High Wind in California, December 26 to 27, 2023

Area Forecast Discussion
National Weather Service Medford OR
920 PM PST Tue Dec 26 2023

.UPDATE...Following the passage of a weak boundary within the warm
sector this evening, winds have started to increase ahead of the
much more robust cold front expected to pass through the area
tomorrow. Around 8:15 this evening at the Medford airport, winds
were calm and the temperature was 48 degrees. In less than ten
minutes, winds became south and were gusting over 30 mph and the
temperature jumped up to 59 degrees.

A few adjustments were made to the forecast this evening to bring
things in line with the current observations, particularly the
overnight lows. With winds increasing and cloud cover overhead,
the typical NBM/bias correct models are not ideal.

For more details on this upcoming wind storm and the rest of the
forecast, see the previous discussion below. -BPN

&&

.AVIATION...27/06Z TAFs...Generally VFR conditions are prevailing
across the region, though there are some isolated areas of IFR/LIFR
in the most sheltered locations. A strong front is approaching the
region tonight, and this will bring strong surface winds to the area
along with the threat of low level wind shear. In general, winds of
35-50 kts from the south will be present at 2000 feet. Winds may
reach 60 kts over the Siskiyou and Cascade ranges. Wind shear looks
to decrease near the end of the TAF period, but will linger over
higher elevation terrain. A warm front will bring chances of
precipitation by Wednesday morning, mainly west of the Cascades.
Expect increasing terrain obscurations along with conditions
lowering to MVFR/low end VFR. /BR-y

&&

.MARINE...Updated 830 PM Tuesday December 26, 2023...Multiple
fronts with strong south winds and multiple heavy swell trains will
pass through the region this week. Storm force south winds and south
gales will persist tonight into Wednesday afternoon. This combined
with heavy swell and very high and steep wind driven seas will
sustain chaotic and dangerous conditions in the short term. Storm
force winds are generally expected beyond 15 nm from shore with
gales expected for all other areas. Very high and very steep wind
driven seas of 18-23 feet are expected during this time. Winds will
ease some Wednesday afternoon, but another shortwave will maintain
gales through Wednesday evening.

Winds ease further Wednesday night into Thursday, but a heavy
southwest swell will build into the waters. This combined with fresh
south swell will maintain very high and very steep and hazardous
seas of 23 to 28 ft at 17-18 seconds Wednesday night through
Thursday. Another round of gales is possible Thursday night into
Friday along with the continuation of very heavy seas. Conditions
may begin to improve over the weekend, but are likely to remain
hazardous into next week./BR-y

&&

.BEACH HAZARDS...Updated 900 PM Tuesday December 26, 2023...Large
breaking waves are expected to continue through at least Thursday,
which will create dangerous conditions along and within the surf
zone, and could inundate beaches and low lying shorelines.

Increasing the hazards of these breaking waves is the occurrence of
King tides. These very high tides peaked early Monday but will
continue higher than normal through Thursday. So far the tides have
come in 0.3' to 1.2' higher at Port Orford and Charleston
respectively. While there still likely won't be coastal flooding,
this does increase the chance just a little more; and so some low
lying areas especially around Coos Bay may see high water from tidal
overflow combined with the action of the high surf.

Regarding high surf, the high surf advisory has been upgraded to a
High Surf Warning through Thursday. Large breaking waves of 20 to 25
ft are expected through Wednesday afternoon. With the arrival of a
very heavy southwest swell, seas will build and this will lead to
dangerously large breaking waves of 25 to 35 ft Wednesday night into
Thursday. This high surf action will increase the inundation of
beaches and increase the risk of beach erosion, especially when
combined with the effects of King tides. /BR-y

&&

.PREV DISCUSSION... /Issued 306 PM PST Tue Dec 26 2023/

SHORT TERM...Tonight through Thursday night.
Our area is in the warm sector, behind a warm front and ahead of
an approaching cold front. Winds across the area, and high surf
at the beaches are the main concerns with this frontal system.
It will primarily be a rain producer for populated areas and the
major passes early Wednesday morning into early Wednesday
evening. Precipitation will be mainly from the coast to the
Cascades, with light amounts as likely extending into northern
Klamath and eastern Siskiyou and far western Modoc counties. The
cold frontal passage is forecast to cause snow levels to dip from
7000 feet now to around 5000 to 6000 feet on Wednesday.
Precipitation of around an inch to two and a half inches is
expected for the coast, heaviest for Curry County. Southerly
downslope winds will limit amounts in the Shasta, Rogue, and
Umpqua valleys to around a tenth to two tenths of an inch.
Otherwise. southwest Oregon amounts of a quarter to three quarters
of an inch will be common. Around an inch of rain is expected for
western and southern Siskiyou County, and up to a tenth of an inch
on the east side.

One of the signs of cold front's approach is the increase of
southeast winds at exposed portions of the higher terrain,
including a most recently reported southeast gust of 39 mph at
Acorn Woman Peak in the Siskiyou Mountains of far southern Jackson
County. The Wind Advisory is about to commence (at 4 PM) for
portions of southwest Oregon, including the southern Rogue Valley
from Ashland to south Medford.

In line with very good agreement in the latest high resolution
data, the main updates this afternoon were to advance the start of
the High Wind Warning for the southern Shasta Valley by a couple
of hours, to 8 PM this evening, and to advance the expiration time
for all wind hazards to 7 PM Wednesday evening. Late tonight, high
winds will also extend across the east side. Please see the High
Wind Warning and Wind Advisory statement /PDXNPWMFR/ for further
details.

The winds should significantly limit the extent of valley low
clouds and fog tonight through Wednesday evening. After a break of
a few hours, a surface trough will pass across late Wednesday
night into Thursday morning with southerly winds picking up a bit
again in the southern Shasta Valley to a windy 20 to 30 mph, not
quite to Advisory levels. Additional showery rainfall of up to
a tenth of an inch will be mainly at the coast and western and
southern Siskiyou County.

The next frontal system is expected to reach the coast late
Thursday night.

LONG TERM...Friday through Tuesday.
The extended forecast starts with strong south to southerly flow
with a mature cyclone spinning well to our west. The upper level
wave will push towards the coast Friday night into Saturday with an
occluded front pushing into the coast.  Model guidance is very
confident in precipitation along the boundary and the ensembles are
fairly confident it will be raining along the coast Friday night
into Saturday. Snow levels are anticipated to star out on the
higher side Friday evening around 7000 feet before lowering as
this relatively cooler air pushes in further inland. The
precipitation really cuts off once this cooler air moves inland,
so snow accumulation is anticipated to be lower over the
mountains. The probability of 4 inches of snow over 24 hours
ending Saturday evening is about 20 % over Crater Lake and 10%
over Mt. Ashland. Not a big snow event even through the snow
levels drop once the cooler air moves in.

As for winds, the difference in pressure across southern Oregon and
northern California will result in more strong southerly flow
through the valleys Friday evening. Compared to the wind event we're
anticipating later tonight into Wednesday, this will not be as windy
based on the NBM probabilities.  The GFS gradient is in line with
this idea as the pressure difference between Medford and Redding is
about 5 to 6 mb, which isn't all that strong.  Inflatable Christmas
decorations seem most at threat here with other loose objects. Right
now we're forecasting wind gusts up to 40 mph in the Rogue and
Shasta valley in addition to higher terrain in eastern Siskiyou
County.

Eventually, the upper wave will push through by Saturday evening
with the probability of precipitation continuing into the weekend.
High pressure really fails to build in the wake of this low as the
atmosphere remains active.  Another wave will push in around Sunday
and Sunday night, but with minimal wind and precipitation activity
in the area.

As we head into Tuesday next week, mean sea level pressure in the
ensemble means suggest a low building in the Pacific around 130W and
sliding in from the northwest.  Given the direction, this would
imply some cooler air is finally on the way. The NBM confirms this
with snow levels down to 4000 feet by 12Z Wednesday of next week. In
addition, the low strength still varies significantly at this point
with a few members strengthening the low down to 990mb and one
member of the ENS out of 50 dropping the low down between 970 and
980 mb.  Some members don't even have a mature cyclone developing.
In any case, it's something to watch in future forecasts.
-Smith

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...High Wind Warning from 4 AM to 7 PM PST Wednesday for
     ORZ029>031.

     Wind Advisory from 4 AM to 7 PM PST Wednesday for ORZ029>031.

     High Surf Warning until 10 PM PST Thursday for ORZ021-022.

     Wind Advisory until 7 PM PST Wednesday for ORZ024>026.

CA...High Wind Warning until 7 PM PST Wednesday for CAZ081.

     High Wind Warning from 4 AM to 7 PM PST Wednesday for CAZ084-
     085.

     Wind Advisory from 4 AM to 7 PM PST Wednesday for CAZ084-085.

PACIFIC COASTAL WATERS...Gale Warning until 10 PM PST Wednesday for
     PZZ350-356-370-376.

     Hazardous Seas Warning from 10 PM Wednesday to 4 PM PST Thursday
     for PZZ350-356-370-376.

     Storm Warning until 1 PM PST Wednesday for PZZ370-376.

&&