Tuesday, July 18, 2017

Flash flooding dangers to continue in western US following deadly flooding in Arizona

By Alex Sosnowski, AccuWeather senior meteorologist
July 18,2017, 8:31:40PM,EDT
 The swimming hole is located in the Tonto National Forest, which is in the mountains northeast of Phoenix.
Another person is feared dead as only four of the 14 people that were swept away were rescued.
The North American monsoon is triggered as moisture, in the form of high humidity, flows northward from Mexico and triggers thunderstorm activity over much of the western U.S. during the summer.
The combination of the isolated, intense storms and arid, rocky terrain is a recipe for disaster as unknowing outdoor enthusiasts venture out into dangerous territory.
Static AP East Verde River, Arizona
Muddy floodwaters of the East Verde River flow under a bridge on Sunday, July 16, 2017, in Payson, Ariz. (AP Photo/Ralph Freso)

During the monsoon season, storms erupt and bring torrential rain to a small area in the western U.S.
Just as similar downpours can bring flash flooding to urban areas, flash flooding can occur in desert locations as well.
Static SW Storms

When this deluge occurs upstream of a canyon, river or dry stream bed, a wall of water can sweep downhill in a matter of minutes, even where the sun may be shining brightly overhead.
In addition to the threat for isolated flash flooding, thunderstorms may also bring frequent lightning strikes and brief strong wind gusts.
It may seem strange to have the risk of flooding in conjunction with the potential for wildfires and dust storms in the same geographical area. However, this often occurs in the West during the summer, since many storms in the West bring little or no rain.
AccuWeather Severe Weather Center: Flash flood advisories
9 dead, 1 missing after flash flood hits popular swimming hole in Arizona
What is a monsoon?

The storms generate lightning, which can spark a wildfire, if the vegetation is dry. While as many as 90 percent of wildland fires in the U.S are caused by humans, much of the remaining causes are from lightning strikes, according to the National Park Service.
As of Monday morning, July 16, 2017, there were dozens of active wildfires in the western U.S., including 18 in Arizona, 16 in Nevada, 13 in California, nine in Colorado, nine in New Mexico and two in Utah, according to the Incident Information System.
Lightning also poses a major threat for those hiking in the mountains.
Similarly, if the ground is dry, gusty winds from the storms can kick up a great deal of dust.
The advancing wall of dust and wind, or haboob, can pose great danger to motorists on the highways in open areas. The visibility can drop to near zero in a matter of seconds.
Static US Late Week

The weather pattern over the interior western U.S. will remain conducive for additional incidents of flash floods and wildfires, while the risk of haboobs will generally continue in desert locations through the balance of the summer and into the early autumn.
However, some fluctuations in the weather pattern over the next few months may cause the risk to decrease or increase from time to time.
The bulk of the storms in the Southwest will focus east of California and Nevada. On occasion, a few storms will erupt over the Sierra Nevada and the Cascades and locations closer to the Pacific coast through the balance of July.

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