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A powerful storm system threatens to bring heavy rain, flash flooding and heavy snow to different parts of the country during the Halloween weekend. The weather could make travel difficult and increase the risk of landslides in the most vulnerable areas.

According to Accuweather, a U.S. private weather company, the phenomenon is tied to an atmospheric river that carries large amounts of moisture from the Pacific Ocean into the United States.

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Which states will be affected by the heavy storms?

The states expected to see the heaviest rain are Arizona, Utah, New Mexico and Colorado, where intense rainfall could trigger flash floods, landslides and rising rivers.

The areas under threat are:

  • Arizona: Phoenix, Tucson and Flagstaff.
  • Utah: Cedar City and Moab.
  • Colorado: Durango and Grand Junction.
  • New Mexico: mainly parts of the northern part of the state.
  • California: Los Angeles and San Diego would see lighter rain, while the Sierra Nevada could get snow.

Across much of the Southwest, rainfall totals of 25 to 100 millimeters are expected, with localized maximums near 278 millimeters in mountain areas.

How long will the storms last and when will they be strongest?

The weather is expected to worsen on Saturday, Oct. 31, while the highest flood risk will run from Saturday night through Sunday, Nov. 1.

In California, the rain will be concentrated mainly from Saturday afternoon and evening into the early hours of Sunday.

Meanwhile, snow in the mountain areas could continue through Monday, Nov. 2, especially in the Wasatch and Rocky Mountains, where 15 to 30 centimeters of accumulation is expected at the highest elevations.

Vista posterior de una persona caminando por la calle bajo la lluvia, protegida por un paraguas transparente mientras de fondo se aprecian las luces difuminadas de la ciudad y el tráfico nocturno.

When will conditions start to improve?

The forecast indicates that the heaviest rain should begin to ease after Sunday, Nov. 1 across much of the Southwest. However, some rain and snow could persist through Monday, Nov. 2.

AccuWeather warns that river flooding and runoff could continue for much of the following week, even after the rain stops. Meanwhile, the system will move into the Great Plains and the Mississippi Valley between Monday, Nov. 2 and Tuesday, Nov. 3, with new storms that could affect eastern Texas, Louisiana, Arkansas and southern Missouri.