textproduct: Eureka

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.