An atmospheric river called the “Pineapple Express” could bring heavy rain to parts of the desert Southwest and southern California during Halloween weekend, a little more than a week after destructive flooding hit parts of Arizona, Nevada, New Mexico and Utah.
This latest round of rain will be powered by a low-end pineapple express, a type of atmospheric river that often fuels heavy rain in the West, especially during a stronger El Niño winter.
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We’ll discuss this trend further after laying out important forecast details this weekend.
When and where will these soaks arrive?
There are two series of torrential rains:
- Friday: West Texas, parts of New Mexico and Arizona
- This weekend: In Southern California primarily, it will increase starting Saturday, peaking early Sunday. In the southwest, rain and some snow in high mountains will increase late Saturday, continue into Sunday, and persist in some areas into Monday.
What are the potential impacts?
Local flash flooding is possible with the first round of rain in parts of West Texas, including Lubbock and Midland.
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A more serious flash flood threat is at play in the desert southwest.
That’s because the ground is still saturated from the 2 to 5 inches of rain that soaked the region last week, triggering significant flash flooding in northern Arizona, New Mexico, southern Colorado and southern Utah.
This is also because some of these areas will experience both spells of rain in just three days.
The map below shows precipitation totals from both systems in the southwest. Areas shaded darker green and yellow may experience more dangerous and widespread flash flooding.
The areas of greatest concern are:
- The north and east sides of metro Phoenix and the Mogollon Rim of northern Arizona
- The Canyonlands of Southern Utah
- Elevations Below Snow Level in Western Colorado
- Any ground ravaged by recent wildfires, especially likely to produce fast-moving debris flows, rocks and mudslides, even with modest rainfall.
Rivers such as the Little Colorado and the Verde could remain high for days after this weekend’s latest deluge, according to the National Weather Service in Flagstaff, Arizona.
If you were planning to go on weekend hikes in parks in southern Utah, southern Colorado, and Arizona, you should consider canceling. Rapid, life-threatening flash floods can sweep through slot canyons quickly.
At higher elevations, this precipitation can fall as snow on the highest peaks of the Rockies and Sierra. This could make travel slippery primarily Sunday through Monday over mountain passes.
A humidity record for the Halloween weekend?
What’s fueling this dip is a southward dive of the jet stream off the West Coast.
This jet stream will carry a ribbon of deep tropical moisture called an atmospheric river toward the southwest.
This atmospheric river tap comes from near Hawaii, which meteorologists call “the pineapple express.”
This is the weather forecast for Halloween weekend, showing an atmospheric river heading toward California and the Southwest.
It’s not a strong version of an atmospheric river or pineapple express, but the humidity fueling these heavy rains could break a record for this time of year.
Total atmospheric humidity in parts of Arizona, Colorado, Nevada and Utah could be 2 to 3 times the late October average, near record levels for Halloween weekend, according to the National Weather Service.
This could be a warning sign, given the trend toward a wetter southwest during the strongest El Niño events, from fall to spring.
Jonathan Erdman is a senior meteorologist at Weather.com and has covered national and international weather since 1996. Extreme and bizarre weather are his favorite topics. Contact him on Blue sky, X (formerly Twitter) And Facebook.
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