COLORADO — A really anomalous surge of the monsoon is set to move out of Arizona and New Mexico and into Colorado overnight and into the day on Thursday. Precipitable water values are forecast to run near 300% of average compared to typical mid-September climatology across portions of the mountains, specifically the San Juans. This kind of moisture surge means high humidity and a good soaking rain for many residents, though the risk of flash flooding in burn scar areas is notably elevated.
San Juan Mountains See the Most Extreme Moisture Anomaly
The most extreme moisture values are expected across the San Juan Mountains, where precipitable water readings could reach approximately 300% of average for this time of year. This represents an exceptionally high moisture surge for mid-September, a period when monsoon activity typically begins tapering off across the Southwest.
Radar imagery already shows extensive rain and storm coverage stretching across Arizona, southern Utah, and into New Mexico, illustrating the scale of the moisture plume currently working its way toward Colorado ahead of the overnight and daytime push.
Front Range and I-25 Corridor Could See 200 Percent of Average
Farther east, across the Front Range, the I-25 corridor, and the eastern plains, moisture values could still reach as much as 200% of average, a significant surge even compared to the more extreme values expected across the higher terrain to the west.
This means communities from Denver to Pueblo and along the broader I-25 corridor should expect meaningfully higher humidity and rainfall potential than typical for this point in September, even if their totals ultimately fall short of what the mountains receive.
Flash Flooding Risk Elevated Across Burn Scar Areas
With such an anomalous surge of moisture moving through, the risk of flash flooding becomes notably elevated, particularly across areas affected by recent burn scars. These areas lose much of their natural ability to absorb rainfall, making them especially vulnerable during heavy rain events like this one…