Storm Warning 1, NWS Medford, OR

Area Forecast Discussion
National Weather Service Medford OR
928 AM PST Tue Jan 3 2023

.DISCUSSION...After the break in the action yesterday, the first
in a series of almost daily fronts is moving into the region this
morning. This first system will have the lowest snow levels, which
be low enough to affect the higher passes over the Cascades and
Siskiyou Summit. However, snow amounts should remain below
advisory amounts. Snow levels then rise above the major passes
Wednesday evening into Thursday, with the biggest concern being
winds. With 700 MB winds in the 60-80kt range over the entire
region at times, this system could see trees toppled and possible
power outages.

&&

.PREV DISCUSSION... /Issued 357 AM PST Tue Jan 3 2023/

DISCUSSION...Satellite imagery this morning shows stratus
covering much of the east side, with some areas of fog developing
in valleys west of the Cascades. High level cloud cover is moving
into the region in advance of the next impulse anticipated this
afternoon and evening. Much like yesterday's system, this front
will weaken as it moves inland and there isn't much precipitation
expected with it. We'll see an uptick in winds again this
afternoon, especially for Shasta Valley and the southern end of
the Rogue Valley as well as east of the Cascades, but winds are
expected to remain below advisory criteria. This impulse will be
followed quickly by a shortwave ridge Tuesday night, so we'll see
a brief break in the weather before the main event of the forecast
period arrives late Wednesday morning. Strong low pressure will
develop offshore between 130 and 140W, sending a warm front
through the region late Wednesday, followed by a cold front late
Wednesday night into Thursday.

Light, warm advection precipitation will move northeastward across
the area during the midday and afternoon Wednesday, but at the same
time, a low pressure center offshore will deepen to around 960mb
(some models show sub-960). This will increase the pressure gradient
across the area, resulting in the main impact from this storm, which
will be strong south to southeast winds. Models continue to indicate
pressure gradients peaking in the -6 to -9 range for MFR-RDD and for
OTH-ACV and gradients of this magnitude tend to result in strong
winds for the region. This combined with 700 mb flow in the 60-80 kt
range, supports the concern for warning level winds along the Coast,
in the Shasta Valley and portions of the East Side. A High Wind
Watch remains in effect for these areas where gusts of 60 to 75 mph
are possible Wednesday afternoon into Thursday morning. In addition,
strong winds are expected across the terrain west of the Cascades as
well as the lower elevations east of the Cascades and the
orientation of these winds will likely equate to strong winds in the
Rogue Valley. So, additional wind headlines will likely be needed
for a larger portion of the area, including here in Medford and over
the East Side.

With respect to precipitation, this looks to be largely a Siskiyou
County system, though periods of heavier rainfall are also likely in
Curry and western Josephine counties Wed/Thu. Southerly upslope flow
will be very strong for an extended period in the Mt. Shasta region,
so there will be plenty of rain/snow there. Snow levels initially as
precipitation arrives will be around 3500 feet, but should rise to
4500-5500 feet during the height of the precipitation Wednesday
night into Thursday. Right now, it looks like 2-4 inches of snow for
areas along I-5 south of Weed, before a changeover to rain. Heavier
wet snow (5-8 inches) is possible along Highway 89 near Snowman
Summit west of McCloud before the changeover to rain occurs there
too. The vast majority of snow accumulation from this system will
occur above 5000 feet down there, so expect lots of fresh snow at
Mt. Shasta ski area. With the changeover to rain expected in the
Shasta region and wet ground from previous storms, there could be
some localized flooding down there since liquid precipitation
amounts are likely to be in the 2-4 inch range. Up in Oregon,
however, since the flow will be so strong and largely from the
south, that will mean lots of downsloping and a lot less
precipitation.

Conditions should begin to improve late Thursday afternoon as winds
weaken, however, gusty winds will continue well into the evening
with showers continuing overnight. We'll catch another break in the
weather early Friday, but the next system moves onshore late Friday
into Saturday. This one will be considerably weaker than the midweek
system and more comparable to what we're experiencing early this
week. More systems follow late Sunday into early next week with
additional impacts possible, especially with continued precipitation
over already saturated grounds. River forecast ensembles are showing
possible rises for portions of the Rogue and Klamath/Pit basins on
Sunday into Monday. Stay tuned for updates as details unfold. /BR-y

AVIATION...03/12Z TAFs...Scattered pockets of IFR to LIFR
conditions have formed in valleys west of the Cascades this morning.
Those lower ceilings should persist a few hours after sunrise and
should clear eventually given the turbulence in the boundary layer
today. Some higher clouds are starting to move in ahead of the next
system. We'll see rain along the coast later this morning with MVFR
conditions more likely along the coast.  More LIFR fog is likely
later tonight west of the Cascades.

-Smith

MARINE...Updated 300 AM PST Tuesday 3 January 2023...Steep seas are
expected into this morning. As a front approaches, gusty south winds
will develop this morning and increase this afternoon and may
approach gale force for a brief period. Also expect very steep,
short period seas to develop on Tuesday. Steep seas and lighter
winds are expected late Tuesday evening and Tuesday night.

Strong low pressure is expected to approach the area from the west-
southwest on Wednesday with a strong front developing offshore.
Models indicate this low will approach the area Wednesday then track
northward along the 130 west longitude off the Oregon and Washington
coasts through Thursday. The front is expected to move into the
waters late Wednesday into Thursday, producing strong south winds and
very high and very steep seas. High end gales are expected, with
storm force winds possible Thursday morning.

Confidence is increasing that very high and very dangerous seas will
continue into Thursday as a long period (15 to 17 second) west-
southwest swell builds into the waters behind the front, combining
with continued very steep south wind seas and fresh swell. Seas of
around 25 to 30 feet are possible on Thursday.

Active weather will then continue, with additional fronts producing
periods of strong winds and very steep seas through the weekend.

-BPN/Smith

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...High Wind Watch from Wednesday evening through Thursday
     afternoon for ORZ030-031.

     High Wind Watch from Wednesday afternoon through Thursday
     afternoon for ORZ021-022.

CA...High Wind Watch from Wednesday afternoon through Thursday
     afternoon for CAZ081.

     High Wind Watch from Wednesday evening through Thursday
     afternoon for CAZ085.

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

     Storm Watch from Wednesday afternoon through Thursday evening
     for PZZ350-356-370-376.

     Hazardous Seas Warning until 8 PM PST this evening for PZZ350-
     356-370-376.

&&

SBN/SBN/SBN