High Wind in Oregon, December 13, 2024

Area Forecast Discussion
National Weather Service Medford OR
1014 AM PST Fri Dec 13 2024

.UPDATE...Updated Aviation Discussion.

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.AVIATION...13/18Z TAFS...A frontal system offshore will move
onshore this afternoon. Strong south winds are developing in advance
of the front and this is causing low-level wind shear risk from the
coast to the Cascades. Winds just above the surface are in the 40-
60kt range (2000 ft AGL), while an inversion or terrain
prevents/blocks the winds from reaching the ground. This could
remain the case at some terminals (Roseburg) through this evening,
but we do think winds will surface here at Medford as the winds
align more with the valley (SE). Wind gusts near or greater than
35kt are possible. Similarly, gusty winds will affect the terminal
at Klamath Falls. We'll be monitoring the winds for potential
airport weather warnings. Strongest winds are expected into this
evening, then should subside overnight into Saturday morning.

In addition, the front is causing reduced ceilings and visibilities,
generally MVFR, but also areas of IFR/LIFR and widespread higher
terrain obscurations in areas of rain/snow. -Spilde

&&

.PREV DISCUSSION... /Issued 254 AM PST Fri Dec 13 2024/

SHORT TERM...through Saturday (December 14th)...The next and
strongest front of the series is approaching the region this
morning. This system will bring strong winds, moderate to heavy
rain/mountain snow, and another significant round of heavy wet
snow to the Mt Shasta region today into Saturday. A surface low
pressure moves from the southwest to the northeast today and
deepens inside of 130 W. This low will move inland into Vancouver
Island tonight and this is a classic wind maker pattern for our
region. Southerly winds are already increasing along the coast and
will continue to increase across the area after sunrise. Forecast
pressure gradients between Medford/Redding peak this
afternoon/evening around -7 to -9 mb and this is a good indicator
for warning level winds along the Coast and in the Shasta Valley
with advisory level winds here in the Rogue Valley. It'll be
breezy everywhere today, but these are the areas we expect the
strongest winds. Additionally, models indicate a 55-65 kt mid
level jet passing over the East Side tonight. The extent of strong
winds across the East Side may be mitigated by limited mixing
that occurs during the overnight period. Currently there is a Wind
Advisory in place for the typical windy locations east of the
Cascades, but it's possible for winds to reach high end advisory
criteria with the highest peaks seeing warning level winds.
Various wind headlines are in place for the aforementioned areas,
and details can be found at NPWMFR.

The other significant impact with this storm will be a heavy wet
snow event in and around the Mt Shasta area late this morning
through Saturday. Moderate to heavy precipitation along the front
will push into the Mount Shasta Area and this in combination with
good upslope flow will enhance precipitation rates in that area.
This combination will result in locally lower snow levels in and
around Mount Shasta Area relative to surrounding areas where 10 to
15 inches of heavy wet snow can be expected below 4000 ft. Above
4000 ft, 1 to 2 ft of snow is expected, including Snowman Summit on
Highway 89. A Winter Storm Warning is in effect for this event, and
additional details can be found at WSWMFR. Outside of the Mt Shasta
Region, snow levels will hover around 4500 ft, so the Cascades and
Siskiyous will also see another round of snow as well. Given this is
southwesterly flow event, these ranges won't be hit as hard as areas
south of the OR/CA border, but snowfall totals will be close to
advisory criteria with this event. However, the borderline criteria
amounts are fairly high up in elevation so we'll hold off on any
winter weather headlines for the Cascades at this point. It should
be noted, however, that there is 60-80% chance of 1"/hr rates up
around Crater Lake and Mt Ashland area this afternoon and evening.

A secondary, weaker low will develop along this front Saturday
morning, then move inland near or just south of Cape Mendocino. This
will maintain precipitation and breezy conditions across the region
through Saturday evening. Though wind and snow are the main impacts
with this system, it should be noted that this will bring a hefty
amount of precipitation to the region as well. Total rainfall
amounts from this morning through Saturday evening will range from 1-
3 inches along the coast and in western/southern Siskiyou County, 1-
2 inches for inland areas west of the Cascades (0.50-0.75" in the
typically downsloped valleys) and 0.50-0.75" across the East Side.
Given the prolonged dry period we're coming out of, the mainstem
rivers are plenty capable of absorbing this moisture so we aren't
expecting any flooding impacts, but ponding of water on roadways is
likely in areas with heavy rain.

The weather will settle some Saturday night into Sunday under a
brief period of upper level ridging, but the active weather isn't
over. Details can be found in the previous Long Term discussion that
follows. /BR-y

LONG TERM...Sunday through Thursday, December 15-19, 2024...

Sunday will be a transition day in the active weather pattern that
will continue into early next week. Short wave upper ridging is
expected to move through the area then, so any showers lingering in
the westerly flow aloft (mainly over the mountains) should come to
an end. As such, this will probably be a good day for travel.

The break will be brief since an active Pacific jet will send
another disturbance in our direction Sunday night into Monday. It
appears rain could redevelop at the coast as early as Sunday
evening, then spread inland overnight into Monday. This system looks
fairly typical for mid-December with some rain/wind at the coast,
gusty south to southeast winds in the Shasta Valley and southern
Rogue Valley and southwest winds over the East side. Some snow will
fall in the mountains (mostly above 3500 feet). We're not expecting
anything too hazardous, but there still could be some minor to
moderate travel impacts. The focus for precipitation will be from
Curry and Josephine counties into far southern and western
Siskiyou County where preliminary liquid amounts look to range
from 0.50-1.50 inches. Amounts drop off farther to the north and
east of these areas with 0.10-0.30 of an inch the most likely
amount in the west side valleys. Most areas east of the Cascades
should have 0.10 of an inch or less. Snow amounts of 2 to 6 inches
appear to be a good bet in the Cascades/Siskiyous as well as the Mt
Shasta region with up to 10 inches in the highest mtns.

After that, confidence is increasing that a pattern change toward
drier weather will occur Tue or Wed of next week. Guidance is fairly
unanimous in showing a warm front offshore pushing toward the coast
Monday night with additional rainfall. Some models (12z
deterministic GFS) bring this front in farther south (into our
area), but most skirt the area with most impacts felt along the
immediate coast and to our north. We'll keep an eye on this to see
if there are any trends to bring this system in farther south, but
for now, we have gone with the NBM/ensemble blend which begins the
drying trend Tuesday. Models then show the warm front lifting north
of the area Tuesday night with an upper ridge building into
California Wednesday. While a weak disturbance could pass through or
over the top of the ridge Wednesday, the storm track likely remains
to our north with ridging to persist Thursday into Friday. Given the
time of year and following multiple precipitation events, this may
mean another round of fog/low clouds for the valleys. -Spilde

MARINE...Updated 200 AM PST Friday, December 13, 2024...Steep
swell dominated seas and gusty winds will continue overnight. Then,
a strong front moving into the area today will bring very strong
south winds, with gusts at or near storm force, and very steep seas
beginning this morning and persisting through Friday night. Models
continue to show that south winds will quickly strengthen this
morning, peaking from mid morning into this afternoon with gusts to
around 55 kt. Winds and seas will lower Friday night into early
Saturday but seas will remain steep through at least Sunday morning.
Another strong front is expected to approach the area Sunday night
and move through Monday. This will bring a potential for gales and
very steep seas. -CC

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...High Wind Warning from 7 AM this morning to 7 AM PST Saturday
     for ORZ021-022.

     Wind Advisory from 4 PM this afternoon to 7 AM PST Saturday for
     ORZ026-030-031.

CA...Winter Storm Warning from 1 PM this afternoon to 4 PM PST
     Saturday for CAZ080-082-083.

     Winter Weather Advisory from 1 PM this afternoon to 4 PM PST
     Saturday for CAZ080-083.

     High Wind Warning from 7 AM this morning to 7 AM PST Saturday
     for CAZ081.

     Wind Advisory from 4 PM this afternoon to 7 AM PST Saturday for
     CAZ085.

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

     Small Craft Advisory from 10 PM this evening to 10 AM PST Sunday
     for PZZ350-356-370-376.

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

&&

MAS