As the summer progresses, a large area of High Pressure over the eastern Pacific Ocean tends to move eastward. This results in a shift in winds to a more southerly direction across the southwestern states, allowing increased moisture to stream northward from the Pacific Ocean and Gulf of California. (This moisture is especially enhanced by tropical storm activity in the eastern Pacific).
For western Colorado and eastern Utah, the Southwest Monsoon generally begins around the second week of July. An area of High pressure usually "breaks away" from the main Pacific ridge and settles in over the Great Basin by June, bringing hot temperatures during June and early July. As this High center moves eastward across the Continental Divide and into the Central Plains, a slightly cooler but much more moist environment will prevail in the southwest flow behind the High. In most years, the monsoon is over by the end of August...but can last as late as October. ( In fact, many areas in southwest Colorado and southeast Utah experience a secondary precipitation maximum in October due to late-season tropical storm moisture that's been carried northward by the monsoonal flow ).
One unofficial "rule-of-thumb" for determining the onset of the Monsoon Season in the American Southwest is to watch the dew point temperature in Phoenix, Arizona . When the average daily dew point there reaches, or exceeds, 55° F for 3 or more consecutive days, monsoonal conditions have arrived.
Once the Monsoon Season is underway, the southwesterly circulation does not produce thunderstorms everyday, but rather consists of a pattern that undergoes a series of "bursts" and "breaks": *
|Burst||A movement of a weak trough in the upper level westerly wind into the southwest U.S. which spreads upper level cold air into the region. In the lower levels of the atmosphere, strong surface heating and southerly winds will transport moisture into the region. This creates intense atmospheric destabilization and leads to widespread thunderstorm outbreaks.|
|Break||An enhanced ridging of the Pacific subtropical High Pressure moves inland, effectively cutting off the moisture flow and stabilizing the atmosphere.|
Recently-burned areas are also subject to flash flooding. High intensity wildfires often melt the resin out of pine needles and other vegetative growth, leaving a hardened, water-resistant layer on the ground. Fires will also consume much of the ground cover, leaving the area prone to overland water flow. Heavy rains in burned areas can often result in not only flash floods, but also mudslides and debris flows.