High Wind and Heavy Rain in California, December 12 to 13, 2024

AREA FORECAST DISCUSSION
National Weather Service Eureka CA
308 PM PST Wed Dec 11 2024

.SYNOPSIS...Rain and moderate gusty south wind will build in through
the evening with a short period of stronger rain rates around
midnight. A stronger storm systems remains on track to hit the area
Friday into the weekend with higher wind and more sustained rain.

&&

.DISCUSSION...The influence of a crossing warm front has been
evident across the area today. Thick midlevel clouds have gradually
built in and lowered, while warm, south winds have increased with
some gusts this afternoon up to 30 mph in more exposed areas. Bands
of rain are clear approaching the area on satellite and radar.
Southerly downslope wind, however, has generated a deep dry layer
near the surface which will likely delay the onset of rain,
particularly for low elevation location. Though rain may starts as
soon as sunset for location like the King Range, measurable rain is
unlikely to occur much before 10 PM around Humboldt Bay.

In any case, impacts tonight and tomorrow should be relatively mild.
Even high end model guidance shows lass than a 10% chance of gusts
much over 45 mph tonight in populated areas. Most likely rainfall
will be between 1 and 2 inches. There may be a period around
midnight tonight with enhanced rain rates up to 0.4 in / hr (20%
chance) as winds rapidly shift northwest and align with the terrain.
This could generate very minor urban flooding, but overall flooding
risk will be minimal.

Precipitation will fall mostly as snow for high elevation sections
of the interior, most notable in Trinity County. While warm air
advection aloft has pushed snow levels above 5000 feet across most
of the forecast area, cool dry air has remained trapped in valleys
around the Trinity Alps. Cooler air combined with strong evaporative
cooling has lead high resolution models to suggest areas of snow
levels as low as 2500 feet. This setup will bring inconsistent snow
totaling 2 to 6 inches along much of highway 3 and highway 299 east
of Weaverville.

A stronger storm system remains on track for Friday into the
weekend. This storm system will be stronger and more sustained. NBM
is showing around a 50% chance of gusts around 50 mph along the
coast at lower elevations. That said, the probabilities drop quickly
closer to 10% when looking at speeds closer to 55 or 60 mph, even
for high elevation locations. Still, such wind speeds are usually
sufficient to generate extra debris on roadways and scattered power
outages.

A sustained 12 to 24 period of moderate rain will follow
the winds into early Saturday with total rainfall mostly around 2 to
4 inches. Such rain amounts will certainly increase the risk of rock
slides and roadway debris. At the moment, no rivers are forecast for
flood stage, though a few (Mad River at Arcata, Russian at Hopland)
are forecast just a few feet below monitor stage. Though current
flood impacts are most likely to be very minor, a slight increase in
the forecast could ramp up the risk. This possibility (likely a 20%
chance) will need to be monitored closely over the next 24 hours.
Timing of rain alongside King Tide will also generally increase the
risk of urban flooding, especially around Humboldt Bay on Saturday
morning.

Precipitation will continue to fall as snow across high elevations.
Most models place even higher snow levels during this second event
with most impacts continuing to be restricted above 3000 feet in
Trinity county. That said, 10 to 20 inches of heavy snow is most
likely at those high elevations.

The more extended forecast remains more uncertain in specifics. Long
range models continue to show a series of storms impacting the area
for the next couple weeks, though the confidence for magnitude is
very low and variable. Certainty is much better for locations
further north into Oregon. /JHW

&&

.AVIATION...VFR conditions currently present at all TAF sites. Light
south wind is keeping clouds at bay. Multiple models continue to
suggest that a low-level jet of 40-50kts will be present over the
coast this evening and tomorrow early in the morning due to the
frontal forcing. This front is also forecasted to bring vicinity
showers and rain which can sporadically cause limited visibilities
in the late afternoon/early evening. Visibilities show a 90% of
remaining over 3SM through the TAF period according to model
guidance. Winds are expected to be southerly and strengthen in the
early afternoon, with gusts possible to ~20kts at coastal
terminals.

&&

.MARINE...Winds today build from the south to Gale Force in the
northern outers with gusts expected around 40kts. These winds are
forced by an approaching cold front that will sweep over the area
through the day. In the wake of this front, winds are expected to
remain strong, yet switch direction as we enter the cold sector of
the frontal system. A low-level jet behind the front at the 925mb
pressure level has the possibility to mix down leading to the Gale
warning remaining in effect until Thursday afternoon.

Waves today will be dominated by a NW swell and southerly wind
waves. This swell is expected to wane through Thursday before
another pulse of NW swell arrives to our waters on Friday. Southerly
wind waves will be present on Friday as well as second frontal
system arrives to our area from the NW Pacific. Waves are expected
to remain fairly large (>10ft sig. height) from today into the
weekend with the peak heights in the high teens to low 20 ft range
on Saturday.

&&

.COASTAL FLOODING...King tide will start to build in Thursday into
the weekend. Tide models show the north spit tide gauge reaching
right around 8.7 feet tomorrow. Southerly wind may increase the tide
slightly, but is unlikely to generate any meaningful coastal flood
impacts. Tide will continue to build, however, as will southerly
winds Friday. High tide is most likely to reach 8.9 to 9.0 feet at
the North Spit early Friday morning. Such heights will generate
minor flooding in low lying areas of Humboldt Bay, on beaches, and
around King Salmon. The greatest risk of coastal flooding will be
high tide Saturday morning. King Tide alone will reach 8.7 feet.

Southerly wind combined with minor storm surge will likely push that
close to 9.0 to 9.2 feet at the North Spit. This will be enough to
generate more moderate flooding in low locations around Humboldt
Bay. Timing with the strongest wind makes any flooding of 255 or 101
around the Bay highly unlikely. That said, there is potential for a
strong westerly swell around High Tide. There is high uncertainty,
but breakers will most likely be anywhere from 16 to 24 feet.
Combined with king tide, there will be some enhanced potential in
locations such as Shelter Cover and around Crescent City, including
near Crescent City. Should the higher end solutions of the swell
(30% chance) pan out, a coastal flood warning may be needed. /JHW

&&

.EKA WATCHES/WARNINGS/ADVISORIES...
CA...
     Winter Weather Advisory until 10 AM PST Thursday for CAZ107.

     Winter Storm Watch from late Thursday night through
     Saturday afternoon for CAZ107.

NORTHWEST CALIFORNIA COASTAL WATERS...
     Small Craft Advisory until 3 PM PST Friday for PZZ450-455.

     Gale Warning until 3 PM PST Thursday for PZZ470.

     Gale Watch from late Thursday night through Friday evening
     for PZZ470-475.

     Small Craft Advisory until 3 AM PST Thursday for PZZ475.

     Gale Warning from 3 AM to 3 PM PST Thursday for PZZ475.

&&

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For forecast zone information
see the forecast zone map online:
https://www.weather.gov/images/eka/zonemap.png