Fire crews from Nevada and Oregon have arrived to help California firefighters battle hundreds of blazes that are darkening the sky over California, leading public health officials to issue air-quality warnings. The lightning-caused fires have charred tens of thousands of acres and forced hundreds of residents to flee their homes, though few buildings have been destroyed, said Daniel Berlant, a spokesman for the California Department of Forestry and Fire Protection, or CALFIRE. "It's just extremely, extremely dry," Berlant said Tuesday. "That means any little spark has the potential to cause a large fire. The public needs to be extra cautious because we don't need any additional wildfires." Elsewhere in the state, residents were ordered to evacuate an area of Monterey County on Wednesday because of a huge blaze that started before the lightning storm. More than 800 wildfires were set by a storm that unleashed nearly 8,000 lightning strikes across Northern California over the weekend. The storm was unusual not only because it generated so many lightning bolts with little or no rain over a large geographical area, but also because it struck so early in the season and moved in from the Pacific Ocean. Such storms usually don't arrive until late July or August and typically form southeast of California.