Flood Watch Covers All of Northern and Central Arizona Through Thursday Evening

The National Weather Service in Flagstaff issued a Flood Watch at 12:37 a.m. MST on Tuesday for all of northern and central Arizona. The watch runs until 11:00 p.m. MST on Thursday, October 22, and it warns that flash flooding caused by excessive rainfall is possible across a wide stretch of the state, from the Kaibab Plateau and Grand Canyon Country south through the Mogollon Rim and the White Mountains.

The list of areas under the watch is long. It includes Marble and Glen Canyons, the Coconino Plateau, the Yavapai County mountains and valleys, the Little Colorado River valley in Coconino, Navajo and Apache counties, the Chinle Valley, the Chuska Mountains and Defiance Plateau, Oak Creek and Sycamore Canyons, the Black Mesa area, and northern Gila County. In plain terms, the watch covers the high country, the canyon country and the desert washes in between, all at the same time.

Why the risk is rising

The weather service says an upper level trough will bring a deep surge of subtropical and tropical moisture into the region. Showers and thunderstorms are expected to develop across much of northern and central Arizona early Wednesday morning and continue through Thursday evening. The stronger thunderstorms will bring a threat of flash flooding, and the weather service says more prolonged rainfall could also push up the flow in streams, rivers and washes.

The forecast for Sedona, which sits on Oak Creek inside the watch area, shows the pattern clearly. The Weather Channel calls for thunderstorms from Tuesday night, with a 90 percent chance of rain on both Wednesday and Thursday. Oak Creek is one of the places the weather service names in the watch, and canyon creeks like it can rise quickly when heavy rain falls upstream.

Where the water can rise fastest

Flash flooding will be possible in creeks, in normally dry washes, at swimming holes and over recently burned areas, the weather service says. Low water crossings may be flooded. Burn scar ground is a special concern in Arizona because soil that has been through a wildfire does not soak up rain the way healthy ground does, so water runs off fast and carries mud and debris with it.

A watch is not a warning. It means the conditions for flash flooding are coming together and flooding is possible, not that it is already happening. The weather service urges people in the watch area to have a way to receive Flash Flood Warnings and to be ready to act if warnings are issued. Dry washes that look safe in the morning can fill in minutes during a heavy storm cell, and a flooded low water crossing can stop a car or sweep it off the road.

What happens next

Rain is expected to build early Wednesday and last through Thursday evening, and the watch expires at 11:00 p.m. MST on Thursday. If storms train over the same ground or a burn scar takes heavy rain, the weather service can upgrade parts of the watch to Flash Flood Warnings on short notice. Anyone driving canyon roads, hiking near creeks or camping near washes in northern and central Arizona this week should check the forecast before setting out and follow updates from the National Weather Service in Flagstaff.