482
FXUS66 KPQR 072153
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
253 PM 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
temps are very likely to persist through the end of the week. No rain
is expected through next weekend.

&&

.SHORT TERM...Sunday night through Tuesday night...The overall
forecast picture has changed little over the last 48 hours, and
remains on track for oppressive heat to persist from the Coast Range
eastward through Tuesday. For those who may not have time to read
through this full discussion, please refer to our key points below
for a quick summary:


KEY POINTS:

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

2. High temperatures will likely approach or reach 105 degrees for
the Willamette and Hood River Valleys Monday and Tuesday.

3. Nights will be slow to cool off in urban areas through Monday
night. Strengthening onshore flow will bring some relief to all
locations Tuesday night into Wednesday, however temps will still be
running above normal.

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 70s
through Tuesday. Trending even cooler Wednesday onward.


Confidence continues to increase that Tuesday night will be the
definitive end to this heat event for inland areas as onshore flow
increases and begins to moderate temperatures. Excessive heat has
already come to an end for coastal communities as observed
temperatures early Sunday afternoon were generally in the 70s from
Astoria to Tillamook. Temps were around 65-70 degrees on the Long
Beach Peninsula. Even cooler temps were being observed south of
Tillamook, especially from Newport to Florence where temps were in
the upper 50s to mid 60s with a broken marine stratus deck over the
coastal waters attempting to push slightly onshore. Similar temps are
expected at the coast Monday, then trending cooler Tuesday through
Thursday as onshore flow persists. Despite the relatively cooler
temps at the coast, excessive heat will persist for inland areas
through Tuesday with widespread high temps generally ranging between
100-105F and morning lows between 60-70F (warmest in urban areas). In
addition, poor overnight relief is expected Monday night with temps
running noticeably warmer when compared to what has been observed
over the past few nights. NBM probabilities for low temps of 70
degrees or warmer are at 40-60% for the Portland metro and Salem, but
much lower elsewhere. For probabilistic guidance from the latest NBM
regarding high temps, refer to the table shown below. Note
probabilities have generally decreased a bit from yesterday.


Mon     Tue Wed
       July 8    July 9July 10
      100  105   100105    100  105
  =========================================
Kelso      65%  20%   55%10%1%   0%
Portland*     90%  45%   80%50%    10%   0%
Salem      90%  50%   80%60%    20%   0%
Eugene      90%  40%   80%30%5%   0%
Hood River    95%  50%   95%65%    20%   0%

The % of reaching 110 or higher is now at or less than 10% for all
areas. Given the way this heat event has been trending, confidence is
high we will NOT reach 110 degrees anywhere in the Willamette Valley.

*Portland is for Greater Portland/Vancouver metro area.


It is worth mentioning that the worst of this heat wave looks to be
Monday through Tuesday, so it will be a good idea to stay hydrated
and out of the direct sun. Do not forget about any pets and/or
outdoor plants you may have, as gardens will need frequent watering
and pets will need plenty of water and shade. It will be best to walk
dogs during the morning hours or late evening when temps will be
cooler and surfaces such as concrete and asphalt will no longer be
hot enough to burn their paws.

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 the 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, so it is now clear this particular record
will not be broken at the Portland Airport.

One little wrinkle to the forecast on Tuesday is some weak vorticity
lifting northward from California, which will enhance instability
over the Cascades. For locations west of the Cascades, forecast
soundings show a fairly impressive stable layer between 900-800 mb,
which will limit surface-based instability for locations west of the
Cascades. While mid-level lapse rates steepen impressively and MUCAPE
values increase, soundings continue to suggest we will not have
enough moisture for thunderstorms to develop. At best, some cumulus
clouds developing along and near the Cascade crest appear to be the
most likely outcome. 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.
However, after some coordination with fire partners and neighboring
NWS WFOs, it appears our live 100-hr and 1000-hr fuels in the higher
Cascades are not quite dry enough yet to support rapid fire spread if
dry/unstable conditions were to materialize over the Cascades on
Tuesday. West of the Cascades, fine fuels are the main concern right
now as those are most dry and ready to burn, however a lack of wind
in the forecast suggests concerns regarding the potential for rapid
fire spread is low. Therefore, there are no immediate fire weather
concerns at this time and the fire weather discussion has been
removed from our Area Forecast Discussion as a result of that.
Instead, please refer to our Fire Weather Planning forecast if you
are searching for more detail regarding our fire weather forecast.
-TK

&&

.LONG TERM...Wednesday through Saturday night...In regards to the
weather forecast for Wednesday through Saturday, there are no major
concerns at this time as varying degrees of onshore flow will
prevent extreme heat from persisting or redeveloping. That said,
models and their ensembles continue to suggest temps will at the very
least remain above normal through Saturday with high temps most
likely winding up somewhere between 85-95 degrees each day. The
probability for high temps above 95F is fairly low Wednesday through
Thursday at around 25% or less over the Willamette Valley and
Portland metro; the one exception to that is on Wednesday when
probabilities range between 30-70%. Conditions are very likely to
remain dry through next Saturday as NBM PoPs are at 0% across all of
southwest WA and northwest OR. -TK

&&

.AVIATION...Little change. Strong high pressure over the region
will maintain clear skies, along with hot temperatures into the
early evening. As a result, expect the potential for high density
altitude effects on takeoffs this afternoon into early evening.

Patchy stratus along the Oregon coast will dissipate this
afternoon. But, as the air mass along the coast cools tonight,
will see patchy IFR stratus and fog re-form.

PDX AND APPROACHES...High pressure will maintain VFR under clear
skies.

&&

.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 this week. Gradients
tight enough to pop gusts to near 20 kt through early this
evening, mainly south of Cascade Head. 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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