Monsoon Storms Could Bring Heavy Rain, Flash Flooding To Tucson Area

Tucson residents should prepare for several more days of monsoon storms that could bring sudden flooding, dangerous winds and sharply reduced visibility across the region. The National Weather Service in Tucson said storms will remain likely through Thursday, with ordinary cells producing roughly a half-inch to 1 inch of rain and stronger storms capable of dropping 1 to 3 inches in a short time. That amount of water can overwhelm streets, washes and low lying areas, especially where storms repeatedly track over the same spot.

The immediate concern is not simply the total rainfall, but how quickly it could arrive. The Weather Prediction Center expanded its excessive rainfall outlook across much of Southeast Arizona, including the area around Tucson and north toward Pinal County. Another forecast placed Southeast Arizona in a moderate flood risk category, meaning storms that stall could produce particularly intense rainfall. Rates could reach 2 to 3 inches per hour, allowing normally passable roads and washes to flood with little warning.

Forecast confidence was limited for Wednesday because morning clouds and leftover showers could interfere with the usual afternoon heating that helps monsoon storms build. That could delay the strongest activity until the evening, rather than eliminating the threat. Additional showers and thunderstorms were also possible overnight, with especially favorable conditions in Pinal, Graham and Greenlee counties. Tucson remained under a storm chance through Thursday, while daytime temperatures were expected to stay in the low 90s.

The risk picture has changed as the storm pattern developed. One early forecast said the National Weather Service had not issued a Flash Flood Watch for the Tucson area because forecasters were still unsure where and when the most intense storms would form. Another regional forecast reported that a watch covered much of south central Arizona, including Pima County, Tucson and surrounding counties. The differing updates reflect a fast changing setup in which alerts can expand or be adjusted as storms become more organized and their paths clearer.

People traveling around Tucson should also account for hazards beyond standing water. Thunderstorms may produce frequent lightning, erratic wind gusts of about 40 to 50 mph and visibility of 1 mile or less in the heaviest rain. Strong outflows can push dust across roadways, while rain can make it difficult to see flooded dips and crossings. The weather service advises drivers never to enter a flooded roadway and urges everyone to stay out of washes during storms. Residents should check current alerts and conditions before leaving, particularly when dark clouds begin building nearby.

Recent storms have already shown how quickly conditions can deteriorate in Southern Arizona. Heavy rain and lightning affected central and southern parts of the state, and flash flooding was reported on roads near Tombstone. Water also covered part of State Route 86 near Sells. The broader monsoon pattern has generated dust storms, sometimes called haboobs, near the Phoenix area, adding another transportation hazard for people driving between communities or through open desert.

The activity is tied to a deep supply of moisture over the region and a larger weather pattern involving a cutoff low pressure system moving out of West Texas. As that system approaches New Mexico, smaller disturbances are expected to encourage afternoon storm development. A dip in the jet stream across the West and Great Basin will then help shift the monsoon northward later in the week, toward northern Arizona, the Grand Canyon, New Mexico and eventually parts of Colorado, Wyoming and Nebraska.

That northward shift could bring some relief to drought stricken areas while increasing flood concerns elsewhere. Colorado, including the Denver area and the Front Range, has areas of extreme and exceptional drought after a very dry winter. Denver and Salt Lake City were reported to be more than 4 inches below average in precipitation for the year. Forecasters cautioned, however, that beneficial rainfall and dangerous flooding can occur in the same storm pattern, depending on where rain falls and how quickly it accumulates.

For Tucson, the pattern is expected to become drier Friday as air moves in from the southwest, although isolated storms could still form, mainly south and east of the city. Lower moisture and instability may limit weekend storms mostly to higher terrain. The cloud cover and rain should also briefly ease the heat, with Friday temperatures expected to remain well below 100 degrees and weekend readings a few degrees below average. Even with the improvement, conditions may remain humid at times, and residents should continue monitoring forecasts until the heavier storm threat has passed.