647
FXUS66 KPQR 121003
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
303 AM PDT Sat Apr 12 2025

.SYNOPSIS...High pressure will be the primary feature for the
next 5 to 7 days leading to a noticeable warm-up by early next
week. Until then however, lingering cooler temperatures during
the overnight hours likely present a period of frost concerns
focused tonight into Sunday morning. Otherwise, little to no
weather related impacts are anticipated moving forward.

&&

.SHORT TERM...Today through Monday night...Early this morning
the axis of a weak shortwave trough is progressing overhead with
radar imagery depicting a few scattered showers moving into the
north oregon and southwest Washington coast/coast range. Going
through the rest of the day, weak and rather shallow instability
driven by surface heating and a slight decrease in temperatures
aloft (~800mb) embedded within northwesterly flow will likely
facilitate isolated pop-up shower activity across a good chunk
of area - albeit focused over the coast, coast range, and
Cascade foothills. CAMs like the HRRR, NAMNEST, UW WRF, etc. are
all in agreement resolving this activity so PoPs were generally
raised from the NBM to at least get a mention of showers into
the forecast for locations where there is decent CAM agreement.
It`s worth noting one of these showers can`t be ruled out over
the inland valleys as well, particularly Salem northward through
the Portland/Vancouver metro during the midday/afternoon hours,
but the majority of the day should end up dry.


Behind the shortwave a broad area of high pressure will move in
and will remain anchored over the Pacific and along the coast.
This high will keep conditions generally settled, and clear.
Before we get to the warmer temperatures Sunday afternoon/Monday
onward, we have to make it through a night of below normal
temperatures. Tonight, skies are forecast to quickly clear out
which will allow for efficient radiative cooling. This combined
with northerly flow aloft and 850 temperatures around freezing
will aid in keeping the column cool enough to inhibit any warmer
air from mixing down. In addition, due to the stagnant position
of the ridge, winds are not expected to be an issue and
therefore more widespread frost is possible, especially in the
rural portions of the southern Willamette Valley, Coast Range
valleys, and the valleys of the Cascade Foothills will be the
most at risk of frost development. Areas that are shaded will
maintain frost longer through the morning. The Upper Hood River
Valley will see at or below freezing temperatures. The challenge
here is just how low in elevation these temperatures will go.
For right now, the highest probability of temperatures at or
below 32 deg F is from Parkdale south, and at the higher
elevations. However, there is around a 30-40% chance for below
freezing temperatures around Odell. Near the Columbia in Hood
River proper chances are low (less than 5-10%). Given the
beginning of growing season and the higher probability, have
maintained the Freeze Warning for Sunday morning. If
temperatures aloft warm a bit more than expected, overall
surface temperatures may become touch warmer as a result, closer
to 33-36 F warmest scenario. However if that does occur, frost
would still be possible.

Sunny skies on Sunday will lead to a warmer Monday. South to
southeasterly flow will become dominate and usher in warmer air
from the desert southwest. 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 85-90%+ 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 again adjusted a couple degrees higher than
the deterministic NBM - it has a habit of slightly under
forecasting springtime high temperatures in these scenarios.
Blended towards the NBM 75th percentile for the Greater
Portland/Vancouver Metro through the rest of the Willamette
Valley. -Schuldt/Muessle


.LONG TERM...Tuesday through Friday night...Ensemble models are
in fairly good agreement resolving the large scale pattern
through the middle to end of the week with consensus for some
degree of persistent ridging. That said, on late Tuesday and
Wednesday there is a small subset of ensembles members (10-20%)
which show a heavily tilted upper-level shortwave trough sliding
southward into the region from British Columbia due to a larger
ridge feature holding further offshore over the Pacific,
however, the vast majority of members from the EPS, GEFS, and
GEPS keep this shortwave shunted towards the Rockies - the
forecast represents this most likely dry scenario. Still
looking at the potential for breezy conditions from the east
returning late week, but no strong winds are expected.
Temperatures will stay fairly stagnant and above normal mid to
late week with the mean high temperatures for the inland valleys
around 70-78 deg F and lows in the mid 40s. No rain expected
until potentially next weekend but the exact details regarding
timing and amounts remain rather murky at this time.
-Schuldt/Muessle

&&

.AVIATION...Generally VFR conditions throughout the period, with
gradually clearing cloud cover. Around a 30-50% chance of MVFR
CIGs developing in the Willamette Valley after 14z through 20z.
Not expecting widespread MVFR CIGs but a terminal may briefly
drop below VFR. Winds continue shifting northerly, around 5-10 kt
for all terminals through the period. Gusts to 20 kt will be
possible for coastal terminals this afternoon.

PDX AND APPROACHES...Likely VFR through the period in onshore
flow. Around a 20-40% chance of MVFR clouds developing after 14z
and dissipating by 19z but impacts would likely b sporadic if
anything develops. Weak winds overnight will increase to between
5-10 kt from the northwest during the day. -Batz

&&

.MARINE....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. There is a a 60-80%
chance for frequent gusts of 21 kt or greater Sunday afternoon and
evening as the thermal low strengthens and increases the pressure
gradient.

High pressure looks to gradually weaken next week, but conditions
should remain relatively benign as models show no indication of
any stronger systems bringing Gales to the region. -Batz

&&

.PQR WATCHES/WARNINGS/ADVISORIES...

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

     Frost Advisory until 8 AM PDT this morning for ORZ121.

     Freeze Warning from midnight tonight to 8 AM PDT Sunday for
     ORZ121.

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

PZ...None.

&&

$$

www.weather.gov/portland

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