The once innocuous-looking cut off low is currently producing some impressive convective activity over the central Bay Area and south to near Santa Barbara. The low seems to be moister than anticipated. Deformation bands on the N/E sides of cut-offs are always tricky in CA, and this time it is causing quite a few forecast headaches. Some small hail and lightning were reported earlier this evening just west of San Jose. I would expect relatively narrow bands of showers, locally heavy, with embedded thunderstorms to continue tonight and likely tomorrow from about San Francisco south to Los Angeles. Isolated showers and thunderstorms are possible as far north as Santa Rosa and as far south as San Diego, however. Suprisingly, some rainfall rates in this developing convection are pretty high, so wouldn’t even be out of the question to see some flashy stream flows or debris flows, esp. invof. the 05/06 burn areas in SoCal/CentCal. Rainfall will be highly variable with this system–generally under 0.25 inch, but locally up to 2 inches (or even a little more). Stay tuned–as the prognosis is changing very quickly.

Models are now indicating that a screaming east Asian jet (nearly 250 kts!!!) will try to undercut this blocking pattern later in the week (see Weather West homepage for an illustration of the current “hybrid” blocking pattern–Rex Block within a larger Omega Block). That’s still a ways out, but certainly bears watching. There could be at least some rain in the offing for the dry weather-weary…maybe.       

More tomorrow.