textproduct: Eureka

SYNOPSIS

A strong surface high pressure system over the eastern Pacific interacting with an intense inland thermal trough will maintain a shallow marine layer along the coast, bringing overnight low clouds and localized dense fog. Inland areas will remain dry and hot under a building upper-level ridge. Triple- digit heat will become widespread across the interior valleys of Trinity, Mendocino, and Lake counties by Thursday.

DISCUSSION

The 500 mb analysis shows a building upper-level ridge over the southwest U.S. extending into California. The persistent coastal marine layer is a result of a tight low-level onshore pressure gradient between the offshore marine high and the interior thermal low. Widespread IFR/LIFR ceilings and some visibility restrictions will impact coastal areas through the morning hours. Expect partial afternoon clearing along the immediate coast as daytime heating deepens the mixing layer slightly, though low stratus will return rapidly after sunset.

Inland areas will experience significant heating as the upper ridge strengthens and moves westward. Subsidence will compress the marine layer, keeping it shallow and confined to the coast and immediate valleys. High temperatures on Wednesday will reach the 90s to near 100 degrees across the interior valleys of Trinity, Mendocino, and Lake counties. By Thursday, triple-digit heat will expand across all interior zones, with the highest values expected in the deepest valley floors.

AVIATION

The marine boundary layer remains firmly established along the coast. KACV and KCEC are experiencing IFR to LIFR conditions due to low stratus and fog, with visibilities lower than 1 mile at times. Clearing to VFR is expected by early Wednesday afternoon before stratus returns after 03z. KUKI will remain consistently VFR with clear skies and typical diurnal wind shifts.

MARINE

The pressure gradient between the offshore high and the inland thermal low is tightening across the waters. Winds south of Cape Mendocino are sustained at 20 to 25 knots, producing steep, wind-driven seas that warrant a continuation of the Small Craft Advisory for the southern inner and southern outer waters.

In the northern outer waters, wind speeds are modeled to increase to 20 to 25 knots with wave heights reaching 9 feet at a short 8- second dominant period late tonight. These waves will be steep and choppy. A Small Craft Advisory may be issued on this shift to address these conditions, or the decision may be deferred to the upcoming midnight shift to see if trends continue. Northern inner waters will remain comfortably below advisory thresholds.

EKA WATCHES/WARNINGS/ADVISORIES

CA... None. NORTHWEST CALIFORNIA COASTAL WATERS... Small Craft Advisory until 3 AM PDT Friday for PZZ455-475.