And occasionally a good bloom occurs when the above conditions appear not to have been met. The latter can happen when an unusually warm rain triggers germination in winter, but the short growing season usually precludes a really good show. When it does happen, the peak typically lasts only two weeks at a given location, sometime between late February and mid April. Most often it happens in early to mid March. Indian Reservation do fairly regularly.
Perennials are less fussy about the timing of rainfall.
Thus a late but wet rainy season can still produce good blooms of penstemon, larkspur, brittlebush, etc. Some plants such as palo verdes, ocotillo, and most cacti flower very year regardless of rainfall. But it is the annuals alone that produce the desert's famous carpets of color.