Monday, July 4, 2011

Monsoon season is on the way



Published: Thursday, June 30, 2011 1:19 AM MDT

BULLHEAD CITY — Monsoon season in Arizona officially started on June 15, but more than two weeks later, residents in Mohave County still haven’t seen a drop of the famous (and in some cases, infamous) monsoon rains.

That could be set to change soon, however. According to the National Weather Service, the Tri-state will begin to see the first vestiges of monsoon season as early as next week, though it could still take another two weeks before enough moist air from Mexico arrives to begin forming any major storm systems here.

“I wouldn’t call it like an onset, more like a ‘slosh’ of monsoon,” said Faith Borden, a meteorologist with the NWS in Las Vegas. “The high is not exactly where we want it, but we are starting to see some moisture coming in from Mexico.”

According to the NWS, the monsoon occurs when intense heat from the sun causes the local air pressure to drop, which in turn causes the winds to shift direction from the west to the south and southeast, beginning to bring moist air from the Gulf of Mexico into the area. The sudden influx of moist air, combined with the heat, causes it to become unstable, eventually forming scattered thunderstorms. As the rains fall, the air becomes even more moist, sparking additional thunderstorms that typically last through the end of September, when the air temperature finally cools off to the point that the winds resume their regular flow.

While meteorologists had speculated about a monsoon “season” in North America for decades, it wasn’t until 2008 that the NWS finally pinned down the June 15 to Sept. 30 calendar it now uses to track the monsoon. Before that, Borden said, monsoon season was determined whenever the average dewpoint rose to 54 degrees for three days straight.

Borden did acknowledge that this year’s monsoon is coming a little later than usual, adding that the real rains might not begin until mid-July. Typically, she said, the Tri-state region gets between three and six inches of rain during the monsoon, though as much as half of that can come from a single powerful storm system.

“It is very fickle, even in a good year,” Borden said. “Typically we look for mid-July in our area, maybe early-July for the Mohave area. However, this year we’re looking for it a couple weeks later — what we’re seeing this next week is what we normally see around the beginning of June.”

All the same, she said, the NWS is calling for scattered showers and thunderstorms as early as Independence Day, meaning the monsoon may not be far behind.

“We’re looking for hot temperatures this weekend, and then next week some gradual shifting of moisture and for the Mohave area, probably some isolated-to-scattered afternoon and evening thunderstorms,” she said.

While the monsoon season gets its name from the torrential rains that soak the Indian subcontinent every year, Borden said that name can be a bit misleading for residents of the high desert.

“Just because we’re in a monsoon pattern doesn’t mean it’s going to rain every day,” she said. “Really, if you get one area that has a pretty good rain event one day, the atmosphere is what we call ‘worked over,’ so you’re not likely to see the rain the following day.”