873
FXUS66 KPQR 231308
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
508 AM PST Sun Feb 23 2025

.SYNOPSIS...Active weather continues through Monday night as a
moderate to strong atmospheric river and multiple frontal
systems bring moderate to heavy rain and periods of strong winds
to the area today and Monday. Greatest flood concerns for
coastal rivers, with ponding of water and rises of creeks and
streams as the main concern inland. Quieter weather expected
during the middle to later part of the coming week.

&&

.SHORT TERM...Now through Tuesday Night...Steady rain is
ongoing across northwest Oregon and southwest Washington early
this morning as a moderate to strong atmospheric river remains
focused on the Pacific Northwest. Rainfall rates have remained
rather modest overnight, with most locations seeing no more than
a tenth of an inch or so per hour, thought parts of the Oregon
Coast Range have approached a quarter inch per hour at times.
Event totals so far range from 1.5-3.0 inches along the coast
and coastal terrain, to 0.5-1.0" in the interior valleys, and
1.0-1.5" in the Cascades. Still expect the heaviest rainfall to
develop later this morning through this afternoon as
precipitable water values remain above 1 inch while IVT values
max out in the 500-750 kg/m/s range along the Oregon Coast.
Rainfall rates will be aided by forcing from a shortwave trough
embedded in southwest flow aloft, along with the next
associated surface frontal boundary moving into the area this
afternoon. Model QPF amounts for today have increased slightly
from previous forecast packages, with coastal areas and the
higher terrain expected to see another 1.25-2.50" of rain
through this evening while the interior lowlands pick up an
additional 0.75-1.25". A few orographically favored spots in the
Coast Range and Cascades could pick up as much as 3.0-3.5"
through this evening. Forecast rainfall amounts have come down
a bit along the south Washington Coast today as the most
favorable atmospheric river metrics become centered along the
central Oregon Coast by this afternoon, which could lead to some
adjustments in the forecast for rivers in southwest
Washington.

Precipitation will taper off fairly rapidly later this evening
as the frontal system departs the area. Expect a lull in
precipitation through tonight as transient shortwave ridging
moves overhead, though light rain may linger across Lane County
overnight as the axis of precipitation drops south into
southwest Oregon. Otherwise, much of the area has a shot to
remain dry late this evening through early Monday before the
final shot of moderate to heavy precipitation Monday into MOnday
night. Rain will quickly spread back into the area from the
south on Monday morning as low pressure system rapidly deepens
offshore, sending the last in the series of frontal systems
across the area. IVT values will increase back into the 250-500
kg/m/s range, supporting another 1-2 inches of rain for coastal
areas while areas east of the Coast Range see an additional
0.5-1.0" through early Tuesday morning. All told, expected QPF
today and Monday coupled with the rain that has already fallen
keeps event total rainfall amounts in line with previous
forecast amounts of 4-6" in coastal and higher terrain areas,
though some spots in the Coast Range will likely reach as high
as 7-8" through early Tuesday morning. Meanwhile, the interior
lowlands remain on track for upwards of 2.5-3.0" and perhaps
slightly higher amounts in some locations. Refer to the
Hydrology section below for details regarding potential flood
impacts from this event.

Wind remains the other primary forecast concern as a pair of
seasonably strong frontal systems crosses the region today and
Monday. The next front will arrive on the coast this afternoon
and looks to bring winds similar in magnitude to yesterday
across the area, with widespread 40-50 mph gusts along the coast
and 25-35 mph gusts in the interior valleys. Could again see
some slightly stronger gusts on beaches and headlands this
afternoon, but do not expect gusts to reach 60 mph with this
system. The probability for stronger gusts as high as 45 mph
remains low in the interior valleys today as the NBM puts
chances at around 10% while higher resolution guidance is
around 10-20%.

Monday`s system still represents the best chance for stronger
winds to impact the area as a strong low pressure system passes
offshore of the Oregon and Washington Coasts Monday afternoon.
Models have displayed good run to run consistency in depicting
a surface low bottoming out around 980 mb as it tracks northeast
towards Vancouver Island by Monday evening, passing
approximately 60-100 nm off the north Oregon Coast late Monday
afternoon. This lends increasing confidence to the forecast,
which is reflected in increasing model probabilities for warning
level winds along the coast and advisory level winds inland. A
High Wind Watch remains in effect for the coast, Coast Range and
Willapa Hills Monday morning through early Tuesday morning,
with probabilities for 60+ mpg gusts sitting at 60-80% for the
immediate coast. Held off on upgrading to a warning at this time
as confidence in warning level winds remains a bit lower for
inland coastal communities and parts of the Coast Range.
Regardless, most of these areas can be expected to see
widespread gusts of at least 45-55 mph Monday into Monday
night. Meanwhile, opted to issue a Wind Advisory for the
Willamette Valley and Portland/Vancouver Metro Monday afternoon
through early Tuesday morning as NBM and hi-res model probs
depict a 40-70% chance for 45 mph gusts from Portland to Eugene
Monday and Monday night. Given saturated soils from ongoing
moderate to heavy rain, these winds will have the potential to
result in downed weakened or dead trees.

Heavier rain and stronger winds will come to an end by early
Tuesday as an upper level trough crosses the region Monday
night, with lighter rain showers possibly lingering across the
area into Tuesday afternoon. The trough will lower snow levels
back down to 3500-4000 feet Monday night into Tuesday morning,
and as such will bring accumulating snow back to the Cascade
passes. Snow amounts look likely to remain below advisory
criteria at this time, but NBM probabilities do show a 30-40%
chance for as much as 6 inches of accumulation through Santiam
and Willamette passes through late Tuesday morning. The forecast
for now calls for 2-5 inches of accumulation at pass level
Monday night into Tuesday. /CB

.LONG TERM...Wednesday through Saturday...Ensemble guidance
continues to heavily favor a ridgier pattern mid to late week,
promoting a potentially warmer and drier pattern across the
region. Although the bulk of ensemble members do keep the area
dry Wed-Fri, the forecast maintains a chance of rain on Thursday
as some solutions depict a weak system moving through the top
of the ridge, but there is good agreement that any precipitation
amounts would remain light across the area. NBM continues to
depict high temperatures in the upper 50s to low 60s int he
interior valleys during the latter half of the week. Guidance
hints at another trough moving into the region next weekend, but
confidence remains low at this time. /CB

&&

.AVIATION...The TAF period will be dominated by widespread
stratiform rain and a mixture of flight conditions from VFR to
IFR. Drops in flight conditions will likely coincide with
moderate to heavy precipitation. This will result in surface
visibilities falling into the 2-5 SM range as well as cigs
lowering below FL025. Higher probabilities exist for IFR
conditions along the coast and MVFR conditions inland through the
TAF period. In addition to the widespread rain, gusty south winds
will continue across the airspace with gusts up to 45 kt along the
coast and 35 kt inland with the exception of KTTD which could see
gusts up to 50 kt. Strongest winds expected starting 20Z Sunday
through 06Z Monday. Also expecting LLWS for KONP starting around
20Z Sunday through 00Z Monday. Overall, conditions will be very
chaotic through the TAF period.

PDX AND APPROACHES...Steady rain along with a mixture of VFR to
IFR conditions expected throughout the TAF period, with a trend
towards predominately MVFR conditions. Gusty south winds will
continue with gusts up to 35 kt, strongest gusts expected
00Z-06Z Monday. /42

&&

.MARINE...Active weather will continue across the waters through
at least Tuesday. Expect gale force wind gusts to continue
through this afternoon, except over PZZ271 (the northern outer
waters) where winds have decreased to around 30 kt. Aside from
this zone, Gale Warnings remain in effect through Sunday evening
to cover the ongoing wind threat. During this time, there will
be around a 20% probability for isolated gusts up to 55 kt for
all zones but PZZ271, which will have around a 20% probability
for gusts up to 40 kt.

Sunday night into early Monday morning, winds across all marine
zones briefly weaken with gusts up to 30 kt. Seas also subside
through early Monday morning towards 11 to 14 feet at 13 seconds,
this will result in steep and choppy seas.

Monday to Tuesday, a strong low pressure system tracking roughly
parallel to the OR/WA coast as it moves northward towards
Vancouver Island is forecast to significantly increase southerly
winds and result in building seas. Starting around mid morning
Monday, southerly winds are forecast to increase to 35-45 kt
sustained with wind gusts up to 55 kt. Expect these winds to
persist through Monday night and ease sometime after early Tuesday
morning. Seas during this time will be very steep and choppy. In
addition to high wind waves, a dominant southwesterly swell will
also result in combined seas building above 20 feet by late
Monday morning and peaking Monday evening through early Tuesday
morning around 25-30 feet at 15 seconds. Seas are not forecast to
fall below 20 feet until late morning Tuesday. Will maintain the
current Storm Warning across all waters, including the Columbia
River Bar. This warning is in effect from Monday morning to
Tuesday morning.

In addition, confidence is high that high surf advisory criteria
will be met with breakers up to 35 ft. A High Surf Advisory has
been issued for this, which is discussed in more detail in the
beach hazards discussion below. /42-TK

&&

.BEACH HAZARDS...There is high confidence for high surf conditions
at the coast from 4 PM Monday to 4 PM Tuesday, which is when a
High Surf Advisory is in effect. This is when large waves and
hazardous surf conditions are expected due to an increasing west-
southwesterly swell, peaking around 30 ft and producing breakers
up to 35 ft. Destructive waves may wash over beaches, jetties, and
other structures unexpectedly. People can be swept off rocks and
jetties and drown while observing high surf. Minor beach erosion
may damage coastal properties and buildings. Higher than normal
water run-up is expected on beaches and low-lying shoreline.

In addition, cannot rule out minor tidal overflow flooding around
high tide on Tuesday, however confidence is low as the outcome
will be highly dependent on exact river levels and if they are
able to reach approximately 80% of flood flow or higher. This is
because the forecast total tide alone is not high enough to
suggest coastal flooding will occur, however background river
levels must also be considered in addition to the total tide
forecast when determining potential flood impacts. As of right
now, it appears we will be just shy of meeting criteria to issue a
Coastal Flood Advisory, but this could change with future forecast
updates if confidence increases. -TK/42

&&

.HYDROLOGY...Ongoing atmospheric river event will be enhanced
by additional systems crossing the region today and Monday,
bringing periods of moderate to heavy rain each day. Factoring
rain that has already fallen, forecast event total rainfall
amounts ending Tuesday morning remain on track for 5-6 inches
along the coast and in the adjacent higher terrain as well as
over the Cascades, with 2-3 inches likely in the interior
valleys. Amounts of 7-8 inches look increasingly likely in
orographically favored parts of the Oregon Coast Range as well
as the High Cascades. Generally expect another 1-2 inches of
rain along the coast and in the mountains both today and
Monday, and up to one inch for the interior lowlands each
period. Latest HREF and NBM guidance continues to indicate the
potential for hourly rain rates near 0.25-0.3 inches at times
along the Coast Range and Cascades.

This will certainly produce rises on area rivers. Expect to see the
largest responses by this evening on some of the flashier
rivers in the Coast Range and Willapa Hills, including the
Grays, Wilson, and Siletz Rivers. Meanwhile, several slower
responding rivers draining from the Coast Range are forecast to
reach action stage Monday into Tuesday. Additionally, a number
of rivers, streams, and creeks in the interior lowlands that
drain from either the Coast Range or the Cascades are now
expected to see rises into at least Action Stage with several
locations peaking at or just under minor flood stage. This
includes the Clackamas River near Estacada and at Willamette
Falls. Significant snow melt in the higher terrain will
contribute to the increase in river levels. This increase snow
melt along with the hourly rain rates will also enhance the
chance of landslides in the higher terrain.

The primary impacts in lowlands will likely be related to ponding of
water on roadways and poor drainage in low lying areas. Do not
currently expect any widespread debris flow or flash flooding
concerns as hourly rainfall rates look to max around around a
quarter to perhaps a half inch, and therefore should remain well
below thresholds on area burn scars through the duration of the
event.

A Flood Watch remains in effect for the entire forecast area
through 4 AM PT Wednesday. Continue to monitor the forecast
closely as relatively minor changes in forecast rainfall amounts
could have significant implications on potential river and
areal flood concerns in the coming days. -CB/HEC

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...High Wind Watch from Monday morning through late Monday night
     for ORZ101>103-106-107.
     High Surf Advisory from 4 PM Monday to 4 PM PST Tuesday for
     ORZ101>103.
     Wind Advisory from noon Monday to 4 AM PST Tuesday for
     ORZ109>118.
WA...High Wind Watch from Monday morning through late Monday night
     for WAZ201-203.
     High Surf Advisory from 4 PM Monday to 4 PM PST Tuesday for
     WAZ201.
     Wind Advisory from noon Monday to 4 AM PST Tuesday for WAZ205-
     206.
PZ...Gale Warning until 7 PM PST this evening for PZZ210-251>253-
     272-273.
     Small Craft Advisory from 7 PM this evening to 7 AM PST Monday
     for PZZ210-251>253-272-273.
     Storm Warning from 7 AM Monday to 4 AM PST Tuesday for PZZ210-
     251>253-271>273.
     Small Craft Advisory until 7 AM PST Monday for PZZ271.
&&

$$

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