Rainier, Oregon Weather Forecast Discussion

277
FXUS66 KPQR 130934
AFDPQR
Area Forecast Discussion
National Weather Service Portland OR
234 AM PDT Sat Sep 13 2025
.SYNOPSIS...Shallow marine stratus and/or fog this morning
should clear out by the early afternoon. The next frontal
system arrives tonight into Sunday, bringing widespread rain
showers. Conditions dry out and warm up again early next week
with the potential for offshore winds Monday night through
Tuesday. Another weak front returns onshore flow and
precipitation chances mid-week.
&&
.SHORT TERM...Now through Monday Night...Satellite imagery as of
early Saturday morning depicts patchy, shallow marine stratus
along the coast, Lower Columbia River Valley, and Coast Range
lowland valleys. The axis of an upper level shortwave ridge is
currently centered over the Pacific Northwest, which allowing
for a much shallower marine layer compared to previous nights.
This is pretty evident from the latest surface observations
along the central Oregon coast where cloud ceilings are around
100 ft or less, essentially becoming fog and reducing visibility
to 1/2 mile or less. If you`ll be driving in fog this morning,
especially along the coast, slow down, use your low beam
headlights, and keep your distance from other vehicles.
Later this morning around 4-7 AM, model soundings suggest that
shallow stratus will also develop across the Willamette Valley
and southwest Washington lowlands as the surface moistens under
the nighttime inversion. Gradual clearing of low stratus and/or
fog should begin around 9-11 AM as daytime heating progresses
and leads to better mixing of the lower atmosphere. We should
see mostly sunny skies by the early afternoon with high
temperatures in the upper 70s to low 80s across interior
valleys and 60s along the coast. By this evening, the
aforementioned shortwave ridge progresses eastward with
increasing cloud cover across the area as the next frontal
system approaches.
Cooler temperatures and widespread light rain showers are
likely (70-80%) by Sunday morning as a shortwave trough and
accompanying cold-front swings through western Oregon and
southwest Washington. While precipitation along the immediate
frontal boundary may be more stratiform in nature, rainfall
quickly transitions to lingering showers by the midday hours.
Forecast rainfall totals from late Saturday evening through
Sunday evening are forecast to be in the 0.10-0.20" range
across most interior valleys, 0.20-0.40" along the coast, and
0.30-0.50" across the Coast Range, Willapa Hills, and Cascades
with the highest relative amounts on the western slopes of these
terrain features. However, confidence in these totals are only
moderate as over the past several model runs guidance has slowly
been trending downward with our storm totals as is often the
case for these early fall systems. That`s not to say large
swaths of the region won`t get any rain at all, but amounts
outside of the west slopes of the coast range appear rather
light. By Sunday evening, the front further weakens and rapidly
exits to the east allowing any lingering shower activity to
taper off overnight into early Monday morning. Shortwave ridging
returns on Monday as will drier conditions and clearing skies
by Monday afternoon.         -Alviz/Schuldt
&&
.LONG TERM...Tuesday through Friday...Monday night into Tuesday
there is high agreement among ensemble members in a return to
above-average 500 mb heights as the ridge axis centers over the
Pacific Northwest. This is in addition to a near surface
thermal trough amplifying across western Oregon with higher
pressure east of the Cascades, leading to an offshore flow wind
pattern and a sharp spike in high temperatures. Afternoon highs
on Tuesday are forecast in the mid 80s across most interior
valleys with upper 60s to low 70s along the coast. There
remains some uncertainty in the exact placement the thermal
trough feature which would determine the strength and western
extent of offshore winds, but for now they don`t appear
particularly strong. Still, a -3 to -5 mb pressure gradient
from KTTD to KDLS Tuesday may yield breezy east winds through
the Columbia River Gorge with gusts up to 25-35 mph east of
Troutdale. This Monday night through Tuesday period needs to be
monitored in the coming days given an expected drop in relative
humidity compared to past days. This offshore flow pattern looks
brief at least, since the thermal trough breaks down Tuesday
night into Wednesday and returns westerly onshore flow.
Wednesday, most ensemble members are in agreement of an upper
level trough in the northeast Pacific approaching British
Columbia. A weak front associated with this system will swing
through the Pacific Northwest and return chances (20-40%) for
rain showers, with the highest chances along the coast and
southwest Washington. Forecast confidence decreases Thursday to
Friday as ensemble guidance struggles with the exact
progression of the upper level pattern. Most ensembles still
maintain general troughing over the area, however, there is high
uncertainty with the exact magnitude, track, and timing of the
trough.      -Alviz/Schuldt
&&
.AVIATION...VFR conditions beneath largely clear skies inland
will trend to MVFR as low marine stratus develops by 10-14z Sat
with calm to light and variable winds. While MVFR conditions are
favored (about 60% chance), there is a 30-40% chance of IFR cigs
around 500-1000 ft or even lower from 12-16z Sat. Additionally,
there`s a 10-20% chance of vis restricted to 1SM or less within
fog during this timeframe, mainly for Portland area terminals and
KEUG. A relatively shallow marine layer is expected, allowing for
an earlier break out to VFR from 17-19z Sat, with winds building
out of the west to southwest at around 5 kt. High clouds begin to
increase after 00z Sun ahead of an approaching cold front with
rain showers expected at inland terminals beginning between 08-12z
Sun.
Along the coast, IFR/LIFR conditions are expected to continue
through the overnight period with cigs near 500 ft, vis restricted
to 1SM or less within fog/mist, and light and variable winds.
Conditions will begin to slowly improve after 15z Sat as winds
build out of the southwest to near 10 kt, with MVFR cigs likely to
persist through much of the day. Cigs will trend down toward IFR
again after 03-06z Sun with cold frontal rain showers arriving
arriving after 05-09z Sun.
KPDX AND APPROACHES...VFR conditions expected to continue through
10-14z Sat with northwest winds of less than 5 kt. MVFR cigs
below 2 kft are most likely to develop between 10-14z Sat, but
there are also 30-40% chances of IFR cigs at 1000-500 ft, and
lesser (around 10-20%) in vis below 1SM within fog. The worst
flying conditions are expected from 13-16z Sat, before the onset
of mixing will begin to improve vis and/or lift cigs. An earlier
break out to VFR is expected thanks to a shallow marine layer,
likely by 17-19z Sat. West winds around 5 kt will continue
through Saturday evening. Increasing high clouds after 00z Sun
ahead of an approaching cold front which will bring rain showers
starting between 08-12z Sun.     -Picard/HEC
&&
.MARINE...A cold front approaches the waters today, moving through
the waters tonight into Sunday morning. South winds expected to
increase this evening to 10-15 kt with gusts to around 20 kt
through Sunday morning with occasional gusts up to 21-25 kts for
the far outer waters around 50-60 NM away from the coast. With
gusts above 21 kts not expected to be widespread, no small craft
advisory is being issued. Seas 6-8 feet at 10-13 seconds expected
through the weekend as a northwesterly swell moves into the
waters.
High pressure returns over the waters Sunday afternoon, shifting
winds northerly again. Pressure gradients expected to tighten over
the waters through Tuesday, and with this, could see some gusts
to around 21 kt during the afternoon and evening hours each day.
Another weak front is expected to move through the waters midweek,
briefly shifting winds west to southwest.     -HEC
&&
.PQR WATCHES/WARNINGS/ADVISORIES...
OR...None.
WA...None.
PZ...Dense Fog Advisory until 11 AM PDT this morning for PZZ210-
     251>253.
&&
$$
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NWS PQR Office Area Forecast Discussion