The southwestern United States is under the watch of escalating weather activity that poses threats of heavy rainfall, flash flooding, and mountain snow over the Halloween weekend. This warning comes after states are already at risk following recent storms.
Threat of Heavy Rainfall in Southern California
According to AccuWeather meteorologists, significant rain could fall on Southern California, contingent on the interaction between multiple storm systems and Pacific moisture. An atmospheric river might develop, channeling tropical moisture from near Hawaii toward the West Coast. This phenomenon, when it forms from Hawaii, is often referred to as a Pineapple Express.
Understanding the Atmospheric River
As per NOAA, atmospheric rivers are narrow bands carrying immense volumes of water vapor, akin to aerial rivers. When this moisture-laden air meets land, especially in mountainous regions, it can unleash intense rain or heavy snow. These systems are valuable in regions suffering from drought but can trigger hazardous conditions if precipitation occurs rapidly.
Storm-induced heavy rainfall can exceed drainage capacities, causing rivers to swell and prompt flash floods. Mountains may see extensive snowfall, while areas with steep slopes or recent burn scars from wildfires are prone to mudslides and debris flows.
Predictions for the Upcoming Weather Systems
An AccuWeather map illustrates potential areas affected by these weather systems through Halloween and into the subsequent week. One weather system is approaching from Alaska; another forms off Washington’s coast, and Tropical Rainstorm Olana near Hawaii contributes to the anticipated moisture.
Although it’s expected that these weather systems will remain mostly distinct while moving down the coast, Southern California cities like Los Angeles, San Diego, and Palm Springs could receive less than an inch of rain. Some mountain slopes facing south and west may get up to 3 inches.
A less likely scenario involves these systems and the tropical moisture merging sooner, resulting in heavier, prolonged rainfall along the coast, which could escalate the risk of flash floods.
Renewed Flooding Threat in the Southwest
Prospects of heavy rain signal renewed flooding risks in parts of Arizona, Utah, New Mexico, and Colorado from late this weekend into early next week. Predicted rainfall is between 1 to 2 inches in many areas, with certain upslope regions in central Arizona, southwestern Colorado, and Utah potentially receiving 2 to 4 inches. AccuWeather forecasts a StormMax of 7 inches in localized areas.
This potential rainfall is alarming, considering the already saturated regions from the recent downpours. Areas in the San Juan Mountains of northern New Mexico and southern Colorado registered 5 to 10 inches of rain in the last week, resulting in flooding and record streamflows.
Further rains could cause new flooding events, affecting cities like Phoenix and Flagstaff in Arizona; Durango in Colorado; and Farmington in New Mexico. Additionally, the Pacific Northwest may see showers, and the Rocky Mountains in Montana, Idaho, Wyoming, Colorado, and Utah could witness snowfall in higher elevations.
Meteorologists will maintain vigilance over the evolving weather pattern as further developments occur regarding the timing, trajectory, and intensity of precipitation.