Map Shows Where Pineapple Express Could Threaten Halloween Weekend

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. Read More on News Newsweek contacted the National Oceanic and Atmospheric Administration ( NOAA ) 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. Heather Zehr, a senior meteorologist at AccuWeather, described the system to Newsweek as a "classic Pineapple Express," explaining that "the moisture [is] being directed from near Hawaii toward the west coast of North America."

However, the system is expected to follow an unusual path. "This system is expected to take an unusual track, with the main plume of moisture likely to be directed into Mexico's Baja California first, then turning north into Arizona and eventually Utah," Zehr said. "Arizona is on track to get the heaviest impacts from this storm with the worst of the rain coming from Saturday night into Sunday," Zehr added. "Rainfall totals of at least an inch are expected across much of the state, and 2-4 inches is likely over most of the higher terrain.

There can be locally higher totals up to an AccuWeather StormMax of 7 inches. Those areas are at greatest risk for flash flooding, but flooding downpours are possible across the state." Utah is also expected to experience significant rainfall, with Zehr forecasting that "many areas in Utah will also get 1-2 inches of rainfall with localized flooding." Southern California is expected to receive some rain, although the distribution is likely to vary considerably.

"There will be a sharp cutoff between areas experiencing heavier precipitation and those receiving much lighter amounts," Zehr said. San Diego could see up to an inch of rain, primarily on Saturday night, before showers begin to diminish on Sunday morning. Downtown Los Angeles is likely to avoid the heaviest rainfall, although some locations across the Los Angeles Basin could receive up to half an inch. Zehr also highlighted potential driving hazards, saying: "Even with lower rainfall totals in some areas, wet roadways will increase the risk of hydroplaning."

Under the more likely scenario, 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. 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.

Newsweek’s reporters and editors used Martyn, our AI assistant, to produce this story. Learn more about Martyn here . Contact Newsweek editors on this story: Matthew Robinson and Emma Lee-Sang .