Debris Flow in California, January 31, 2021

Area Forecast Discussion
National Weather Service Medford OR
347 AM PST Sun Jan 31 2021

.SHORT TERM...A moist frontal system is located near the Pacific
northwest coast with a significant upper level trough to the
west. GOES satellite imagery shows a plume of moisture directed
towards the Oregon coast. Models and ensembles remain on track
showing this front remaining stalled near the coast today and
tonight, then will gradually track inland Monday through Tuesday
morning.

With this pattern expect significant precipitation, especially
along the coast and into Josephine and Siskiyou Counties. Current
models indicate moderate to heavy rain of around 3 to 6 inches
along the southern Oregon coast today through Tuesday morning and
4 to 9 inches of precipitation in the coastal mountains in Curry
County. Josephine and western and southern Siskiyou counties are
expected to receive around 2 to 5 inches of precipitation during
this time.

Snow levels are expected to be around 5500 feet elevation for
western portions of the areas, especially near the coast and just
inland, where significant precipitation is likely. Elsewhere
expect snow levels of around 4500 to 5500 feet today and Monday.
Snow levels will trend lower Monday evening through Tuesday
morning. The main exception to these snow levels is in the Mount
Shasta area where lower snow levels of 3500 to 4000 feet are
forecast in south- central and southeast Siskiyou County near
Mount Shasta. Additional snow accumulations in the higher
mountains above 4500 feet and in the Mount Shasta area above 3500
feet. Moderate to heavy snow is expected through Monday in and
near Mount Shasta with the potential for periods of one inch per
hour snowfall rates over the higher terrain above 4500 feet
elevation today then for elevations above 3500 feet Monday through
Tuesday morning. Of note, precipitation rates will lower some
this evening and tonight across southern Siskiyou county before
increasing substantially Monday and Monday night. Snow levels may
fluctuate higher and times, especially today and early Monday.

An extended period of moderate to locally heavy precipitation
from the coast into Josephine and western Siskiyou Counties
combined with saturated soils and additional runoff from snowmelt
is forecast to bring significant river rises to mainstem rivers
and a potential for small stream and creek flooding. Additionally,
urban areas could see significant ponding of water in low lying
areas or areas with poor drainage. A flood watch remains in effect
through Monday night. Please see the FFAMFR for details.

In addition to precipitation, gusty winds will continue across
the area with strong winds in the Shasta Valley, portions of the
Rogue Valley in Jackson County, over the mountains and into areas
east of the Cascades, including the Summer Lake area. Strong winds
are likely today and moreso on Monday. Gusts to 60 mph are
expected in the Shasta Valley today and Monday and across the
higher terrain and Summer Lake areas east of the Cascades Monday
and Monday night.  The southern Rogue Valley is expected to see
winds gusting to 45 mph today and Monday. High wind warnings and
advisories remain in effect. Please see the NPWMFR for details.

.PREVIOUS LONG TERM (ISSUED 343 PM PST Sat Jan 30 2021)...Tuesday
through Saturday...Although the entirety of the long term portion
of the forecast has been freshened with the latest data, much of
what was written yesterday still holds true. The main changes were
to the details of Tuesday/Tuesday night and Thursday night. Low
and mid level clouds will still be present most of the time from
Tuesday into Friday morning.

Compared to the previous depiction of the upper low shifting
inland on Tuesday, and east of our area on Tuesday night, there is a
pronounced well agreed upon shift to a slower movement with
splitting of the trough. This has the biggest impact upon the
forecast for central California. For our area, although still
relatively meager compared to what occurs tonight through Monday
night, the amount and duration of lingering showers and snow showers
on Tuesday has been adjusted higher. This activity will be focused
upon southern Siskiyou County with around 2 to 4 inches of snow
forecast for Tuesday morning in the vicinity of Mt. Shasta with a
snow level around 3700 to 4000 feet. Up to an additional inch is
possible Tuesday afternoon. Showers and snow showers will continue
to taper off into Tuesday night with only a slight chance, mainly
for the higher terrain.

For Wednesday, our weather comes back into closer agreement with
yesterday's expectations. The impact of the longwave trough on our
weather will be minimal and we transition into a new pattern that
will persist into the weekend. It is comprised of a broad ridge
offshore and a series of disturbances (ranging in strength from very
weak to nearly moderate/typical) moving over the top of the ridge
then from northwest to southeast from the Alaskan coast across
the Pacific Northwest. The main point of uncertainty during the
long term relates to the strength of these disturbances, which
will impact temperatures and especially wind speeds and the
probability of precipitation.

The first shortwave disturbance is still expected Wednesday, and
still looks very weak. But, it has trended a bit stronger to warrant
the mention of a slight chance to chance of very light precipitation
for southwest Oregon and northern Klamath County , with the best
chance for an inch of snow in eastern Douglas County above 2000 feet
(around Toketee Falls and Diamond Lake).

Our area is likely to be between disturbances on Thursday, but low
level moisture and stratus will linger in valleys, especially in
southwest Oregon. West winds are expected to be breezy on the east
side, mainly in the afternoon and evening.

As mentioned above, the probability of precipitation late Thursday
night in southwest Oregon and northern Klamath County was increased
above the NBM/blended model solution with the ECMWF and a slight
majority of the ECMWF ensemble members shifting to a
stronger/colder/wetter depiction of what looks to be the strongest
of the disturbances. As such, the snow level was lowered to 3500 to
4000 feet and up to an inch of snow forecast for the southern Oregon
Cascades. On the wet and strong end of the range of model solutions,
it is plausible to receive a more widespread though still light
precipitation event with lower snow levels (down to 2500 feet) and
as much as 3 inches of snow for the southern Oregon Cascades.

Another break between shortwaves is most likely on Friday into
Friday night, although ridging and drying of the air mass is
expected to be stronger compared to Thursday. This would lead to a
higher probability of afternoon sunshine, but also slightly tighter
pressure gradients on the east side with breezy northwest winds
again.

Uncertainty increases rapidly and is very high regarding the timing
and strength of the next shortwave to follow the track and slide
inland over the top of the Pacific ridge. But, a majority of members
are focused on Saturday night into Sunday, and the ECMWF again
trends toward the stronger end of the range of solutions. -DW

&&

.AVIATION...For the 31/12Z TAFs...Showers are starting to fall along
the coast and some areas west of the Cascades this morning.  Also,
many surface stations in the larger valleys are reporting strong
winds with gusts approaching 35 knots at times.  We did issue an
airport weather warning for Medford as surface winds did approach 35
knots briefly this morning.  Those strong southeast winds are
expected to continue for most of the morning hours.  Otherwise,  VFR
ceilings are most likely, except for some MVFR ceilings in the
heavier showers along the coast.  This trend will continue for the
majority of the TAF period.

-Smith

&&

.MARINE...Updated 330 AM PST Sunday 31 Jan 2021...South gales are
expected to continue through this afternoon, and although gales
should come to an end at that time, winds will remain gusty through
Monday afternoon. Meanwhile, very steep and hazardous seas are
ongoing, resulting from a combination of high and very steep wind-
driven seas and west-northwest swell.  In fact, some of these seas
will approach 20 feet later this morning due to the long fetch and
sustained gales across the Pacific Northwest. Then, seas will
gradually diminish late this evening into Monday morning, and are
not likely to drop below 10 feet until late monday or even Tuesday
morning.

Once conditions improve, relatively calm winds and seas are expected
for much the rest of the week, although there is the possibility
that a system or two could bring more wind and steep seas to the
area this weekend.

-BPN/Smith

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...Wind Advisory until 4 AM PST Tuesday for ORZ030-031.
     High Wind Watch from Monday morning through Monday evening for
     ORZ030-031.
     Flood Watch through late Monday night for ORZ021-022-024.
     Wind Advisory until 1 PM PST Monday for ORZ026.
     Winter Weather Advisory until 4 AM PST Monday above 4500 feet in
     the for ORZ024-028.

CA...Wind Advisory until 4 AM PST Tuesday for CAZ085.
     High Wind Watch from Monday morning through Monday evening for
     CAZ085.
     Flood Watch through late Monday night for CAZ080.
     Winter Weather Advisory until 4 AM PST Monday above 4500 feet in
     the for CAZ080.
     Winter Storm Watch from late tonight through Tuesday morning
     above 4000 feet in the for CAZ080.
     Winter Weather Advisory until 4 AM PST Monday above 3500 feet in
     the for CAZ082-083.
     Winter Storm Warning until 4 AM PST Monday above 4500 feet in
     the for CAZ082-083.
     Winter Storm Watch from late tonight through Tuesday morning
     above 3500 feet in the for CAZ082-083.
     High Wind Warning until 4 PM PST Monday for CAZ081.

Pacific Coastal Waters...Flood Watch through late Monday night for PZZ350-356.
     Small Craft Advisory from 1 AM to 10 PM PST Monday for
     PZZ350-356-370-376.
     Gale Warning until 4 PM PST this afternoon for
     PZZ350-356-370-376.
     Hazardous Seas Warning until 1 AM PST Monday for
     PZZ350-356-370-376.

CC