The record-breaking heat wave roasting the West is expanding its grip on Thursday, with the focus of the triple-digit heat shifting into California — particularly the Central Valley and desert regions.

Why it matters: Across the West, the combination of record heat, preexisting drought conditions, and dry lightning strikes from afternoon thunderstorms threatens to ignite numerous wildfires Thursday.

• Fires have already been burning in Montana, Arizona and other states affected by the heat wave.
• More than 40 million people across the West and Southwest are likely to see temperatures in their area exceed 100°F during the next several days.

Details: The sprawling area of high pressure, known as a heat dome, is drifting west and now has the Golden State in its grip. High temperatures of up to 110°F are forecast for inland portions of Southern California, northward into the Central Valley, and even stretching toward portions of the San Francisco Bay area and northern parts of the state.

• The operator of the California electric grid has issued a "Flex Alert" for Thursday, asking residents to reduce their electricity use at peak times of day, as it works to balance spikes in power demand with supply.
• Due to the drought, the state cannot rely on importing as much hydroelectric power as it normally would. This could prompt natural gas plants to be turned on during peak periods.

Threat level: According to the National Weather Service, temperatures across the West will be 10°F to 25°F above average for this time of year. Forecasters are not holding back in their use of language, either: "Dangerous and potentially deadly heat will continue throughout the Southwest and Intermountain West," forecasters wrote in an online discussion.

• "Residents enduring the abnormal heat are urged to stay hydrated and avoid extended periods of time outdoors if possible." High temperatures in the Sacramento region could peak at about 113°F. Inland portions of the L.A. and San Diego metro areas will sizzle, while cool breezes off the Pacific keep coastal sections relatively cool.
• The area at greatest risk for wildfires on Thursday is in southern Wyoming, due to the combination of heat, dry soil and high winds.
• However, California, Nevada and the Four Corners states are also at an elevated risk, particularly due to the likelihood of dry thunderstorms erupting in Northern California. Such storms produce cloud-to-ground lightning, but only light amounts of rain, leading to wildfires.

The big picture: The West is enduring its most extensive and severe heat wave since the start of this century, and the heat wave and drought are intensifying one another. Given the record levels of soil dryness, more solar radiation goes into heating the air, rather than evaporating moisture in soils. This pushes air temperatures higher, to levels more commonly seen in July or August, if at all.

• Climate change is likely playing a prominent role in this event. One of the most robust conclusions of climate science is that heat waves are becoming more intense and longer-lasting as the climate warms overall.
• In recent years, there has also been a trend toward stubborn and sprawling heat domes, which block storm systems and keep hot weather locked in place for days at a time.
• This heat dome extends from the U.S.-Mexico border to the Canadian border.