450
FXUS66 KPQR 152310
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
410 PM PDT Mon Jul 15 2024

.SYNOPSIS...Above normal temperatures will persist for inland
portions of SW Washington and NW Oregon through the weekend,
though onshore flow should help to guard against more extreme
temperatures. Flow aloft will turn more southerly Tuesday,
potentially carrying moisture and instability northward from
California. This will lead to about a 10-20% chance of
thunderstorms Tuesday afternoon into Tuesday night, best chances
over the higher terrain.

&&

.SHORT TERM...Monday through Wednesday...Temperature
observations as of 2PM Monday are around 4 to 10 degrees cooler
then this time yesterday for most low lying inland locations,
and similar to a few degrees cooler for the rest of the region.
The area of most cooling is the northern Willamette Valley as
well as along and north of the Columbia River due to the
overnight and early morning cooler marine air intrusion along
the Columbia River. Still, temperatures are expected to peak in
the upper 80s to low 90s this afternoon for the Willamette
Valley and only low 80s along the I-5 corridor in SW Washington
north of Clark County.

Temperatures are expected to heat up again on Tuesday as the flow
aloft becomes more southerly and 850 mb temps jump to around 23-25C,
with the potential of a few locations within the interior valleys
reaching triple digits. Latest NBM guidance has increased
chances for 100 degree temps for the Willamette Valley to 40-60%,
except a 60-70% chance from Salem to Canby. The Portland metro
area, as well as the Columbia River Gorge and Hood River Valley,
remains in the Major HeatRisk category for heat related
illness. The complicating factor is flow will be turning
southerly, which brings the potential for additional high level
cloud cover, but even if temperatures are a few degrees cooler
than currently forecast, the relief would likely be minimal as
the influx in moisture would result in slightly more humid
conditions than we are accustomed to in the Pacific Northwest.

The other forecast challenge will concern a cutoff low near the
California coast as models continue to depict it becoming dislodged
by the amplification of the pattern and moving northward as a
negatively tilted shortwave trough Tuesday into Tuesday night. This
will bring concern for lightning strikes as mid level moisture and
instability are advected northward into our area, yielding surface
based convection Tuesday afternoon transitioning to more of an
elevated nocturnal threat overnight Tuesday into Wednesday morning.
Ensemble based probabilistic guidance is still somewhat underplaying
the thunder threat as it tends to struggle with elevated
convection concerns. However, soundings indicate some CAPE in
the 700-500mb elevated layer tomorrow afternoon and into
Wednesday morning. The best location for convection is across
the Oregon and south Washington Casacdes and the coast to the
Coast Range, but we are monitoring the potential for elevated
instability over the inland valleys tomorrow night, as well.
Any lightning will obviously present heightened fire weather
concerns given the recent prolonged stretch of hot and dry
weather.

In the wake of the upper shortwave trough, there is good agreement
of a stout southerly marine push at the surface late Tuesday into
Wednesday. The upper disturbance will likely deepen the marine layer
with substantial onshore flow bringing the potential for increased
marine stratus and somewhat cooler temperatures for Wednesday,
albeit mid-to upper 80s for highs, with at least a 50% chance of
temps exceeding 90 degrees for Salem to Portland. DH/HEC

.LONG TERM...Thursday through next Monday...Warm and dry conditions
expected through the remainder of the week and into the weekend.
Ensembles as well as WPC 500 mb clusters show good agreement of
upper level ridging building back westward with 500 mb heights
climbing towards towards 595 dam for the latter part of this week,
through the weekend and into Monday of next week. An upper level
trough off the coast towards the middle of next week could provide a
bit of a reprieve for the coast, but there is a relatively low
probability at this time. Expect inland valley highs remaining
around 85-95, the coast around 65-75 and the Cascades 75-85. /42

&&

.AVIATION...High pressure will maintain VFR conditions under
clear skies through the rest of the evening with northerly to
northwesterly winds. Expect wind gusts up to 25 kt along the
coast, and gusts up to 20 kt inland. Winds weaken below 10 kt
tonight.

High confidence for marine stratus returning along the coast
tonight after 03-05z Tue, returning IFR/MVFR conditions or lower.
Guidance also suggests a 15-30% chance of MVFR stratus returning
to the Portland/Vancouver Metro Area again after 12z Tue. These
probabilities may be higher as MVFR stratus has been observed at
KPDX/KTTD in the mornings even though previous chances have been
low. Thus, MVFR cigs were included in the recent TAF package for
KPDX/KTTD for tomorrow morning. Any low stratus that develops
inland should clear up by 17-18z Tue, and by 21-22z Tue for
coastal locations.

PDX AND APPROACHES...VFR conditions under clear skies through most
of the TAF period. Chance for low stratus returns for the
Portland-Vancouver Metro Area around 12-16z Tue. Northwesterly
winds through the rest of the evening around 7-9 kt, further
weakening tonight.    -Alviz

&&

.MARINE...High pressure over the NE Pacific will maintain gusty
northerly winds through Tuesday morning. Expect gusts up to 25 kt
through late evening, mainly for the waters south of Cape Falcon.
Tonight, winds will weaken below small craft conditions as winds
turn slightly offshore along the immediate coast. Therefore, the
Small Craft Advisory for the inner waters (out 10 NM) between Cape
Falcon and Florence, OR will only be valid through 2 AM PDT
Tuesday. Meanwhile, small craft conditions will continue over the
outer waters (10-60 NM) through mid-morning, with the Small Craft
Advisory for these waters valid through 9 AM Tuesday. Seas will
mainly be wind-driven and choppy at times, between 4-6 feet.

Note that a weak low pressure system off the California coast will
lift north along the Oregon coast Tuesday night into Wednesday
morning, resulting in a slight chance (15-20%) of thunderstorms
over the waters. Lightning and/or localized gusty winds (up to
low-end Gales) could impact the waters if thunderstorms do
develop. -Alviz

&&

.FIRE WEATHER...A combination of environmental factors has
prompted a fire weather watch to be issued for areas east of the
Coast Range, including the Willamette Valley, the Cascade
Foothills and portions of the Oregon and Washington Cascades.
Hot and dry conditions are expected Tuesday afternoon with highs
reaching into the mid to upper 90s across the Willamette Valley
and upper 80s to low 90s in the Foothills and Cascades. RH
values will drop below the critical threshold with values
expected in the 15-30% range during the afternoon. While wind is
not expected to be a factor tomorrow, a surface low riding
north/northeast along the Oregon Coast will bring an influx of
mid-level moisture to the region along south/southeast winds.
This influx of moisture will support elevated instability which
could support convective initiation across the Cascades and into
the Willamette Valley. Given thunderstorms are expected to be
elevated in nature, not much precipitation will be associated
with them at the surface which will not help improve the dry
conditions. Fuels in the Willamette Valley are the driest but
fuels are also conducive for wildfires in the Foothills and
Cascades. The main threat will be fires started by lightning
strikes associated with thunderstorms. The thunderstorm threat
will be greatest Tuesday evening and into the overnight hours,
mainly from 00-12Z and will linger into Wednesday afternoon and
evening mainly across the WA634/635 zones. -Batz

&&

.PQR WATCHES/WARNINGS/ADVISORIES...

OR...Heat Advisory from 11 AM Tuesday to 8 AM PDT Wednesday for
     ORZ111-112-120-122.

WA...Heat Advisory from 11 AM Tuesday to 8 AM PDT Wednesday for
     WAZ206-207-209-210.

PZ...Small Craft Advisory until 2 AM PDT Tuesday for PZZ252-253.

     Small Craft Advisory until 9 AM PDT Tuesday for PZZ272-273.
&&

$$

www.weather.gov/portland

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