Lake Merwin Washington Weather Forecast Discussion

859
FXUS66 KPQR 072150
AFDPQR
Area Forecast Discussion
National Weather Service Portland OR
250 PM PDT Tue Oct 7 2025
.SYNOPSIS...Warm, dry, and sunny conditions today will give way
to cooler, cloudier, and wetter weather beginning tomorrow and
continuing through the weekend. Low pressure meandering offshore
will bring persistent chances for rainfall, most notably as the
system moves inland and overhead late Friday into Saturday.
&&
.DISCUSSION...Now through Monday...Heights aloft have begun to
fall ahead of an upper-level trough deepening as it swings
southward off the coast of British Columbia while upper-level
ridging shifts east of the Rockies. At the surface, thermal
troughing along the coast is similarly shifting inland, with
pressure gradients supporting earlier easterly flow reversing
to yield increasing onshore flow through this evening. This
change of airmass will bring much cooler temperatures and
increased cloud cover to the region beginning Wednesday.
As the trough pinches off to form a closed upper low atop a
deepening surface low centered west of the central Oregon coast,
chances for rainfall will increase through the remainder of the
workweek. There remains some uncertainty in the position of
these vertically-stacked lows, which will affect when rainfall
may begin across the region. At this time, there is a 30-40%
chance rainfall may begin as early as Wednesday night along the
coast and across the western slopes of the Cascades, and around
a 50% chance for inland valley locales by Thursday evening,
although these chances are sensitive to the position of the
low. There is high confidence, 70-90% chances, in rainfall
across the region by Friday afternoon and continuing through
much of Sunday as consensus remains high that the broad area of
low pressure will move inland and overhead through the weekend.
For the period from 5 AM Friday through 5 AM Monday, a
reasonable low-end rainfall accumulation, or the rainfall
amount with a 75% chance of exceedance, is 0.55-0.75" along the
I-5 corridor, and 0.75-1.5" along the coast and in the higher
terrain of the Coast Range and Cascades. Conversely, the
reasonable high-end accumulation (25% probability of exceedance)
is 1-1.5" along the I-5 Corridor and 1.5-3" along the coast and
in the higher terrain of the Coast Range and Cascades. Or in an
alternative frame, the chances of exceeding 0.5" and 1" of
rainfall in the 72-hour period are, respectively, 80-90% and
35-55% along the I-5 corridor, and 90-100% and 75-90% along the
coast and in the higher terrain of the Coast Range and Cascades.
As the aforementioned low moves overhead, temperatures will
continue to trend cooler across the region, with the snow level
falling to 4500-5000 ft by Sunday morning. This will allow snow
to accumulate in the High Cascades, however only light and
minimally-impactful accumulations are expected at pass level.
Frost Advisories may need to be issued in the Upper Hood River
Valley as well as valleys within the Cascades and Coast Range
as overnight lows fall into the 30s through the weekend.
Elsewhere, temperatures will remain cool with daytime highs in
the 50s and 60s and overnight lows in the 40s. -36
&&
.AVIATION...Dry, southwest flow aloft as high pressure shifts
east. VFR with clear skies will continue through at least this
afternoon along the coast and much of tonight inland. An upper
level trough dropping south from the Gulf of Alaska will push a
weak front toward the area this evening. Expect increasing clouds
with MVFR marine stratus pushing onto the coast by 03z Wed. As the
front progresses inland, expect increasing clouds with CIGs
lowering. There is around a 30% chance of MVFR CIGs across
northern portions of the Willamette Valley, including KSLE to
KPDX, while higher chances (70-80%) for MVFR exist across the
southern Willamette Valley between 12-18z Wednesday. Winds have
turned onshore NNW at the coast, while gusty east winds near KTTD
and the western CR Gorge are expected to continue through at
least 00z Wed.
KPDX AND APPROACHES...VFR and clear skies through tonight. Gusty
east winds should remain east of the terminal from KTTD into the
western CR Gorge today, while KPDX sees more variable winds 5 kt
or less. Increasing clouds likely by 12z Wednesday with a 20-30%
chance of MVFR CIGs. /02
&&
.MARINE...Low pressure shifts inland later today as a weak cold
front approaches the waters from the north, causing winds to
become more north to northwesterly. Winds are expected to
increase through tonight, with more widespread gusts to 25 kt
across the outer coastal waters. A Small Craft Advisory remains
in effect through 5 AM PDT Wednesday. Seas around 3 to 4 ft are
expected to gradually build to around 7 to 8 ft tonight, becoming
more steep and choppy.
An area of weak low pressure approaches the coastal waters on
Wednesday while winds are expected to weaken, followed by
offshore flow on Thursday as the surface low continues to drop
south. Seas are likely to linger around 5 to 7 ft through the end
of the week. Still some uncertainty where the surface low hangs
out at on Friday, but will likely push inland by Saturday. /02
&&
.PQR WATCHES/WARNINGS/ADVISORIES...
OR...None.
WA...None.
PZ...Small Craft Advisory until 2 AM PDT Wednesday for PZZ210.
     Small Craft Advisory until 5 AM PDT Wednesday for PZZ251>253.
     Small Craft Advisory until 5 AM PDT Wednesday for PZZ271>273.
&&
$$
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NWS PQR Office Area Forecast Discussion