636
FXUS66 KPQR 211758
AFDPQR

Area Forecast Discussion...UPDATED
National Weather Service Portland OR
956 AM PST Fri Feb 21 2025

...Updated aviation and marine discussions...

.SYNOPSIS...
Ridge over the area shifts east as a low pressure system moves
into the region. This system will bring a series of frontal
systems into the Pacific Northwest through the weekend and into
early next week. The first will arrive on Saturday night, with a
second on Sunday evening. These systems will be very wet and
bring heavy rainfall. Localized flooding, especially along the
coast, is possible. Gusty winds with these series of frontal
systems are expected. Another frontal system will arrive on
Monday, but confidence of the track and timing is low at this
time.

&&

.SHORT TERM...Now through Sunday...A high pressure ridge over
the region continues to move east as a low pressure system
approaches from the NE Pacific. With high pressure still within
the region, will see cloud breaks this morning, potentially
leading to some patch fog development. With current sky cover
consisting of broken overcast, there is low to moderate
confidence for some patch fog development. If any fog is to
develop, it will only stick around for a few hours.

Through today, the low pressure system over the NE Pacific will
move closer to our region and setup to bring a series of
frontal systems through the weekend into early next week. By
this afternoon, expect some relatively light rain to move
through the region, primarily SW Washington for inland areas and
along the SW Washington and NW Oregon coast. Specifically,
expect 0.05 to 0.25 inches within the valley lowlands (10-20%
chance to exceed 0.25 inches) and 0.25 to 0.50 inches along the
coast (20-40% chance to exceed 0.50 inches), highest near
Tillamook. Southerly winds along the coast will begin to
increase overnight into Saturday, with gusts up to 25-35 mph at
coastal sites and up to 40 mph along the beaches and headlands
by late tonight (40-60% chance to exceed 35 mph at coastal
sites and similar chances to exceed 40 mph at beaches and
headlands). While this Friday front will be weaker, it is still
part of the main low pressure system in the NE Pacific.
Considering how weak the Friday front will be and the many more
frontal systems to come, the frontal system on Saturday will be
considered the first system of the series from this expected
atmospheric river. Based on ensemble guidance, it is shaping up
to be a moderate atmospheric river that will be over an extended
period of time (Saturday and Sunday).

Saturday, the first front of the series of systems from this
atmospheric river will arrive around the afternoon. Southerly
winds will increase further, with expected wind gusts up to
30-40 mph along the coast and gusts up to 45-50 mph at beaches
and headlands. Valley lowlands will likely see wind gusts up to
25-30 mph, with infrequent gusts up to 35 mph possible.
Precipitation through the region will be much higher than
Friday. For precipitation accumulation from 4AM Saturday to 4AM
Sunday, expect 0.5 to 1.0 inches of rainfall for the valley
lowlands (30-50% chance to exceed 1 inch) and 1.25 to 2.00
inches along the coast (20-40% chance to exceed 2 inches). The
inland valley lowlands could see lesser rainfall amounts due to
the potential for southerly winds becoming southwesterly,
resulting in a weak rain shadow effect.

Sunday, the second front of the series of systems will arrive
around the afternoon. This front looks to have more robust
rainfall amounts for the lowland valleys than the Saturday
system. For precipitation accumulation from 4AM Sunday to 4AM
Monday, expect 1.0 to 1.5 inches within the valley lowlands
(10-20% chance to exceed 1.5 inches) and 1.25 to 1.75 inches
along the coast (10-30% chance to exceed 1.75 inches), highest
from SW Washington to NW Oregon, and higher near Tillamook.
Through Sunday, the low pressure system over the NE Pacific that
is fueling these series of fronts will move a bit south. This
will cause a north- south pressure gradient, causing winds to be
more southerly and less likely to shift southwesterly, so a
rain shadow effect is not expected. Southerly coastal winds will
remain elevated but about 3-5 mph less compared to Saturday.
The valley lowlands will see similar wind gusts to Saturday,
around 25-30 mph, but with increased Columbia River Gorge wind
gusts up to 55 mph. Sunday overnight into Monday morning, wind
gusts will let up a bit, down to around 30 mph along the coast
and in the valley lowlands.

With snow levels above 5500 ft Saturday and Sunday, precipitation
will fall as rain over the Coast Range, Cascade foothills, and
Cascades, resulting in the potential for landslides in areas,
especially with weakened trees. Overall impacts will be combined
with the threat of localized ponding and urban flooding, and
river flooding - especially along the coast). See the Hydrology
Section below for more details.
~Hall

.LONG TERM...Monday through Wednesday...Some models are showing
a deepening low pressure system moving into the North Pacific
from the south. This system would bring even more rain, but
gustier winds. Ensemble models are struggling to realize this
system, but based on meteograms, the most likely range (25-75th
percentile) is anywhere from 30-45 mph inland, and up to 65 mph
along the coast. Confidence is incredibly low at this point as
the main low pressure center is splattered over the northeast
Pacific. As we see how these warm fronts shake out, we will have
a better idea for Monday. But, now is the time to prepare for
the potential strong winds.
-Muessle

&&

.AVIATION...Radar, satellite, and surface observations as of 1730z
Fri depicts VFR conditions with high-level clouds (> 10kft) and
light rain approaching the north Oregon coast as a weak warm front
brushes far northwest OR and southwest WA. Conditions will trend
VFR with high broken clouds through 00z Sat. Tonight, CIGs will
fall toward low-end VFR (4-6 kft) as the trailing cold front moves
through the region. This weakening front will bring more
widespread rain showers tonight into early Saturday morning and
MVFR CIGs along the coast. Between 09-15z Sat, there will be a
10-30% chance for MVFR CIGs within the Willamette Valley.

Winds will generally be southerly for most terminals, with gusts
to 20-25 kt along the coast. Meanwhile, KTTD-KDLS offshore
gradients will support easterly winds continuing through the
Columbia River Gorge and into the eastern Portland Metro, with
gusts also to 20-25 kt.

PDX AND APPROACHES...Expect mid to high level CIGs through 06z
Sat. CIGs fall to low-end VFR thresholds as a cold front moves
through tonight and returns rain showers. 20-40% chance for MVFR
CIGs or lower between 09-18z Sat. Easterly winds generally under
10 kt today, increasing to 10-15 kt overnight.        -Alviz

&&

.MARINE...UPDATE: Seas have not increased as fast as previously
forecast overnight. This is because easterly winds have
persisted longer than forecast, which is counteracting the incoming
westerly swell. Winds are forecast to turn more southerly by this
afternoon, which should support building seas. The previous
discussion continues below.

A series of systems will bring elevated winds and seas across all
waters through the weekend and into the start of next week. The
first frontal system will move into the region today, bringing
southerly winds with gusts up to 30 kt and seas building to 12 to 16
ft at around 14 seconds. As a result will maintain the current
Hazardous Seas Warnings for all waters and the Columbia River Bar
through Saturday morning.

The next, more potent frontal system will bring southerly Gales
starting Saturday morning through at least Saturday night, with
gusts up to 45 kt and seas of 14 to 17 ft. Also, it should be
noted that isolated gusts up to 55 kt will be possible on
Saturday. Therefore, have upgraded the Gale Watch to a Gale
Warning. There will likely be a very brief lull on Sunday (3-6
hours), but it might not be long enough to rate a drop in the
already issued hazards as by Sunday afternoon the next system will
likely bring another round of Gales across the waters.

There looks to be longer break between systems starting late
Sunday afternoon through around late Monday morning. Will continue
to monitor as the system on Monday looks to be the strongest, but
also looks to be tracking further north than the previous
systems.         -Alviz/42

&&

.HYDROLOGY...A series of frontal systems will bring heavy rainfall
to northwest Oregon and southwest Washington this weekend and
potentially Monday. Rain will begin Saturday afternoon, and
intensify overnight through Sunday. This will lead to rises on
rivers and creeks, especially along the coast on February 22nd
through the 23rd. Main stem rivers may be delayed to respond, and
flooding may occur beyond this time frame.

There is around a 20-25% chance of the Wilson River, the Grays
River, and other localized quick responding coastal rivers of
reaching minor flood stage on Sunday and potentially Monday.
Confidence in flooding is low as the location of heaviest
precipitation will greatly depend on where the warm front sets
up. In the case that this system over performs, more widespread
flooding is possible.

Heavy rainfall along the Cascades is expected and combined with
recent snow accumulation, there is elevated landslide risk.

Heavy rainfall may also lead to ponding of water, general
lowland flooding, and localized urban flooding, especially in
low-lying areas or other poor-drainage areas. Heavy rainfall
will also increase the potential for landslides in areas of
steep terrain and debris flows over areas that have been burned
by wildfires over the past few years.     -Hall/Muessle

&&

.PQR WATCHES/WARNINGS/ADVISORIES...

OR...None.
WA...None.
PZ...Hazardous Seas Warning from 7 AM this morning to 7 AM PST
     Saturday for PZZ210-251>253-271>273.

     Gale Warning from 7 AM Saturday to 7 AM PST Sunday for PZZ210-
     251>253.

     Gale Warning from 7 AM Saturday to 4 AM PST Sunday for
     PZZ271>273.

&&

$$

www.weather.gov/portland

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