Winter Storm in Oregon, March 9 to 10, 2023

Area Forecast Discussion
National Weather Service Medford OR
918 AM PST Thu Mar 9 2023

.UPDATE...

The current forecast of the incoming atmospheric river appears to
be on track. Widespread rain is expected to fall across most of
northern California and southern Oregon this morning through
Friday afternoon. Updated model guidance does show that
downsloping may delay rainfall in west side valleys. Winter and
wind headlines for potentially hazardous issues have been issued.
Additionally, the Forest Service has issued an Avalanche Warning
for areas around Mount Shasta as fresh snow falling on top of
existing snowpack could hazardous conditions. -TAD

&&

.AVIATION...09/12Z TAFs...Isolated to scattered showers are expected
early this morning with snow levels around 2000 feet, though
conditions will predominantly remain VFR.

Upper level winds will increase this morning. With light to moderate
surface winds, low-level wind shear is forecast at North Bend,
Roseburg and also Medford through around 18Z this morning.

Winds are expected to surface at Medford (from the SE) and increase
at Klamath Falls (SSE) later this morning and increase through this
afternoon. Wind gusts of 30-35 kt are expected for Medford and
35-45 kt for Klamath Falls.

A strong front will produce precipitation and MVFR ceilings at the
coast this morning, expanding across the area into this evening with
terrain becoming obscured. Downsloping will prevent precipitation in
the Rogue Valley until evening. At Klamath Falls, there will be some
melting of snow as it initially falls this afternoon and early
evening, there is a high probability for accumulating snow of around
a couple of inches on the runway with blowing snow and significant
visibility reductions at Klamath Falls tonight. -DW

&&

.MARINE...Updated 800 AM Thursday, March 9, 2023...South to
southeast gales with a cold front will strengthen today and persist
into this evening with very steep and high seas. Winds shift to the
southwest and ease a bit behind the front later tonight. But, seas
will remain high and very steep through Friday. Conditions are
expected to briefly improve Saturday, then another strong front is
possible on Sunday. Yet another system may follow closely behind
Sunday night into Monday night. -DW/CSP

&&

.PREV DISCUSSION... /Issued 339 AM PST Thu Mar 9 2023/

DISCUSSION...Amounts of rain have varied between 0.01" and 0.15"
along the coast as some stratiform rain moves in ahead the main
front attached to the low spinning off the coast. Further inland,
a lot of this activity will serve the purpose of eroding the
~5000 foot dry "surface" layer observed in yesterday's afternoon
sounding before heavier precipitation returns with the main front
later today (we'll have to see how much has been eroded come our
12z sounding!).

Tonight, the main edits to the forecast included reducing the
chance for rain in the west side valleys this morning and
afternoon. This was due to downslope induced rain-shadowing from
35+ kt 700 mb winds. The chance for rain in the valleys increases
after the front moves through, some time around 4 PM. This
rain-shadowing will mean we probably miss out on 0.1" to 0.3"
that is forecast during the downslope period (we leave this in to
represent what could happen if the winds aren't strong enough to
cause the downsloping; which is still possible in a couple of
outlying solutions).

Another change to the forecast was to add a winter weather
advisory for areas above 2500 feet across Eastern Curry,
Josephine, Jackson and Eastern Douglas County. This advisory is
out from this evening into Friday morning. It was a little tricky
too because snow levels start out around between 2500 and 3000
feet, rise to 3500 to 4000 feet in Jackson, Douglas County & 4000
to 5000 in Curry, Josephine County, before falling back to around
2500 feet. This means that there will be a period where some sleet
will mix in with the snow and partial melting may occur right
around 2500 to 3000 feet. This melting/sleet before colder air
moves in could lead to some re-freezing and ice development before
it gets covered by more snow. This was one of our main reasons
for putting out the advisory, with the other being the combination
of wet, heavy snow and winds gusting to 30 to 45 MPH. The wind
and snow combination could be enough to bring down weakened trees
and tree limbs.

Lastly, wind speeds west of the Cascades were lowered by 5 to 10
mph Friday morning and afternoon. Previous speeds would have
suggested a widespread advisory would have been needed even in
parts of the Umpqua Basin. The last few model runs have been
trending downward, so this seemed an appropriate adjustment for
the valleys. Exposed ridges could still be gusty, but again, even
these locations have been trending down, so here too small
reductions were applied.

Beyond those changes, the forecast was still very much on track.
We also checked on the winds at the coast and the need for a high
wind warning there. Peak wind gusts could be very close to the 58
mph criteria, there are some outlying solutions showing this, but
overall the forecast looked to display the most likely solution.
The Curry County coastline and mountains would be the place we
would observe warning strength wind gusts if they were to occur.
And the other winter/snow headlines still looked good to. We
evaluated extending the warning in the Mt. Shasta area but, the
wasn't enough evidence that this was needed to take action.

The other forecast update to share is on the river flooding
concerns, or in this case, a reduction in concern. The latest
forecast from the NW River Forecast Center now has the Deer Creek
at Roseburg peaking below action stage, at around 7 feet. Flooding
concerns still remain in Curry, Coos, and Josephine County.
Details on this can be found in the Hydrologic Outlook headline
product.

Extended discussion from Wednesday afternoon...LONG TERM...Sat
March 11 to Wed March 15...The weekend begins with a brief period
of dry weather in between wet systems. There may be some lingering
showers around Saturday morning, particularly over northwest
Douglas and Coos Counties and over the Cascades, but any
precipitation will be light. The majority of the region will have
a dry, partially cloudy day, with cool daytime temperatures.

Early Sunday another atmospheric river system approaches the coast,
with pockets of energy riding through zonal flow aloft, leading to
periods of generally light rain. South to southwesterly flow means
that rain will begin in Northern California early Sunday before
moving northwards into Oregon Sunday morning. Rain amounts during
this period will be heaviest in Northern California and over the
Coastal Range, while those of us in sheltered west side valleys and
east of the Cascades will see some downsloping effects from this
flow pattern (and thus less precipitation). Unlike the previous
week, this southwest flow with associated warm air advection means
that snow will be relegated to higher elevation, with snow levels
starting at around 4000 feet and rising to 5000-6000 feet by the end
of the weekend.

Rain and high elevation snow continues into the work week as an
upper level trough approaches the coast. Winds will pick up ahead of
this, especially at the coast and over higher terrain. Southeast to
southwest winds means that north-to-south oriented valleys (Rogue
and Shasta Valleys) will likely get some good gusty winds as well.
There could be periods of heavy rain, especially in N. Cal and over
more western counties. Snow levels will be around 5000 feet for the
brunt of this, lowering to 3000-4000 feet as the pattern turns more
showery. We'll be keeping an eye out for flooding potential during
this period, as higher snow levels and heavier rain means that some
of our lower elevation snow pack could start to melt, inundating
some of the rivers.

While there is a fair amount of model consensus on this general
pattern, there are differences in the timing of the upper trough and
thus the increased precipitation. The ECMWF is so far favoring a
more progressive system that moves into the region more quickly Late
Monday morning, while the GFS shows heavier precipitation starting
early Tuesday. At this range there's plenty of uncertainty about the
exact timing and details of this, but the overall synopsis is that
we'll have wet weather Sunday morning through early next week.
Wednesday through at least Thursday will likely be another dry
period, with perhaps some light lingering showers earlier on, before
another wet system arrives over the weekend. -CSP

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...Winter Storm Warning until 4 PM PST Friday for ORZ029>031.

     Winter Weather Advisory until 4 PM PST Friday for ORZ029>031.

     Winter Weather Advisory from 10 PM this evening to 4 PM PST
     Friday for ORZ023>026.

     Wind Advisory from 1 PM this afternoon to 7 PM PST Friday for
     ORZ026.

     Winter Storm Warning until 4 PM PST Friday for ORZ027-028.

     Winter Weather Advisory until 4 PM PST Friday for ORZ028.

CA...Winter Storm Warning until 4 PM PST Friday above 3000 feet for
     CAZ080.

     High Wind Warning from 1 PM this afternoon to 1 PM PST Friday
     for CAZ081.

     Winter Storm Warning until 4 PM PST Friday for CAZ082-083.

     Winter Storm Warning until 4 PM PST Friday for CAZ084-085.

     Winter Weather Advisory until 4 PM PST Friday for CAZ084-085.

PACIFIC COASTAL WATERS...Gale Warning until 10 PM PST this evening
     for PZZ350-356-370-376.

     Hazardous Seas Warning from 10 PM this evening to 4 PM PST
     Friday for PZZ350-356-370-376.

&&