Following days of mostly mild summer temperatures in the Bay Area and Sacramento Valley, forecasters say temperatures this week will climb significantly, with the hottest day of the year possible on Thursday or Friday.

Temperatures over the past week have been cooler than normal in the Sacramento Valley and Bay Area, largely due to the influence of the marine layer. This week in both the Sacramento area and the inland portions of the Bay Area, there is widespread moderate to major heat risk expected from Thursday through Saturday, with limited overnight relief, the National Weather Service said in its forecast discussion. 

A building dome of high pressure centered in the Four Corners area of the U.S. Southwest will expand westward during the second half of the week, the Weather Service said. That will bring temperatures well above normal, and by Thursday and Friday, potentially reaching 104 degrees in the Sacramento area, as well as near triple digits or low 100s in some inland Bay Area locations.

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Further south in the San Joaquin Valley, the Weather Service declared an extreme heat watch from Friday morning through Sunday morning, warning of dangerously hot conditions with high temperatures up to 106 degrees possible. Closer to the Nevada border,  an extreme heat warning was in effect from Wednesday through Saturday

In addition, there are isolated chances of mountain thunderstorms, increasing from Friday to Sunday, along the Sierra from Interstate Highway 80 southward from monsoonal moisture

A gradual cooling trend was expected to begin by late Saturday, forecasters said.