Because of its proximity to the Pacific Ocean, the North Coast has only two seasons, the rainy season from November to April and the dry season the rest of the year. But even though the area is at almost the same latitude as Chicago, the winter temperatures are much warmer due to influence from the Pacific Ocean.
A simple explanation for the dry season is that from spring through fall, a giant bubble of air called the North Pacific High parks itself off the coast, deflecting the weather around it and allowing the North Coast to bask in sunshine. This bubble breaks up during the winter, allowing storms to come ashore during the wet season.

FORTUNA'S MICROCLIMATE
But what makes Fortuna's weather so unique is the combination of the prevailing NW wind and the local topography. Upwelling brings cold ocean water to the surface. This cools and humidifies the air above it. As the warm inland air rises, the cool and humid ocean air is pulled onshore to take its place. This air greatly influences the Eel River Delta temperature, causing fog and low temperatures. Fortuna is located in a protected corner of the Delta, and...

Fortuna circled, view towards the North. The prevailing wind is from the top left corner.
 - Image courtesy of Dr. William Bowen - California Geographical Survey.

...Fortuna is protected from that cold and persistent NW wind by that range of bluffs shown in the picture above. As well as raising the ambient temperature, the bluffs also offer protection from the ocean fogs so that residents can get a tan. This happy combination of factors allows the area to enjoy one of Northern California's finest horticultural climates. The sun shines through the thin fog overhead, and it's very much like living in a giant greenhouse.

FORTUNA TEMPERATURES AND RAINFALL
Summer lows are in the fifties but highs seldom get into the eighties. There's little rain. November through April lows are in the thirties with highs in the mid fifties and sixties with around forty inches of rain spread over those months. There's an occasional early morning frost if the winter sky is clear, but it seldom freezes. The only time the weather gets radical is when the prevailing pattern changes and the winds come from inland for a few days.

THE REST OF THE STORY...
The weather changes dramatically when you travel a few miles upriver or cross the hills surrounding the delta. It's not at all unusual for Fortuna to enjoy seventy degree temperatures while temperatures twenty miles inland are in the nineties or higher.
The last major snow in Fortuna was in 1990, but the surrounding hills show snow every winter.
Because of the area's many microclimates, climate and rainfall can vary immensely. The town of Honeydew, located about twenty miles to the southwest and at the end of a box canyon facing the ocean, averages nearly four times as much precipitation as Fortuna, over twelve feet(!) of rainfall per year.

Data for Scotia (6 miles SSW) is smoothed using a 29 day running average.