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Think It's Wet Now? July Starts Colorado's Monsoon Season

Jim Hill
Rainy windows and low visibility were a common sight across Colorado during an record wet spring. More could be on the way as monsoon season starts in the state.

July 10 is an important date for Colorado. You're forgiven if you don't know why, most people don't. It's the beginning of the Colorado monsoon season – and 2015 is no different.

"That's based on the last 35 years or so of record taking, and you try to spot the first day of the heaviest rain, and July 10th is when that tends to happen most often," said National Weather Service meteorologist Todd Dankers.

The monsoon - a hallmark of late summer weather in Colorado - will be wetter than usual in 2015, Dankers thinks. The storms resulting from tropical moisture from the Gulf of Mexico and the Pacific coast that set up almost daily in July and August could be more severe, thanks in part to a record wet spring.

"With all the moisture that's in place, over Colorado already, it could be a wetter than normal monsoon because the pattern will be right and then the Gulf moisture will be moving in, as well as moisture coming out of our soils through transpiration from plants and just evaporation from the rain that's been falling over the last couple months," Dankers said.

"The more moisture the more potential energy of the storms."

More wet weather could mean more of the hyper localized storms that plunged areas of Denver into torrents of rain and hail in June. According to Dankers, Denver International Airport reported 2.53 inches of rain, about 0.55 inches above normal. Closer to the city, the Stapleton location reported around 7.5 inches of precipitation in June alone, and that's before the monsoon season started.

"We do have the thunderstorm season ahead of us, we'll have to look out for more of those localized storms," he said. "The ironic thing about Colorado is that weather systems can come from any direction, some of our heaviest thunderstorms come from a northwesterly pattern."

Denver's historic July 1990 hail storm, which happened 25 years ago this month (July 11) tallied over $1 billion in damages and was caused by a storm from a northwesterly flow, just like the isolated storms in the Denver area in June 2015.

The wet weather will continue for Colorado, but this time from a different direction. The record precipitation and the Gulf of Mexico moisture will feed the monsoon – and increase the possibility of tornadoes.

"Moisture is a fuel that makes the air mass unstable. The more moisture the more potential energy of the storms," Dansker said. "It's a delicate balancing match. If you add more moisture you may not get enough temperature. And then if you take away the moisture you'll get the temperature but you won't have the moisture to fuel the thunderstorms. And then from time to time, it will be just right and you'll wind up with strong thunderstorms, large hail and possible tornadoes."

It's not uncommon for Colorado's monsoon season to last through Labor Day weekend.

"Of course we've seen it hold strong through the middle of September, like in 2013," Dansker added.

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