President Obama warned Americans to stay safe during an ongoing heat wave as heat advisories remained in effect in at least 15 states and some places braced for temperatures that could top 110°F.

Advisories in effect Thursday from the National Weather Service warned residents across the Midwest and South to stay in cool locations and remain hydrated. The heat index—a measure incorporating heat and humidity—is expected to exceed 105°F in the majority of those places. Extreme heat could spread to parts of the Eastern seaboard over the weekend.

President Obama was briefed on the coming heat wave by his chief science advisor John Holdren earlier this week, according to the White House. “Stay safe as it heats up: Drink water, stay out of the sun, and check on your neighbor,” Obama said on Twitter.

This week’s heat wave—the biggest to hit this year—was caused by a high pressure system known colloquially as a heat dome that traps heat close to the Earth’s surface. The high temperatures could stretch from as far southwest as Southern California to the Northeast. “There’s a very large bubble that’s working its way across the nation,” said Sean Sublette, a meteorologist at Climate Central, earlier this week. “The atmosphere travels in waves—waves that are up and waves that are down. Up waves are allowing a lot of hot air to come up through the equator.”

This heat wave contributes to the likelihood that this year will be the warmest on record.

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Write to Justin Worland at justin.worland@time.com.