Rainfall events such as the one that just occurred in the Antelope Valley are typical for the region. Topographical boundaries of the Mojave Desert include the Tehachapi Mountains to the west, as well as the San Gabriel Mountains and San Bernardino Mountains to the south, placing the Antelope Valley in a rainshadow.
A rainshadow is a region having little annual rainfall because it is sheltered from rain-bearing winds by a range of hills, with local rainfall averaging less than 8 inches annually. Most of the annual rain falls between November and April, leading up to a spectacular display of spring wildflowers.
When it does rain, accumulation of water on the ground surface is common because ground absorption capacity is limited by dry desert soil. During rainfall, water tends to run off of alluvial slopes and creeks and collects in low places like dry lake beds or in washes called arroyos.
Cycles of El Niño tend to bring more rainfall to the Mojave Desert than usual, increasing the risk of flash flooding. Flash floods can be produced when slow-moving or multiple storms occur over the same area. When storms move faster, flash flooding is less likely since the rain is distributed over a broader area.
The Antelope Valley frequently experiences flash floods and standing water from storms, such as the El Niño event of 1914 that led to much of downtown Lancaster being deluged with water. This single storm dropped an average of 1 inch of rainfall in the area, brought in by a tropical cyclone off of the Pacific that had moved northward from Baja California into the desert areas.
This 1914 storm, known by early residents as “the Great Flood of 1914”, led to a flood that washed out the train tracks at the Lancaster Train Depot. During the 1910s, Lancaster experienced a series of bad flooding events due to overflow problems at Amargosa Creek and Littlerock Creek, but the event of 1914 was the most severe at the time.
KCET, Rain Shadow Desert: Why the Rain Often Skips the Desert