Phoenix, Arizona — Another round of heavy rain could threaten parts of the Desert Southwest and Southern California over Halloween weekend as a weather system known as the “Pineapple Express” brings tropical moisture into the region. The developing pattern could trigger flash flooding in areas still recovering from destructive rainfall earlier this month.
The latest forecast calls for two rounds of precipitation, beginning Thursday and Friday across parts of western Texas, New Mexico and Arizona before spreading into Southern California and other parts of the Southwest over the weekend.
Forecasters are particularly concerned about areas where the ground remains saturated from recent storms, increasing the potential for rapid runoff and dangerous flooding.
When and Where Will the Heavy Rain Arrive?
The developing weather pattern is expected to bring two rounds of soaking precipitation across the Southwest.
The first round is forecast for Thursday and Friday, affecting western Texas and parts of New Mexico and Arizona. Localized flash flooding is possible in western Texas, including areas near Lubbock and Midland.
The second round is expected to intensify over the weekend.
In Southern California, rainfall could increase Saturday and peak early Sunday. Across the broader Southwest, rain and some high-elevation snow could develop late Saturday, continue through Sunday and linger in certain areas into Monday.
The timing and intensity of rainfall could vary by location, making updated local forecasts important for anyone planning outdoor activities or travel.
Why Is the Southwest Facing Another Flooding Threat?
One of the biggest concerns is the condition of the ground following recent storms.
Parts of Arizona, Nevada, New Mexico and Utah experienced destructive flooding after receiving approximately 2 to 5 inches of rain last week. Additional rainfall could overwhelm areas where the soil has not had enough time to absorb or drain the earlier precipitation.
When the ground is already saturated, even another period of moderate to heavy rain can produce substantial runoff. Water can move quickly into low-lying areas, washes, drainage channels and narrow canyons.
Some locations could also receive both rounds of precipitation within roughly three days, increasing the risk of repeated flooding.
The areas of greatest concern include the north and east sides of the Phoenix metropolitan area, the Four Corners region and the foothills and higher elevations of San Diego County.
Residents in these areas should watch for flash flood warnings and avoid driving through flooded roads or flowing water.
Why Is It Called the ‘Pineapple Express’?
The Pineapple Express is a type of atmospheric river that transports moisture from the tropical Pacific toward the western United States.
In this case, a southward dip in the jet stream off the West Coast is expected to help draw a concentrated band of tropical moisture into the Southwest. The moisture source near Hawaii is what gives the pattern its familiar nickname.
Atmospheric rivers can deliver substantial amounts of precipitation over relatively short periods. Their effects depend on the amount of moisture they carry, the speed at which they move and how the moisture interacts with local terrain.
The developing system is described as a relatively weak atmospheric river rather than an especially powerful version of the Pineapple Express. However, the amount of moisture available in the atmosphere could still be unusually high for late October.
According to the National Weather Service information cited in the forecast, atmospheric moisture over parts of Arizona, Colorado, Nevada and Utah could reach two to three times the average levels for the end of October, approaching record amounts for Halloween weekend.
That does not necessarily mean every location will experience record rainfall, but it highlights the potential for significant precipitation in some areas.
Hiking and Burn Scar Areas Face Additional Risks
People planning Halloween weekend trips to national parks, mountain areas and other outdoor destinations should pay particular attention to the forecast.
Slot canyons in southern Utah, southern Colorado and Arizona can become dangerous when flash floods develop upstream. Water can rush through narrow passages with little warning, leaving hikers with limited opportunities to escape.
Anyone facing a flash flood warning should move immediately to higher ground and stay out of canyons, washes and drainage channels.
Areas affected by recent wildfires also face an elevated risk. Burned ground may absorb water poorly, allowing rain to generate fast-moving runoff, mud and debris flows even when rainfall amounts are not especially large.
Outdoor plans should be reconsidered wherever heavy rain or flooding is forecast, particularly in remote locations where emergency assistance may take time to arrive.
Mountain Snow Could Affect Travel
Not all the precipitation is expected to fall as rain.
Winter Weather Returns Before Halloween as Snow Forecast Expands Across Western Mountains
At higher elevations, some moisture could fall as snow in the Sierra Nevada and other mountainous areas. The forecast suggests that slippery travel conditions could develop over mountain passes, mainly Sunday into Monday.
Drivers crossing higher terrain should watch for changing road conditions, reduced visibility and possible delays. Conditions may differ significantly between lower valleys and mountain routes.
Travelers should check current forecasts and road advisories before leaving, particularly if their routes cross elevated areas.
Could This Weather Pattern Signal a Wetter Winter?
The incoming system is also drawing attention because of the potential amount of tropical moisture moving into the region near the end of October.
Stronger El Niño patterns have often been associated with wetter conditions across parts of the Southwest during the fall-to-spring period. However, an individual storm does not establish how the entire winter will unfold.
The developing atmospheric river could provide an early indication of how moisture-rich weather patterns may affect the region, but longer-term conditions will depend on the broader climate pattern and the sequence of storms over the coming months.
For now, the immediate concern is the potential for another round of rain to affect communities that recently experienced flooding.
Are you planning to travel, hike or attend Halloween events in Arizona, Southern California or elsewhere in the Southwest this weekend? Share your local forecast and let us know whether the incoming rain could affect your plans.
