Forecasting Location and Magnitude of Thunderstorms Across Mountain Ranges and Deserts During Monsoon Flow in Southern California

Thunderstorms commonly form in the Desert Southwest and southern California during the monsoon season. The monsoon is a seasonal shift of upper level winds, normally occurring from July to September in response to strengthening upper level high pressure near the 4-corners United States, which allows for a transport of subtropical moisture from Mexico. The increase in moisture results in destabilization over the region and when diurnal upslope air rises and converges over mountain ranges the result can be deep moist convection, especially during daily peak heating. The typical monsoon thunderstorm has a pulse-like evolution and produces brief heavy rain and gusty wind before rapidly decaying. However, on occasion these thunderstorms can be multi-cellular organized, intense, and persistent resulting in severe flash flooding, damage and fatalities. Traditional practices for forecasting the occurrence of these thunderstorms tends to be anticipated by general broad topography initiation near climatology, convective temperature and random occurrence during the afternoon surface moisture flux convergences along higher terrain.

This study will demonstrate how monsoon thunderstorms initiation can be forecast using prevailing mid to upper-level wind flow to identify which aspect of a mountain range will favor diurnal low level convergence. The timing of the initiation, determining potential for thunderstorm formation in lower elevations and adjacent deserts, and thunderstorm persistence can be better predicted using the expected height of the level of free convection. Finally, using high resolution numerical weather prediction a forecaster can also add critical detail to the amount of rainfall and expected impact. Determining the prevailing steering wind flow that alters the low level convergence which allows air parcels to obtain the level of free convection, will aid in forecasting initiation and be useful for determining sustainability and magnitude.