Debris Flow in California, February 1 to 2, 2021

Area Forecast Discussion
National Weather Service Medford OR
851 PM PST Sun Jan 31 2021

.DISCUSSION...A plume of moisture is aimed at the forecast area
this evening and this is bringing moderate to heavy rainfall
across portions of Curry, Josephine and western Siskiyou counties.
Moderate rainfall has also occurred in Douglas and Coos counties
today. 24 hour rainfall totals so far range from 0.75" to 1.25" in
Douglas, Coos and western Siskiyou counties, while 1.50" to over
2" has been recorded in Curry and Josephine counties. The heaviest
rainfall is expected to shift slightly westward, becoming more
focused along the coast tonight, then finally push inland Monday
morning and afternoon. Meanwhile, moderate to heavy rainfall will
continue to bring significant rises on area creeks, streams and
rivers in western Siskiyou County as well as Coos, Curry, and
Josephine counties. An areal flood watch remains in effect through
late Monday night. In addition, the Coquille River at Myrtle Point
could reach flood stage Monday night so a flood watch remains in
effect and details can be found at FFAMFR.

In addition to the rainfall, a plethora of other weather hazards
are expected Monday as the front moves inland, and even before
then. Ahead of the front, strong gusty winds are expected for the
windy spots (Shasta Valley, East Side) as well as portions of the
Rogue Valley south of Medford tonight into Monday. Winter weather
will become a concern as well Monday morning, especially for the
southern portions of Siskiyou County, including the Mt Shasta City
region and the Siskiyous around Mt Ashland. Details can be found
at WSWMFR. While the Cascade passes will see some snow
accumulations, amounts will be less than the previously mentioned
areas because of the south/southwesterly flow of this system. All
of this is covered well by the going forecast and no major updates
are needed. For more details on Monday's weather, as well as the
weather to follow for the remainder of the week, see the previous
discussion below. /BR-y

&&

.AVIATION...For the 01/00Z TAFs...A slow moving front will continue
to produce areas of moderate to heavy precipitation with a mix of
MVFR and VFR west of the Cascades through Monday. Meanwhile, gusty
southerly winds, low level wind shear, and mountain obscuration will
be widespread across the area, and mainly VFR is expected east of
the Cascades. -DW/BPN

&&

.MARINE...Updated 800 PM PST Sunday 31 Jan 2021...The last in a
series of strong fronts will produce gales and very steep seas as it
moves through the coastal waters tonight. Winds will ease overnight
once the front moves inland, but seas will remain elevated and very
steep through Monday morning as residual wind seas gradually
diminish. Steep swell dominated seas will then continue through
Monday night.

Weaker fronts will move onshore Tuesday, Wednesday, and Thursday
night, and are likely to produce periods of small Craft Advisory
conditions. Offshore high pressure and a trough near the coast will
develop Friday and persist through the weekend. -BPN

&&

.PREV DISCUSSION... /Issued 317 PM PST Sun Jan 31 2021/

SHORT TERM...This afternoon through Tuesday...A broad trough of
low pressure off of the Pacific Northwest coast continues to spin
moisture into the area this afternoon. An inland surge of
precipitation today across the interior west side to the Cascades
is now re-configuring northwestward, orienting more coastward with
time. We've had reports up around 2 inches of rain in 24 hours
near O'Brien in Josephine County as of 3 PM PST.

Snow levels have risen across the forecast area to approximately
4500 feet this afternoon, though they might be as high as 5500
feet in some areas. This includes in the Mount Shasta area, where
precipitation has come to an end for now in Mount Shasta City and
on Snowman's Summit.

As the precipitation refocuses along and near the coast tonight
through Monday morning the main focus will be heavy rainfall
along and near the coast and winds out ahead of it. Please note
that the Areal Flood Watch remains in effect across much of
western Siskiyou, Josephine, Coos, and Curry counties through
4 AM PST Tuesday morning. We've had some reports of ponding water
beginning in some areas of Josephine County, but no flood reports
as of the writing of this discussion.

The South Fork of the Coquille River at Myrtle Point and Powers
are expected to reach Action Stage later Monday into Monday night,
with Minor Flood Stage expected at the Myrtle Point.
Additionally, the Coquille at Coquille is expected to go to action
stage early Tuesday. We will continue to closely watch the Chetco
River, but, as of now, only significant rises are expected on it
and action stage is not expected to be reached.

Monday afternoon and evening the frontal boundary is expected to
move eastward along with the heavy precipitation with it. Snow
levels will initially be in the 4500 to 5500 foot range as it
arrives, but then are likely to lower into the 3,000 to 4,000 foot
range as it moves through. Guidance still has a wide range of snow
levels as the front moves through, but went with the NAM12 to a
bit lower as the heaviest precipitation with the front moves
through. In short, we expect a period of moderate to very heavy
snowfall as the front moves through, heaviest in the Mount Shasta
area and mountains of Siskiyou County, and less heavy in the
Oregon Cascades. Expect that there's likely to be a period of
moderate to significant road impacts on I-5 later Monday
afternoon into the evening as the front moves through. ~BTL

LONG TERM...Tuesday night through Sunday night...The beginning
portion of the long term bears watching for future updates.

The weather story  for Tuesday night through Wednesday has gotten
more interesting, with a now higher confidence that high
temperatures on Wednesday will be the coldest of the week, and also
an increased probability of precipitation for Tuesday night through
Wednesday. The key questions with a lower degree of certainty at
this juncture are the snow level and precipitation amounts.

The snow level falls a blend of models and is forecast to fall from
3000 feet to around 2000 feet on Tuesday night, then rise back to
around 2500 with the heating of the day on Wednesday. Regarding
snowfall amounts, widespread amounts of a dusting to an inch are
forecast above 2500 feet, and 2 to 5 inches in the southern Oregon
Cascades. The 12Z ECMWF leans toward the stronger/wetter/colder side
of the range of solutions and is focused upon late Tuesday night and
Wednesday morning, the 12Z GFS toward the weaker/drier side, while
the 12Z ECMWF ensemble members are about evenly split between
impactful and non-impactful snowfall. Meantime, the 18Z GFS data has
very recently arrived. It has trended slower (with a focus upon day-
time Wednesday lingering into early Wednesday evening), but is
stronger than its previous run, and still weaker than the ECMWF.
Quite simply, uncertainty is higher than usual this near to an
event, higher than we would like it to be. The widest range in
potential impact relates to the 3 passes along I-5 between Grants
Pass and Canyonville. About an inch of snow is forecast there, but
there is a risk of several inches of snow and also a potential of no
snow/for the snow level to remain above these passes.

A couple of days ago, the bulk of model solutions were progressive
in exiting the upper trough from our area with a rapidly diminishing
probability and quantity of precipitation. Yesterday, the mean
ensemble solutions caught on to a slower and splitting pattern with
our area generally between the northern and southern branches, with
some meager risk of light showers above 3000 feet. Lo and behold,
with the passage of another day, confidence has grown in a continued
slowing of the upper level pattern, with a more stretched trough
from northeast (over western Canada) to southwest (farther offshore
of California). As such, this places our area in a location to be
affected by the next wave dropping southeast from Alaska in the cold
northern branch.

Ridging is still expected to strengthen over the eastern Pacific
later Wednesday night into Thursday for a break between
disturbances, but widespread valley stratus during the night and
morning hours. Models have nearly universally trended weaker with a
shortwave moving southeast down the front side of the ridge Thursday
night through Friday. As such, the probability of precipitation has
been lowered from the previous forecast. Also, the high temperature
forecast on Friday is likely to be tricky/dependent on the extent
and timing of clearing of valley stratus. The ridge is expected
to build inland Friday night through Saturday night. Model
agreement regarding a dry forecast during this period has
increased. That agreement diminishes for Day 7/Sunday, but the
mean solution suggests a weak wave weakening the ridge as it
passes relatively harmlessly north of our area with a slight
cooling trend and increase in high level or mid and high level
cloud cover to accompany another episode of night and morning
valley stratus. -DW

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...High Wind Warning from 4 AM Monday to 4 AM PST Tuesday for
     ORZ030-031.
     Wind Advisory until 4 AM PST Tuesday 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 from 1 PM to 10 PM PST Monday above 4500
     feet in the for ORZ028.

CA...High Wind Warning from 4 AM Monday to 4 AM PST Tuesday for
     CAZ085.
     Wind Advisory until 4 AM PST Tuesday for CAZ085.
     Flood Watch through late Monday night for CAZ080.
     Winter Storm Warning from 10 AM Monday to 10 AM PST Tuesday
     above 4000 feet in the for CAZ080.
     Winter Weather Advisory from 10 AM Monday to 10 AM PST Tuesday
     above 3000 feet in the for CAZ082-083.
     Winter Storm Warning from 10 AM Monday to 10 AM PST Tuesday
     above 3500 feet in the for CAZ082-083.
     High Wind Warning until 4 PM PST Monday for CAZ081.

Pacific Coastal Waters...Small Craft Advisory until 7 AM PST Tuesday
     for PZZ350-356.
     Gale Warning until 4 AM PST Monday for PZZ350-356-370-376.
     Hazardous Seas Warning from 4 AM to 1 PM PST Monday for
     PZZ350-356-370-376.
     Small Craft Advisory from 1 PM Monday to 7 AM PST Tuesday for
     PZZ370-376.

MNF/BTL/BPN

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE MEDFORD OR
915 PM PST SUN JAN 31 2021

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0315 PM     HEAVY RAIN       2 WSW OBRIEN            42.06N 123.73W
01/31/2021  E2.30 INCH       JOSEPHINE          OR   TRAINED SPOTTER

            AS OF 3 PM PST.

&&

MNF

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE MEDFORD OR
916 PM PST SUN JAN 31 2021

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0845 PM     HEAVY RAIN       2 SSW OBRIEN            42.05N 123.71W
01/31/2021  M3.00 INCH       JOSEPHINE          OR   TRAINED SPOTTER

            STORM TOTAL SINCE MIDNIGHT.

&&

MNF