958
FXUS66 KPQR 111001
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
301 AM PDT Fri Apr 11 2025

.SYNOPSIS...Through the first half of the weekend a cooler weather
pattern persists across the region culminating in a concern for
frost, primarily Saturday night into Sunday morning. Fortunately,
conditions rapidly swing back to warm and dry during the early
to middle portion of next week with little to no appreciable
impacts going forward.

&&

.SHORT TERM...Today Through Sunday...Early this morning the
region sits in a weakly forced post-frontal environment with
broad WSW flow aloft as our latest weather disturbance (the
front on Thursday) departs well to the east. Radar imagery
depicts some isolated to scattered light showers across the
western slopes of the Cascades/Cascade foothills, and to some
extent the Willapa Hills/Coast range, but overall conditions
remain mostly dry through today outside of a stray light shower
or two for locations with terrain forcing.

A secondary upper-level shortwave trough likely clips the
Pacific Northwest late Friday night into Saturday morning
providing a slight boost to shower coverage although guidance
keeps the best changes (20-50%) pinned to the north Oregon Coast
range, Willapa Hills, and the north Oregon Cascades through the
south Washington Cascades. Well need to watch the timing of
this feature closely as a cooler airmass overhead proceeding
the shortwave, defined by -4C to -5C 850mb temperatures, has
the capacity to drop surface temperatures into the mid 30s
possibly even near freezing for valley locations with enough
clearing beforehand. Uncertainty surrounding the extent of cloud
cover makes this a low confidence scenario in picking out which
areas may experience temperatures low enough for frost impacts.
Most likely a few spots in the coast range valleys, and south
Willamette Valley, briefly drop into the mid 30s with minimal
impacts - certainly something to monitor. At least there is
higher confidence in clearing on the lee side of the Cascade
crests by saturday morning, particularly the upper Hood-River
valley, where a Frost Advisory has been issued.

Once the shortwave trough on Saturday quickly progresses
eastward both deterministic and ensemble guidance are in
agreement in the development of a high pressure ridge beginning
Saturday night into Saturday morning. This likely leads to
widespread clearing skies and lighter winds winds across the
region setting the stage for efficient radiative cooling, local
boundary layer decoupling, and thus below normal overnight
temperatures. The latest NBM depicts a 40-70% chance for
temperatures to drop into at least the mid 30s away from the
coast and inner Portland metro. Larger areas of frost appear
more likely as a result when compared to the night prior. Given
the set-up and deterministic/ensemble consistency the last
several forecast cycles have decided to issue Frost Advisories
for the Coast range, coast range valleys though the inland
valleys into the Cascade foothills excluding some portions of
the immediate Portland/Vancouver metro where probabilities to
drop into the mid 30s are less favorable. Meanwhile, the upper
Hood River Valley appears to have a very high chance (80%+) of
experiencing freezing temperatures Saturday night so have
continued with the Freeze Watch - itll likely need to get
upgraded in the near feature should guidance stay the course.
Headed through the rest of Sunday the aforementioned ridge
building overhead will facilitate a decent diurnal temperature
range as highs generally reach into the mid to upper 60s across
the inland valleys. This is over a 30 degree swing from the
forecast morning lows. Dry and calmer weather prevails
accompanied by developing offshore flow. -Schuldt

.LONG TERM...Monday through Thursday...Ensembles are in
excellent agreement shortwave ridging continues the warming
trend into Monday. Confidence is extremely high for 70s to
materialize on Monday with the probability of them occurring at
80-90%+ at most sites in the Willamette Valley and other inland
valleys per the NBM. With the axis of a weak thermal trough
passing overhead on Monday in addition to weak warm advection
aloft, high temperatures were adjusted a couple degrees higher
than the deterministic NBM - it has a habit of slightly under
forecasting springtime high temperatures in these scenarios.

Uncertainty in the extended forecast does begin to grow a bit
towards the middle of next week, but is still relatively
limited. Most of the global ensemble members suggest ridging
temporarily flattening with above average heights still remaining
over the region Tuesday and Wednesday. While temperatures
appear most likely to slowly cool during this period due to the
flattening ridge, temperatures should still remain above
normal. With that said, there is ~10% of the global ensemble
membership that bring a deeper shortwave trough into the
northern Pacific Northwest that would result in a return to a
cooler and wetter weather pattern. Given more than majority of
ensemble members suggest dry weather and mild temperatures
continuing, kept the NBM forecast as is in the extended which
keeps temperatures well above average and keeps mentionable (15%
or higher) rain chances out of the forecast. The majority of
ensemble members then rebuild a broader upper ridge back
overhead for the end of the week.  -Schuldt/Neuman

&&

.AVIATION...Generally VFR conditions through some MVFR CIGs in the
central Willamette Valley, impacting KSLE and KUAO. MVFR CIGs may
linger over the next few hours before winds turn northerly and
advect drier air into the region. Winds currently fairly weak and
variable under 5 kts throughout the area, but will begin picking
up around 18z Fri out of the north. Conditions should become VFR
for all terminals by the afternoon and maintain into the overnight
hours.

PDX AND APPROACHES...VFR conditions through the period, with a
broken deck gradually rising from around 4000-5000 ft to 8000-1000
ft by the end of the TAF period. Winds currently weak and variable
around 5 kts, increasing to 10 kts around 18z and shifting
northerly at that time. -Batz/JLiu

&&

.MARINE....A westerly swell will push seas back up to near 10 ft
today, particularly across the northern waters. Wave period around
8 to 10 seconds will lead to steep seas, hazardous to small
craft so will maintain a Small Craft Advisory to cover this
hazard through the afternoon. A much weaker front will drop
southeastward across the waters Friday night, but the odds for
wind gusts climbing up towards 21-25 kt is less than 10%.

There is high confidence high pressure will build over the Pacific
Northwest and thermally induced lower pressure will develop over
California and the southern Oregon coast late in the weekend. This
will lead to a Summer-like wind pattern with building northerly
winds across the waters during this time. Winds will generally be
strongest off the central coast of Oregon and weakest off the
south Washington coast during this time. Expect winds to peak
across the inner waters during the late afternoon and evening
hours, and gusty northerly winds to persist across the outer
waters during the overnight hours.

High pressure looks to gradually weaken next week, but there are
few signals among the weather models of any major fronts crossing
the waters that would produce Gale Force or higher winds. -Batz

&&

.PQR WATCHES/WARNINGS/ADVISORIES...

OR...Frost Advisory from midnight Saturday night to 8 AM PDT Sunday
     for ORZ104>110-113>119-123>125.

     Frost Advisory from midnight tonight to 8 AM PDT Saturday for
     ORZ121.

     Freeze Watch from late Saturday night through Sunday morning for
     ORZ121.

WA...Frost Advisory from midnight Saturday night to 8 AM PDT Sunday
     for WAZ202>205-208.

PZ...Small Craft Advisory until 5 PM PDT this afternoon for PZZ210-
     251>253-271>273.

&&

$$

www.weather.gov/portland

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