Another wet cutoff low for CA; winter begins in earnest before the end of October?

Hello, folks. I do apologize for the most recent gap between posts–things have been rather crazy around here lately.

It has been a wild few weeks of weather across the state, especially in Southern California. We went from one of the coldest summers ever to experiencing some of the hottest temperatures temperatures ever recorded in the Los Angeles Basin. Less than a week later, widespread rain, thunderstorms, and record-cold temperatures were the rule over Southern California. At the same time, more northern parts of the state stayed entirely dry through the event, leading to the unusual situation that Southern California may begin the winter wet season wetter than Northern California (all of this in a strong La Nina year).


More interesting weather is on the way. In fact, a pattern similar to the one that brought all the active weather last week is once again developing across the far Eastern Pacific.

4 km GOES water vapor imagery depicting subtropical moisture plume off CA coast

A new cutoff low will form tomorrow west of San Francisco and move onshore over the Bay Area on Sunday and Monday. To the east of the low, diffluent flow and increasing mid-level lapse rates will act on an existing subtropical moisture plume to produce some elevated convective precipitation. It’s not clear at the moment how widespread thunderstorm activity might be, but I do expect some lighting strikes to be observed by Sunday afternoon at the latest. The shower and thunderstorm threat (along with much cooler temperatures) will spread south and east to cover the entire southern half of California for Monday/Tuesday. The NAM and recently the GFS have been indicating some pretty significant precipitation totals in some of the same places that saw a lot of rain last week, so this will be worth keeping an eye on.

GFS depiction of the cutoff low expected to impact CA weather this week

Beyond midweek, uncertainty in the forecast increases dramatically. The ECMWF indicates the potential for yet another cutoff low, interacting with yet another plume of subtropical moisture over CA, while the GFS is not so emphatic with the late-week precip changes. Both models, however, indicate that the jet stream will begin to buckle over the far Eastern Pacific beginning next weekend, potentially leading to significant rainfall over at least the northern portion of the state. It’s a bit too early to get very excited, but we are just about reaching that time of year…

And what of La Nina? Well, the current strong episode continues to chug along in the Eastern Pacific. SST anomalies have continued to increase (become more negative) and expand in coverage, though the rate of decline has (as expected) slowed in recent weeks.