The Dynamics of Heavy Rain in Southern California Associated with Moisture from Dissipating Hurricane Dolores

The North American Monsoon is a significant source of summer precipitation in the Western United States and Mexico. This can be seen in a secondary or in some cases even a primary peak in monthly precipitation. The airmass that provides the moisture for this precipitation has its source in the tropical Eastern Pacific, the Gulf of California, and to a lesser extent, the Gulf of Mexico. In Arizona this results in the "airmass thunderstorms" that are ubiquitous over the higher terrain during monsoon season. This moist airmass spills over into Southern California, also, and initiates thunderstorms over the mountain ranges and more rarely over the neighboring desert floor and inland valleys. In Southern California the monsoon has a somewhat different character, though. Substantive rain is more common with easterly waves (inverted troughs) accentuating the monsoon flow or with the most prolific rain producers of all, moisture from dissipating tropical cyclones.

The rain associated with these dying tropical systems can be substantial, and even though they don't occur every year, the rainfall they produce can be heavy given the time of year and is a major contributor to the mean rainfall in the area. Here we examine one such case, rain associated with the northward flood of moisture from Hurricane Dolores.

Hurricane Dolores occurred from July 11-18, 2015 and reached Category 4 on the Saffir-Simpson scale. However, it is not its direct effect that we examine here, but the copious rainfall induced in Southern California on July 18-20, which broke many precipitation records for the month of July and caused localized flooding, including destruction of a portion of Interstate 10 in the Mojave Desert. Here we look at the significance of the effects of Dolores in Southern California as well as the synoptic setup. To better understand the dynamics of the situation, we have run a retrospective Weather Research and Forecasting (WRF) model for the time period. It is a nested model with the outer domain capturing the long-range dynamics of the situation and a high-resolution inner domain to understand the fine structure driving the precipitation.

We also examine the significance of the off-season rainfall and speculate about how the climatology of these storms may change in a warming environment.