National Weather Service Area Forecast Discussion for Long Beach, WA USA

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Area Forecast Discussion...updated
National Weather Service Portland, OR or
952 pm pst thu nov 30 2023

updated aviation...

.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.

&&

.short term...through sunday...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 and increases the chance of river flooding and urban
flooding. focusing first on current observations as of 330 pm pst
thursday, areas of light rain were being observed across southwest wa
and northwest or as a weak pacific frontal system continues moving
through the area. however, precipitation was falling in the form of
accumulating snow in the cascades (mainly above 3000 ft), and in the
central columbia river gorge, including stevenson and hood river.
there have also been reports of very light freezing rain near larch
mountain, resulting in a light glaze of ice on road signs but not on
roads.

models continue to suggest cascade snow will become heavy friday
morning as an upper level shortwave trough and attendant surface
front push eastward over the area and bring another round of enhanced
moisture. periods of heavy cascade snow and blowing snow will then
continue through saturday afternoon/evening before snow levels
rapidly rise to above 6000-7000 ft saturday night (resulting in a
transition to rain except on the volcano peaks). expect dangerous
travel conditions for anyone attempting to travel over the cascade
passes friday through saturday; a winter storm warning remains in
effect to cover this threat. qpf/snow amounts have increased a bit
and the cascades should now see 10-30 inches of snow above 3000 feet,
except up to 40 inches possible above 4500 feet. winds will gust to
around 50 mph at times, resulting in blowing snow and reduced
visibilities.

for the rest of northwest or and southwest wa, precipitation will
fall in the form of heavy rain capable of producing river flooding
and urban flooding. the period of heaviest rainfall is expected
saturday night through sunday as a moderate atmospheric river takes
aim at southwest wa and northwest or. forecast rain amounts and
details regarding the flooding potential are discussed below in the
hydrology discussion.-tk


.long term...sunday night through wednesday night....the remnants of
saturday/sunday`s atmospheric river will bring more rain sunday
night, however rain amounts should be much lighter as the system
falls apart over the area. snow levels will be approximately
6000-7000 ft by this point, and snow won`t be a concern anywhere but
the volcano peaks. ensemble guidance is in strong agreement regarding
yet another atmospheric river event on monday, but little confidence
exists regarding its exact path. 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. uncertainty remains high
going into mid/late next week. -jliu

&&

.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. 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 20-40% 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 5%. in fact, coastal rivers have a 10-25% 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

&&

.aviation...evening radar imagery shows scattered showers across
the area as one front departs east of the cascades and another
quickly approaches from offshore. variable mvfr/ifr cigs being
observed across the area as of 06z, with calm winds helping to
support a localized pocket of dense fog in the central valley from
kmmv-kuao. expect this fog to lift and flight conditions to
improve back towards mvfr as the next frontal system approaches.
rain associated with this front should reach the coastal terminals
09-10z and the spread inland through 12z fri morning, accompanied
by gusty south winds and predominant mvfr conditions. frontal
precipitation will clear the area from west to east 15-18z fri,
resulting in another period of post frontal showers and variable
mvfr conditions through fri evening.

** the new aviation weather center website is live. the new
 website can be found at aviationweather.gov **

kpdx and approaches...variable mvfr/ifr conditions with fl008-012
cigs this evening in post frontal showers. next frontal system
approaches the terminal 10-12z fri with steady rain, mvfr flight
conditions and gusty s-sw winds. front will clear the terminal
15-17z fri and yield another round of post frontal showers with
variable mvfr/vfr conditions through the end of the period. /cb

&&

.marine...a cold front will continue to move through the waters
tonight bringing strengthening southerly winds to small craft
criteria through at least tonight. seas will build towards 7-9 ft
by tonight/friday morning. friday and into the weekend, a series
of progressively stronger systems will move through the waters,
resulting in a very active weather pattern through the start of
the upcoming week. for the friday system, winds could gust up to
45 kt across the waters; therefore, a gale warning has been issued
from late tonight into friday morning, followed by hazardous seas
across all waters as seas build towards 14-17 ft. saturday and
sunday, there still remains a 50-60% chance of wind gusts
exceeding 40 kt over the waters, with a higher probability for
these winds on saturday night/sunday. in addition, seas will
continue to build towards 15-19 ft starting saturday and remaining
elevated through at least monday as the very active weather
pattern persists. a brief break will be possible on
tuesday/wednesday,
but models show another system starting to impact the region by
wednesday and that system could cause seas to build towards 18-20
ft. /42/alviz

&&

.beach hazards...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 14-18 ft with a dominant
period of 15-18 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.

&&

.pqr watches/warnings/advisories...

or...winter weather advisory until 4 am pst friday for cascades in
     lane county-northern oregon cascades.

     winter storm warning from 4 am friday to 10 pm pst saturday for
     cascades in lane county-northern oregon cascades.

wa...winter weather advisory until 4 am pst friday for south
     washington cascades.

     winter storm warning from 4 am friday to 10 pm pst saturday for
     south washington cascades.

pz...small craft advisory until 4 am pst friday 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.

     gale warning from 4 am to 10 am pst friday for coastal waters
     from cape shoalwater wa to cape foulweather or out 10
     nm-columbia river bar-coastal waters from cape shoalwater
     wa to cape foulweather or from 10 to 60 nm.

     hazardous seas warning from 10 am friday to 1 am pst saturday
     for coastal waters from cape shoalwater wa to cape
     foulweather or out 10 nm-columbia river bar-coastal waters
     from cape shoalwater wa to cape foulweather or from 10 to
     60 nm.

     gale warning from 4 am to 7 am pst friday for coastal waters
     from cape foulweather to florence or out 10 nm-waters from
     cape foulweather to florence or from 10 to 60 nm.

     hazardous seas warning from 7 am friday to 1 am pst saturday
       for coastal waters from cape foulweather to florence or out 10

     nm-waters from cape foulweather to florence or from 10 to
     60 nm.

&&

$$

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