446
FXUS66 KEKA 200756
AFDEKA

AREA FORECAST DISCUSSION
National Weather Service Eureka CA
1256 AM PDT Sun Sep 20 2026

.KEY MESSAGES...
* High pressure maintains unseasonably warm interior temperatures
  Sunday, while compressed marine stratus keeps coastal zones cooler.

* Strong subsidence keeps the marine layer flattened overnight,
  anchoring low stratus and patchy fog along the immediate shore.

* Steep, wind-driven seas maintain Small Craft Advisory conditions
  across the outer zones and southern inner waters through Monday.

&&

.SYNOPSIS...Broad 500 mb high pressure centered over the Pacific
Northwest maintains strong subsidence and warm interior weather
through the weekend. An offshore closed low positioned south of the
forecast area drifts eastward into Central California and Nevada by
Tuesday. This upper-level pattern preserves a compressed marine
layer along the coast into Monday before weak offshore flow develops
Tuesday night into Wednesday morning as the upper low tracks inland.


&&

.DISCUSSION...At 500 mb, broad ridging centered over the Pacific
Northwest extends across Oregon and Northern California, with height
fields over Northwest California slightly disrupted along the
western periphery by a closed cut-off low off the Central California
coast. Mid-level deformation north of this feature combines with
subsidence along the western flank of the ridge to compress the
marine boundary layer along the coast.

At the surface, the Pacific High opposed by an interior thermal
trough extending through the Central Valley maintains a tight zonal
and onshore pressure gradient. This gradient continues to drive
fresh to strong northerlies across the coastal waters.
Inland, boundary-layer compression supports warm afternoon
temperatures on Sunday. Probabilistic guidance indicates an 80 to 95
percent probability of high temperatures exceeding 90 degrees across
interior Trinity County valleys, while interior Mendocino and Lake
counties reflect probabilities between 40 and 70 percent. Under this
ridge setup, interior Lake County valley thermal profiles remain
equal to or hotter than inland Trinity and Mendocino counties due to
strong subsidence aloft and isolation from marine airflow.
Probabilities for wetting rain (greater than 0.10 inches) remain at
0 percent across all interior and coastal zones through midweek
under the continental airmass.

As the 500 mb closed low tracks inland into Nevada Tuesday into
Wednesday with its trough axis extending into Southern California,
the surface thermal trough shifts toward the coast. This
configuration turns low-level flow easterly to northeasterly
starting late Monday night, peaking Tuesday night, and persisting
through Wednesday morning. Daytime solar heating will maintain
brief, diurnal upvalley breezes during the afternoon on Tuesday. The
nocturnal offshore flow disrupts the coastal marine layer, yielding
a 40 to 60 percent probability of coastal clearing on Tuesday, with
sky conditions turning mostly clear Tuesday night and coastal high
temperatures warming into the upper 60s to mid-70s. High pressure
rebuilds over the Pacific Northwest late in the week, re-
establishing seasonal marine stratus along the coast.


&&

.AVIATION...A compressed marine layer holds low stratus and fog
over coastal corridors tonight. At KACV, probabilistic guidance
indicates a 50 to 60 percent probability of ceiling heights falling
below 500 ft (LIFR) between 09Z and 17Z Sunday, with a 30 to 40
percent probability of visibility dropping under 3 SM (IFR) and a 15
to 25 percent probability of visibility under 1 SM (LIFR). At KCEC,
probabilities for ceiling heights below 500 ft peak between 50 and
65 percent from 07Z to 17Z Sunday, with a 30 to 50 percent
probability of visibility under 3 SM and a 20 to 30 percent
probability of visibility under 1 SM. A 20 to 30 percent probability
of afternoon clearing exists Sunday as boundary layer mixing
temporarily lifts marine stratus. Regional terminal KUKI will remain
comfortably VFR under clear skies with typical diurnal valley
breezes.


&&

.MARINE...The zonal and onshore pressure gradient between the
offshore Pacific High and the interior thermal low maintains fresh
to strong northerlies across the waters.

For the Northern Inner Waters, light to moderate north winds and
moderate combined seas will keep conditions below advisory
thresholds.

Across the Northern Outer Waters, building northerly
winds will generate steep, short-period wind waves overnight,
pushing wave steepness and wave heights into Small Craft Advisory
criteria by early Sunday morning.

For the Southern Inner Waters, fresh to strong northwest breezes and
steep wind waves maintain hazardous conditions for small craft
through Monday afternoon.

Across the Southern Outer Waters, fresh to
strong northerly winds and steep short-period seas will peak with
localized gale-force gusts, maintaining steep, hazardous seas
through early next week.


&&

.EKA WATCHES/WARNINGS/ADVISORIES...
CA...
None.
NORTHWEST CALIFORNIA COASTAL WATERS...
     Small Craft Advisory until 3 PM PDT Monday for PZZ455-475.

     Small Craft Advisory until 3 PM PDT Monday for PZZ470.

&&

$$

NOTE: The full forecast discussion is now regularly issued between
10 am/pm and 1 pm/am. Additional updates will be issued as needed.

Visit us at https://www.weather.gov/eureka

Follow us on Facebook and Twitter at:
https://www.facebook.com/NWSEureka
https://www.twitter.com/NWSEureka

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