196
FXUS66 KPQR 200432 AAA
AFDPQR

Area Forecast Discussion...UPDATED
National Weather Service Portland OR
932 PM PDT Tue Aug 19 2025

Updated aviation and fire discussions

.SYNOPSIS...
Onshore flow and near average temperatures Wednesday
will give way to building high pressure and produce a multi-
day stretch of hot temperatures and a mix of moderate and major
heatrisk for inland valleys Friday and Saturday. There is a
10-20% chance that the hot temperatures persist through Tuesday.

&&

.SHORT TERM...Now through Saturday Night...Water vapor
satellite imagery reveals a shortwave trough over the northeast
Pacific and a shortwave ridge over the southern Rocky Mountains.
This will maintain low level onshore flow into Wednesday and
keep temperatures near to slightly above average.

Global ensembles are in good agreement the aforementioned Four
Corners high pressure will build northwestward into the Pacific
Northwest later this week into this weekend. This will result in
rather dramatic heat-up for inland valleys Thursday into
Friday. Before this happens we will have a period of overlapping
northerly winds and low relative humidity across the southern
half of the Willamette Valley Thursday afternoon and early
evening. A Fire Weather Watch has been issued given the fuels in
this zone primarily consist of grass and shrubs and will
sufficiently dry out by this time.

Flow will turn easterly across the Cascades and Columbia River
Gorge Thursday night and result in the thermally induced surface
trough spreading northward across the region on Friday. NBM and
global ensemble guidance is in good agreement most inland
valleys will be in the upper 90s to around 100F Friday and
Saturday with widespread moderate heatrisk across northwest
Oregon and southwest Washington. Pockets of major heat risk are
now in the forecast for the Portland/Vancouver metro and
Columbia River Gorge where confidence in warmer overnight low
temperatures exist. Either way, extra precautions will need to
be taken for those planning to run or walk in these hot
temperatures. Have a plan to make sure you can stay hydrated and
can easily cool off with cold wet towels or rags. The reason
major heatrisk is not more widespread at this point is that low
temperatures are still forecasted to be in the low to mid 60s at
most locations. However, there is a 30-40% chance that low
temperatures only end up in the mid to upper 60s and major
heatrisk becomes more widespread.



.LONG TERM...Sunday through Tuesday...Ensembles are in general
agreement temperatures will gradually lower Sunday through the
middle of the next week as high pressure gradually shifts
eastward and onshore flow increases across the region. It should
be noted that approximately 10-20% of the global ensemble
guidance suggests Friday and Saturday`s hot temperatures will
persist through Monday and/or Tuesday. Again, this is not the
most likely scenario, but it`s not implausible either.


&&

.AVIATION...Low stratus begins to build along the coast tonight,
bringing chances of LIFR to IFR conditions to the south of KTMK
and IFR to MVFR to the north of KTMK. Moderate to high confidence
that conditions at coastal terminals will deteriorate between now
and 06Z Wednesday and persist until 18Z Wednesday. VFR conditions
will return thereafter. There is a 20-30% chance that conditions
at KONP will persist past 18Z WEdnesday, although conditions would
likely be improved to IFR to MVFR by then. Additionally, coastal
winds will remain northwesterly and under 10 kt until 18Z
Wednesday, then increase to 10-15 kt with gusts up to 25 kt
thereafter.

This marine stratus may push up the Columbia River and into the
Portland/Vancouver metro and possibly Troutdale between 12-17Z
Wednesday, with low confidence (10-30%) of MVFR CIGs during this
time. Otherwise, inland sites should maintain VFR conditions
through the rest of the TAF period, with northwesterly winds under
10 kt.

PDX AND APPROACHES...VFR conditions will continue through most of
the TAF period. There is the potential (10-30%) for MVFR CIGs
to develop between 12-17Z Wednesday as coastal stratus pushes
inland. Could also affect KTTD. By 18Z Wednesday, high confidence
for VFR conditions through the rest of the TAF period.
-Hall

&&

.MARINE...A fairly typically summertime pattern is expected
moving through the rest of the work week into the weekend as an
area of high pressure sits over the northeastern Pacific. On
Wednesday the pressure gradient across the waters will begin to
tighten helping to facilitate slightly stronger northerly winds
as the day goes on. Given the increasing probabilities for 21
knot or greater gusts come Wednesday late afternoon evening
through Thursday across the inner and outer waters south of Cape
Falcon, a Small Craft Advisory was issued. It`s possible this
advisory will need to be extended for the outer waters through
the end of the week as northerly winds likely persist.
Otherwise, seas generally stay around 3-6 feet through the rest
of the week with a dominate period of 8-10 seconds. -Schuldt

&&

.FIRE WEATHER...A thermal trough will form starting on Thursday.
As is common with this pattern, northerly to northwesterly winds
will amplify. Within the southern Willamette Valley, these
increased winds will combine with very low humidity and thus,
create weather conditions that would promote rapid fire spread.
As it stands, there is around a 50% chance that hazardous fire
weather conditions may manifest. These conditions are described
as a relative humidity of 25% or lower, and winds of 10-15 mph
with gusts greater than 25 mph. These conditions must coincide
for 4 hours or more. Winds are borderline in their duration
which could be a limiting factor. As stated in the previous
discussions, hot and dry conditions will prevail through the
week with the lowest humidity expected Friday afternoon. During
this time, winds will be offshore aiding in warming and drying.
Cannot rule out gusty easterly winds through the Columbia River
Gorge on Friday but at this point, confidence is incredibly low
on whether they will meet critical wind criteria or not.
-Muessle


&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...Extreme Heat Watch from Friday morning through late Saturday
     night for ORZ112-120>122.
     Fire Weather Watch from Thursday afternoon through Thursday
     evening for ORZ685.
WA...Extreme Heat Watch from Friday morning through late Saturday
     night for WAZ209-210.
PZ...Small Craft Advisory from 5 PM Wednesday to 5 AM PDT Friday
     for PZZ252-253-272-273.
&&

$$

www.weather.gov/portland

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