Intense sunshine, warm temperatures near the earth’s surface and sluggish winds in the upper atmosphere can combine to produce very heavy rains and frequently lightning in Colorado from the middle of July through mid-August, according to the Colorado State University state climatologist.
The last 12 days of July and first 12 days of August historically have a distinctly higher probability of producing intense rains than other times of year – dropping three or four inches of rain in one or two hours, said Nolan Doesken, state climatologist who runs the Colorado Climate Center at Colorado State.
Some of the biggest storms and worst floods in the state during this “monsoon season” have occurred in northeastern Colorado, but other parts of Colorado are susceptible as well, Doesken said. Southwest Colorado often gets heavy local downpours during this period.
Notable storms that have occurred in this "window" in recent decades include:
• The Big Thompson Canyon flash flood of July 31, 1976
• The catastrophic Cheyenne, Wyo., flash flood of August 1, 1985
• The Fort Collins flash flood of July 28, 1997; and
• The Pawnee Creek flash flood near Sterling, CO of July 29, 1997.
“The sky will give us some clues,” Doesken said. “When dew point temperatures climb to 60 degrees or above, the chance for very heavy rains increases. This doesn’t happen too often in Colorado, but when it does, we can see and feel it. When clouds cling close to the ground and have dark, solid cloud bases, that is a sign of copious moisture.
“Many of our summer storms have very high cloud bases. These storms can still be severe, but will likely not produce very heavy or long-lasting rains,” Doesken said. “It is the solid, low-based thunderstorms to look out for. If you see very thick, well-defined curtains of rain coming from the bottom of these clouds straight down to the ground, these are likely the very heavy rains we are most concerned with. If these storms move slowly or continue to redevelop in the same place, these are more ingredients for flash flooding.”
Why this time of year is prone to intense rain:
1) Plentiful solar energy to initiate convection (warm air rising buoyantly). While the days are beginning to get shorter and the sun is a bit farther south, it is still not far from summer solstice when we get maximum solar energy.
2) The troposphere – the lower atmosphere from the surface up several miles into the air where thunderstorms form – reaches its warmest temperatures this time of year. The amount of water vapor the atmosphere can hold is a non-linear function of temperature. Warm air can hold much more water vapor than air at cooler temperatures. Abundant moist air is a prerequisite for very heavy rains.
3) The North American monsoon circulation most reliably pumps subtropical warm, moist air into Colorado from the south and southwest this time of year. The extent of this wind pattern (a monsoon is a wind pattern, not just rain) is greatest in late July and early August.
4) Upper-level winds a few miles above ground level, which move quickly most of the year, are sluggish in late July and August (i.e. the jet stream is weak and usually displaced north over Canada). Therefore, when storms form, they tend to move slowly, allowing heavy rains to fall in the same place for longer periods of time.
“Extremely heavy rains during this time period tend to be fairly localized and not widespread,” Doesken said. “While extreme flooding can occur, it tends to be short-lived and over fairly small watersheds. Large river flooding – such as the Colorado River, Rio Grande, Arkansas and South Platte – is unlikely, but smaller tributaries may have localized severe flash flooding.”
For updated information on heavy rain potential, go to http://www.weather.gov/.