Giant green stems with budding yellow flowers greeted hikers along a narrow path beneath the soaring Santa Monica Mountains on a recent drizzly day. This is where, just seven months ago, the worst fire in Los Angeles County history swept through, destroying more than 1,000 homes and blackening miles of hillsides and canyon. But thanks to one of the wettest seasons in years, rains have transformed the fire zone back to life with great speed. And all those flowering black mustard plants point to a looming disaster once the rains finally end and Southern California shifts to its dry, hot, windy summer and fall.