North American monsoon surge to bring flash flood, wildfire risks to western US
A surge of monsoon moisture will bring downpours and flash flooding to parts of the Southwest, while dry thunderstorms heighten the wildfire risk in the Northwest into the weekend.
Fire crews were called to make multiple rescues in Tucson, Arizona, as heavy rain prompted flooding on Sept. 14.
A significant surge of moisture from Mexico will unleash numerous showers and thunderstorms across the Rockies into this weekend. Meanwhile, a separate storm will trigger mainly dry thunderstorms in parts of the Northwest.
The North American monsoon, which brings southerly winds carrying humid air from Mexico, the Gulf and the tropical Pacific Ocean, has been active at times this summer. However, its rainfall has been sporadic, leaving large portions of the Colorado River Basin without significant precipitation.
That has changed and will continue to evolve over the course of the week, at least across part of the basin, as a plume of deep moisture streams northward from Mexico.
In less than six hours on Wednesday afternoon and evening, Mesa, Arizona, received 3.2 inches of rain, which is more than one-third of the city's average annual rainfall. Drenching rain reached much of Arizona, New Mexico, western Colorado and southeastern Utah on Wednesday.
Looking ahead into Friday, repeated downpours can quickly overwhelm normally dry washes, low-lying roads and other flood-prone locations. In the higher terrain, intense rainfall can also trigger debris flows, while gusty thunderstorm winds may produce blowing dust in drier areas. The debris flows may be most substantial in recent wildfire burn scar areas, but they are not limited to those locations.
The flooding threat will expand northward Thursday into Friday as showers and thunderstorms extend from Arizona and New Mexico through Colorado, Wyoming and Montana. The greatest flood risk will remain across portions of Colorado and New Mexico, with flash flooding remaining a concern and debris flows especially possible in burn scars.
Hikers and campers should avoid canyons, arroyos and small streams because storms miles upstream can send sudden surges of water downstream.
Away from the plume of moisture streaming out of Mexico, a relatively dry storm will continue moving across parts of Oregon, Northern California and northern Idaho on Thursday.
This storm will bring cold air higher in the atmosphere while temperatures near the ground remain warm. The resulting atmospheric instability will produce thunderstorms with little rain, gusty winds and small hail. Lightning from these mainly dry thunderstorms could ignite new wildfires.
The weather pattern is likely to bring a general 1-4 inches of rain over a broad area in eastern and northern Arizona, New Mexico, Colorado and southern and eastern Utah. Within this zone will be pockets where 4-8 inches of rain is likely with an AccuWeather Local StormMax™ rainfall of 12 inches.
Much of the region from the Rockies to the Pacific Coast could benefit from soaking rain without causing flooding. Drought conditions are especially severe across the Colorado River Basin, with vast areas experiencing severe to exceptional drought.
Low flows on the Colorado River have contributed to historically low water levels in Lake Powell and Lake Mead during mid-September. Both reservoirs support hydroelectric power generation at their respective dams. Lake Powell is now at only 21% of capacity, while Lake Mead is at 27%.
The rain into the first part of the weekend over the region should lead to runoff into the Colorado and Rio Grande river systems in the region. However, it could take several days for that surge to reach the main stems.
This map is a product of the United States Geological Survey (USGS)
A push of dry air from the Pacific is forecast to sweep from west to east over much of the region this weekend.
"The current influx of moisture from the North American monsoon may produce one of the last major surges of showers and thunderstorms this year," AccuWeather Senior Meteorologist Dave Houk said. "However, some additional pulses are likely in the coming weeks. The tropics remain a wild card, especially if a storm moves northward along Mexico’s west coast."
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