634
FXUS66 KPQR 152340
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
340 PM PST Mon Dec 15 2025

.SYNOPSIS...A continued active pattern will bring multiple
weather hazards through the next week including heavy rain,
urban, small stream, and river flooding, gusty winds, Cascade
snow, and hazardous seas.

&&

.SHORT TERM...Now through Wednesday Night...The region remains
beneath the storm track as repeated shortwave impulses bring
rounds of rain and winds through the next two days. An initial
wave approaching the coast was responsible for earlier rainfall,
however showers continue, particularly over areas of terrain,
where sufficient instability exists above the surface layer to
support narrow bandlets of rainfall. These features should trend
lower in intensity and coverage from north to south through this
evening as the shortwave moves onshore. A strong low-level jet
reaching 60-70 kt only a few thousand feet above the ground
couped with this convective environment has allowed for strong
wind gusts to reach the surface, particularly along the coast
and in areas of terrain so far today. As the jet both weakens
and moves inland, the risk for further strong wind gusts will
decrease into this evening, but isolated occurrences may
continue in exposed areas of high terrain. As such, the High
Wind Warning along the coast has been allowed to expire, but the
Wind Advisory in effect for the Oregon Coast Range and Willapa
Hills will remain in place through 4 PM this afternoon.

The overnight period will offer a brief respite before another
wave brings renewed rain and wind to the region Tuesday into
Wednesday. This system is expected to be more potent than the
first, with a deep surface low tracking inland near the north
end of Vancouver Island and a strong trailing cold front
extending southward to the Oregon coast. Another shorter-
duration period of high vapor transport will be associated with
this atmospheric river, resulting in a relatively brief period
of rain across the region, but one that could nonetheless bring
heavy enough rain rates to continue a risk of flooding in poor
drainage areas, especially given continued high saturation
levels in soils across the region. An additional risk from this
next system will be further strong winds. Again, just ahead of
the cold frontal boundary, a low-level jet reaching 60-70 kt at
925 hPa will move onshore through Tuesday night. As today,
periods of strong wind gusts reaching down to the ground appear
likely, and there is a 30-50% chance of wind gusts of 55 mph or
higher along the immediate coast, and similar or slightly higher
chances in higher terrain and particularly along the Cascade
crest. Chances for gusts reaching 45 mph in valley areas remains
around 10-25%, however an isolated gust over 50 mph also cannot
be ruled out. Additional wind hazards may need if confidence in
occurrence of these winds increases.

Wednesday will bring another relative lull in the pattern, with
brief drying and a break from strong winds through much of the
day. As the front sags southward, a cooler air mass will arrive
in its wake with temperatures closer to seasonal norms but
still about 5 degrees above normal. Snow levels falling to 3000
ft or even slightly lower will allow for some snow to accumulate
as pass level through Wednesday and Wednesday night. This colder
period is when precipitation amounts will be reduced between
systems, so snow amounts remain unremarkable, with a 40-60%
chance of 6" of more of accumulation at Cascade passes. There is
a 10% chance of snow levels falling to 2000-2500 ft. -36


.LONG TERM...Thursday through Monday...Another moderate to
strong atmospheric river takes aim at the region Thursday into
Friday, bringing additional rain on top of elevated rivers and
saturated soils. Initially, heavy rain rates raise the risk for
urban or small creek and stream flooding on Thursday, while the
risk for river flooding increases late Thursday into Friday as
the abundant runoff makes its way downstream. As is typical with
ARs, the highest precipitation totals are likely to be over the
coastal terrain and across the Cascades, and the warm dynamics
will also favor increasing snow levels during the heaviest
precipitation. Rainfall forecasts have tended to shift slightly
between forecast updates, but guidance remains in good agreement
that the AR will impinge on the northern OR coast before
tracking southward toward the southern OR coast by Thursday
night. Unlike recent ARs which have been directed moreso at the
Washington coast, the highest rain totals are currently focused
across the southern half of the region, generally south of
US-26. Chances to exceed 2" of rainfall Thursday and Thursday
night increase from 30% to 50% south along the I-5 corridor from
Kelso/Longview to Eugene, and from 50% to 75% in the Coast Range
and Cascades. Chances to exceed 4" of rainfall are 5-15% across
inland valleys and 20-40% in areas of higher terrain.

This amount of precipitation over only 24 hours would support
river flooding as saturated soils will allow most of the rain to
make its way into area rivers as runoff. Chances for Minor to
Moderate river flooding continue to increase, with the highest
confidence in flooding for rivers draining the central OR Coast
Range, both toward the coast and into the Willamette River.
Those who live near areas prone to river flooding should keep a
close eye on the forecast moving forward, and additional
information can be found in the Hydrology discussion below.

The veritable parade of weather systems does not look to let up
this weekend and into next week. While confidence in forecast
details is low at this lead time, the active pattern and
persistent chances for rain will most likely continue. -36


&&

.AVIATION...Radar imagery as of early Monday afternoon depicts
lingering showers as a system moved through the area. This is
bringing a mix of MVFR and low-end VFR CIGs across all
terminals, with some IFR/LIFR CIGs along the coast. Expect
this trend to continue through 03z Tue, with CIGs briefly
improving to predominately VFR this evening. High confidence
(80% chance or greater) CIGs drop back down to IFR or lower
along the coast after 12-15z Tue as the next system moves in.
During this same time frame, inland terminals also have a 20-40%
chance of dropping back down to MVFR.

Southwesterly winds are gusting to 25-30 kt along the coast and
20-25 kt across the Willamette Valley as of early Monday
afternoon. Winds around 2000 ft are around 40-50 kt, which will
create a strong crosswind especially for E-W aligned runways and
some low level speed shear resulting in turbulence. Winds across
the area weaken overnight, and become breezy again after 12-15z
Tue with the next system with gusts to 20-25 kt across the
region.

KPDX AND APPROACHES...Mix of high-end MVFR and low-end VFR CIGs,
with CIGs trending more VFR this evening. South-southwesterly
winds gusting up to 25 kt through the afternoon. Low level speed
shear is possible due to 40-50 kt winds around 2000 ft. Winds
weaken below 10 kt after 00-03z Tue, with breezy southerly winds
returning after 12-15z Tue. -10

&&

.MARINE...Observations at buoy 46029 and 46050 as of early
Monday afternoon show southerly winds with gusts to 30-35 kt
and seas hovering around 14-16 ft at 9-10 sec. Should be seeing
seas peak around 16-18 ft this afternoon with seas unlikely
rising above 20 ft (only a 1-2% chance of exceeding 20 ft). High
confidence that winds will gradually weaken this afternoon,
decreasing to 10-15 kt with gusts up to 20 kt late this evening.
Seas may remain steep and choppy during this brief lull;
therefore, a Hazardous Seas Warning is in effect across all
waters including the Columbia River Bar from 4 PM this afternoon
to 4 AM Tuesday. Seas fall to 10-13 ft by late Tuesday morning.

The brief lull ends early Tuesday morning when another frontal
system approaches the waters. This will return breezy south-
southwesterly winds with widespread gusts ramping up to 25-30 kt
between 4 AM-10 AM Tue, then 35-40 kt between 10 AM-4 PM Tue,
and strong Gales with gusts up to 48 kt arriving after 4-7 PM
Tue. For the inner waters out 10 NM including the Columbia River
Bar, there is a 20-30% chance for isolated Storm force wind
gusts up to 55 kt due to the potential for a coastal jet. It`s a
marginal set-up for a coastal jet since guidance isn`t really
showing much of an inversion in the low-level atmosphere to
squeeze the strong winds toward the surface. Either way, it will
still be very windy. Behind the front, winds turn westerly
around 1-4 AM Wednesday. Seas will also re-build to 14-16 ft
Tuesday evening, and peaking and holding around 18-20 ft at
10-11 sec between 1 AM-1 PM Wednesday. Another Gale Warning will
likely be issued for this system this afternoon.

Going forward, an active weather pattern continues through the
week bringing additional winds and wave hazards to the coastal
waters. Seas are very likely to remain elevated above 10 ft
(greater than 90% confidence) from Wednesday through Friday as
additional weather systems move onshore. At least models are
beginning to hint as some larger breaks between weather
disturbances next weekend into the following week, but at this
7-8 day time-scale, the confidence in this slowdown
materializing is only low to moderate at this time. -10/27

&&

.HYDROLOGY...Abundant rainfall is a relatively short period from
Thursday through Thursday night atop saturated soils while area
rivers remain high following well above normal rainfall thus
far in December has resulted in an elevated risk for flooding
late this week. As is typical in atmospheric rivers, the highest
rainfall totals are likely to be over areas of terrain including
the Coast Range and Cascades, while lesser totals can be
expected across inland valleys. Nonetheless, rainfall is likely
to be measured in inches across most of the region. Unlike
recent other ARs, this system looks to favor a more southern
placement along the central OR coast of these highest rainfall
totals, which may allow some river basins to better handle these
rainfall amounts without yielding flooding. Other areas to the
north within the Coast Range in Lincoln, Tillamook, Benton, Polk,
Yamhill, and Washington Counties may see more overlap between
recently observed heavy rainfall and additional heavy rainfall
with this upcoming system. Similarly, river basins covering the
central OR Cascades in Lane and Linn Counties have a slightly
more optimistic outlook than north into Marion and Clackamas
Counties, but that does not mean flooding cannot happen in all
of the aforementioned areas. Luckily, where rainfall has so far
been the heaviest across southwestern Washington and far
northwestern Oregon may see the lowest totals from this system,
although even 1-2 inches of rain over 24 hours may be enough to
cause additional flooding in these very saturated areas.

At this point, the rivers with the highest confidence of
reaching flood stage are those which drain the central Oregon
Coast Range, including the Wilson, Trask, Siletz, and Siuslaw
draining toward the coast, and the Marys, Luckiamute, Yamhill,
and Tualatin draining toward the Willamette. The chances for
these rivers to reach at least Minor flood stage are 25-55% at
this lead time. Rivers draining the Cascades, including the
Mohawk, Santiam, Pudding, Molalla, Clackamas, and Sandy, have
slightly lower but still significant chances of 15-45%. These
rivers will begin to rise later on Thursday as heavy rain
continues, not cresting until Friday in most cases. The
Willamette River main stem itself also has a 5-10% chance of
reaching flood stage above Willamette Falls, and slightly higher
chances in tidally-influenced portions downstream of Oregon
City, however runoff will take longer to reach the Willamette
and flooding would be delayed later than in its tributaries. A
Flood Watch has been issues across all of northwestern Oregon
and southwestern Washington from 4 AM Thursday through 4 AM
Saturday to highlight these risks.

These river forecasts remain very sensitive to rainfall
forecasts, and interested parties should expect changes as the
event nears. The latest river forecasts across the region can be
found at www.water.noaa.gov/wfo/pqr. -36

&&

.PQR WATCHES/WARNINGS/ADVISORIES...

OR...None.

WA...None.

PZ...Hazardous Seas Warning until 4 AM PST Tuesday for PZZ210-251>253-
     271>273.

     Small Craft Advisory from 4 AM to 4 PM PST Tuesday for PZZ210-
     251>253-271>273.

     Gale Warning from 4 PM Tuesday to 10 AM PST Wednesday for PZZ210-
     251>253-271>273.

&&

$$

www.weather.gov/portland

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