827
FXUS66 KPQR 041046 CCA
AFDPQR

Area Forecast Discussion...Corrected
National Weather Service Portland OR
330 AM PDT Thu Jul 4 2024

.SYNOPSIS...The hottest weather so far this year will develop by
Friday, lingering through the weekend as high pressure strengthens
aloft. The heat will be prolonged and dangerous with multiple days
expected to reach 100 degrees for most inland valleys. Nights will
struggle to cool off, leading to minimal overnight relief, especially
in urban areas. This heat wave will continue through at least Sunday,
with some potential for it to last several days into next week. No
rain is expected through the middle of next week.

&&

.DISCUSSION...Today through next Thursday...Well, it is often said
across the Pacific Northwest that the Fourth of July is our
unofficial start of peak summer weather, and Mother Nature appears
to be right on time this year. A major heat wave will begin to
develop today, then continue through the weekend for inland portions
of SW Washington and NW Oregon.

KEY POINTS:

1. A major heat wave will occur for SW Washington and NW Oregon
except for the immediate coast. Temperatures begin to heat up today,
with dangerously hot conditions continuing through at least Sunday.

2. The hottest days will likely be Saturday and Sunday.

3. This heat wave will likely be remembered more for its duration
than its intensity. Temperatures appear unlikely to reach the
extremes encountered during the June 2021 Pac NW heat event. However,
three to five consecutive days of 100-107 degree heat with little
relief at night is still dangerous, especially to those sensitive to
heat struggling to find a place to cool off.

4. 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.

5. Dry northerly winds will accompany the hot temperatures along the
Coast Range and Willamette Valley, resulting in rapid drying of fine
vegetation. Fire danger will increase rapidly as fuels dry out.


Forecast models finally appear fairly locked on to the weather
scenario expected to play out over the next few days. We had always
known it was going to get hot as a strong upper ridge built over the
region - the question was how hot. Fortunately, it appears the most
extreme hottest model solutions have fallen off the table with the
00z model/ensemble suite. NBM probabilistic guidance now suggests
there is only a 20% chance anywhere in our forecast area will reach
110 deg F with this heat wave - with the best chance being in the
Tualatin Valley Sunday afternoon. On the flip side, it is now a
near-certainty (80-100% chance) that much of the Willamette Valley
will reach 100 degrees by Sunday. This should happen for most of our
inland valleys Friday afternoon, with intense heat persisting with
our upper level ridge all the way through the weekend.

As of the 07z NBM run, here are temperature probabilities for the
Willamette Valley on Sunday, where/when the most intense heat is
expected:

Greater than or equal to 100 degrees: 80-100%
Greater than or equal to 105 degrees: 40-70% (highest north)
Greater than or equal to 110 degrees: 5-20% (highest north)
Greater than or equal to 115 degrees: Less than 5%

As recent studies have shown, night time temperatures are often more
important 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 100-107 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 30-40% chance of lows at KPDX failing to
reach 70 degrees Saturday night, but the chance is likely much higher
than that for much of Portland due to microscale effects like the
urban heat island effect which can make temperatures slower to cool
off at night.

Part of the reason models have backed off from the more extreme hot
solutions is the NW flow aloft and associated shortwave energy
providing some resistance to the ridge strengthening and bulging
northward across the Pac NW. How this NW flow and the northern
periphery of the ridge evolve over the weekend will be very important
to whether or not this heat wave gets reinforced for a second week.
We have seen multiple occasions this year where westerly flow in the
NE Pacific "bridges the gap" across the northern periphery of a
ridge, causing it to be weaker than it was depicted in early model
runs. Either way, all WPC clusters suggest the upper level ridge axis
shifts east of the Cascades by Tuesday, which should allow at least
some onshore flow by midweek. With 500 mb heights still well above
normal, the cooling influence of any offshore flow will be limited
east of the coast range due to a shallow marine layer. So... although
it should begin to cool down Tuesday night, temperatures will very
likely remain above normal inland through most if not all of next
week.  Weagle

.FIRE WEATHER...Increasingly hot weather will combine with dry and
gusty afternoon northerly winds this afternoon/evening and again
Friday afternoon/evening as thermal low pressure strengthens over SW
Oregon. While fuels are not quite ready for our fire weather zones to
be Red Flag Warning eligible, these conditions will quickly dry out
vegetation over the coming days. Forecast guidance suggests winds
will be lighter Saturday, but it will still be very hot, dry, and
unstable with thermal low pressure overhead. Those working out on the
line today or any day through this weekend should pay special
attention to staying hydrated, as the expected conditions can easily
cause heat-related illness or worse.  Weagle

&&

.AVIATION...Little change in the pattern, with some coastal marine
stratus. The only terminal impacted for the rest of the night will
be KAST, which will see IFR cigs continuing through much of the
night, lifting around 16-18z Thu. VFR conditions are expected
afterwards for the rest of the TAF period. Central/southern
coastal terminals (KONP) tonight will mostly just remain VFR,
though there`s around a 25% chance of intermittent MVFR cigs at
times before 16z. Inland terminals will just remain VFR through
the night, with NNW winds between 5-8 kts.

Daytime hours will just remain VFR for all terminals, and winds
will gust right up to 15-20 kt during afternoon and evening hours.

PDX AND APPROACHES...VFR conditions throughout the TAF period.
Wednesday afternoon brings NW winds up to 10 kt starting at 20z
Thu. Winds weaken going into Thu night. /JL

&&

.MARINE...High pressure over the northeast Pacific with a
thermally induced low pressure over California/Oregon and the
Great Basin. This pattern will persist through the remainder of
the week and likely into the start of next week. Small Craft
Conditions continue to develop across all the waters each
afternoon and evening, with weakening winds in the second half of
each night and early mornings going forward. Expect gusts up to
30 kt with similar conditions each afternoon and evening as winds
will closely follow a diurnal cycle. 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 from noon today to 11 PM PDT Sunday for
     ORZ104>125.

     Heat Advisory from noon today to 11 PM PDT Sunday for
     ORZ126>128.

WA...Excessive Heat Warning from noon today to 11 PM PDT Sunday for
     WAZ202>210.

     Heat Advisory from noon today to 11 PM PDT Sunday for WAZ211.

PZ...Small Craft Advisory until 5 AM PDT Friday for PZZ272-273.

&&

$$

www.weather.gov/portland

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