High Wind in California, December 25, 2020

Area Forecast Discussion
National Weather Service Medford OR
941 PM PST Thu Dec 24 2020

.DISCUSSION...A winter storm will affect our area from Friday
morning through Saturday morning. The next system of note in a
new series of disturbances looks to arrive Tuesday night.

The forecast was updated to upgrade the Gale Warning
over the coastal waters to a Storm Warning on Friday into Friday
evening. Also, the snow level, precipitation/snowfall amounts, and
winds were adjusted to reflect input from new, recently arrived
model data. Most notably, in line with the 00Z GFS and Blend of
Models there was a slight adjustment higher of snow levels in
southern Siskiyou County late Friday night from 3500 feet up to
4000 feet. As such, the highest probability for snow near Mount
Shasta City will be during the evening, from around 4 PM to 10
PM.

Late this afternoon, an update was also issued to adjust coastal,
Rogue Valley, and Shasta Valley winds such that (in addition to
the Wind Advisory for the area from Paisley to Summer Lake) it
warranted adjusting or issuing a High Wind Warning for the coast
south of Cape Arago, a High Wind Warning for the Shasta Valley,
and a Wind Advisory for the Rogue Valley south of Medford.

Please see the previous discussion below for further details.

&&

.AVIATION...For the 24/18Z TAFs...VFR conditions will persist into
this evening. LIFR/IFR cigs and MVFR/IFR vis in fog or low clouds is
expected to develop in the Umpqua Valley tonight. Isolated LIFR/IFR
are possible in other valleys west of the Cascades. However, an
approaching strong front will bring increased southeast winds aloft
to the area and allow some light breezes in valleys which will limit
fog and LIFR/IFR cigs/vis. So, expect mainly VFR across the area
through Friday morning.

Low level wind shear (LLWS) will become a concern  for areas west of
the Cascades and along the coast late tonight into Friday morning as
winds aloft increase ahead of the front. Strong winds are expected
to develop at the surface along the coast south of Cape Blanco, east
of the Cascades, in the Shasta Valley and in the southern portions
of the Rogue Valley by Friday morning into Friday afternoon.

The front will bring rain into the coast Friday morning with areas
of MVFR cigs, rain and MVFR cigs will become widespread along the
coast in the early afternoon. Precipitation will spread inland
during the day Friday. This is expected to result in some areas of
MVFR cigs and mountain obscurations. -CC

&&

.MARINE...Updated 800 PM PST Thursday 24 Dec 2020... Southerly
winds strengthen tonight into Friday, reaching gale force by tonight
and peaking on Friday with strong gales and gusts to storm force.
Strongest winds are expected mianly from Cape Arago southward . with
winds peaking in the morning and afternoon. Seas will increase
overnight and become very steep and hazardous. Then very steep and
hazardous, wind-driven seas will build further on Friday, peaking
Friday morning and afternoon at around 15 to 24 ft.  Winds ease late
Friday evening and overnight, but seas will remain very high and
steep into Saturday morning.

Guidance continues to trend lower with the high and long period
swell that was expected Saturday. Now it looks like a more typical
northwest swell train of 10 to 12 ft at 14 to 15 seconds. With that
said, seas will transition to swell dominated Saturday and remain
high and steep well into next week. There will be a break in the
weather Sunday into at least Tuesday morning, with the next front
possible by Tuesday afternoon. -CC

&&

.PREV DISCUSSION... /Issued 410 PM PST Thu Dec 24 2020/

SHORT TERM...Through Saturday Night...Christmas will be marked the
arrival of a strong winter storm. Through tonight, expect dry
weather with clouds across the area. High clouds have thickened
across the region faster than expected, which means that we will not
be reaching the forecasted high temperature for the day. Also
adjusted the lows for some west side valleys down a few degrees
because even though the high clouds will hold some heat in, there
isn't much to hold in.

Did increase the winds along the southern portion of the Rogue
valley, including Ashland. winds in the  Klamath basin were also
increased some. To help put some of these winds into perspective,
the NAEFS ensemble awareness table puts the return interval for
surface winds along the coast at values of 1 in every 5 years along
southern Oregon. Expect that beaches and headlands to be
exceptionally blustery. Also along the southern coast there exists a
slight chance for a thunderstorm offshore Friday night into early
Saturday morning as the cold pool moves overhead. Chances are, no
thunder will actually occur; but a slight chance is warranted. If a
thunderstorm or heavy shower does occur, expect small hail to fall
and accumulate, possibly creating slippery conditions for Highway
101 along the coast.

Have included some rain shadow effect for the Rogue, Scott, and
Shasta valleys in the lead up to the storm in the PoPs. Rain will
make it's way into these locations eventually, where 0.38", 0.70",
and 0.40" are forecast respectively. Rain totals along the southern
Oregon coast will be similar to a weak AR, with the coast range
forecast to receive up to 3" and between 1.5" to 2" at lower
elevations like Brookings and Gold Beach. With the south flow the
Coos County coast and higher terrain of central and western Douglas
County will receive less rain than a normal westerly system. In
the northern Coast Range up to 1.75" is possible and about 1" in
the Douglas county higher terrain. At lower elevations along the
coast amounts between 0.95 and 1.10" are forecast. Further inland
at Roseburg and Elkton, 0.55" and 0.76" are forecast
respectively.

Snow amounts have stayed the same with a slight increase in
confidence that lower snow levels around 3500 ft are possible in the
Mt. Shasta area, with localized lowering to 3000 ft with in strong
snow showers with 1"/hr snowfall rates. With such a windy system,
blowing snow will create reduced visibilities along exposed and
long open fetches of road with fresh snow. It will be best to get
any needed holiday travel done before noon if possible.

Conditions will improve quickly Saturday morning and most showers
will be tapering off by noon and winds will ease during the early
morning. The exception for the winds will be on the east side
where some moderately gusty winds could still persist into the
afternoon. We will also enter a stretch of fairly cold nights.
-Miles

LONG TERM...Sunday morning until Friday morning...

Ensemble guidance suggests there is a very high probability the next
low will miss us and move into central California late Sunday into
Sunday night.  The probability of precipitation is about 20 percent
for southern Siskiyou County and southern Curry County as this low
moves through.  Drier air and east winds will move into the forecast
area Sunday into Monday.

Then, we'll see the next system impact the region sometime around
Tuesday and Wednesday.  The deterministic models have been varying
when they bring the warm front through.  The latest round of
guidance suggests Tuesday afternoon and evening.  However,  ensemble
data suggests that it could very well be later around Wednesday. In
any case,  we'll likely be under some type of atmospheric river (AR)
event mid week.  Snow levels will most likely shoot up to the 6500
foot level when the initial warm air pushes onshore before falling
down 4500 feet around Thursday. -Smith

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...Wind Advisory from 10 AM Friday to 4 AM PST Saturday for
     ORZ030-031.
     High Wind Warning from 6 AM to 4 PM PST Friday for ORZ021-022.
     Winter Weather Advisory from 10 AM Friday to 10 AM PST Saturday
     above 4500 feet in the for ORZ027-028.
     Wind Advisory from 7 AM to 7 PM PST Friday for ORZ026.

CA...Winter Storm Warning from 10 AM Friday to 10 AM PST Saturday
     above 3500 feet in the for CAZ080-082-083.
     High Wind Warning from 7 AM to 7 PM PST Friday for CAZ081.

Pacific Coastal Waters...Storm Warning from 7 AM to 7 PM PST Friday
     for PZZ350-356-370-376.
     Gale Warning until 7 PM PST Friday for PZZ350-356-370-376.
     Hazardous Seas Warning until 10 PM PST this evening for
     PZZ350-356-370-376.
     Hazardous Seas Warning from 7 PM Friday to 1 AM PST Saturday for
     PZZ350-356-370-376.

DW/CC