Halloween Weekend Storms Could Bring More Flash Flooding to the Southwest

The first Pineapple Express of the season is headed toward the West Coast, threatening to rain out the Halloween weekend. Here is a look at what forecasters are saying could be a soggy stretch of days for much of the western U.S.

Pineapple Express Brewing for the West Coast

A pair of storms brewing in the Pacific will raise the risk of flooding rainfall for the West Coast this weekend, amplified by what is known in meteorological terms as the Pineapple Express. One storm is forecast to form off the coast of Alaska, while another system will set up off the Washington coastline. Both of these storms will draw up tropical moisture from near Hawaii and send it into the western U.S.

What is left of Tropical Rainstorm Olana in the Central Pacific will drive this plume of moisture traveling from near the Hawaiian Islands to the northeast. The atmospheric river is forecast to dump heavy rain to the lower elevations and snow to the higher terrains. Plumes of deep moisture that originate in the tropics and travel to the West Coast are called the Pineapple Express due to their origins near Hawaii.

The rain is poor timing for parts of the Southwest that have been dealing with immense moisture over the last several days. This includes much of Southern California. How much rain hits Southern California and the Desert Southwest will depend on how the two storms interact with the atmospheric river.  

The most likely forecast models show the storms and their associated moisture separating as they zip along the Pacific coastline. This scenario would limit the total amount of rain that falls across Los Angeles, San Diego, and Palm Springs. However, even small amounts of rain will cause travel disruptions at times. Areas that see more potent rain bands could once again be under the threat of localized flooding and debris flows. This threat will be the highest across recent burn scars.

This track would translate to less than an inch of rain for most of the major urban areas in Southern California. The south- and west-facing mountain slopes could pick up totals between 1 and 3 inches, supporting the development of flash flooding and mudslides.

The less likely scenario sees the two storms and the surge of tropical moisture merging sooner. This would result in a widespread storm that stalls out as it reaches the West Coast. Forecasters are warning that this development would mean heavier rain for Southern California beginning late in the day Saturday and persisting into Sunday, just in time for all of the Halloween festivities.  

Four Corners Under Threat of Renewed Flooding

While Southern California would dodge the worst of the impacts in the more likely scenario, it will be the opposite story for the interior West. This track would put the two storms and the tropical moisture in the same place over this region, leading to a renewed risk of flooding across the already saturated region.

The Four Corners region was the target of several days of flooding rainfall late last week. Several major rivers in Arizona, Utah, New Mexico, and Colorado are experiencing major water rises. The incoming moisture will exacerbate these impacts into early next week.

Several inches of rain could fall over a short time period across the interior West. Communities that experienced flooding over the weekend will be the most vulnerable to the next wave of moisture. Some parts of the San Juan Mountains between the border of New Mexico and Colorado notched 5 to 10 inches of rain last week.

Urban areas such as Phoenix and Flagstaff, Arizona; Durango, Colorado; and Farmington, New Mexico, could all be under the umbrella of flooding rainfall later this weekend and into the early part of next week. This dynamic weather maker is also forecast to bring rain showers to the Pacific Northwest. Lastly, the higher and middle terrains of the northern and central Rockies will be cold enough to see snow showers.

This system will continue to chug along to the east early next week, bringing up moisture from the Gulf and depositing it across the Plains and Mississippi Valley.

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