120
FXUS66 KPQR 051023
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
321 AM PDT Fri Jul 5 2024

.SYNOPSIS...Long duration and dangerous heat wave begins in earnest
today, with inland high temperatures expected to warm several degrees
over Friday`s already-very warm temperatures. Temperatures will be
slow to cool each evening, especially in urban areas. The coast will
be the place to go to escape the heat this weekend, as beaches will
be cooled by afternoon sea breezes. Little change in this overall
pattern is expected through Sunday, and latest forecast models are
strongly suggesting the heat will persist through Monday if not
Tuesday. Confidence increases in some degree of cooling as onshore
flow strengthens midweek, but forecast models suggest strongly that
above normal temps will persist all of next week. No rain is expected
through at least the middle of next week.

.DISCUSSION...Today through next Friday...Overall, not much has
changed forecast-wise over the past 24 hours. Southwest Washington
and northwest Oregon will experience a dangerously persistent heat
wave, with at least the next four days expected to approach or exceed
100 degrees for the inland valleys. Adding to the concern is that
temperatures will be slow to cool off each night, with urban and
suburban areas only dipping below 70 degrees for a few hours (if
that) before heating back up the next morning. Rural outlying areas
will see a little more relief with overnight lows dipping closer to
60 degrees, but evenings will be uncomfortably warm even there.

KEY POINTS:

1. A major heat wave begins in earnest today for SW Washington and NW
Oregon except for the immediate coast.

2. This heat wave will likely be remembered more for its duration
than its intensity. Inland valleys can expect maximum temperatures of
100 to 105 each afternoon Friday, Saturday, Sunday, and Monday. There
is a possibility the heat will continue even longer than that.

3. Nights will be slow to cool off, especially for urban and suburban
areas. Of most concern are highly urbanized areas where temperatures
will be even slower to cool off due to the urban heat island effect.
Once temperatures warm above 70 degrees this morning, some inner
Portland neighborhoods may not see temperatures fall below 70 degrees
again until Monday morning, or perhaps even longer.

4. The combination of multiple consecutive days with triple-digit
heat and only modest relief at night will make this heat wave
particularly dangerous. Those sensitive to heat will struggle each
night to recover from the hot days if they do not have access to
adequate cooling. The longevity of this event will likely compound
and exacerbate impacts.

5. The coast will be the place to go to cool off, though the cooling
influence of the ocean will only extend a few miles inland.
Daytime sea breezes should keep coastal communities mostly in
the 70s and 80s for highs.

As recent studies have shown, overnight temperatures are often
more important than daytime temperatures during a heat wave, because
warm nights can limit the ability to cool off and reset for the next
hot day. This compounding effect of multiple hot days with warm
nights will exacerbate the the threat of heat-related illness,
especially for those who are sensitive to heat and lack the
ability to cool off. In that sense, this several day-long
stretch of 100-106 degree highs and lows struggling to fall
below 70 degrees is potentially more dangerous than just a day
or two of intense 110 degree heat followed by a quick cooldown.
NBM suggests a 30-50% chance of lows at KPDX failing to reach 70
degrees Saturday night, Sunday night, and Monday night.
Unfortunately this chance in reality is likely much higher than
that for inner portions of the Portland/Vancouver Metro due to
microscale effects like the urban heat island effect which can cause
temperatures to cool slower than they normally would at night.

Looking at the big picture, strong high pressure aloft anchored off
the California coast is amplifying rapidly just off the US West
Coast. Meanwhile there is weak shortwave energy moving into Montana
from western Canada on NW flow aloft. This NW flow aloft has lingered
long enough to provide some resistance for the amplification of the
upper ridge, which is a big reason the hottest model solutions have
mostly fallen off the board. This NW flow aloft is expected to
diminish by Sunday. On one hand, this will allow slightly hotter
temperatures aloft to build northward into the northwestern US, but
on the other hand it will also allow the 500 mb ridge axis to shift
slowly eastward. As the Columbia Basin heats up, thermal low pressure
will eventually become stronger there than west of the Cascades,
encouraging the onshore flow which should bring an end to this
episode of excessive heat. That said, even with the onshore flow
temperatures will be well above normal as we move into midweek.
Substantial positive 500 mb height anomalies are likely to keep the
marine layer too shallow to have a dramatic impact on inland
temperatures. Latest NBM probabilistic guidance shows no more than a
30 percent chance of high temps lowering to early July normals
(generally the lower 80s) for the Willamette Valley by next Friday.
So while the intense heat may end by midweek, it will still be very
warm for several more days after that.Weagle/Schuldt

&&

.FIRE WEATHER...Increasingly hot weather will combine with dry
and gusty afternoon northerly winds again this afternoon/evening as
thermal low pressure strengthens over SW Oregon tightening surface
pressure gradients, especially across the southern Willamette Valley.
While fuels today are not quite ready for our fire weather zones to
be eligible for a Red Flag Warning, the hot and dry weather will
rapidly dry out vegetation over the coming days. Unfortunately
overnight relative humidity recoveries appear poor over the higher
terrain for several nights moving forward, further working to chip
away at fuel moisture. Forecast guidance still suggests winds will be
lighter Saturday, but it will still be very hot, dry, and unstable
near the surface with thermal low pressure overhead. Those working
out on the line any day through this weekend into early next week
should pay special attention to staying hydrated, as the expected
conditions can easily cause heat-related illness or worse.
-Schuldt/Weagle

&&

.AVIATION...VFR conditions expected to continue throughout the
time period. With increasing high pressure, marine stratus has not
developed at the coast, and chances of MVFR conditions or below at
the coast are <5%. Inland sees no chance of MVFR or below
conditions.

Winds are currently essentially calm throughout the TAF area, but
will increase going into Friday morning. From around 20z Fri
onwards, northerly gusts at the coast could reach up to 25 kt.
Inland, northerly gusts up to 20 kt will be possible at that time.
Winds will calm again going into Friday night, by around 05z Sat.

PDX AND APPROACHES...VFR with clear skies. Winds currently light
and variable, but increase around 20z Fri. Expect northerly gusts
up to 20 kt to be possible Friday afternoon and evening. Winds
decrease back to being light and variable around 05-06z Sat. /JL

&&

.MARINE...The synoptic pattern has high pressure over the eastern
Pacific paired with a thermally induced low pressure over
California/Southern Oregon and the Great Basin. This pattern will
persist through the weekend and likely into the start of next
week. Therefore, expect Small Craft conditions across all waters
with gusts up to 30 kt. Winds continue to follow a diurnal cycle,
reaching Small Craft Advisory criteria each afternoon and evening,
decreasing each night and morning. Seas will be predominately
wind-driven resulting in 5-8 ft at 8-10 seconds throughout the
week.

&&

.PQR WATCHES/WARNINGS/ADVISORIES...

OR...Excessive Heat Warning until 8 PM PDT Monday for ORZ104>125.

     Heat Advisory until 8 PM PDT Monday for ORZ126>128.

WA...Excessive Heat Warning until 8 PM PDT Monday for WAZ202>210.

     Heat Advisory until 8 PM PDT Monday for WAZ211.

PZ...Small Craft Advisory from 2 PM this afternoon to 2 AM PDT
     Saturday for PZZ251>253-271.

     Small Craft Advisory until 8 AM PDT Saturday for PZZ272-273.
&&


&&

$$

www.weather.gov/portland

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