872
FXUS66 KEKA 212358
AFDEKA

AREA FORECAST DISCUSSION
National Weather Service Eureka CA
458 PM PDT Mon Sep 21 2026

.KEY MESSAGES...

* Unseasonably warm and dry conditions continue inland through Wednesday,
  with a 90% probability that Lake County reaches the highest
  temperatures across Northwest California.

* Marine clouds along the coast will slowly thin out through
  Tuesday evening, leading to a 50% chance of afternoon sun.

* A passing cold front brings a high chance (70-90%) of light rain
  to Del Norte and northern Humboldt late Thursday night into
  Friday, with lower probabilities (under 20%) south into
  Mendocino and Lake counties.

&&

.SYNOPSIS...High pressure off the Pacific Northwest combined with
an interior thermal low will maintain warm, dry conditions across
inland valleys through Wednesday. Meanwhile, a shallow marine
layer along the coast will slowly weaken through Tuesday evening
before an approaching cold front introduces cloud cover and a high
chance of measurable rain to Del Norte and northern Humboldt
counties late Thursday night into Friday. /MH


&&

.DISCUSSION...An upper-level shortwave trough off the Central
California coast continues to weaken the ridge aloft, drawing
high cirrus across the region. At the surface, dry conditions
remain driven by the zonal onshore pressure gradient established
between the eastern Pacific surface high and the interior thermal
low. Strong atmospheric subsidence paired with complete isolation
from marine air will cause Lake County valley highs to equal or
exceed peak readings in Trinity and Mendocino counties, reaching
88 to 94 degrees through Wednesday.

Model ensembles depict a brief shortwave ridge axis moving
rapidly across the region early Thursday morning, bringing a
transient positive height anomaly aloft before height falls
overspread the forecast area ahead of an incoming cold front late
Thursday night into Friday morning. Ensemble probabilities for
12-hour measurable precipitation peak late Friday morning at
70 percent across Del Norte, northern Humboldt, and northern
Trinity counties. Probabilities drop to 30 percent across
southern Humboldt and central Trinity, falling under
10 percent across Mendocino and near 0 percent in Lake County.
Behind the boundary, 850 mb temperatures drop significantly,
bringing an 80% likelihood of daytime highs settling in the lower
to mid-70s over the weekend. Valley frost potential behind the
front remains below 15% due to persistent boundary-layer mixing.
/MH

&&

.AVIATION...Strong low-level coastal inversions maintain ceilings
hovering between 400 and 700 feet AGL (IFR to LIFR thresholds)
alongside patchy drizzle at coastal terminals KACV and KCEC. High-
resolution ensembles indicate a 50% probability of partial
clearing near KCEC by Tuesday evening as low-level flow weakens
the inversion top. The terminal at KUKI will remain VFR (ceilings
above 3,000 feet AGL and visibilities exceeding 5 miles) through
the period under high cirrus and light, terrain-driven surface
winds. /MH

&&

.MARINE...Steep, short-period wind waves continue to dominate the
coastal waters as the zonal onshore pressure gradient drives
fresh to strong northerly flow across the outer shelf. Buoy 46213
displays 7-foot seas with an 7-second dominant period, while Buoy
46014 reports 6-foot seas at 8 seconds. While wave heights remain
below 9 feet in the Northern Inner Waters, short wave periods of
maintain hazardous wave steepness across all outer zones.

In the Northern Inner Waters, north winds will average 10 to 15
knots, keeping Small Craft Advisory conditions localized, though
steep waves near 6 feet at 8 seconds will occasionally spill into
areas south of Trinidad Head. In the Southern Inner Waters, Small
Craft Advisory conditions persist as northerly winds of 15 to 25
knots with gusts to 35 knots build steep seas to 6 to 11 feet
through Wednesday evening. In the Southern Outer Waters, north
winds will build to 25 to 30 knots with near-gale gusts up to 35
knots Tuesday morning, pushing total wave heights to 8 to 13 feet
at 8-second periods. In the Northern Outer Waters, north winds of
15 to 25 knots and 7 to 10-foot steep seas will gradually subside
mid-week. If current wave steepness trends continue, later shifts
may want to consider extending headline coverage for the northern
outer waters. /MH


&&

.EKA WATCHES/WARNINGS/ADVISORIES...
CA...
None.
NORTHWEST CALIFORNIA COASTAL WATERS...
     Small Craft Advisory until 9 PM PDT Wednesday for PZZ455.

     Small Craft Advisory until 9 AM PDT Wednesday for PZZ470.

     Small Craft Advisory until 9 AM PDT Thursday for PZZ475.

&&

$$

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

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