Fire weather no longer–summer is replaced immediately by winter

The last few days have seen a rapid transition of weather patterns in California. Hot and extremely dry conditions have given way to moist and downright cool temperatures. The most notable feature on current satellite pictures is the extensive subtropical moisture plume ploughing into Central CA from the SW. This plume has brought some rather unexpected areas of rain (mainly light) and some embedded thunderstorms today to areas south of Santa Cruz and north of Los Angeles. This plume will continue to cause some weak convective activity tonight and for the early part of the day on Monday. The real story, however, is the approaching Gulf of Alaska trough set to impact most of CA by Tuesday. This trough is expected to dig deeply south off the Northern California coast during the next 48 hours and begin to move onshore near the San Francisco Bay Area. As this occurs, shower activity will develop over most parts of Norcal. Most of the precipitation is expected to be convective in nature, as the air aloft will likely be quite cold for this time of year. Extra dynamics as a result of a jet streak near the Bay Area may give an added push to convective activity here. Thunderstorms are certainly a possibility, and so rainfall amounts could be locally heavy-ish (1.25 inches or so in a couple of areas). Details will become clearer as the event gets closer. Winter seems to be on our doorstep already…