Flood in Oregon, December 4 to 5, 2023

Area Forecast Discussion...CORRECTED
National Weather Service Medford OR
412 AM PST Mon Dec 4 2023

...Corrected Aviation Section...

.DISCUSSION...We're in a relative break between atmospheric river
systems today, with limited precipitation in the region. Good
mid- level moisture and westerly flow is creating conditions that
support enhanced (orographic) shower activity over the south OR
Cascades and Curry County mountains, but elsewhere occasional
light rain is all you can expect through at least Tuesday morning.
Low temperatures this morning will be a couple degrees warmer
than yesterday's for most inland areas due to continued low-level
southwesterly flow bringing warmer (relatively) air into the area.
Similarly, afternoon high temperatures will be similar to if not
a couple degrees warmer than yesterday's. This warm flow will also
continue to raise snow levels to above 9,000 feet by late this
morning, so besides OVER the peak of Mt. Shasta all of our
precipitation today will fall as rain.

Heavy rainfall in the last few days have led to rapid rises in
small streams and creeks. As rain has ended, most of these
streams will drop briefly today before the next system rolls in,
with the exception of the Coquille River at Coquille, which has
reached full capacity/bankfull as of writing this and is forecast
to reach minor flood stage today, and our flood warning for this
remains in effect.

Another atmospheric river will slowly move onshore Tuesday, moving
inland late Tuesday through Wednesday. This will bring widespread
moderate to heavy precipitation to the region, with rain starting
along the Coos County coast and northwards Tuesday morning, then
gradually spreading southward to include the Curry coast and
coastal mountains Tuesday afternoon into Tuesday night. Areas
further inland can expect to see precipitation late Tuesday into
mid- Wednesday. Areas along the coast can expect to see another
2-5" of rain out of this, with about 1-2" forecast further inland
in western Josephine and west/central Douglas counties. This heavy
rainfall will lead to additional rises in streams and rivers in
this area, and a flood watch remains in effect for Coos, Curry,
western Douglas and western Josephine counties through Wednesday
for potential flooding of rivers, creeks, streams, and other low-lying
and flood-prone locations.

Winds will pick up again as the atmospheric river moves in,
especially Wednesday morning and afternoon, as it brings a 45-55
kt 700 mb jet into the area. This will result in gusty southwest
winds over ridges, especially over the Klamath/Curry County
Mountains and typically windy areas east of the Cascades such as
the Summer Lake/Winter Rim area and the Warner Mountains.

Snow levels will once again drop through the day Wednesday as the
stream of more tropical moisture cuts off and a trough moves in
aloft, bringing a cooler air mass. Snow will return to the
mountains (Cascade, N. CA Siskiyou, and Warner ranges) by early
Wednesday afternoon. The progressive pattern continues into the
end of the work week, as another trough and associated frontal
system moves through the area Thursday.
-CSP

&&

.AVIATION...04/12Z TAFs...Terminals are precipitation free for the
time being, but we are still seeing low ceilings across the
terminals. This has resulted in areas of MVFR conditions (or close
to at least) and this will be the case for most of this cycle.
Medford could see visibilities drop to IFR later this morning and
once again Tuesday morning. Precipitation will return to the area
again today, but should be less coverage today/tonight (compared to
the last couple days) and hence the omission of any prevailing
rain at the terminals outside of Klamath Falls (KLMT). Even then,
confidence is on the lower side for KLMT to see rain showers
actually impact the terminal.
-Guerrero

&&

.MARINE...Updated 200 AM Monday December 4, 2023...An active period
of weather will continue this week over the waters. Gusty south to
southwest winds will be common, along with southerly wind waves and
long period west swell.

Winds will increase through the morning hours today, and seas will
remain steep once again through Tuesday morning. Gales are expected
again north of Gold Beach today through Tuesday morning. In
addition, seas are expected to become very high later today around
15 to 19 ft and will continue to build to a peak around 16 to 22 ft
Tuesday into early Wednesday morning. During this same time period,
wind speeds are forecast to come down relatively speaking. However,
since seas are still expected to remain very high, this will result
in the need for a Hazardous Seas Warning across all the waters
Tuesday through Wednesday morning. Additional disturbances Wednesday
and Thursday will not be as strong, with high and steep seas slowly
diminishing into Friday.
-Guerrero

&&

.BEACH HAZARDS...Active weather continues this week. A
series of fronts will move through the region during this time along
with multiple moderate to high swell trains. An extended period of
sustained wave action is expected through Tuesday. Breaking waves of
15 to 20 ft will coincide with higher than normal ocean levels,
leading to an increased risk of beach erosion. Additionally, there
is a minor risk of sneaker waves as these swell trains build into
the waters. Confidence is low, however, considering wind wave
interference is likely to limit, but not completely negate the
sneaker threat.

Late Tuesday afternoon into Wednesday morning, very large breaking
waves of 20 to 30 ft are expected due to large long period west
swell. In addition, another moderate risk for sneaker waves will
also be present Tuesday evening. If venturing to area beaches during
this time, use caution when visiting and remember to never turn your
back on the ocean!
-BPN

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...Beach Hazards Statement through Wednesday morning for ORZ021-
     022.

     Flood Watch through Wednesday afternoon for ORZ021>024.

CA...None.

PACIFIC COASTAL WATERS...Small Craft Advisory until 10 AM PST Tuesday
     for PZZ356-376.

     Hazardous Seas Warning from 10 AM Tuesday to 4 AM PST Wednesday
     for PZZ350-356-370-376.

     Small Craft Advisory until 4 AM PST early this morning for
     PZZ350-370.

     Gale Warning until 10 AM PST Tuesday for PZZ350-356-370-376.

&&