Cold Weather Advisory 4, NWS Eureka, CA

AREA FORECAST DISCUSSION
National Weather Service Eureka CA
316 AM PST Sat Dec 14 2024

.SYNOPSIS...After a second round of enhanced rain crosses the area
this morning, rain showers will quickly diminish through the
afternoon, giving way to widespread valley fog this evening. A
second weaker though still gusty and rainy system will impact the
area Sunday into Monday. Calmer and drier conditions will then
return through midweek.

&&

.DISCUSSION...A moderate to strong atmospheric river has crossed the
area form north to south over the past 24 hours. SOutherly winds
peaked Friday afternoon with gusts of 40 to 60 mph in exposed areas,
generating isolated power outages. Generally 1.5 to 4 inches of rain
has fallen across the area. Much of the rain fell along a
concentrated band behind the surface old front that brought rain
rates of 0.3 to 0.5 in/hr sustained for a couple hours. Though not
extreme, such rain rates were enough to generate widespread impact
reports including rockslides, mudslides, and minor urban flooding.

As of the early morning hours, the strongest core of rain has moved
south and east of the area with calmer period of light shower
building in. Most high resolution models show a brief period of
stronger showers around sunrise that will most likely generate 0.5
to 1.2 inches of additional rainfall across the area. Such rain
rates may briefly increase flooding concern again, especially in
near coastal areas that will be feeling the influence of King Tide
combined with yesterdays rain. Additional urban and small
stream flood impacts, however will be minor.

Much of the precipitation for Trinity County has so far fallen as
snow, especially above 3000 feet. Though now saturated air combined
with weakening upslope cooling have helped create rising snow
levels, snow is still expected to fall with the shower this morning
at high elevations, bringing a further 3 to 6 inches of snow along
high elevations of highway 3.

Showers will gradually wean through the day. With slight instability
aloft, there is a very slight chance (15%) that some showers could
produce infrequent lightning right along shore, but both dry air
aloft and very shallow instability make any widespread thunder
exceedingly unlikely. Skies will mostly clear by the evening. Though
there will be dry air advection to help cool and dry out the area,
recent rain will promote widespread valley fog and mist, most likely
inhibiting the formation of any meaningful frost Saturday night into
Sunday.

Another, weaker system will approach the area on Sunday into Monday.
A cold front associated with this system will promote yet another
round of gusty south winds. Most models show winds being slightly
weaker than this past round with most likely peak gusts in exposed
areas between 40 and 50 mph. NBM shows a 40 to 60% chance of gusts
over 45 mph in populated areas such as Crescent City and Kneeland.
Chances are much lower (20 to 30%) around Humboldt Bay and in any
locations in Mendocino and Lake Counties. Moisture transport is also
much weaker ahead of this front with only 1 to 3 inches of rain most
likely across the area. Such amounts will only be enough to generate
very transient flooding concerns, though enhanced risk of rockslides
will continue to be a threat. Upsloping and cold air advection will
most likely once again help drop snow levels in the Trinity Alps to
near 3000 feet. Overall accumulation will be light, however, with
snowfall on Highway 3 and the highest peaks of 299 of 3 to 6 inches.

The vast majority of cluster ensemble members show general ridging
Tuesday through Wednesday, which will generally calm and dry out
conditions. Unlike the start of December, however, there is high
confidence that this ridge will be short lived. Essentially all
ensemble members show a deep trough across the eastern PAcific
around next weekend. Most models (50 to 70%) show this trough
pulling a moderate atmospheric river, similar to the current set of
storms, right across NW California. About 25% show the system push
moisture further south which would actually leave the north coast
unusually cool and dry with general offshore flow. /JHW

&&

.AVIATION...Gust winds and passing rain showers are expected today
through the daylight hours in the wake of the passing frontal
system. Winds will be lighter today before picking up tomorrow when
another frontal system enters the coastal waters. Inland terminals
will also be subject to strong winds Sunday/Monday. Conditions are
forecasted to remain above MVFR through the TAF period.

&&

.MARINE...Small craft advisory conditions are in effect in the wake
of the frontal system that brought gales and even some localized
areas of storm force winds to the waters yesterday. Winds today will
remain relatively moderate from the NW and should diminishing
slightly through the day. Gale force southerlies are forecasted to
build into the waters again Sunday afternoon as another frontal
system enters our waters. These strong winds should last through the
evening.

Waves are currently holding at a fairly steep NW swell and
wind waves are degrading from the recent high coastal wind. On
Monday, wind waves will ramp back up to steep creating more chaotic
seas. A large NW swell will build in Monday night into Tuesday
increasing sea heights substantially.

&&

.HYDROLOGY...As mentioned in the main discussion above, rainfall
overnight supported numerous minor to moderate flooding impacts
along the North Coast most in the form of landslides and road
debris. The greatest impacts where observed late the evening when a
band of enhanced rain rates crossed the area.

With 2 to 5 inches of rain in most main river drainages, main stem
rivers are quickly rising early this morning. Several rivers such as
the Russian RIver at Hopland and Navarro River at Navarro are
expected to reach and maintain minor flood stage through the morning.
Such stages usually lead both rivers to impact local highways.
Additionally other rivers such as the Garcia River and Scotts Creek
are both rushing towards to points (around 10 feet each) where they
often impact Highway 1 and some local roads respectively.

River impacts are less further north, but the Eel River at
Fernbridge and the Mad River at Arcata both remain on track to hit
Action stage late this morning or early afternoon, the chance for
either reaching flood stage remains exceedingly small (less than
10%). River levels will gradually fall this evening. The second
round of lighter rain on Monday is not currently forecast to have
any strong impact on river levels, with the ensemble river forecasts
showing very low (less than 10%) chances of any rivers reaching
flood stage. /JHW

&&

.COASTAL FLOODING...Peaking high tides today combined with heavy
rainfall and high surf could bring coastal flooding across the
Northwest California coast through the day. Affected areas could
include: King Salmon and other low lying areas around Humboldt Bay,
Pebble Beach Rd and Highway 101 near Crescent City, low lying areas
in Point Arena and Fort Bragg, and low lying areas along Highway 1
on the Mendocino coast. Heavy rains across the coast yesterday could
exasperate flooding and creeks and streams could back up as high
tide approaches. Tides are highest today, and even though the
strongest of the winds will have decreased, conditions remain
favorable for coastal flooding.

&&

.EKA WATCHES/WARNINGS/ADVISORIES...
CA...
     Coastal Flood Advisory from 8 AM this morning to noon PST
     today for CAZ101.

     Flood Watch until 3 PM PST this afternoon for CAZ103-104-
     106-109>115.

     Coastal Flood Warning from 8 AM this morning to noon PST
     today for CAZ103-109.

     Winter Storm Warning until 4 PM PST this afternoon for
     CAZ107.

NORTHWEST CALIFORNIA COASTAL WATERS...
     Coastal Flood Warning from 8 AM this morning to noon PST
     today for PZZ415.

     Small Craft Advisory until 9 AM PST Monday for PZZ450-455.

     Small Craft Advisory until 3 PM PST Sunday for PZZ470.

     Gale Watch from Sunday afternoon through Monday morning for
     PZZ470.

     Small Craft Advisory until 9 PM PST Sunday for PZZ475.

     Gale Watch from Sunday evening through Monday morning for
     PZZ475.

&&

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

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Special Weather Statement
National Weather Service Eureka CA
928 AM PST Sat Dec 14 2024

CAZ109>115-142130-
Southwestern Mendocino Interior-Southern Lake County-Mendocino
Coast-Northern Lake County-Southeastern Mendocino
Interior-Northwestern Mendocino Interior-Northeastern Mendocino
Interior-
928 AM PST Sat Dec 14 2024

...Gusty North-Northwest Winds This Morning...

An area of low pressure is moving onshore over Sonoma and Mendocino
counties this morning. Gusty north-northwesterly winds are expected
behind this low with gusts between 35 and 45 mph in Mendocino and
Lake counties through the early afternoon.

Locations impacted include...
Ukiah, Fort Bragg, Willits, Lakeport, Point Arena, Upper Lake,
Laytonville, Albion, Middletown, Ukiah Municipal Airport, Clearlake,
Barkerville, Philo, Hopland, Saratoga Springs, Loch Lomond, Anderson
Springs, Glenhaven, Soda Bay, and Gualala.

LAT...LON 3885 12282 3876 12352 3881 12360 3895 12374
      3905 12369 3934 12383 3955 12376 3974 12383
      3975 12291 3958 12289 3958 12274 3928 12276
      3922 12266 3921 12252 3906 12249 3893 12234
      3890 12242 3881 12237 3871 12246 3867 12262

RPA