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By Emma Marsden
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A map shows a weather system drawing tropical moisture toward the southwestern United States which could bring heavy rain, flash flooding and mountain snow over Halloween weekend, with several states facing the risk of renewed flooding after recent storms.
The developing weather pattern could also bring significant rainfall to Southern California, although the extent of the threat will depend on how separate storm systems combine with moisture moving across the Pacific, according to AccuWeather meteorologists.
The setup could produce an atmospheric river, a long, narrow corridor of concentrated water vapor that transports moisture from tropical regions toward the West Coast. If the corridor stretches from near Hawaii to the western United States, it could develop into what is commonly known as a Pineapple Express.
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Newsweek contacted the National Oceanic and Atmospheric Administration (NOAA) and AccuWeather via email on Friday for comment.
AccuWeather
What Is a Pineapple Express?
A Pineapple Express is a type of atmospheric river that carries warm, moisture-rich air from the tropical Pacific, often from the region around Hawaii, toward the West Coast of North America.
Atmospheric rivers are sometimes described as rivers in the sky, according to NOAA, because they transport large amounts of water vapor through relatively narrow bands in the atmosphere. When this moisture reaches land and encounters weather systems or mountainous terrain, it can produce prolonged periods of heavy rain and substantial snowfall at higher elevations.
Although these systems can provide valuable rainfall to drought-prone regions, they can also create dangerous conditions when large amounts of precipitation fall over a short period.
Heavy rainfall can overwhelm drainage systems, cause rivers and streams to rise rapidly and trigger flash flooding. Mountain areas can experience heavy snowfall, while steep slopes and locations recently affected by wildfires may be particularly vulnerable to mudslides and debris flows.
AccuWeather Map Shows Where Heavy Rain Could Hit
The map from AccuWeather shows the potential development and movement of the weather system, highlighting areas that could experience significant precipitation over Halloween weekend and into early next week.
Two storm systems are expected to play a role in drawing tropical moisture toward the western United States. One is moving in from Alaska, while another is developing off the coast of Washington state. Tropical Rainstorm Olana, located near Hawaii, could also contribute to the moisture plume.
AccuWeather meteorologists say the more likely scenario would see the storms and their associated moisture remain somewhat separated as they move along the Pacific coast.
Under that outcome, rainfall across major Southern California metropolitan areas, including Los Angeles, San Diego and Palm Springs, would be limited, with most receiving less than an inch. However, some south- and west-facing mountain slopes could receive up to 3 inches.
A less likely scenario would see the storms and tropical moisture combine earlier, potentially creating a larger and slower-moving system near the coast. That could bring heavier and more persistent rainfall to Southern California, increasing the possibility of widespread flash flooding.
Southwest Faces Renewed Flooding Risk
Parts of the Southwest could face a more significant rainfall threat, with Arizona, Utah, New Mexico and Colorado among the states at risk of renewed flash flooding, debris flows and substantial river rises from late this weekend into early next week.
Rainfall totals of 1 to 2 inches are expected across much of the region, while some upslope areas of central Arizona, southwestern Colorado and Utah could receive between 2 and 4 inches. AccuWeather's Local StormMax forecast is 7 inches.
The additional rainfall is particularly concerning because some areas have already experienced significant precipitation in recent days.
Parts of the San Juan Mountains in northern New Mexico and southern Colorado received between 5 and 10 inches of rain over the previous week, contributing to flooding and record streamflows at some monitoring stations.
Further downpours could cause renewed flooding, while cities including Phoenix and Flagstaff in Arizona, Durango in Colorado and Farmington in New Mexico could experience flooding downpours.
The wider storm system is also expected to bring showers to the Pacific Northwest and snow to higher elevations of the Rocky Mountains, including parts of Montana, Idaho, Wyoming, Colorado and Utah.
Forecasters will continue to monitor the developing weather pattern as the timing, track and potential rainfall amounts become clearer.
Contact Newsweek editors on this story: Matthew Robinson and Emma Lee-Sang
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