From Daniel Swain, Weather West:
A pretty unusual large-scale atmospheric pattern over the northeastern Pacific Ocean and inland over the Southwest will contribute to a strikingly intense and persistent statewide heatwave this week across California. Indeed, extreme and in many cases record-breaking heat will actually extend across a pretty broad chunk of the American SW in the coming days–not just California. Heat will begin building today, but will become noticeably oppressive in some places by Monday. The heat will continue to intensify Tuesday, Wednesday, and possibly through Thursday before reaching a very impressive peak–especially across NorCal. Widespread daily record highs are expected, especially in NorCal, on multiple successive days (likely in some spots from Mon-Thurs). Right now, the high-resolution models are suggesting that this week’s extreme heat will be most anomalous across portions of the San Francisco Bay Area, where I would not be entirely surprised to see some monthly (not just daily) heat records fall. But ensemble forecasting tools are also suggesting that 850mb temperatures could approach/exceed record levels across an even broader region by Wed/Thurs–so there is some potential for this upcoming heat event to “outperform” even these pretty lofty predictions.