The National Weather Service placed large sections of the central and southern Rockies and parts of the Great Basin under severe-thunderstorm and flash-flood alerts over the weekend, capping a three-day stretch of volatile late-summer weather. Offices in Boulder, Reno, Phoenix and Flagstaff issued dozens of rapid-fire warnings on 22–24 August, citing storms capable of producing golf-ball-size hail, damaging winds and cloudbursts intense enough to trigger life-threatening floods. In Colorado, repeated warnings swept from the Front Range into the eastern plains. Boulder and Jefferson counties were under back-to-back advisories on Saturday, while Lincoln, Kit Carson and El Paso counties remained in a severe-thunderstorm watch through 9 p.m. MDT Sunday. The Boulder office cautioned that cells near Aspen Park and Arriba contained hail up to 1.75 inches in diameter, and a flash-flood warning was posted for El Paso County until 5:30 p.m. Farther west, the Reno office issued a flash-flood warning that covered Nevada, Sierra and Washoe counties until 4:30 p.m. PDT Sunday, along with successive severe-thunderstorm warnings for the Tahoe corridor and communities along the California-Nevada line. Additional alerts stretched into Arizona and California’s Imperial Valley, where the Phoenix office warned of flooding in Imperial County until 6:30 p.m. PDT. While the bulk of the activity has concentrated on the Rockies and Great Basin, the outbreak’s reach extended coast-to-coast. Tornado warnings were posted for Duval County, Florida, late Saturday and for Wayne and Cayuga counties in upstate New York on Sunday afternoon. Forecasters said the hazardous conditions are being driven by monsoonal moisture colliding with a stalled frontal boundary. The weather service urged residents across the affected regions to monitor updates, move indoors at the first sign of lightning and avoid low-lying roads that could flood within minutes.