastal areas will likely see temperatures
peak on Sunday, while most areas across the interior will see
temperatures peak on Monday as a thermal trough situates itself
along the I-5 corridor. Afternoon highs across the region will
climb into the 90s to 100-110 degree range. Sunday night into
Monday morning looks to be the time frame where overnight lows
will be the warmest, with temperatures only dropping down into the
low to mid 70s for much of the area. Overnight lows for areas
close to the coast will be a roughly 10 degrees cooler given the
marine influence- but will be warm nonetheless- and mainly in the
mid 60s. These warm overnight lows will offer little to no relief
for areas overnight.

Thus, an excessive heat warning remains in effect for much of
western Washington. Those without adequate access to cooling or
proper hydration will be at particularly high risk for heat-
related illnesses through the weekend and into early next week.
Some important tips to remember as we dive into this prolonged
period of hot weather:

-Avoid strenuous activities and limit your time outdoors,
 especially during the peak heating times of the day.

-Make sure to stay hydrated by drinking plenty of water, even if
 you do not feel thirsty.

-Check on family, friends, and neighbors-especially those who do
 not have the proper access to cooling.

.LONG TERM /TUESDAY THROUGH FRIDAY/...An upper level low
originating from Northern California will be a feature to keep an
eye on over the next few days as it tracks northward across the
Pacific Northwest's coastal waters on Tuesday. Although this
feature looks to remain offshore, it should help to deepen the
marine layer as it progresses northward. Increasing onshore flow
at the surface will start to bring some relief temperature-wise to
the interior on Tuesday, with afternoon highs dropping to the
upper 80s to low 90s. Areas along the coast look to see a 15-20
degree cool down on Tuesday, with high temperatures expected to
top out in the low 70s. Additional diurnal pushes of marine air
Wednesday into late week will help to moderate temperatures some,
however afternoon highs will still run above normal for June and
July standards. 14

&&

.AVIATION...Clear skies.  Winds aloft are light easterly this
morning and those will turn to the southeast tonight and remain
light. 19

KSEA...Clear skies, northerly afternoon and evening breeze to
10kt.

&&

.MARINE...Northerly flow trending more to the northeast through the
weekend then for the coastal waters the flow will turn southerly on
Monday as the thermal low combined with a weak low pressure system
moving south to north. The coast will likely see a squirt of
southerly winds with fog and low clouds--a southerly surge at some
point, probably overnight Sunday night or Monday morning.

The inland waters will see rather light winds--tending to be light
northeast flow in the late night and morning hours and then a
diurnal northwest breeze for the afternoon and evening. That is to
day, a typical summertime land sea breeze circulation should develop
each day despite the strong thermally induced low pressure over the
region.

Our hot thermal low regime over the weekend will see pressure
gradients turn onshore Monday afternoon and night as the thermal low
shifts east of the Cascades and as that happens advisory level winds
will be possible each evening, especially in the Strait of Juan de
Fuca.

Westerly swell 4 to 6 feet. 19

&&

.FIRE WEATHER...Record-setting heat on tap for this weekend along
with unstable conditions and borderline wind speeds will make for
concerning times from a fire weather perspective. At the time of
this writing, ERC values in most of our fire wx zones have not yet
reached the 90th percentile, which would trigger thoughts in the
direction of Red Flag conditions. ERC values are close or meeting
these values in the two Cascade zones, 658 and 659. So the RFW
discussion splits in two, watching 658 and 659 to see if they meet
criteria for Dry and Unstable conditions /as widespread Haines 6
values expected Sun and Mon/ and watching the fuels for the other
zones. Would not be surprised if a Fire Wx headline for the Cascade
zones made it into the forecast, possibly for Monday, but will need
to evaluate incoming 12Z data before making any decisions.
Regardless, it is safe to say that localized Red Flag conditions
will be possible there and caution is highly encouraged.

Regardless of ERC values, it is safe to say that finer fuels in the
area, grass and shrubs, have certainly cured and will do so further
as current heat persists. This creates a prime environment for
roadside and brush fires and as such, drivers are highly encouraged
to check and secure anything that may be dangling from any trailers
or undercarriages, as these may create sparks. Additionally, any
smokers are strongly encouraged to extinguish and leave butts in car
ashtrays, as flinging them out the window could also lead to
roadside fires.  18

&&

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

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Excessive Heat Warning until 9 PM PDT Monday for Bellevue and
     Vicinity-Bremerton and Vicinity-Cascades of Pierce and
     Lewis Counties-Cascades of Snohomish and King Counties-
     Cascades of Whatcom and Skagit Counties-East Puget Sound
     Lowlands-Everett and Vicinity-Hood Canal Area-Lower
     Chehalis Valley Area-Olympics-Seattle and Vicinity-
     Southwest Interior-Tacoma Area-Western Skagit County-
     Western Whatcom County.

     Excessive Heat Warning from 2 PM this afternoon to 9 PM PDT
     Monday for Admiralty Inlet Area-Central Coast-Eastern
     Strait of Juan de Fuca-North Coast-Western Strait of Juan
     De Fuca.

PZ...None.
&&

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