A very unusual pattern is currently shaping up for the northern half of CA and much of OR during the next few days. An unseasonably strong trough and associated cold front is currently approaching the CA coast. A strong vorticity maximum and associated wave is enhancing the southern end of the front, the strongest part of which will impact Mendocino, Sonoma, and Marin counties tomorrow. Rain will fall from WA state south to about Monterey County tomorrow morning along the coast, and some showers will survive the inland trek, especially over higher terrain. Although this is not a particularly unstable situation, there remains a possibility of isolated thunderstorms as far south as the Bay Area as steep lapse rates do exist (it’s summer–lots of insolation, and pretty cold air aloft at 850 mb) and because this system has very strong dynamics for this time of year. Precipitable water values just off the CA coast are currently in the 1.5 to 2 inch range, which is almost unheard of during July. Coastal CA does occasionally see light precipitation events in summer, but these are almost always associated with monsoonal moisture surges or remnant tropical moisture and are exclusively of a convective nature. NorCal sometimes sees light Pacific stratiform events as late as June in certain years (like 2005), but never in the latter half of August. For many, it will be a welcome change in the weather during a season which is usually pretty monotonous in this part of the world. Also, assuming that there is no lightning associated with the system, the precip in far NorCal may actually be enough to (very temporarily) assuage fire weather fears. The increased winds associated with frontal passage may not be accompanied by precip in the Sierras, however, which could ultimately serve to spread the current fires. Beyond Thursday, a ridge looks to build back in from the south and may lead to a renewed monsoonal surge. Rather interesting for July in CA, that’s for sure…