A flash flood watch will be in effect from noon to midnight Thursday in the western half of Utah, from the central part of the state to the southern border.
The National Weather Service’s watch extends up into the Tooele Valley, where wildfire burn scars — including the more than 4,000 acres burned in the Jacob City Fire in July — are particularly prone to flooding.
The affected area includes St. George, Zion and Bryce Canyon national parks, Cedar City, Milford, Richfield, Nephi, Manti and Delta.
Showers and thunderstorms are expected, and excessive runoff may result in debris flows on burn scars and flooding in slot canyons, normally dry washes, slickrock areas and other low-lying, flood-prone locations.
The threat won’t end on Thursday night, however. The weather service warns that a “moist and unstable air mass will keep a daily threat of thunderstorms in place” through the weekend across much of Utah.
Flash flooding is “probable” at most of Utah’s national parks and national monuments on Thursday — Bryce Canyon, Capitol Reef, Grand Staircase-Escalante, Natural Bridges, Grand Gulch and Zion — and “possible” at Arches, Canyonlands, Glen Canyon and San Rafael Swell.
The National Weather Service forecasts that flooding is “probable” at all 10 national parks and monuments on Friday.
The flash flood watch does not include the Wasatch Front, although precipitation is expected there. In Salt Lake City, there is a 60% chance of showers and thunderstorms on Thursday afternoon, a 40% chance on Friday afternoon, and a 50% chance on Saturday and Sunday afternoon, falling off as afternoons turns to evenings. And there is a chance of still more rain on Monday, Tuesday and Wednesday.
Expect high temperatures from the mid-80s to about 90, and overnight lows from the mid-60s to about 70.
The forecast for southern Utah is similar, just a few degrees warmer. In St. George, there is a 40% chance of afternoon showers and thunderstorms on Thursday, Friday, Saturday and Sunday — falling off in the evenings — and more rain is possible on Monday, Tuesday and Wednesday. Daytime high temperatures will be in the mid- to upper 90s, with overnight lows in the mid-70s.
According to the Utah Department of Environmental Quality, all monitored areas across the state will see yellow/moderate air quality on Thursday, Friday and Saturday.