Fox News Weather Center

Southwest US Monsoon Unfolding with Rainfall, Lightning

A building weather pattern in the West will bring needed rainfall and even flooding to some areas, but also the risk of new wildfires.

The developing pattern over the western United States favors the expanse of an annual phenomenon known to locals as "The Monsoon."

The monsoon favors thunderstorms that bring localized torrential rainfall and also those without much rain, but deliver frequent lightning strikes.

Increasing humidity levels have and will continue to result in an uptick in the number and expanse of thunderstorms.

While the storms bring life-giving rainfall to normally arid areas of the region, they also bring the risk of flash flooding and the ignition of new wildfires.

Monsoon Outlook

According to Paul Pastelok, head of the long-range forecast team, "Initially the thunderstorms will focus over the Four Corners states but will expand outward to the West, this week into next week."

At the local level, it is impossible to say which areas will be hit by rainfall sufficient enough to cause flooding and which places could be facing dry lightning and new wildfires.

"The area from northern California to eastern Oregon, eastern Washington and much of Idaho will remain relatively free of storms over the next couple of weeks," Pastelok said.

A disturbance is forecast to drift westward this weekend (July 13-14) into the following week.

"The disturbance is likely to enhance the thunderstorm activity within the monsoon pattern from West Texas to New Mexico, Arizona, southern Nevada and part of Southern California," Pastelok added.

Similar setups in the past have resulted in torrential rainfall and flooding.

However, even before that disturbance arrives, some downpours will reach these areas.

According to Western Weather Expert Ken Clark, "We expect thunderstorms to reach Phoenix and Las Vegas this week, as well as some of the desert areas in California."

While the storms bring a risk of starting new fires, the high humidity works as an aid to firefighters. The higher moisture makes the brush more moist. The more moist brush make it a bit more difficult for existing fires to spread rapidly and lowers the chance of a lightning strike starting a new fire.

Sporadic downpours may also serve as a natural firebreak.

Isolated torrential rainfall caused by the storms can also be very dangerous. A storm miles away can lead to a flash flood hours later downstream. Dry stream beds in the region, called Arroyos, can rapidly fill with water, sweeping away unsuspecting motorists and children playing in the vicinity.

Some of the storms that bring little or no rainfall can also kick up a considerable amount of dust, which can pose another sudden problem for travelers in the region.

For much of the Southwest, the monsoon-driven thunderstorms represent the bulk of the moisture received on an annual basis. Few winter storms release as much moisture as a single, drenching thunderstorm. Much of that winter moisture falls in the form of snow over the mountains.

Wildfire Outlook

According to the National Interagency Fire Center, during 2012 there were 67,774 wildland fires across the nation, which burned 9.3 million acres. As of July 8, for the year so far, there have been nearly 25,000 wildland fires, which have burned 1.9 million acres.

"The prime fire season for the West in general spans August to October," Clark stated. "During September and October, Santa Ana winds and hot weather are a particular problem in Southern California."

By the late summer and early autumn, vegetation has had all summer to dry out.

Last year, drought over the Plains, Midwest and in parts of the East and South during the winter, spring and early summer contributed to a high number of fires early on. This year so far much of the same area has been abnormally wet, which has contributed to a lower number of fires nationally.

While conditions are likely to stay wet in much of the Eastern and Central states into the summer, the national number of wildland fires, driven by conditions in the West will tend to catch up to last year's levels.