Is Kingman in for a wet September?

A rainbow arcs over the wash passing under Bank Street in July 2013.

A rainbow arcs over the wash passing under Bank Street in July 2013.

KINGMAN - Entering the last month of Arizona's monsoon season, it's highly unlikely that Kingman will top the 5.7 inches of rain recorded in 2014, the highest precipitation total in 15 years.

Kingman received 3.49 inches through the first three months of this season, including 1.08 inches in August, state climatologist Nancy Selover of Arizona State University reported Tuesday. That's nearly an inch above the average of 2.58 inches, putting us on track for a wetter-than-normal monsoon season, she said.

Kingman picked up two-thirds of an inch on Aug. 7, one-fourth of an inch on Aug. 13 and nearly one-tenth on Aug. 25, as measured at Kingman Airport.

"It's been near average," Selover said from her Tempe office. "Up in the northern counties, they're ahead in Flagstaff and the mountains and even in southeast Arizona. The whole southwest desert in the lower elevations has been really dry, from Lake Havasu City through Phoenix and down west of Tucson."

The western part of the state saw heavy rainfall early in the monsoon season, washing out a bridge on Interstate 10 in California, but that was the biggest drop the area has received this year, Selover said.

"I expected more of those hurricanes and tropical storms to move north this year, like last year, but so far only one has done so, and that was in late June," she said. "The wetness of this year's monsoon is largely due to the very wet June. But hopefully we can add some more to it with a wet September."

Drier forecast

Selover said she expects Arizona to get more rain in September, which was a month of hammering thunderstorms a year ago. Mohave County received up to 6 inches of rain in some areas, washing out roads and sending public works crews scrambling for repairs.

"There's a tropical storm off the coast of Mexico that's supposed to come in here this weekend," Selover said. "It would be nice if it comes up the southwest desert. Yuma could sure use it and up the Colorado River to Parker and Lake Havasu City."

Don't count on it, said Daniel Berc, meteorologist with the National Oceanic and Atmospheric Administration in Las Vegas. The system is more of a "tropical depression," and won't be affecting this area, he said.

"It's pretty rare that a tropical storm comes up this way and gives us any significant impact," Berc said Wednesday from Las Vegas.

This year's monsoon season is more typical than the previous two years, which were really busy, he said. Mohave County saw quite a bit of monsoon moisture from Hurricane Blanca in June, but September is going to be a little drier, Berc believes.

"Right now, we're not looking at much," the meteorologist said. "Our models only go out so far. We only model 14 days and to be honest, I don't trust them that far out. We produce a seven-day forecast. Beyond that, it's, 'Which model do you trust?' "

If storms off Mexico bring tropical moisture like last year, September could be wetter than normal, ASU climatologist Selover said. The warm water that affected the monsoon season last year and somewhat this year is along the west coast of Mexico, not in the central Pacific Ocean.

Monsoon formation

The monsoon season, which runs from June 15 through Sept. 30, is critical to Arizona, bringing about half the state's annual rainfall total.

Monsoon rainfall can vary tremendously between distances of just a few miles, and mountainous areas tend to receive the most precipitation.

It's especially important for southern Arizona, as winter storms generally don't bring significant precipitation that far south, Selover said. Central Arizona is about evenly split between summer and winter precipitation, while northern Arizona gets roughly two-thirds of its rainfall in the winter.

Since the water off the California coast is a cold current, moving south along the coast from the Aleutian Islands, there is generally not much evaporation, so the western United States has little atmospheric moisture to work with for precipitation development, the ASU climatologist explained.

In the summer, high pressure moves away from the southern California coast and a high-pressure system sets up over the four-corners area of Arizona, Utah, Colorado and New Mexico. Air rotates clockwise around high pressure, so this pattern brings warm, moist air north from Mexico into Arizona.

That feeds thunderstorms across the state. When the sun retreats back toward the equator in September, the moist air also tends to retreat, unless tropical storms form. The monsoon circulation pattern breaks down, and is replaced with a westerly flow of relatively dry air from the Pacific Ocean.

El Nino is strengthening, which means the central Pacific sea surface temperatures are warmer than normal, and it appears to be getting even warmer, Selover said. The strongest El Nino was in 1997 when sea surface temperatures were almost 3 degrees Centigrade above normal.

"NOAA is saying this is the strongest one since 1997, but 1997 was not an especially wet year for most of Arizona," Selover noted. "However, it certainly has the potential to be a very wet winter. If we get cold fronts coming down from the arctic, we could enjoy a good snow pack in the higher elevations of eastern and northern Arizona. Of course, this assumes El Nino continues to strengthen, or at least remains as strong as it currently is."