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864
FXUS66 KPQR 221009
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
300 AM PDT Sat Jun 22 2024

.SYNOPSIS...
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After another hot day for inland valleys on Saturday,
cooler temperatures will return Sunday into Monday as onshore flow
strengthens. Conditions will remain mostly dry through Tuesday aside
from areas of light rain and drizzle Saturday night into Sunday
morning for the coast, west slopes of the Coast Range and Cascade
foothills. The eastern Portland metro may see a little light rain as
well, but confidence is low. Precip chances become more widespread
Wednesday into Thursday as an upper level trough moves inland, which
will also bring the return of cooler temperatures.
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&&

.DISCUSSION...
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SATURDAY THROUGH FRIDAY NIGHT...Satellite imagery from
early Saturday morning revealed clear skies across the region aside
from a few passing high clouds. The exception was along the coast
from Coos Bay to Tillamook where a low marine stratus deck was in
place. However, the HREF suggests this stratus deck will scatter out
later this morning and bring mostly sunny conditions to the coast
before low clouds return to the coast late Saturday
afternoon/evening. As such, another cool day is in store for the
coast with highs in the 60s. With plenty of sunshine inland and very
little change to the weather pattern, expect another warm day with
high temps in the mid to upper 80s. Cannot completely rule out highs
of 90 degrees or warmer as the NBM is showing a 20-40% chance from
Salem to the Portland metro. However given observed surface temps
over the past two days have been in the upper 80s/lower 90s for most
inland valley locations and 850 mb temps on Saturday are 1-2 deg C
cooler, highs in the mid to upper 80s seems spot on. This is also in
line with the deterministic NBM, which has verified very well over
the past couple of days.

Beyond Saturday, models and their ensembles continue to depict
strengthening low-level onshore flow and increasing low-level
moisture Saturday night into Sunday as a weak cold front and
attendant upper level shortwave reach the coast. The latest suite of
hi-res model guidance shows light rain or drizzle beginning at the
coast/west slopes of the Coast Range Saturday evening before
expanding into the Cascade foothills and eastern Portland/Vancouver
metro Saturday night/early Sunday morning. NBM PoPs remain too low
based on ensemble guidance, forecast soundings, and the overall setup
in place (moist westerly upslope flow with saturated conditions in
the boundary layer; it still appears the marine layer will deepen to
1-1.5 km according to NAM/GFS soundings). Therefore decided to
increase NBM PoPs by blending in CONSAll guidance. In addition to the
light drizzle/light rain in the aforementioned areas, expect
expansive cloud cover Sunday morning across nearly all of northwest
OR and southwest WA. With the increase in cloud cover/stronger
onshore flow, Sunday will be cooler with highs most likely in the low
to mid 70s across the interior lowlands (except 60s at the coast).
Cannot completely rule out highs in the lower 80s if morning cloud
cover scatters out quickly enough, but the probability of this
occurring has decreased to 1-5%.

Less cloud cover on Monday, but still seasonable with highs in the
60s at the coast and 70s inland. A very brief warm-up is still in
store on Tuesday as a transient shortwave ridge passes through the
region, sending high temps back into the 80s for inland valleys. The
warmest ensemble members show high temps near 90 degrees over the
Willamette Valley, however the chance of this occurring is around 10%
or less. Even if some areas do briefly warm to 90 degrees Tuesday,
conditions will most definitely trend cooler Wednesday into Thursday
as an upper level trough over the northeast Pacific moves into
western WA/OR. This trough will also bring a chance of rain, however
confidence is low regarding the exact timing and exact rain amounts
given the large degree of model spread evident. NBM 6-hr PoPs
currently peak on Thursday between 20-60%, lowest from Florence to
Eugene to Oakridge and highs across southwest WA. The probability for
24-hr rain amounts in excess of 0.25 inches (from 5pm Wednesday
through 5pm Thursday) is at 10-30%, except less than 10% to the south
of Salem, south of Lincoln City, and east of Eugene.  -TK
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&&

.AVIATION...
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VFR prevailing, except for IFR/LIFR cigs for much of
the coast. Coastal stratus is more patchy between KTMK and KAST as
of 10Z Sat, but models and satellite trends indicate it will fill
back in shortly. CONSShort model indicate the coastline will remain
IFR/LIFR through the day with cigs below 1000 ft, but locations just
inland should see improvement to VFR or high end MVFR later this
morning. But that may be short lived as a weak front approaches this
afternoon. HREF probabilistic guidance has high chances (80-90%
chance) for IFR spreading to the coast 21-23Z Sat. Then after the
front shifts further inland, cigs chances for IFR decrease with MVFR
becoming dominant. Inland areas remain VFR until about 04-06Z Sun
when clouds start filling in on the Cascades then back build toward
the valley with about a 40-60% chance for MVFR cigs reaching valley
terminals after 09Z Sun.

Wind generally less than 10 kt through the day.

PDX AND APPROACHES...VFR with mostly clear skies today, then
increasing mid level clouds around 45-60 ft after 06Z Sun. There
is around a 60% chance for cigs 2000-3000 ft after 09Z Sun. Light
northwesterly winds early increasing to 5 to 9 kt after 22Z Sat.
/mh
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&&

.MARINE...
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High pressure gives way to weak front moving through the
waters today. Winds are expected to remain under 20 kt. General
seas around 4 to 6 ft with a 6 to 7 second period, but increasing
westerly swell to around 8 ft is expected by Sunday afternoon.
High pressure gradually returns for the start of next week. /mh
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&&

.PQR WATCHES/WARNINGS/ADVISORIES...

OR...None.
WA...None.
PZ...None.

&&

$$

www.weather.gov/portland

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