Some forecast scenarios call for some monsoon moisture starting next week
By Bob Berwyn
SUMMIT COUNTY — An early monsoon season may bring some moisture relief to parched and fire-prone Colorado as soon as next week — but there’s also a chance that smoke from regional wildfires could inhibit the formation of thunderstorms during the summer rainy season.
“Next week may be the peak of fire danger, then there is a hint the monsoon may come early … it’s going to be very interesting to watch,” said Boulder-based NOAA scientist Klaus Wolter, a researcher with the Earth Systems Research Laboratory.
The Rocky Mountain monsoon season usually starts in mid-July and lasts for about a month, with moisture from the Gulf of Mexico and subtropical eastern Pacific streaming into the region from the south.
Since formation of the monsoonal flow is partially dependent on large-scale regional surface heating, a year with early snowmelt, — like this year — can sometimes bring an early start to the season.
And in the larger-scale weather pattern, a shift from La Niña to El Niño boosts the chances of monsoon rains, Wolter said.
“The possible onset of a La Niña can be good for us along the Divide and Eastern Plains, but my concern is, this year is very similar to 2002, when the efficiency of the monsoon season was reduced,” Wolter said, referring to what he called a “feeble” monsoon, despite generally favorable conditions. Similar conditions prevailed in 2000, when, after a seemingly promising setup, the monsoon fizzled.
Simply put, a regional blanket of haze may prevent the intense, localized heating needed to form the most productive thunderclouds, Wolter explained. Additionally, the haze may also become trapped in mid-atmospheric inversion, three miles high, that makes it harder for the thunderclouds to build vertically to their full potential.
“It reduces the sunshine that reaches the ground … that’s not a good thing if you’re trying to create thunderstorms,” he said, emphasizing that, at this point, the impact of haze on the monsoon is still a “back-of-the-envelope” concept.
Most existing climate models don’t account for those tiny aerosol particles from the fires (and other sources), though they have improved recently, according to Wolter. If you don’t calculate the impact of the aerosols, it can lead to over-forecasting he said.
Looking back at some of the data from 2002 suggests that the total water delivery from the path of thunderstorms that formed that year was greatly reduced from what was expected.
And the source of the haze doesn’t have to be from local wildfires in Colorado. Depending on regional winds, fires in other states could have similar impacts. On the other hand, if the season fires up early, some strong rains could rinse the atmosphere, he said.
One clue came in 2000, when Wolter’s wife, a field biologist, was using an instrument to measure the strength of the sunlight.
“She thought the instrument was broken. It was reading 20 percent less,” he said.
The impact of haze is the only reasonable explanation he’s seen for years like 2000 and 2002, when the monsoon failed after all the ingredients for good moisture were in place.