High Wind in Washington, December 2, 2023

Area Forecast Discussion...Updated
National Weather Service Portland OR
855 PM PST Fri Dec 1 2023

Updated aviation...

.SYNOPSIS...A set of atmospheric rivers will move over the
region through the weekend. The next system (the weaker of the
two) arrives Saturday, with the second trailing on Sunday.
Higher confidence in the next arriving on Monday. Will see
heavy rain, warmer temperatures, and gusty winds with each
system. Snow will dissipate on Saturday through the Cascades.

&&

.SHORT TERM /THROUGH MONDAY/...A very dynamic system over the
weekend as a series of atmospheric rivers (AR) move over the
Pacific Northwest. Starting aloft, a low pressure system drops
down over southeast Alaska which will encourage westerly flow
aloft. This low will push a weaker cold front over the area,
though it will still have ample energy and precipitation
associated with it. This next system currently has around 150%
of the normal precipitable water content which falls in line
with the weak AR trend. However, this is not an overly robust
system in comparison to the ones that will follow. IVT values a
tool used to describe the intensity of an AR) sit at around 500
kg/ms which coincides with a weak to moderate AR. Will see
continued rainfall below 3000 ft, and some lingering snow at
higher elevations in the Cascades. The Winter Storm Warning
remains in effect, though snow levels are rising considerably.
Highest snow amounts will be above 4500 ft where an additional
20-30 inches of snow is possible. Winds will also be a concern
with this next system, though they are falling well below any
hazard criteria. The pressure gradient from Portland to Eugene
shows around a
-9mb difference between the two sites which coincides with
 gusts around 35 mph or so from the south. This will occur
 Saturday morning.

Moving into Sunday, the next AR arrives. This system has much
more support aloft, and wrap around warmer air from the Pacific.
With the warmer system incoming, temperatures will rise
considerably so all areas will be at or above freezing...both
during the day and night. At 850 mb (around 5000 ft)
temperatures will be around 5 deg C which supports warmer air
through the column. This also shows that at the mountain tops,
snow melt is likely. Rainfall amounts are still quite variable. 24-hr
ensemble forecasts shows the area around the Siuslaw Natl
Forest at a widespread 4 inches, with the Cascades 4-5 inches.
The bulk of the rain will fall along the Coast Range and the
Cascades. Due to the instability and isentropic lift, have
trended on the high side for the precipitation accumulation
forecast. Will also note that the WPC has included our area in a
marginal to slight excessive rainfall forecast.

With the excessive rain and the melting snow, flooding is
likely. Please see the hydrology section below for more
information.

The next AR arrives on Monday into Tuesday. This system has the
potential to be the strongest of the systems thus far. As it
stands, it is well into the moderate to strong category. There
have been a few shifts though as models like the ECMWF have
shifted it a bit further north. This means that the main target
of the precipitation would be from Marion County northward and
along the ranges. For more information about this event, read
the long term discussion. 							-Muessle

.LONG TERM /MONDAY EVENING THROUGH FRIDAY/...Agreement is good
regarding another AR event on Monday evening/night, with fair
confidence regarding IVT values between 700-950 kg/(ms). Rainfall
with this event looks to be slower moving compared to the weekend's
AR events, but with possibly higher overall QPF over a wider range
of time. In the first 24 hrs of rainfall (4pm Mon to 4pm Tue), the
Portland metro area sees around a 50% chance of rainfall amounts
exceeding 1.5". Agreement is improving regarding the system tracking
slightly further north, with the Washington parts of our
forecast area and northern Coast Range seeing 85+% chance of
exceeding 1.5". The southern parts of our forecast area will
see lighter rainfall, with only a 20-40% chance of exceeding
1.5". River flooding continues to be an issue on Sunday with
continued rainfall and snowmelt from the mountains; see the
Hydrology section for more details.

We continue to see active weather throughout Tuesday and Wednesday
as we remain in an unsettled post-frontal showery regime. Rain
amounts look lighter during this period, with somewhat warmer
temperatures that will slowly decrease. Deterministic ECMWF and GFS
runs are still hinting at a weak system developing Thursday, but
this is unlikely to meet Atmospheric River criteria, and overall QPF
currently looks weak. However, many ensemble members don't show this
feature, and being so far out, uncertainty is still high for this
system.													-JLiu

&&

.AVIATION...Latest batch of showers moving through the area is
producing variable VFR/MVFR cigs at the terminals this evening, with
winds starting to pick up out of the south ahead of the next front
approaching the coast. The front will reach the coast 06-08z and
then spread inland 08-10z this evening, bringing another round of
steady rain and more predominant MVFR conditions, along with south-
southwest winds gusting as high as 35 kt at the coastal terminals
and 25-30 kt at the inland terminals. Front will clear the coast by
12z and the Willamette Valley by 15z Sat, with post frontal showers
and variable MVFR/VFR flight conditions returning. Conditions will
gradually trend closer to VFR after 18-21z Sat through the end of
the period.

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

KPDX AND APPROACHES...Predominantly VFR in scattered rain showers
this evening with cigs below FL040. Next frontal rain band will
arrive at the terminal around 10z Sat with MVFR conditions and a
period of south winds gusting 25-30 kt. Front will clear the
terminal around 15z Sat, yielding post frontal showers and
variable MVFR/VFR cigs trending more solidly VFR after 18z Sat.
/CB

&&

.MARINE...Seas around 13-16 ft at 12-15 seconds through tonight.
Therefore, a Hazardous Seas Warning is now in effect for all
waters except PZZ253 and PZZ273 through late tonight.

On Saturday, another system will bring another round of Gale
force gusts up to 45 kt with seas building to 16-18 ft with a
period of 13-17 seconds. 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. 		-42/Alviz

&&

.HYDROLOGY...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. Forecast
precipitation amounts remain on track from prior forecast packages,
with rain amounts for the entire weekend still expected to range
between 3.0-5.0 inches along the coast, 4.5-7.0 inches over the
Willapa Hills and Oregon Coast Range, and 2.50-4.0 inches across the
Willamette Valley, Portland/Vancouver metro, and interior lowlands
of southwest WA. Additionally, 4.0-7.0 inches of rain is expected
over the Cascades, Cascade foothills, and western Columbia River
Gorge, with 2.5-3.5 inches expected for central Columbia River Gorge
and Upper Hood River Valley. Additional rainfall will occur early
next week, however uncertainty remains high regarding exact rain
amounts from Monday onward.

Given the aforementioned rainfall amounts expected this weekend,
river levels will be rising quickly. Many rivers across northwest OR
and southwest WA still show 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 now around 5-10%. Many coastal rivers still show
closer to a 40-60% chance of minor flood stage and 15-30% chance of
reaching major flood stage; this includes 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/CB

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...Flood Watch through Tuesday afternoon for ORZ001>008-010-012.
     Beach Hazards Statement from late tonight through late
     Wednesday night for ORZ001-002.
     High Wind Watch from late Saturday night through Sunday
     evening for ORZ006>008.
     Winter Storm Warning until 10 PM PST Saturday for ORZ011-013.
WA...Flood Watch through Tuesday afternoon for WAZ020>022-039-040.
     Beach Hazards Statement from late tonight through late
     Wednesday night for WAZ021.
     High Wind Watch from late Saturday night through Sunday
     evening for WAZ039.
     Winter Storm Warning until 10 PM PST Saturday for WAZ019.
PZ...Flood Watch through Tuesday afternoon for PZZ251.
     Beach Hazards Statement from late tonight through late
     Wednesday night for PZZ251.
     Gale Warning from 10 PM this evening to 6 AM PST Saturday for
     PZZ210-251>253-271>273.
     Hazardous Seas Warning from 6 AM to 4 PM PST Saturday for
     PZZ251>253-271>273.
     Small Craft Advisory until 10 PM PST this evening for PZZ210-
     253-273.
     Small Craft Advisory from 6 AM to 4 PM PST Saturday for PZZ210.
     Hazardous Seas Warning until 10 PM PST this evening for PZZ251-
     252-271-272.
&&

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