Forecasting Location and Magnitude of Thunderstorms Across Coastal Regions, Mountain Ranges and Deserts During the Southwest Monsoon in Southern California

Thunderstorms commonly form in the Desert Southwest and southern California during the monsoon season.  The Southwest 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 their persistence resulting in severe flash flooding damage and fatalities.  While less frequent, the thunderstorms can be rooted in elevated layers and have high impact on the coastal region not climatologically favored for lightning or heavy rain during the warm season.  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 advance of subtropical moisture and subtle synoptic-scale ascent can also be a boundary for elevated thunderstorm initiation.  The elevated thunderstorm  impacts include intense and underestimated lightning activity along the coast, closed beaches or cancelled activities, abnormal runoff during the  typical dry season, ignition of wildfires, microscale downburst winds, and even injuries and fatalities at beaches.  The potential and timing of surface-based and elevated thunderstorm formation along coastal areas, in lower elevations and adjacent deserts, as well as thunderstorm persistence can be better predicted using the expected height of the level of free convection.  For strictly terrain driven deep moist convection, 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 of potential impact.  Finally, using high resolution numerical weather prediction a forecaster can also add critical detail to the amount of rainfall and expected impact.