Heavy Snow in Oregon, February 23 to 24, 2023

Area Forecast Discussion
National Weather Service Medford OR
248 PM PST Thu Feb 23 2023

.DISCUSSION...The latest satellite image shows plenty of "popcorn"
configuration of clouds over the waters and are moving from west to
southwest to east to northeast into our area. In the lower
elevations, especially below 1500 feet, snow showers that do occur
the rest of the afternoon will melt on the roads. The exception will
be in heavier showers where they could result in brief slushy roads,
then once the heavier showers let up in intensity or move away with
sun breaks, any snow on the roads will once again melt.

A cold upper and surface low pressure currently off the northwest
Oregon coast will continue to drop south off the Oregon coast during
the afternoon and evening hours, with plenty of showers rotating
around the low just about everywhere, but especially along and west
of the Cascades.

The low will move south into northern California this evening and
night at which point the focus for moderate to heavy snow will be in
northern California, specifically around the Mount Shasta Area, and
eastern through southeast Siskiyou County However, the lift and
upslope flow will develop from about Weed south to the Mount Shasta
Area, including Mount Shasta City, the Mount Shasta ski area, and
areas northeast of Mount Shasta towards Tennant. This will result in
moderate to heavy snow in these areas with snowfall rates pushing
into the 1 to 2 inch per hour range south of Weed, and along highway
89 tonight. Also, moderate to strong winds in the south end of the
Shasta Valley near Weed in combination with snow could result in
reduced visibilities, blowing and drifting snow, and occasional
whiteout conditions.

However, this won't be the only area that will get impacted.
Portions of the west side, including southern Coos, Curry, southern
Douglas, Josephine and Jackson Counties could be at risk for
accumulating snow early this evening. A look at several high res
guidance and several pieces of data from SPC HREF suggest as the
upper and surface low drops off the California/Oregon coast, there
could be some return flow around the eastern fringes of the the
upper low that could result in additional showers in these areas.
However this should be primarily from this evening until shortly
after midnight tonight. Given snow levels are all the way down to
near seas level, felt it was best to issue a Winter Weather Advisory
for tonight. Flurries and occasional light snow are possible in the
portions of the Rogue Valley where we don't have the Winter Weather
Advisory that could leave a light dusting resulting in slick roads,
but again most of the concern should be during the first half of the
night, and it's even possible that little or nothing happens due to
downsloping and some drying moving in from the north.

Another area of concern will be east of the Cascades, mainly along
highway 97 due to lift and upslope flow this evening into most of
Friday morning. Given that it's already cold, snow ratios will be
higher, so it won't take much to snow to pile up there, and a Winter
Weather Advisory has been issued. The combination of snow and winds
could result in reduced visibilities and blowing and drifting snow.

Meanwhile a colder air mass will be moving into the area and there
could be some locations that could break record lows Thursday night
into Friday morning, including the coast. This despite ample cloud
cover over most of the area.

Friday, the upper and surface low will slowly move south along the
California coast. Upslope flow precipitation will persist in the
Mount Shasta area, with moderate to heavy snow lingering into the
morning hours, then precipitation should gradually diminish during
the afternoon hours Friday. Meanwhile light to moderate snow will
persist east of the Cascades, mainly along highway 97 Friday
morning, then precipitation will diminish and end Friday afternoon.

Dry weather is expected Friday night with decreasing cloud cover.
Low temperatures Friday night could even be colder, especially where
is fresh snowpack with numerous record lows being broken once again.
-Petrucelli

.LONG TERM...Saturday, Feb. 25 through Thursday March 02...
More low elevation, valley snowfall? That is what the extended
pattern has us wondering. Overall, the pattern isn't too dissimilar
to the past couple of weeks and the snow that resulted from it. This
has us thinking of low elevation snow as a valid possibility again.

A period of weak ridging will slowly transition on Saturday back
towards a troughing pattern. The brief ridging will, while it lasts,
allow afternoon temperatures to climb around normal for the end of
February. Temperatures west of the Cascades should be in the upper
40s to low 50s, while east of the Cascades we are anticipating
temperatures in the low 40s. The moderate temperatures will also be
associated with a decaying east wind, which will turn onshore west
of the Cascades and southerly east of the Cascades by that
afternoon.

Then Sunday into Monday there is very strong model agreement that
another low will drive south out of the Gulf of Alaska. Beginning
early Sunday morning this low brings a high 60-100% chance for
rain/snow to SW Oregon and NorCal. With west-northwest steering
flow, only Lake County due to the shadowing effects of the Cascades,
has lower chances for precipitation.

Ahead of the initial front the snow level is forecast to be
between 3500 and 4500 feet, lower west of the Cascades. Then as
the front moves through and cold air funnels in, the snow level is
expected to lower to between 1300 and 2000 feet. At this range in
the forecast we lack hi-resolution model data and as a result the
valleys don't get resolved well. Even so, the trend for snow
levels near west side valley floors is very evident on pattern
recognition alone. In addition, overnight low temperatures in the
valleys are expected to be below freezing, which should allow for
at least a brief window for snow to accumulate; just in time to
have some impact on the morning commute before possibly turning
back over to rain. For areas east of the Cascades, we are
anticipating all snow and though generally light, gusty winds of
45-60 mph on Sunday in combination with any existing snow plus the
fresh snow, could create travel difficulties due to reduced
visibility from blowing snow.

A secondary low should then get wrapped into this main trough and
bring another wave of cold air and moisture into the region. Snow
levels remain mostly the same with this secondary low through Sunday
until Monday night and then they are forecast to lower further below
1000 feet. The highest confidence for travel impacts from snow will
be in the Kalmiopsis Wilderness, the Siskiyous, and Mt. Shasta
region when a brief window of southwest flow Monday afternoon
triggers orographic lift driven snowfall; which even this far out is
looking like it could have snowfall rates approach 1"/hr. This is
also the window where the East Side is most likely (40-70%) to pick
up new snowfall. As northwest flow returns overnight, snow in the
areas just mentioned decreases, but snow showers across the west
side become more prevalent. Amounts will vary quite a bit because of
the nature of scattered showers but, west side valleys have a 10-30%
chance for 0.1" or more of snow through the Tuesday morning commute
timeframe (60% around the southern end of the Illinois valley).
Meanwhile the main passes along I-5 and over the Cascades have a 40-
90% chance for at least 1" or more of snowfall. And not to get too
excited, but, there is a 10% chance for at least 0.1" of snow along
the coast again (mostly the Curry County coast).

With the low off the PNW coast still churning on Tuesday, shower
activity will continue, although it will be steadily diminishing.
The snow level is forecast to remain steady in the 1200 to 1500
foot range. By Tuesday night ridging builds in as the trough
sweeps over the Southwest. During this time period of Tuesday
through Wednesday some record temps will be possible. Both minimum
temperature and low-maximum temperatures could be challenged
along the S. OR coast and in NorCal. East of the Cascades, a
return to single digit temperatures overnight is anticipated, with
sub-freezing temperatures across the West Side.

With the door still open another trough looks to move out of the Bay
of Alaska and will begin to impact the area at the end of the
forecast period on Thursday. This is where the guidance really shows
some strong variation -thankfully it is the end of the forecast
period. The leading solution that takes up 72% of the ensemble space
shows moist northwest flow that would bring renewed snowfall across
the region. The remaining ensemble data shows more of a split flow
pattern that would be more wet for central and southern California,
but relatively dry for us. And seeing as how our california climate
sites are the only ones above normal precipitation for this water
year (since Oct. 1) and the 72% chance would not be dry for them, it
is to the benefit for everyone else this water year that the leading
solution is more wet.
-Miles

&&

.AVIATION...23/18Z TAFs...Low pressure digging south off the Oregon
Coast will provide a cold and moist onshore flow providing snow
showers the coastal Range and generally to the western and southern
tier of the regional terrain. As time progresses this afternoon and
night, flow transitions and increases from the south with coastal
showers lessening and upslope snow becoming heavy over northern
California. For North Bend (KOTH) and the coast, look for widespread
MVFR ceilings and visibility in light rain or rain/snow with
scattered IFR through the day transitioning to lesser precip and
improving conditions overnight. Also, despite some continuing
terrain obscuration, expect an improving trend inland for Roseburg
(KRBG) and Medford (KMFR) as some east to southeast winds channel in
the Rogue and Umpqua Valleys. Sub-VFR impacts of showers and
ceilings should be mainly over here however. East of the Cascades,
look for gusty south to southeast winds and scattered snow showers
that may provide intermittent MVFR cigs/vsby. Stavish

&&

.MARINE...Updated 230 PM Thursday, February 23, 2023...Steep
northwest swell will continue to slowly diminish tonight. A cold
area of low pressure will move south and track just to the
west of the coastal waters. This low pressure will create
southwest winds early tonight.  As the low migrates south, winds
will shift to the northeast by Friday morning.  These
northeast winds will strengthen on Friday and create high and steep
seas in the outer waters.  Most of the seas will be wind driven and
will be hazardous to smaller crafts. The active weather pattern will
persist through next week although there will be a lull in activity
Saturday night. -Miles/Sargeant

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...Winter Weather Advisory from 7 PM this evening to 7 AM PST
     Friday for ORZ021-022.

     Winter Weather Advisory from 7 PM this evening to 7 AM PST
     Friday for ORZ023-024-026.

     Winter Weather Advisory from 7 PM this evening to 10 AM PST
     Friday for ORZ029-030.

CA...Winter Storm Warning until 10 AM PST Friday above 2000 feet for
     CAZ080.

     Winter Weather Advisory until 10 AM PST Friday for CAZ080.

     Winter Storm Warning until 10 AM PST Friday for CAZ081>083.

     Winter Weather Advisory until 10 AM PST Friday for CAZ081-083.

PACIFIC COASTAL WATERS...Small Craft Advisory until 10 AM PST Friday
     for PZZ376.

     Small Craft Advisory until 4 AM PST Saturday for PZZ350-356-370.

     Hazardous Seas Warning from 10 AM Friday to 4 AM PST Saturday
     for PZZ370-376.

&&