By Reginald Stanley. Posted December 24, 2023, 3:42 PM.

A slow-moving cutoff low pressure system moving south along the California coastline last week brought widespread rainfall to much of the region, as well as isolated thunderstorms and mild temperatures.

A shortwave trough passing through the region on Monday kicked off last week's wet weather pattern, producing mostly light rain totals across much of Southern California. Most communities remained dry on Tuesday, before a surge of heavy precipitation and thunderstorms swept through the region Wednesday evening. This event was marked by southerly flow, with waves of precipitation moving in almost a straight line from south to north, as the responsible cutoff low curved near Point Conception. The cutoff low then stalled near Santa Barbara, sending a deluge of subtropical moisture into Santa Barbara and Ventura counties and producing localized flooding there, also prompting a brief tornado warning for Oxnard. WeatherCurrents' station in Simi Valley received particularly heavy rainfall at that time.

Following mostly dry conditions on Thursday, the cutoff low finally shifted eastwards over Southern California proper on Friday and Saturday. Spectacular displays of lightning were reported over parts of San Diego County late Friday evening as another line of thunderstorms formed over coastal waters off Southern California, and later moved ashore. These thunderstorms were preceded by very gusty winds across the region Friday night. Many inland and valley locations received heavy rainfall around and after midnight Friday, lasting several hours along with gusty winds. The warm nature of the storm kept snow levels high, mainly over 7500 feet above sea level. Residual showers lingered across parts of the region for the remainder of Friday, as the cutoff low finally exited the region to the east towards Arizona.

Precipitation amounts were initially forecasted to be quite high across Southern California, including the deserts - however the erratic movement of the cutoff low led to the storm largely underperforming across the region for areas south and east of Los Angeles County. As a result, storm totals across Orange, San Diego, San Bernardino and Riverside counties ended up being roughly 50 percent of initial predictions. In the WeatherCurrents network, most communities recorded cumulative amounts ranging between 0.5 and 1 inch by Saturday. Simi Valley, northwest of Los Angeles, was a spectacular exception to this rule, having received over three inches total from the storm, owing to its location where the cutoff low had stalled. Elsewhere, Moreno Valley recorded the highest total inland with 1.06 inches, while the lowest total recorded from the storm occurred in Anza with 0.47 inches. In Temecula Valley's Wine Country, 0.84 inches was reported through Friday (courtesy of Jim Sappington). Rain gauge problems continued in Perris, Riverside (Orangecrest) and French Valley. Substituted rain totals from nearby Wunderground stations were used where considered accurate. In Perris, manual rain gauge totals on-site were used instead.

Weak high pressure has moved in over the region in wake of the cutoff low's departure, leading to dry, mild conditions through the middle of the week. A trough entering the region late in the week may bring a chance of light precipitation going into the weekend.

Here are the rain totals for the WeatherCurrents network and associates:

1. Orangecrest and French Valley totals substituted from nearby Wunderground stations due to rain gauge malfunctions.
2. Perris totals taken from manual rain gauge on-site through Friday.