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Summer rains could be boosted by evaporating snow
A National Weather Service graphic shows what a typical post-La Niña monsoon could look like.
By Bob Berwyn
SUMMIT COUNTY — You might want to get your outdoor yah-yahs out of the way early the next few days, as the National Weather Service is predicting an early onset of the monsoon season, with a good chance for afternoon thunderstorms through the rest of the week. Wednesday could be the wettest day, as a plume of subtropical moisture takes direct aim at the high country, but scattered daily rain can be expected right on through next weekend.
Specifically, the National Weather Service says there is a 40 to 50 percent chance of rain through Friday, with highs in the low 70s each day and overnight lows in the mid-40s, fairly close to seasonal averages. The record high for July 5 is 84 degrees, set in 2001. The record low, a chilly 24 degrees, set way back in 1931. Frisco’s all-time record high for July is 89 degrees, a reading that hasn’t been reached since 1939.
The U.S. southwestern monsoon season occurs when winter and spring’s jet stream-driven westerlies retreat to the north. Instead of being dominated by incoming cyclonic storms off the Pacific, the weather in the Southwest and the Rockies is influenced by the clockwise rotation of air around a big area of high pressure parked in the center of the country, often over Texas. The rotation draws moist air northward from the Gulf of Mexico, the Gulf of California and the eastern Pacific.
As the intense summer sun heats that moist air, it rises, building towering thunderclouds that condense and generate precipitation. The monsoon season generally begins in mid-July and lasts for two to three weeks.
How long and strong this year’s monsoon season will be in Summit County is not clear, said climatologist Klaus Wolter, with the University of Colorado’s Climate Diagnostic Center. Wolter recently released his latest three-month outlook, which includes a dry summer forecast for the eastern half of Colorado.
But Wolter said his forecasting skill for Summit County and the high country west of the Continental Divide is lower. In very general terms, summers following strong La Niña years are not always the best for monsoons, he explained. For example, the 1995 monsoon “died on the vine,” he said.
For the immediate future, Wolter said this early anemic monsoon could be reinforced by evaporating moisture from the remaining high country snowpack.
In the longer-term, Wolter is still predicting better-than-average odds that — after a summer vacation — La Niña will return for an encore performance late this year. But he doesn’t expect a repeat of the banner snow year in Colorado. Historically, strong La Niñas tend to linger for another year, but are not as powerful on the second round.
A 90-day outlook from the National Weather Service Denver also suggests that post-La-Niña monsoons tend to shift westward, favoring the Great Basin, including Nevada and the east slope of the Sierra Nevada, but once again, it’s never clear in advance how far west the shift will go and whether western Colorado will remain in the monsoon zone.
