266
FXUS66 KPQR 020455
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
955 PM PDT Tue Apr 1 2025

.SYNOPSIS...Cool and showery conditions will remain in place
through Wednesday as upper level troughing remains over the
region, with high temperatures in the 50s. Chilly overnight lows
bring frost potential Wednesday night and Thursday night,
albeit fog is favored over frost Wednesday night while frost is
favored over fog Thursday night. Expect warmer and drier
conditions with highs near or above 70 degrees Friday into the
weekend.

&&

.SHORT TERM...Now through Thursday Night...Satellite and
surface weather observations from early to mid Tuesday afternoon
showed scattered light rain showers developing over southwest WA
and northwest OR in response to daytime heating. Expect showers
to decrease in coverage Tuesday evening as heating wanes,
albeit some isolated showers will likely remain. Most showers
should be light, however the strongest showers could produce
brief periods of moderate rain. Model soundings continue to
suggest these showers will be vertically shallow with a minimal
threat of hail and/or thunder. There could be a few cells that
overperform and produce one or two flashes of lightning and/or
hail smaller than the size of peas, however the probability for
thunder is generally less than 15%. Note some locations will
stay dry through the remainder of the day due to the hit-or-miss
nature of these showers.

Showers will become more widespread on Wednesday as a shortwave
trough and associated vort max traverses the region in northwest
flow. Similar to Tuesday, thunder is unlikely to occur with
these showers due to a lack of surface-based instability. If a
few breaks in cloud cover occur, there could be a very isolated
weak thunderstorm or two that produce one or two flashes of
lightning and small hail, however probabilities for thunder
remain less than 15%. Expect precipitation amounts between a
tenth and a quarter inch along the coast and around a tenth of
an inch or less inland. Minimal snow accumulations are expected
for the Cascade passes, with snow levels hovering around
2500-3000 feet.

Models suggest showers will linger over the area Wednesday
evening before becoming isolated and mainly confined to the
coast and mountains Wednesday night. The NBM does suggest cloud
cover will attempt clearing out after midnight, however some
model soundings show a deep layer of cloud cover lingering
through the night, particularly the NAM. The HREF ensemble mean
for cloud cover also shows minimal breaks in low cloud cover.
Given the showery activity that is set to occur Wednesday
morning through the evening, the ground will be wet overnight.
This will tend to favor the formation of fog rather than frost
in the lowlands, assuming cloud cover clears out enough to allow
for air temps to cool down to the dew point temperature. While
frost cannot be completely ruled out in the Willamette Valley
and Cascade foothills, it appears it would be quite patchy in
coverage. As such, Frost Advisories likely won`t be needed
Wednesday night unless the forecast changes. Thursday still
looks to be a day of transition as dry northerly flow develops
behind the departing upper level trough, bringing mainly dry
conditions with seasonable temperatures.

Frost is more likely to occur Thursday night as the ground will
be relatively drier at that point, surface dew point
temperatures will be slightly lower, winds will be calm and
skies will be completely clear through the night with widespread
lows in the mid 30s. Suspect at least some zones will eventually
need a Frost Advisory, but will hold off for now until
confidence in exact overnight lows is higher. -TK


.LONG TERM...Friday through Monday Night...After another chilly
start to the day on Friday with potential areas of frost, expect
rapidly warming temps through the afternoon thanks to plentiful
sunshine and an incoming upper level ridge over the coastal
waters. The NBM 50th percentile seems reasonable for high temps
on Friday, suggesting widespread highs well into the 60s, except
around 70 degrees in the Portland/Vancouver metro. This makes
sense as the metro will be under the influence of low-level
offshore flow with easterly winds increasing through the western
Columbia River Gorge into the eastern metro.

Models and their ensembles show the ridge axis moving inland
over western WA/OR on Saturday while low-level offshore flow
remains in place. As such, temps will be even warmer on Saturday
with widespread highs in the low to mid 70s. Even coastal
locations will likely flirt with 70 degrees thanks to offshore
flow. Probabilities for highs of 80 degrees or warmer have
decreased to 0-1% on Saturday, suggesting highs in the upper 70s
is a reasonable high-end solution.

Despite high confidence in the forecast for Friday and Saturday,
forecast confidence remains low on Sunday. The uncertainty on
Sunday is being driven mainly by model timing differences in the
breakdown of the aforementioned ridge and subsequent arrival of
the next upper level trough and associated cloud cover and rain
ahead of it. The GEFS/GEPS ensemble mean for 6-hr QPF both show
measurable rain beginning over western WA/OR sometime Sunday
morning, while the ENS mean holds off until the
afternoon/evening. To complicate matters further, all three
ensemble systems have some members showing rain holding off
until Sunday night. This is resulting in a very large degree of
model spread for high temps Sunday, which the NBM 10th-90th
percentile shows well, depicting high temps ranging anywhere
from the upper 50s to near 80 degrees. If rain moves in early in
the day, highs will likely be in the upper 50s to lower 60s. If
rain moves in during the middle part of the day, highs would
likely be in the mid 60s to around 70 degrees. If rain holds off
until Sunday night, expect highs in the mid to upper 70s.

Despite being further out in time, model spread actually
decreases on Monday compared to Sunday. This is due to higher
confidence the aforementioned ridge will for sure be to our east
by Monday at the latest, resulting in a relatively cooler
weather pattern with chances for showers. 6-hourly PoPs
currently range between 40-60% through the day Monday. -TK

&&

.AVIATION...An initial weak trough has exited eastward out of
the region with isolated light rain showers continuing across
the Cascades, while a second weak trough approaching the coast
from the northwest. Renewed coverage of rain showers has begun
at AST and will reach southward to ONP and eastward to inland
terminals by 08-10Z Wed. MVFR cigs remain favored at AST/ONP
within onshore flow and during showers, with low-VFR cigs at 3-4
kft more likely from Portland-area terminals south through the
Willamette Valley. Light showers are not anticipated to restrict
vis below 6SM, however brief periods of MVFR vis within heavier
showers cannot be ruled out. As showers progress to the
southeast through the morning, increased diurnal mixing will see
cigs lift to 4-5 kft, before showers end by 00-06Z Thu as
ridging begins to build over the Northeast Pacific. Winds will
remain generally out of the west at less than 5 kt tonight,
before increasing to 5-10 kt by 18Z Wed and turning out of the
northwest to north after 00Z Thu.

PDX AND APPROACHES...Latest radar trends continue to bring the
next round of showers toward PDX around 09-10Z Wed, although
impacts at the terminal are not expected aside from brief MVFR
cig/vis possible if a heavy shower moves directly overhead,
however latest guidance suggests the majority of heavier showers
will be focused to the east and southeast, nearer to the
Cascades. Following a minor lull in shower activity through
the late morning, additional diurnal showers are expected
through the afternoon, although lifting cigs will make MVFR
conditions less likely despite increased shower coverage. Again,
brief restricted vis remains a possibility in the low-
probability case a shower moves overhead. Showers will quickly
decrease in coverage by 00-03Z Thu, with dry conditions expected
thereafter. Northwest to west winds of 5 kt or less are
expected overnight, increasing to 5-10 kt Wednesday afternoon,
and diminishing again through Wednesday evening. -Picard

&&

.MARINE...High pressure over the northeast Pacific and lower
pressure inland will lead to modest westerly, northwesterly and
northerly winds through the work week. Seas will steadily
decrease through the end of the week. High pressure will shift
over the waters early in the weekend before giving way to a cold
front late in the weekend. There is a 20% chance that Gale
Force southerly wind gusts occur ahead of the front sometime on
Sunday, but a greater than 90% chance that seas will climb into
the teens by late Sunday or early Monday as a southwesterly to
westerly swell push into the waters.

Finally, we are in a period of stronger ebb currents. The
Wednesday  morning ebb is in our very strong category and the
Thursday morning ebb is in the strong category.

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...None.
WA...None.
PZ...Small Craft Advisory from 6 AM to 10 AM PDT Wednesday for
     PZZ210.
&&

$$

www.weather.gov/portland

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