Southwestern U.S. Faces Threat of Heavy Rain and Flash Flooding
A developing weather system is drawing tropical moisture toward the southwestern United States. This could result in heavy rain, flash flooding, and mountain snow over the Halloween weekend. Several states face potential renewed flooding after recent storms.
Moisture Threat to Southern California
Southern California might receive significant rainfall. The extent of this threat depends on how storm systems interact with moisture moving across the Pacific. According to AccuWeather, this setup could form an atmospheric river—a concentrated moisture pathway. If this pathway extends from Hawaii to the U.S., it may develop into a Pineapple Express.
Understanding Pineapple Express
A Pineapple Express carries warm, moist air from the tropical Pacific, often near Hawaii, to North America’s West Coast. Atmospheric rivers, like the Pineapple Express, transport large amounts of water vapor across narrow atmospheric bands. When reaching land or mountains, they bring prolonged heavy rain and snowfall.
These systems can benefit drought-prone regions with necessary rainfall. However, they also pose dangers when rainfall is excessive in a short time—overwhelming drainage systems and causing rivers to rise abruptly. Mountain slopes and areas previously affected by wildfires are especially prone to mudslides and debris flows.
Potential Impact Areas
AccuWeather’s map indicates areas with potential heavy precipitation during Halloween weekend and early next week. Two storm systems may direct tropical moisture toward the western U.S. One moves from Alaska; another forms near Washington. Tropical Rainstorm Olana near Hawaii may also influence moisture levels.
A likely scenario suggests the storms and moisture remain somewhat separate, limiting rainfall in major Southern California areas like Los Angeles, San Diego, and Palm Springs to under an inch. However, some mountain slopes could receive up to 3 inches. A less probable scenario would involve earlier combination of the storms and moisture, possibly resulting in heavier rain near the coast and greater flash flooding risk.
Risks in the Southwest
The Southwest, including Arizona, Utah, New Mexico, and Colorado, faces a higher threat of substantial rainfall. This region risks renewed flash flooding, debris flows, and significant river rises late this weekend into early next week. Areas could receive 1 to 2 inches of rain, with central Arizona, southwestern Colorado, and Utah getting 2 to 4 inches. AccuWeather’s Local StormMax predicts up to 7 inches.
Additional rainfall is concerning for areas with recent heavy precipitation. The San Juan Mountains in northern New Mexico and southern Colorado saw 5 to 10 inches of rain last week, causing flooding and record streamflows at monitoring stations.
Further rain could lead to renewed flooding in regions like Phoenix and Flagstaff in Arizona, Durango in Colorado, and Farmington in New Mexico. The broader storm system may also bring showers to the Pacific Northwest and snow to Rocky Mountains high elevations, affecting Montana, Idaho, Wyoming, Colorado, and Utah.
Forecasters are closely monitoring the weather pattern as details about timing, track, and rainfall amounts emerge.