Winter Storm Watch 2, NWS Eureka, CA

AREA FORECAST DISCUSSION National Weather Service Eureka CA 312 PM
PST Wed Jan 3 2024

.SYNOPSIS...Scattered showers are expected through the end of the
week with a few weak frontal passages. Valley fog is likely
overnight as interior temperatures drop. The first storm with
potential for snowfall at relatively lower elevations will arrive
early on Saturday, impacting travel through Sunday.

&&

.DISCUSSION...The axis of a high amplitude trough has moved
east of the PNW, allowing the bulk of precipitation from this
morning's frontal passage to taper off into scattered coastal
showers partly cloudy conditions. A brief period of ridging is
expected late this evening into Thursday, however this generally
progressive pattern will continue through the end of the week in the
form of shortwave driven weak frontal boundaries. Model guidance is
in general agreement on rain totals between 0.15 to 0.25 inches in
Del Norte and Northern Humboldt counties by 06Z Friday, with
slightly higher accumulations concentrated at higher elevations.
While temperatures should remain moderate in coastal regions,
interior Humboldt and Trinity valleys will dip below freezing
tonight and Thursday night, allowing for potential for widespread
valley fog. Lack of high level cloud cover and moist boundary layer
conditions following Thursday's precip will make radiational cooling
and persistent fog development more favorable by early Friday
morning, especially as winds calm and shift northerly ahead of the
next system.

The primary concern for this weekend's storm is still the timing and
amplitude of the cold upper level trough into northern California.
Long range deterministic models are generally in agreement that
widespread impactful precipitation will arrive very early Saturday
morning. 700 MB temperatures are consistently near or below -10C,
keeping snow levels between 2,500 and 3,500 ft through Saturday
night. The bulk of the precipitation is likely to be finishing up by
Saturday evening, with potential for snow totals of 10-15 inches in
the Trinity Alps and high elevations of Del Norte county by Sunday
morning. Higher elevations in interior Humboldt and Northern
Mendocino may see 5 to 8 inches in the same timeframe. The potential
for higher totals in Southern Humboldt and Mendocino counties will
depend on how far the trough extends into the region in correlation
with any changes to the QPF. Regardless, this storm will be
impactful for anyone traveling in Del Norte, Trinity and Interior
Humboldt and Mendocino counties.

Winds for this system are still forecasted to be marginal compared
to what we've seen over the past few weeks. The latest run of NBM is
only grazing 30KTS on exposed ridges and coastal headlands.
Precipitation is still the primary hazard, and will likely require
an advisory within the next day or two. Another cold trough is
looking to potentially dip into the PNW late next week, although the
depth of that trough is still uncertain.

&&

.AVIATION...Widespread rain/showers continued across the coastal
areas
today with the TAF sites having reported observations
averaging MVFR conditions; occasional VIS dropped into IFR.
SE-S winds were briefly breezy at CEC this morning and basically
light elsewhere. Inland fliers had not only showers to contend with,
but also snow showers over higher terrain...especially over eastern
Humboldt and Trinity Counties. Even with building shortwave
ridging, an embedded, weak disturbance will allow for rain to keep
on keeping on (in some capacity) across the North Coast through
early Thursday. Interior Mendocino, which includes UKI has been
improving through the day with MVFR-VFR Cigs. /TA

&&

.MARINE...A prominent west northwest swell has filled in this
afternoon as overall wind speeds continue to decrease.

A strong low off the Alleutian Islands generates a new long period
northwest swell group forecast to arrive late Thursday night into
Friday and will generate more wind and wave induced small craft
advisory conditions, maybe even pushing hazardous seas, especially
for the southern outer waters, lasting into Saturday. A weak cold
front will generate increased north wind speeds for Friday, then a
stronger reinforcing cold front will move through for Saturday,
introducing stronger west to northwest winds. Expect wind gusts near
30 KTS south of Cape Mendocino and much of the southern outer waters
Friday. Then much more widespread 30KT wind gusts, especially for
outer waters.

With the northwest swells come expected high surf for Friday into
Saturday, with forecast breaker waves potentially reaching 20-22
feet Friday into Saturday. Combined seas of at least 16-18 feet are
expected for Friday into Saturday night as well.

Yet another longwave northwest swell arrives for late Monday night
into Tuesday of next week.

&&

.EKA WATCHES/WARNINGS/ADVISORIES...
CA...
None.
NORTHWEST CALIFORNIA COASTAL WATERS...
     Small Craft Advisory until 9 AM PST Thursday for PZZ450-455-
     470-475.

&&

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