textproduct: Eureka

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.