Prolonged Heatwave Breaks, Thunderstorm Threat Increases in Northern California

The focus is on a mid to upper-level low advancing along the California coast, driving a plume of monsoonal moisture from the south.Photo byNWS

The prolonged heatwave affecting Northern California is finally breaking, but it brings an increased threat of thunderstorms and dry lightning as monsoonal moisture moves in from an offshore low-pressure system. Inland temperatures are expected to trend down for the first half of next week before rising again.

The focus is on a mid to upper-level low advancing along the California coast, driving a plume of monsoonal moisture from the south. This moisture increase, particularly in the mid to upper levels, combined with a very dry surface layer, presents conditions conducive to strong "dry" showers and thunderstorms, primarily in Lake, Eastern Mendocino, and Trinity counties. Interior Humboldt is also likely to see thunderstorm activity.

Thunderstorm chances will increase through the evening, with the highest confidence for activity in the eastern regions from Lake to Trinity County, especially this afternoon and evening. Early morning thunderstorms have already been observed in coastal Mendocino and Southern Humboldt County. The threat will continue into Monday, particularly in Trinity County, before a shortwave trough brings drier conditions on Tuesday.

The ongoing Red Flag Warning highlights the fire weather threat due to dry thunderstorms.