Winter Storm in Oregon, March 4 to 5, 2023

Area Forecast Discussion
National Weather Service Medford OR
945 PM PST Fri Mar 3 2023

.Updated Aviation Discussion.

.AVIATION...04/06Z TAFs...The main concern for the TAF period will
be low level wind shear at Roseburg and Medford. There is evidence
that supports borderline low level wind shear at North Bend for at
least a few hours and this was added to the TAF.

Meanwhile, at the coast and just offshore, ceilings will vary
between MVFR and VFR as showers move in from the west. In the
absence of showers, ceilings will improve to VFR. The timing and
duration of when ceilings will change category will be a challenge
tonight, so watch for updates. As the front moves closer to the
coast, ceilings are likely to lower to IFR towards 11z along with
lower visibility. Moderate to strong south winds are likely later
tonight into tomorrow morning, then winds will shift southwest late
in the morning.

Inland west of the Cascades, VFR ceilings are expected through
tonight, then will lower during the morning hours with Roseburg
lowering first, followed by Medford. Visibility is also expected to
lower, especially in areas that get moderate to heavy precipitation.
Also the terrain will be obscured most of the day Saturday. There's
some evidence pointing towards moderate southwest winds in Medford
for a brief period of time after the front passes through towards
late Saturday morning.

East of the Cascades, VFR ceilings will continue through early
tomorrow morning. Ceilings and visibility will lower around mid
morning as light snow moves into Klamath Falls. The lower conditions
will also include moderate to occasionally strong winds. -Petrucelli

&&

.DISCUSSION...This evening, very northwestern Oregon has rain
while southern Oregon is dry. Our next system is still on track
to arrive tonight as the upper level low continues southward. The
winter weather products begin at 4 AM, and models are still in
agreement with how this system will affect the area. The main area
of higher impacts still resides along the Oregon Coast and
western Siskiyou County. Tonight and through late morning, parts
of the higher terrain in Curry and Siskiyou counties could see 1
to 2 inch per hour snowfall rates. With the current forecast of
the front and precipitation in place, lower snow levels at 1,000
to 2,000 feet support an inch in parts of the Rogue Valley early
Saturday morning. More details on the forecast can be found in the
previous discussion, and our winter product information can be
found at WSWMFR.

&&

.MARINE...Updated 830 PM Friday, March 3, 2023...Several of the
buoy observations are showing south winds gradually increasing.
South winds will continue to increase over time tonight as a strong
front moves closer to the waters. The front will move into the
waters overnight tonight, then moving inland early Saturday morning.
South winds tonight into early Saturday are expected to reach gales
mainly between 3 nm and 30 nm from shore. Winds will quickly shift
to the west and diminishing behind the front, but moderate winds will
continue through Saturday. At the same time seas will remain steep
all day Saturday, and a larger northwest swell will move in Saturday
afternoon. Another item to keep in mind will be isolated
thunderstorms as a colder air mass with 500m b temperatures between
-34 and -36 C move over the waters.

Steep seas will expected to continue Saturday night into Monday,
with another round of moderate to strong south winds developing
Sunday night into Monday morning. -Petrucelli

&&

.PREV DISCUSSION... /Issued 416 PM PST Fri Mar 3 2023/

DISCUSSION...

Short Term...Tonight through Sunday Night...Some light showers
are appearing on radar this afternoon,mainly along the coast,
throughout the Umpqua Basin, and into the Cascades. These will
taper off this evening, but are not likely to completely end until
the next system arrives.

This next system will be the main event. A large upper level
trough and associated surface front will dig south from the Gulf
of Alaska, arriving offshore of southern Oregon late tonight into
early tomorrow morning. As the occluded front pushes through the
region through the morning and early afternoon, moderate to heavy
precipitation is expected along the coast and across the West
Side to the Cascades. With cold air already in place, and snow
levels expected to remain between 1000 and 2000 feet initially,
widespread winter weather impacts are expected, similar to those
seen during the system earlier this week.

The highest initial impacts will be along the Oregon Coast and
western Siskiyou County, with moderate to heavy rainfall
(0.75-2.00 inches) expected along the coast and heavy snowfall
over mountain ranges in Curry and western Siskiyou counties.
Snowfall accumulation rates in these higher elevation areas could
exceed 1 inch per hour Saturday morning. Curry and Siskiyou
mountains are forecast to get 1-2 feet of snow by Saturday
evening, while the Mount Shasta region is expected to get over 6
inches of snow. Enhanced precipitation should keep the snow level
lower in the Illinois Valley, where over 6 inches of snow is
expected in this period as well. The Cascades and Siskiyou
Mountains, including the higher passes near Lake of the Woods,
Highway 62 north and Union Creek and around Crater/Diamond Lakes
could get over 8 inches of snow by Saturday afternoon. Inland
valleys (such as the Umpqua and Rogue) could see an inch or so of
snow through Saturday afternoon. Gusty winds in some areas, namely
over higher terrain, within the Shasta Valley, and across the Eats
Side, could produce periods of blowing snow and reduced
visibilities as well.

Snow levels rise to about 1500 to 2000 feet Saturday afternoon,
and while precipitation will decrease in intensity, it will not
end, with showers continuing through the evening. Saturday
night into Sunday morning, another wave of energy will pass
onshore as it orbits the offshore upper low. A surface low is
expected to pass to our south, but many of the models show the
mid-level low passing onshore and tracking northwest over our
area. The exact path of these two features will play heavily into
the location and intensity of precipitation. The result will be
another round of heavy precipitation along the southern Oregon
coast into northern California due to the resulting upslope flow.
As this is set to occur during the overnight and morning hours,
snow levels will once again be low, down to about 500 feet or
potentially lower. As a result, area valleys, and even portions
of the coast, could easily pick up some snow accumulations until
snow levels rebound to about 1500 feet and temperatures warm
through the afternoon. Showers then continue through Sunday night,
although intensity and amounts will taper off significantly from
those earlier in the day.

It should also be mentioned that with such a robust low offshore,
a few thunderstorms are possible along the coats and into the
coastal ranges Saturday into Sunday.

More detail and information on area winter impacts and snowfall
amounts can be found in the Winter Weather Warnings and
Advisories, found at PDXWSWMFR. -BPN

LONG TERM...Monday through Thursday...Despite the fact that a cold
upper low remains parked off the coast, Monday may not be that
eventful for most of our area as much of the dynamics remains
offshore and south of us. With cold, moist, onshore flow, we will
continue to see moderate orographic precipitation in the coastal
mountains, but just light, nuisance type precipitation elsewhere.
Snow levels will vary considerably during the day - ranging from 500
feet or so at night to 2,000 feet during the day - and so will any
resulting impacts from snow.

There is still fairly low confidence in the details for Tuesday and
Wednesday, but guidance all shows the low moving onshore during the
period. As a result, we'll see an increase in precipitation, and
with snow levels continuing to drop below 1,000 feet at night, west
side valley snow impacts will remain possible, especially at
night and during any periods of heavier precipitation.

Long range ensemble guidance is in pretty good agreement in breaking
down the blocking pattern that's been bringing us all this cold,
snowy weather later this week. As the aforementioned low moves
inland, there will probably be a day or two of ridging and
relatively drier, milder weather late this week. But beyond that,
there is an increasing probability of a transition to much warmer,
wetter southwest flow aloft by next weekend. How exactly that shapes
up is still uncertain, but it does introduce the possibility of
warmer rains on top of all the snow we've had in the mountains. This
would become a concern for higher main stem river flows and the
flooding concerns that usually brings. That's still quite a ways
out, but worth watching. -Wright

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...Winter Weather Advisory from 4 AM Saturday to 4 PM PST Sunday
     for ORZ024-026.

     Winter Weather Advisory from 4 AM Saturday to 4 PM PST Sunday
     for ORZ029>031.

     Winter Weather Advisory from 4 AM Saturday to 1 PM PST Sunday
     above 1000 feet for ORZ021-022.

     Winter Weather Advisory from 4 AM Saturday to 4 PM PST Sunday
     above 1000 feet for ORZ023-025.

     Winter Storm Warning from 4 AM Saturday to 4 PM PST Sunday above
     1500 feet for ORZ024.

     Winter Weather Advisory from 4 AM Saturday to 4 PM PST Sunday
     for ORZ027-028.

CA...Winter Storm Warning from 4 AM Saturday to 4 PM PST Sunday for
     CAZ080-082.

     Winter Weather Advisory from 4 AM Saturday to 4 PM PST Sunday
     for CAZ081.

     Winter Storm Warning from 4 AM Saturday to 4 PM PST Sunday for
     CAZ083.

     Winter Weather Advisory from 4 AM Saturday to 4 PM PST Sunday
     for CAZ083.

     Winter Weather Advisory from 4 AM Saturday to 4 PM PST Sunday
     for CAZ084-085.

PACIFIC COASTAL WATERS...Small Craft Advisory until 4 AM PST Monday
     for PZZ350-356-370-376.

     Gale Warning until 7 AM PST Saturday for PZZ350-356-370-376.

&&