From the Fourth National Climate Assessment, released (for some reason) on the day after Thanksgiving by the Trump administration. Folks, I’ve just started reading the Southwest section, but I must say, for SoCal and other hot places in California, in particular, this looks like very bad news.
Under the higher scenario (RCP8.5), climate models project an 8.6°F (4.8°C) increase in Southwest regional annual average temperature by 2100.23 Southern parts of the region could get up to 45 more days each year with maximum temperatures of 90°F (32°C) or higher.23 Projected hotter temperatures increase probabilities of decadal to multi-decadal megadroughts,61 ,62 ,69 ,70 which are persistent droughts lasting longer than a decade,69 even when precipitation increases. Under the higher scenario (RCP8.5), much of the mountain area in California with winters currently dominated by snow would begin to receive more precipitation as rain and then only rain by 2050.71 Colder and higher areas in the intermountain West would also receive more rain in the fall and spring but continue to receive snow in the winter at the highest elevations.71
Tragically, the RCP8.5 [emissions] scenario is the pathway we are on at the present moment. Here is a mapping of the warming to date, with a caption below. Note that Ventura County, despite its coastal location, has already warmed as much as all but a very few locations in the entire Southwest.
Figure 25.1: Temperatures increased across almost all of the Southwest region from 1901 to 2016, with the greatest increases in southern California and western Colorado.23 This map shows the difference between 1986–2016 average temperature and 1901–1960 average temperature.23 Source: adapted from Vose et al. 2017.23