A low-pressure system brought significant rain to Southern California on March 10th and March 11th. At the time of this presentation, the system was right at the peak of its strength and beginning to degenerate. Part of the reason for this was the placement of the subtropical jet stream as seen below in figure 1, the mid-level water vapor satellite image. The jet looked to be positioned just south of California going into Arizona. It is possible to spot because though not shown in the image, the clouds in the area where the jet was were moving faster than the other clouds in the region. Additionally, a slight ridge was beginning to form on top of transverse bands, which are ripple-looking clouds close to the jet. You can see them if you look across middle west Texas. A map of upper-level wind patterns confirms this in the second panel of figure 1.
Fig. 1: (top panel) A mid-level water vapor satellite image (source: https://whirlwind.aos.wisc.edu/~wxp/goes16/wv/goes16_namer.html) from 11 March 2021 at 12:10 UTC, and (lower panel) is a wind mat at 250 mb with sea level pressure contours in black, 1000-500 mb thickness in red and blue dashed lines, and the wind speed shaded based on the color bar (source: http://www.atmos.albany.edu/student/abentley/realtime/standard.php?domain=northamer&variable=mslp_jet)
California is currently in a drought, and they are almost to the end of a drier than average wet season. Due to the drought gripping the state, any precipitation is welcomed at this point. This was a good soaking rain over the course of multiple days. There were a few mudslides reported nonetheless. Snow was even recorded at higher elevations in the mountains. Both Los Angeles and San Diego recorded over half an inch of rain, and San Diego was 0.43 inches above average for the rainfall expected for the March 11th. Below in figure 2 is the forecasted amount of rain for the areas.
Fig. 2: This is a map of 24-hour Qualitative Precipitation Forecast totals. It was issued March 10th at 08:32 UTC and forecasted for 12:00 UTC March 10th to 12 UTC March 11th. The color scale indicated California could receive anywhere from 0 to 1.25 inches of rain depending on the region. (source: https://www.wpc.ncep.noaa.gov/archives/qpf/display_maps.php?prodtype=issued&proddate=03/10/2021&prodtime=12&allsent=no&imagetype=color&actualprods=94q)
This system had output most of its precipitation by March 11th, but some precipitation remained. The progression of the system over radar was fascinating to watch because it did not look too impressive. The key to remember is if it were a system over the Midwestern plains or the east coast, the system would not mean much. Because it is California, people had to be reminded to drive more carefully than they were used to and prepare for a rain shower like the southeast U.S. prepares for a snow shower, minus purchasing the milk and bread. Here is a clip of a meteorologist giving the weather report on March 10 just before 17:40 PST (01:40 UTC) (source: https://www.foxla.com/video/909367)