Temperatures that are approaching or exceeding record levels across the West have prompted the National Weather Service to issue a rare excessive-heat warning, with highs in the Colorado River Basin nearing or surpassing 120 degrees.
On Wednesday, a weather station south of St. George measured a high of 118 degrees - an all-time record for the state. Thursday, the same station posted a 117-degree high, said Salt Lake City-based Weather Service forecaster Brandon Smith.
Utah technically escaped that excessive-heat warning because its heat index wasn't high enough, Smith said. Under the national standard, the heat warnings are issued only when temperatures above 80 degrees, dew point temperatures greater than 60 and relative humidities higher than 40 percent combine.
That simply doesn't occur in arid Utah. "It can be 120 degrees out and if there's not much humidity, it won't affect you much," Smith said.
Tell that to the folks in St. George, Mesquite, Las Vegas and Lake Mead. Weather stations there on Thursday registered highs between 115 and 122 degrees, just shy of Death Valley's 122 to 126.
"It is definitely hot, there is no question. But we expect that in July and August," said Russ Clove, manager of a Marriott hotel in St. George.
Red-flag fire conditions covering the eastern half of Utah were expected to expire Thursday night, opening the way for a different type of hazard as thunderstorms develop.
The Weather Service said the storms will bring heavy rain and frequent lightning, mostly in the high country. But continued hot weather in the western part of the state and the Wasatch Front could bring red-flag warnings along with the thunderstorms, whose dry lightning can spark fires.
The Weather Service issued the excessive heat warning to alert the public to the dangers of prolonged heat waves. Only winter cold - not earthquakes, hurricanes, tornadoes or floods - claims more lives than heat, meteorologists say.
With the summer sun and heat comes ozone pollution, and this week has been no exception. On Wednesday, several air-quality monitoring stations exceeded the daily health allowance for ozone, said state Division of Air Quality spokesman Bob Dalley, who oversees the monitoring network.
The trend appeared to continue on Thursday, when readings at the Great Salt Lake, in Cottonwood and West Valley City all recorded ozone levels exceeding the health standard for at least short periods.
"Red" ozone advisories were issued for Salt Lake, Davis, Utah and Weber counties, and sensitive groups - the young, the old and people with heart and lung problems, in particular - were urged to avoid prolonged exertion outdoors.
"We've got at least the next three days where we shouldn't see much difference," Dalley said.
Temperatures in Salt Lake City are expected to tie a record 102 on Friday. In hot, sunny conditions like these, pollution from engines "stews" and increases ozone levels.
* JUDY FAHYS contributed to this report.
Though low humidity can lessen heat impacts, temperatures are taken in the shade under light breezes. That means exposure to full sun can increase the heat hazard by 15 degrees. Hot, dry winds contribute to dehydration and lead to heatstroke or heat exhaustion. For those reasons, the Weather Service has issued some common-sense advisories:
* Drink plenty of water during extreme heat, especially while outdoors.
* Never leave children or pets in vehicles, even for just a few minutes.
* Check on elderly relatives and neighbors.
* While hiking, pay attention to changing weather conditions and seek shelter from storms.