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-- Discussion containing changed information from previous version are highlighted. --
000
FXUS66 KPQR 011117
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
317 AM PST Fri Dec 1 2023

.SYNOPSIS...A pair of atmospheric rivers will bring periods of heavy
rain to the area through the weekend, heaviest Saturday night through
Sunday. Periods of heavy snow will also occur in the Cascades for
elevations above 3000 ft through Saturday before snow levels rapidly
rise above 6000-7000 ft Saturday night. There is the potential for a
third atmospheric river early next week, however uncertainty is high
with this system. Aside from dangerous travel conditions over the
Cascade passes, the main concern this weekend into early next week is
the potential for river flooding and urban flooding.

&&

.DISCUSSION...
-- Changed Discussion --
THROUGH THURSDAY...A very active weather pattern is in
store for northwest OR and southwest WA as a series of at least two,
if not three, atmospheric rivers impacts the area this weekend into
early next week increasing the chance of river flooding and urban
flooding.

Radar as of 2 AM shows widespread rain for elevations below 3000 ft
and snow for areas above. Accumulations over the last 24 hours shows
about 0.5" along the coast, 0.75" along the Coast Range, 0.25" across
the interior lowlands, and around a 0.15" across the Cascades.
Webcams show snow along Hwy26 near Ski Bowl, Hwy20 at Santiam Jct and
Pass, and ORE58 at Willamette Pass.

An upper level trough is swinging its attendant surface front towards
the B.C., Washington, and Oregon coastline. Multiple shortwaves
embedded along the front are visible in the GOES low level water
vapor channel which will bring enhanced instability along the coast
and therefore chances of thunderstorms during the first part of the
day Friday. CIRA`s Advected Layer Precipitable Water (ALPW) product
shows the 850-700mb layer within the warm sector of the approaching
front containing 0.25" and the SFC-850mb layer containing 0.50". This
enhanced moisture layer will push onshore Friday morning bringing
widespread rain to the area. Cold air advection will continue Friday
through Saturday as southwest WA, northwest OR remain on the north
side of the jet stream. Postfrontal westerly winds will drive
orographically enhanced snow showers which will bring 10-30" of snow
across the Cascades above 3000 ft and up to 40" for areas above 4500
ft. Winds will gust as high as 50 mph across the higher terrain
resulting in reduced visibility from blowing snow. Therefore, the
Winter Storm Warning remains in place through 10 PM Saturday.

Snow levels will be on the rise Saturday as the warm sector of a low
in the Gulf of Alaska pushes northeast into the region. This is
moving in sooner than previous model runs which could end the Winter
Storm Warning early but will leave the expiration time in place for
now.

By Saturday night snow levels across the Cascades will be around 6000
ft and ample moisture will be moving in. High temperatures Sunday
will be a balmy 58-60 degrees, 10 degrees above normal, across the
lowlands within the warm air mass. WPC QPF model guidance suggests
the heaviest period of rainfall will be between 10 PM Saturday
through 4 PM Sunday. Heavy rain will be capable of producing river
and urban flooding. Forecast rain amounts and
details regarding the flooding potential are discussed below in the
Hydrology discussion.

Ensemble guidance is in strong agreement regarding yet another
atmospheric river event Monday into Tuesday. IVT values range from
600-900 kg/(ms), with a start time sometime during Monday daytime
hours. Although confidence is low, ensemble guidance seems to
generally indicate the path of this system being slightly north of
our CWA. Current NBM guidance indicates around a 50% chance of 24hr
rainfall (Mon 4am - Tue 4am) exceeding 1", down to 20-30% In the
southern Willamette Valley. The northern Cascades, northern Coast
Range, and Willapa Hills see 90-100% chance of >1" rainfall, while
the southern Cascades and Coast Range are closer to 30-40%. With
heavy rainfall from preceding days, additional rainfall, and snowmelt
runoff from rising snow levels, river flooding continues to be a
concern along non-mainstem rivers Sunday into early next week. See
the Hydrology discussion below for more details.

Yet another atmospheric river is possible Tuesday into Wednesday,
bringing mild and rainy conditions and continued chances for river
flooding. That being said, uncertainty remains high with this system
as models struggle to resolve exactly where the axis of heaviest
rainfall will occur. In addition to this system, the deterministic
ECMWF and GFS runs are showing yet another possible system late
Wednesday night going into Thursday morning, but many ensemble
members aren`t displaying this feature. -BMuhlestein
-- End Changed Discussion --

&&

.HYDROLOGY...
-- Changed Discussion --
A series of at least two, if not three, atmospheric
rivers will bring periods of widespread heavy rain to northwest
Oregon and southwest Washington this weekend into early next week.
The first system will also bring heavy Cascade snow for elevations
above 3000 feet through Saturday evening before snow levels rapidly
rise above 6000-7000 feet Saturday night. This means mountain
snowmelt will also be a contributing factor in regards to river
flooding concerns Sunday into early next week as rain will be falling
over a fresh snowpack. The period of heaviest rainfall is currently
expected to occur Saturday night through Sunday. Total rain amounts
for the entire weekend are expected to range between 3.0-5.0 inches
along the coast, 4.5-6.5 inches over the Willapa Hills and Oregon
Coast Range, and 3-4 inches across the Willamette Valley,
Portland/Vancouver metro, and interior lowlands of southwest WA.
4.0-7.0 inches of rain is expected over the Cascades, Cascade
foothills, and western Columbia River Gorge. 2.5-3.5 inches of rain
is expected for central Columbia River Gorge and Upper Hood River
Valley. Additional rainfall will occur early next week, however
uncertainty is high regarding exact rain amounts from Monday onward.

Given the aforementioned rainfall amounts expected this weekend,
river levels will be rising quickly. Most rivers across northwest OR
and southwest WA have anywhere from a 30-50% chance of reaching at
least minor flood stage, except for the mainstem Columbia and
Willamette Rivers where the chance of reaching minor flood stage is
around 10%. Most coastal rivers will have a 40-60% chance of minor
flood stage and 20-30% chance of reaching major flood stage; this
includes the Willapa River near Willapa, the Naselle River near
Naselle, the Nehalem River near Foss, the Wilson River and Trask
River near Tillamook, the Nestucca River near Beaver, and the Siletz
River at Siletz. To view current and forecast river stages for any
river gage location across southwest WA and northwest OR, visit
https://water.weather.gov/ahps2/index.php?wfo=pqr

Lastly, urban flooding is also possible this weekend, especially in
low-lying areas with poor drainage. If you have gutters and/or storm
drains that still need to be cleared out, now is the time. Any
motorists with travel plans this weekend should also allow extra
time for their commute as hydroplaning will be a risk due to ponding
of water on roads.  -TK
-- End Changed Discussion --

&&

.AVIATION...
-- Changed Discussion --
Current radar imagery and observations as of 11z Fri
show rain moving into the region associated with an approaching
frontal system with a mix of MVFR/VFR cigs. As this front moves
inland this morning, expect predominately MVFR (locally IFR) cigs
and vis at any given terminal through 15-17z Fri. From 18z Friday
through the evening, rain will turn showery in a post-frontal
environment and cigs will vary between MVFR and low-end VFR.
Expect southerly to southwesterly winds to gust up to 20-25 kt at
the coast and Willamette Valley. After 18-21z Fri, winds will
gradually weaken to 5-10 kt.

** The new Aviation Weather Center website is live. The new
 website can be found at aviationweather.gov **

KPDX AND APPROACHES...MVFR thresholds through 18z Fri as a frontal
system moves into the area and brings rain. After 18-21z Fri,
expect a mix of MVFR and VFR cigs through the evening. Southerly
winds up to 10-15 kt will gradually weaken in the afternoon.
-Alviz
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&&

.MARINE...
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A frontal system will move through the waters early
Friday morning, strengthening winds to Gale force criteria across
the waters through mid-morning Friday. Seas around 10-13 ft this
morning will gradually build toward 13-15 ft/11-12 seconds by
Friday afternoon. Therefore, a Hazardous Seas Warning will follow
the Gales Friday afternoon-evening for most areas except the
coastal waters between Cape Foulweather to Florence, OR.

On Saturday, another system will bring an additional round of
Gale force gusts up to 45 kt with seas building to 16-18 ft.
Sunday through early next week, a very active weather pattern
persists, with a 70-80% chance for additional rounds of Gale
force gusts over the waters. Seas will remained elevated through
at least Monday to mid-next week and peak up to 18-20 ft. A brief
break will be possible Tuesday-Wednesday, but models show another
system approaching the waters by Wednesday that would maintain
elevated seas.    -Alviz/42
-- End Changed Discussion --

&&

.BEACH HAZARDS...
-- Changed Discussion --
- High Sneaker Wave threat Saturday-Wednesday

A high sneaker wave threat will be present along the OR and WA
coasts starting this Saturday and continuing through at least next
Wednesday. The cause is a series of systems coming from the
central Pacific Ocean and moving towards the Pacific NW. Expect
wave heights to slowly build towards 15-19 ft with a dominant
period of 14-17 seconds.

It is important to not focus on the highest wave heights as a
representation of peak sneaker wave threat; the period between the
waves is almost more important as it represents the energy being
carried by the swell. Sneaker waves will drive up further on the
beach, when compared to waves of similar height. This can
potentially catch beachgoers off guard. In addition, waves can
easily lift and move large logs on the beach and knock people/pets
off of jetties. Therefore, have issued a Beach Hazard Statement
along our CWA`s entire coast starting early Saturday morning and
through at least early Wednesday morning. -Alviz/42
-- End Changed Discussion --

&&

.PQR WATCHES/WARNINGS/ADVISORIES...

-- Changed Discussion --
OR...Winter Weather Advisory until 4 AM PST early this morning for
     Cascades in Lane County-Northern Oregon Cascades.

     Winter Storm Warning until 10 PM PST Saturday for Cascades in
     Lane County-Northern Oregon Cascades.

WA...Winter Weather Advisory until 4 AM PST early this morning for
     South Washington Cascades.

     Winter Storm Warning until 10 PM PST Saturday for South
     Washington Cascades.

PZ...Gale Warning from 10 PM this evening to 10 AM PST Saturday for
     Coastal waters from Cape Shoalwater WA to Florence OR out
     10 NM-Columbia River Bar-Coastal waters from Cape
     Shoalwater WA to Florence OR from 10 to 60 NM.

     Small Craft Advisory until 10 PM PST this evening for Columbia
     River Bar.

     Small Craft Advisory until 4 AM PST early this morning for
     Coastal waters from Cape Shoalwater WA to Florence OR out
     60 NM.

     Gale Warning until 2 PM PST this afternoon for Coastal waters
     from Cape Shoalwater WA to Cape Foulweather OR out 10
     NM.Coastal waters from Cape Shoalwater WA to Cape
     Foulweather OR from 10 to 60 NM.

     Hazardous Seas Warning from 2 PM this afternoon to 10 PM PST
     this evening for Coastal waters from Cape Shoalwater WA to
     Cape Foulweather OR out 10 NM.Coastal waters from Cape
     Shoalwater WA to Cape Foulweather OR from 10 to 60 NM.

     Small Craft Advisory from 10 AM this morning to 10 PM PST this
     evening for Coastal waters from Cape Foulweather to
     Florence OR out 10 NM-Waters from Cape Foulweather to
     Florence OR from 10 to 60 NM.

     Gale Warning until 10 AM PST this morning for Coastal waters
     from Cape Foulweather to Florence OR out 10 NM-Waters from
     Cape Foulweather to Florence OR from 10 to 60 NM.
-- End Changed Discussion --
&&


&&

$$

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