918
FXUS66 KPQR 071108 CCA
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
405 AM PDT Sun Jul 7 2024

.SYNOPSIS...Northwest Oregon and southwest Washington remain within
the grip of a prolonged heat wave that will persist through Tuesday.
The strong ridge of high pressure aloft responsible for our heat wave
will slowly migrate eastward over the coming days. As it does so,
thermal low pressure will focus on areas east of the Cascades,
allowing onshore flow to strengthen Tuesday night and Wednesday. This
will bring an end to the record hot temperatures, but above normal
temperatures are very likely to persist through the end of the week.
No rain is expected through next weekend.

&&

.DISCUSSION...Today through Saturday...The overall forecast picture
has changed little over the last 48 hours, and remains on track for
oppressive heat to persist Coast Range eastward through Tuesday.
Confidence continues to increase that Tuesday night will be the
definitive end to this heat event as onshore flow increases and
begins to moderate temperatures. Coastal communities generally had
high temps in the 80s Saturday; some of these towns may remain stuck
in the 70s today as thermal low pressure strengthens over the
Willamette Valley and draws sea breezes onshore. The marine layer has
deepened slightly over the past 24 hours according to the Astoria PSL
profiler, which adds confidence to at least slight cooling along the
coast this morning. Marine stratus has also developed near the coast
overnight, but mostly appears limited to areas with colder SSTs due
to upwelling. That said, the perfect sunny beach days way be ending
for the coast, as the daytime sea breezes may advect some of this
stratus onshore.

KEY POINTS:

1. Record or near-record heat will persist inland through Tuesday.

2. High temperatures will likely approach 105 degrees for the
Willamette and Hood River Valleys Monday and Tuesday. For most of
these valleys, latest probabilistic guidance suggests a 40-70% chance
of reaching 105 degrees each day.

3. Nights will be slow to cool off in urban areas through Monday
night. Strengthening onshore flow will bring some relief Tuesday
night into Wednesday.

4. The prolonged and compounding nature of this heat wave will make
it increasingly hard for people, pets, and plants to cope with the
heat. This will especially be the case for those with medical
conditions that make them sensitive to heat.

5. Coastal areas will remain cooler, with highs generally in the mid
70s to mid 80s today and Monday, then cooler for Tuesday.


It is important to impress on everyone that aside from the coast, we
have not yet seen the worst of this heatwave. The most brutal period
of heat appears to be Monday through Tuesday for the Willamette
Valley and Lower Cowlitz eastward. Today`s temps will be similar to
the last two days, but models have been very insistent on temps
climbing another 2-4 deg F for Mon-Tue. Additionally, Monday night
appears to offer the least overnight relief from the heat, with NBM
probabilities of lows 70 deg F or higher at around 60-90% for the
Portland metro area and even 30-60% probabilities for much of the
Willamette Valley. So... folks need to treat this heatwave like a
marathon and not a sprint. Pace yourself, try to limit outdoor
exposure to the cooler morning or night hours. Most importantly, stay
hydrated. Heat-related illness can sneak up on you, and this is the
type of heat that puts just about anyone at risk. Also, don`t forget
about your pets: hot surfaces such as concrete and asphalt can easily
burn paws. Best to take your pets out during the cooler morning and
night hours, if possible.

It appears much of the uncertainty we were dealing with a couple days
ago regarding the duration of the heatwave has been cleared up, now
that models show reasonable agreement on downstream features such as
the recurving of Beryl in the Gulf of Mexico. This will make room for
our upper level ridge to expand and migrate eastward, focusing the
most intense heat east of the Cascades. As the Columbia Basin becomes
hotter than the west side, thermal low pressure will strengthen,
pulling increasing onshore flow across much of western WA/OR and
through the Columbia Gorge Tuesday night and beyond. The onshore flow
will bring some moderation in temperatures, pulling them down from
record territory. That said, NBM probabilistic guidance still
suggests a 30-60% chance of the Willamette and Hood River Valleys
climbing to 95 deg Wednesday. Our forecast leaned toward NBM 25th
percentile temperatures Wednesday onward, as an approaching weak
upper trough should weaken subsidence and allow the marine layer to
expand some.

For the Willamette Valley, the record number of days in a row with
highs temps of 100 degrees of warmer is 5 days, set in mid-July 1941
at Portland airport. Many areas in the Willamette Valley/Clark County
have seen 4 days in row a few times, such as back in August 2023, or
2015 or in 1981. For this heatwave, it looks like 4 days of 100+ deg
heat is a good bet at Salem and Eugene, but will see if the record 5
days is reached. Portland has just missed 100F by one degree the past
two afternoons.

Here are the odds of reaching 100+/105+ deg at select cities, based
on the 07z iteration of the NBM on July 7th via the 1D Viewer:

                Sun          Mon         Tue         Wed
               July 7       July 8      July 9      July 10
          100  105   100105    100  105    100  105
  ========================================================
Kelso      50%  <1%   70%15%   80%  30%     0%   0%
Portland*     90%  20%   95%50%   99%  70%    10%   0%
Salem      95%  40%   99%70%   99%  80%    10%   0%
Eugene      95%  25%   95%50%   90%  40%     5%   0%
Hood River    85%  15%   95%65%   99%  80%    25%   0%

The % of reaching 110 or higher is <10% for all areas, except on
Tuesday when there is a 15-30% chance across the Willamette Valley
(best chance in Salem, second best chance in Portland).

*Portland is for Greater Portland/Vancouver metro area.


One little wrinkle to the forecast Monday night into Tuesday is some
weak vorticity lifting northward from California, which will enhance
instability over the forecast area. While mid-level lapse rates
steepen impressively, it looks like we will lack the moisture needed
for thunderstorms. The steepening lapse rates in the presence of
thermal low pressure may end up being more of a fire weather concern,
as it can promote deeper columns in any fires which may start.
Meanwhile, fuels are drying out impressively, and all our fire zones
now appear dry enough to be eligible for Red Flag Warning
consideration. We considered issuing a Fire Weather Watch for our
inland areas Monday through Tuesday, but opted to allow the day shift
to coordinate with our fire partners regarding the status of fuels.
Weagle

&&

.AVIATION...Upper level ridge shifts over the region with light
westerly flow aloft. High pressure will maintain clear skies and
VFR conditions for inland sites through the period with hot
daytime temperatures. As a result, expect the potential for high
density altitude effects on takeoffs during peaking heating in the
afternoon hours. Air mass along the coast cools later tonight
into Sunday morning, with pockets of IFR stratus and fog forming
just onshore before clearing later in the morning.

PDX AND APPROACHES...High pressure maintains VFR conditions
through the period. Light winds becoming northwest 5-7 kt by the
afternoon. /DH

&&

.MARINE...Not much in way of change. Thermal low pressure just
onshore extending southward along much of the coast. Meanwhile,
high pressure remains anchored well offshore. This will maintain
northerly winds on the waters into early next week. Gradients
tight enough to pop gusts to near 20 kt in afternoons/evenings,
but that will be generally south of Cascade Head Sunday. High
pressure builds on Monday, tightening pressure gradients with wind
gusts up to 25 kt possible by Monday afternoon. Seas mostly a mix
of wave fetches: one at 1 to 2 ft from the southwest, with the
other from the west- northwest at 2 to 4 ft. As such, overall
combined seas stay in the 3 to 4 ft range into early next week.

&&

.PQR WATCHES/WARNINGS/ADVISORIES...

OR...Excessive Heat Warning until 10 PM PDT Tuesday for ORZ104>125.
     Heat Advisory until 10 PM PDT Tuesday for ORZ126>128.

WA...Excessive Heat Warning until 10 PM PDT Tuesday for WAZ202>210.
     Heat Advisory until 10 PM PDT Tuesday for WAZ211.

PZ...None.

&&

$$

www.weather.gov/portland

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