Visitors to Grand Canyon National Park are being warned about extreme heat that will plague the popular destination early next week after a recent increase in heat-related incidents in the inner canyon, including the deaths of three hikers.

The U.S. National Weather Service issued an extreme heat watch at the Grand Canyon for midday Monday through Tuesday, forecasting temperatures that could reach or exceed 110 degrees Fahrenheit (43 degrees Celsius) at the low-elevation Phantom Ranch.

Hikers are “strongly advised” to avoid hiking in the middle of the day, the U.S. National Park Service said in a release this week following a “recent influx of heat-related incidents.”

An extreme heat watch was in effect on June 16 when two hikers, aged 67 and 68, were found dead on the North Kaibab Trail, which NPS describes as the most difficult of the major inner canyon trails. NPS said they appear to have succumbed to symptoms of heat-related illness.

A third person, 72, died June 12 along the South Kaibab Trail after becoming ill from the heat, NPS said.

About 90 miles (145 kilometers) south, Oak Creek Canyon visitors and residents were evacuated late Friday as a wildfire burned hundreds of acres just north of Sedona, Arizona.

Extreme heat increases risk of hiking at the Grand Canyon

Park and weather officials alike emphasize to Grand Canyon visitors that hiking conditions can be deceiving. Temperatures at the rim of the Grand Canyon are often 20 to 25 degrees F cooler than what hikers will experience at the bottom of the canyon.

“It’s just a hot place at the bottom of the Grand Canyon,” said Justin Johndrow, a meteorologist with the National Weather Service office in Flagstaff, Arizona. Johndrow warned that the region is approaching the hottest period of the year before the rain during monsoon season later in the summer offers some relief from the heat.

Hikers may have cooler temperatures and an easier time going downhill to start the descending trails, but they face an intense climb of thousands of feet in elevation and much hotter bottom-of-the-canyon temperatures to get back up. Those conditions can cause heat illness symptoms to sneak up on visitors.

“That’s very strenuous even on a mild day,” Johndrow said of the hike back up to the rim. “Throw in temperatures of 105 to 110 degrees and that causes some pretty bad problems.”

Park officials advised visitors to avoid strenuous hiking in the middle of the day.

Above-average temperatures and red flag warnings are across the western U.S.

Much of the western U.S. from the Rockies to the Pacific Coast were getting above-average temperatures Saturday and anticipating even hotter weather early next week. Officials also warned the prolonged dry, hot weather and relatively low humidity increased the risk of fire danger throughout the area.

At least a dozen agencies were working to combat the fire burning a few hundred acres north of Sedona, Arizona, the city’s fire department said on social media. The U.S. Forest Service issued evacuations for visitors and residents near Oak Creek Canyon, and about 30 miles (48 kilometers) of the adjacent state highway was closed in both directions.