723
FXUS66 KPQR 050348 CCA
AFDPQR

Area Forecast Discussion...UPDATED
National Weather Service Portland OR
848 PM PDT Thu Jul 4 2024

Updated Aviation Discussion...

.SYNOPSIS...Its going to get hot! A long duration, impactful,
and historic heatwave is expected to develop by Friday,
lingering through the weekend into early next week as an
anomalous high pressure strengthens aloft. Across the inland
valleys anticipate an almost 4-5 day stretch of near to above
triple digit temperatures. Nights will struggle to cool off,
leading to minimal overnight relief, especially in urban areas.
At least guidance has been trending towards the return of
somewhat cooler temperatures around the middle of next week,
albeit staying above normal. No precipitation is expected
through the middle of next week.

.DISCUSSION...Today through next Thursday...As we enter the
unofficial start of summer across the Pacific Northwest
(commonly joked about as July 4th/5th) Mother Nature has outdone
herself as we embark into a prolonged stretch of dangerously
hot conditions for inland portions of SW Washington and NW
Oregon. Daily high temperature records will likely be broken for
many inland sites going forward.


KEY POINTS:

1. A major heat wave will occur for SW Washington and NW Oregon
except for the immediate coast. temperatures peak Friday
through Monday, and now potentially even into Tuesday, with
dangerously hot conditions.

2. The hottest daytime conditions will likely be Saturday,
Sunday, and Monday.

3. This heat wave will likely be remembered more for its
duration than its intensity. Exploring historical weather data
at Portland, Salem, and Eugene reveals there have only been 3-5
comparable events in which there was a 4-5 day streak of
consecutive 100 degree days. Four to five consecutive days of
100-106 degree heat with little relief at night is dangerous,
especially to those sensitive to heat struggling to find a place
to cool off. The longevity of this event will likely compound
and exacerbate impacts.

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 as fuels
dry out over the weekend into early next week.


A strong upper-level ridge of high pressure has begun to build
into Western CONUS with guidance continuing to show better
agreement regarding the finer details of the heatwave expected
to last into early next week. One of biggest changes today has
been the extension of near to above 100 degree temperatures into
at least Monday of next week. Fortunately, it appears the most
extreme model solutions suggesting 110+ temperatures have
continued to wane. Latest NBM probabilistic guidance now
suggests there is only a 10-20% chance anywhere in our forecast
area will reach 110 deg F with this heat wave - with the best
chance being in the Tualatin Valley extending towards the
Portland metro Sunday afternoon. Still, this is just a minor
consolation to the added duration of heatwave into at least
Monday maintained by most deterministic and ensemble guidance.
Models begin to show the axis of the strong upper-level ridge
slowly meandering eastward Tuesday into Tuesday night but have
delayed its shift slightly. Should it stay on the latest project
track it wont be enough for any appreciable cool down across
our inland areas with NBM probabilistic guidance now showing a
40-60% chance to meet or exceed 100 degrees Tuesday afternoon -
the current forecast reflects this with high temperatures in the
upper 90s to near 100 degrees.

As of the 13z 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: 80-95%
Greater than or equal to 105 degrees: 50-60% (highest north)
Greater than or equal to 110 degrees: 5-20% (highest north)
Greater than or equal to 115 degrees: 0-3%

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 100-106 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 much 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.

Part of the reason models continue to back off from the more
extreme hot solutions is the NW flow aloft and associated
shortwave energy over Canada 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 into Monday and Tuesday of next week will be
paramount as 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 Wednesday, which should allow at least some onshore
flow during the midweek. With 500 mb heights still well above
normal and the rather slow progression of the upper-level
pattern, the cooling influence of any offshore flow will be
limited east of the coast range due to a shallow marine layer.
Although it should begin to cool down on Wednesday temperatures
will very likely remain 10-15 degrees above above normal inland
through most if not all of next week. Its worth noting the
normal high temperature for this time of year should be around
80-82 degrees from Portland to Eugene. -Schuldt/Weagle

&&

.FIRE WEATHER...Increasingly hot weather will combine with dry
and gusty afternoon northerly winds this afternoon and again
Friday 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 Red Flag Warning
eligible, these conditions will rapidly dry out vegetation over
the coming days. Unfortunately overnight relative humidity
recoveries appear poor over the higher terrain 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...Widespread VFR under clear skies as high pressure
builds across the airspace. Only a 10-20% chance of some low
stratus developing along the coast between 12-17Z Friday.
Generally northerly winds 6-8 kt for inland terminals, with gusts
up to 20 kt until around 05-06Z Friday. Similar timing for coastal
locations, but gusts up to 30 kt are expected. These afternoon to
evening conditions are expected to reoccur through the weekend,
with gusts expected to return at inland and coastal terminals
around 18-20Z Friday.

Given the hot daytime temperatures, which are forecasted to be
above 100F for an extended period of time, high density altitude
issues could present a problem starting 20Z Friday.

PDX AND APPROACHES...VFR under clear skies.  winds 4-8 kt with
gusts up to 20 kt expected to end around 06Z Friday. Expect a
return of similar gusts around 21-23Z Friday. -JH

&&

.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 will subside below Small Craft
overnight for all zones but PZZ272 and 273. However, the elevated
winds will return each afternoon and evening. 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 until 11 PM PDT this evening for PZZ251>253-
     271.

     Small Craft Advisory until 11 AM PDT Friday for PZZ272-273.
&&

$$

www.weather.gov/portland

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