iSeeChange: Eyes of the Storm - Monsoon Forecasting

Jul 16, 2013

Norvan Larson, Forecaster at the National Weather Service Forecasting Office in Grand Junction
Credit Travis Bubenik, KVNF

If you've followed the weather for even the past few days, daily whether predications have been pretty, well, predictable: sunny in the morning, cloudy in the afternoon, a chance of rain as the day wears on and the sun starts to drop.

The Monsoon season has arrived in Colorado, the annual time when hot, high pressure in the atmosphere moves east across the Continental Divide and cool, moist air comes trailing in behind it. It's a reliable weather pattern, but exactly how reliable? 

Norvan Larson, a forecaster for the National Weather Service  (NWS) at its Grand Junction office, says the southwestern monsoon doesn't really follow a strict timeline, so predicting its behavior isn't exactly easy. 

"It's hard to look at the monsoon as being anything official," Larson says. "It's hard to nail down if you're thinking of the Indian Ocean monsoon; it's not as predictable and persistent. The bursts and breaks can be any amount of time, really. There are no rules for it." 

The "bursts and breaks" he refers to are the monsoon's cycles, the few days or couple weeks of daily rains, followed by brief dry spells. 

Each year around early July, winds in the southwestern United States pull moisture from the Pacific Ocean, or the Gulf of Mexico, up through the plains and into Western Colorado. But Larson says even that basic pattern can vary from year to year. Larson explains that this year's first monsoonal burst came from the west, an unusual "long way around" as he calls it. 

Nolan Doesken, the State Climatologist based at Colorado State University in Fort Collins, says predicting monsoon behavior is a tricky business. 

"Often the best forecast is simply, 'What is the expectation from climatology?' In other words, what have past years shown us?" explains Doesken. "It is easy to predict two to five days in advance, difficult to predict two to five weeks in advance." 

So if it's that hard to predict monsoon rains week to week, you can imagine the challenge climatologists face in studying how larger-scale monsoon cycles are affected by global climate change. And then when scientists do study those changes, the results can seem almost ironic. 

"It's been thought, theorized and slightly proven that widespread drought conditions over the western U.S. may actually accentuate the monsoon," says Doesken. He says hotter, drier conditions may actually help draw moisture into Western Colorado. Hotter temperatures trigger the wind patterns that bring on monsoon rains, so during times of drought, we may actually see more rain during the monsoon season. Thankfully, that can help balance out the drought's harsh effects on agriculture. 

"Even after a year with poor snow pack, below-average stream flow, below-average reservoir and stream water supplies, you can still come in with a good, damp monsoon season," Doesken says. "And without making up those deficits, you can still have a comfortable summer."

Despite the wide variety in each year's monsoon season, one thing does remain pretty stable: the threat of mudslides or floods. That's especially true in summers like this year's, with heavy wildfire activity in steep terrain. Larson says he's familiar with some areas that are prone to flooding, even without wildfires, but that when massive fires like the West Fork Complex do burn through these areas, mudslides and floods can be a constant threat for years after the fire is put out. 

Case in point for Larson was the Missionary Ridge Fire of 2002. He says during that fire, less than half an inch of rain an hour over the fire's burned-out area caused problems, mainly because there was no vegetation left to hold back flowing water. 

"The soil was charred so hard, it basically created almost like an asphalt covering. The rainfall just ran right off of it," says Larson, adding that the NWS monitors these areas closely after fires, since rain can behavior "profoundly" different over charred terrain. 

While the nature of monsoons can and usually does change from one year to the next, Doesken says there isn't much evidence yet that monsoon cycles are changing on a larger scale. 

"The monsoon has always been a bit erratic, it still is, and the main thing we've been able to see is a tendency towards warmer temperatures and higher summer evaporation rates," he says. "In terms of precipitation from the monsoon, we have not seen what I'd call a systematic trend, just a lot of year to year variation."