Small Craft Advisory 137, NWS Eureka, CA

AREA FORECAST DISCUSSION
National Weather Service Eureka CA
315 PM PST Wed Dec 18 2024

.SYNOPSIS...Mild temperatures and patches of dense fog are
expected tonight through Friday. A storm system will approach
late Friday bringing strong south winds by evening, followed by a
bout of heavy rain Saturday morning. A parade of progressively
stronger and wetter atmospheric river storms will impact Northwest
California with heavy rain, strong winds and high surf over the
weekend and next week.

&&

.DISCUSSION...Prime hazards going into tonight and Thu will be
patches of dense fog both along the coast and interior valleys.
Warm front offshore may generate some sprinkles or light rain
this afternoon and early evening, otherwise dry and mild weather
conditions are forecast to continue for much of the area for the
remainder of the week.

Potential wind impacts will start to emerge late on Friday as the
first in a long series (at least 6) of atmospheric river storms
approaches the North Coast. GFS, ECMWF and NAM12 indicate a 925mb
speed max around 60kt developing in advance of the frontal boundary
as early as Friday afternoon and evening. Probability for gusts
more than 50 mph increases (>80%) for the higher elevations of
Humboldt and Del Norte, particularly the King Rang and SW Humboldt
County, by Friday evening. Strong winds with gusts over 40 mph
for lower elevations may not materialize for lower elevations, at
least for the Humboldt Bay area and Eel Delta, until early Saturday
morning, based on NBM probabilities for gusts > 40 mph.

Heavy rain is then expected early Saturday morning with frontal
passage. We may have some rapid rises on small streams and creeks
and perhaps some localized road closures due to the saturated
soils and quick run-off. The short duration of heavy rain rates
with this front should limit the potential for flooding. There
are some signs for shallow low-topped convection, mostly out over
the coastal waters and near the coast and this will need to be
watched. Swift storm motions should preclude any flash flooding.

A parade of atmospheric river storms will impact the area late
into the weekend into all of next week. Confidence is high we
will get another round of moderate to heavy rain Sunday and
probably strong winds. The duration and magnitude are still not
completely certain, but once again this second AR will set the
stage for a longer duration and perhaps stronger AR Monday through
Tuesday. The threat for flooding will increase with each of these
events next week. Stay tuned. DB

&&

.AVIATION...Behind the light rainfall observed this morning from a
weak frontal passage, the skies are beginning to clear. High
pressure ridging is quickly amplifying behind the front. The
residual moisture, increased subsidence, and clearing skies will
create an environment favorable for fog development tonight. The fog
may be dense at times. Southeast winds will develop tonight, though
light, the offshore flow may be in the 3 to 4 kt range to the coast
and enough to disrupt the fog. The timing and potential improvement
in visibility is highly in question. If the winds are lighter to
the coast the fog may prevail longer into Thursday morning. Surface
winds will be light and generally from the south or southeast on
Thursday.

&&

.MARINE...Southerly winds are continuing to ease this afternoon
through the waters. Seas are dominated by a mid period westerly
swell. A long period WNW swell will fill in tonight, and will become
the dominant wave group as the mid period swell fades. The lull in
winds will be short as the first of a parade of strong frontal
systems begin to impact the region. This will create periods of
strong southerly winds and generate large, long period westerly
swells this weekend and through next week.

Winds increase slightly Thursday with a weak, clipping front. Only a
small portion of the far outers may reach 20 kts. Thursday night
will mark the begging of increasing trend in southerly winds. Gale
conditions look to be reached as early as Friday morning. Southerly
winds will continue to strengthen Friday through Saturday morning.
Winds gusts may near or reach 50kts along a strong frontal boundary,
currently modeled from late Friday through Saturday morning, but the
timing may shift a bit. Winds will quickly subside behind the front,
likely late Saturday. A large, long period westerly swell will build
later Saturday behind the front, in the 17 to 20 ft range.

&&

.BEACH HAZARDS...A long period WNW swell will fill into the waters
tonight, becoming the dominant wave group as an existing mid period
westerly swell subsides. This will create a moderate risk for
sneaker waters from early Thursday morning, with a lingering threat
through Friday.

&&

.EKA WATCHES/WARNINGS/ADVISORIES...
CA...
None.
NORTHWEST CALIFORNIA COASTAL WATERS...
     Small Craft Advisory until 9 PM PST this evening 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