PHOENIX — A powerful surge of Pacific moisture is aimed at the Southwest as two storm systems threaten heavy rain and dangerous flash flooding heading into Halloween weekend. Arizona and nearby states could see widespread downpours. Atmospheric moisture could reach as high as 325 percent of normal across parts of southern Arizona. One storm arrives Friday, with a possibly stronger surge on Sunday.
Atmospheric River Aims at the Southwest
Satellite and moisture imagery shows a long, narrow plume of moisture stretching from the Pacific Ocean near Hawaii toward the Southwest. This kind of plume is known as an atmospheric river. It carries large amounts of water vapor that can fall as heavy rain when it reaches land.
The plume is shown sweeping across Southern California and into Arizona. The deepest moisture, in red and orange, sits over southwestern and central Arizona. Moisture also spreads north into Nevada, Utah, and Idaho.
Southern Arizona Sees the Most Moisture
Moisture levels could reach up to 325 percent of normal in parts of southern Arizona. That includes areas near Phoenix and Tucson. Air holding that much water can produce very heavy rainfall rates.
Heavy rain can quickly overwhelm desert ground and drainage systems. Normally dry washes can fill with fast-moving water in minutes. Urban areas with limited drainage could see street flooding.
Two Storms Bring Repeated Rounds of Rain
The first storm is expected to arrive on Friday. It could bring showers, thunderstorms, and locally heavy rain to Arizona and Southern California. Mountain snow is possible at the highest elevations…