Hazardous Seas Warning 1, NWS Eureka, CA

AREA FORECAST DISCUSSION
National Weather Service Eureka CA
532 AM PST Tue Jan 2 2024

.SYNOPSIS...Rain and high mountain snow are forecast to increase
today through this evening as a cold front moves into Northwest
California. Showers and isolated thunderstorms will follow
tonight. Showers are then forecast to taper off during the
day on Wednesday. Progressively colder storms with lower snow
levels are expected Friday through Sunday. More precipitation
is probable early to mid next week.

&&

.DISCUSSION...The hits keep on coming. The next storm in the
parade of storms was just offshore with the surface frontal
boundary inside 130W early this morning. Cold core aloft and mid
level cyclonic circulation was trailing the front just outside
130W. A smattering of lightning strike has been under and around
the cold core as well. The trough is forecast to split, with the
primary center heading NNE over Oregon offshore waters while the
southern branch pivots right toward Cape Mendocino this evening.

Some light spotty precip will occur through mid morning in the
upglide preceding the surface boundary. Heaviest precipitation
rates will arrive to the coast about 20Z or noon and then spread
inland during the afternoon. Minor flooding of low lying areas and
sharp rises on streams, creeks and rivers is certainly possible
with short duration heavy rain rates (over 0.30in/hr). None of the
main stem rivers are forecast to reach monitor stage. Unless the
boundary stalls, the steady eastward progression will mitigate any
risk for flooding.

Heavy snowfall rates (greater than 1 to 2in/hr per HREF) will
also develop this afternoon over the Trinity mountains with travel
impacts likely at Scott Mountain Pass (5400 ft) on highway 3. The
highest rates will most likely occur late this afternoon and last
for 3 or 4 hours. We started the winter storm warning a couple of
hours earlier to account for lighter accumulations and errors in
timing. The heaviest snowfall rates will most likely fall off
after midnight. Road impacts will persist even after the heavy
snow ends, so the winter storm warning above 4500 feet in NE
Trinity County goes until 3 AM.

The next item of interest is the potential for convection and
isolated thunderstorm primary out over the waters and near the
coast as the cold air aloft (-30C at 500mb) creates weak
instability. Thermodynamic profiles over land areas do not look
very conducive for thunderstorms with meager CAPE and low
equilibrium levels. SREF and HREF calibrated guidance indicates
low probabilities - mostly near the coast and over the waters.

Showers will wind down on Wed evening and overnight as a ridge
builds behind the trough. Models differ with the depth and
timing of light precip in advance of the next shortwave trough on
Thu/Fri. Forecast confidence for more rain or snow on Thu or Fri
is low. Models differences increase and ensemble spreads grow
greater as we head into Friday and the weekend. Deeper trough
expected Saturday with possible lower snow levels and greater
snow impacts for the mountain passes, western slopes of Humboldt
and Del Norte Counties and communities in Trinity county. The
highest QPF does not appear to line up with snow levels below 3000
ft, however. There is still uncertainty regarding how much snow
will occur for the lower elevations, 1500-2500 feet, if any at
all. Once snow levels get that low, the QPF appears to drop off
considerably and it will just get cold with only light snow
amounts or flurries - if any. Parade of frontal systems will
likely continue into early to mid next week, bringing more
precipitation. Stay tuned.

&&

.AVIATION...Fog briefly formed in interior valleys early this morning,
however an approaching front has already begun to lift this layer
and promote low-level mixing. Cigs should remain low, particularly
in Mendo and Lake, but vis should see improvement through the
morning. Meanwhile, the approaching system will increase southerly
winds through this morning and drive LLWS over the North Coast.
Winds reach a crescendo with the main frontal passage around
noonday, along with lower cigs/vis and moderate rainfall. Showery
rain persists through the rest of the afternoon and into the
evening as additional weak waves move onshore, bringing
intermittent MVFR to IFR into tonight.

&&

.MARINE...GALE FORCE southerly winds and steep seas spread across
the coastal waters this morning as a front approaches. Winds weaken
tonight, then sharply accelerate and turn westerly early Wednesday
morning as another boundary sweeps across the waters.  Winds weaken
again and turn southerly Thursday ahead of another system, slowly
building then quickly switching back to northerly Friday as a quick-
moving front sweeps southward. Meanwhile, a fresh W swell quickly
fills in Wednesday, with combined seas building into the high teens.
This fresh swell will quickly decay overnight Wednesday, with
quieter seas expected Thurs.

Model discrepancies lead to low confidence in the specifics of the
wind forecast during the day Fri and heading into this weekend at
the moment...although it looks like there may be a period of
blustery westerly winds behind a frontal passage sometime between
Friday afternoon and early Saturday morning, then there is generally
good agreement that winds will turn northerly Sunday as high
pressure fills in behind the passing low.

Meanwhile, a large WNW swell is forecast to fill in Friday morning.
A majority of the energy from this swell will be from a storm that
is currently undergoing rapid intensification around 50N, 175W, and
is forecast to continue to deepen over the next 18 hours. Current
guidance is depicting this swell filling in very quickly Friday
morning, reaching the low teens at 20 seconds early Friday morning,
then peaking Friday afternoon with wave heights in the mid teens
around 18 seconds or so. A couple of other, more local systems
spinning around offshore during Friday will likely add to the sea
state this weekend, although details on these still remain
blurry...however it is looking likely that the large swell Fri will
receive some reinforcement over the weekend, it just remains unclear
how much. In any case, it looks likely that seas will be at least
around the mid teens Friday afternoon into this weekend. /TDJ

&&

.EKA WATCHES/WARNINGS/ADVISORIES...
CA...
     Wind Advisory until 4 PM PST this afternoon for CAZ102-
     104>106.

     Winter Storm Warning from 1 PM this afternoon to 3 AM PST
     Wednesday for CAZ107.

NORTHWEST CALIFORNIA COASTAL WATERS...
     Small Craft Advisory until 7 AM PST this morning for PZZ450-
     455.

     Gale Warning from 7 AM this morning to 3 PM PST this
     afternoon for PZZ450-455.

     Gale Warning until 3 PM PST this afternoon for PZZ470.

     Small Craft Advisory until 5 AM PST early this morning for
     PZZ475.

     Gale Warning until 3 PM PST this afternoon for PZZ475.

&&

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Preliminary Local Storm Report
National Weather Service Eureka CA
845 AM PST Tue Jan 2 2024

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0800 AM     Non-Tstm Wnd Gst 8 W Comptche            39.29N 123.75W
01/02/2024  E25 MPH          Mendocino          CA   Public

            Large tree leaning into roadway of east
            bound lane of Comptche Ukiah Road.

&&

JMM

Preliminary Local Storm Report
National Weather Service Eureka CA
113 PM PST Tue Jan 2 2024

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

1253 PM     Landslide        7 SSE Hopland           38.88N 123.05W
01/02/2024                   Mendocino          CA   Law Enforcement

            Rock slide, reported by CHP. Heavy rain in
            the vicinity.

&&

JMM