September floods in Boulder County, Colorado--monsoons

The monsoon setting over Colorado

According to the National Weather Service, as of 8:20 a.m. this morning, Boulder has received 9.64 inches of rain this month, breaking the old record of 5.50 inches set back in 1940. Most of this rain fell overnight. The heavy rain resulted from warm moist air flowing in from the south colliding with cool moist air coming from the north and east. The pattern is expected to continue through the end of the week.
           According to the National Weather Service, the "southwest monsoons" usually begin around the second week of July when an area of high pressure breaks away from the main Pacific ridge and settles in over the Great Basin, bringing hot temperatures during June and early July. This high center moves eastward across the Continental Divide and into the Central Plains (more or less what you see in the graphic above), followed by a slightly cooler but very moist environment brought in by southwesterly flow behind the high. The monsoon is typically over by the end of August, but can continue as late as October. To quote the NWS site "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."
         The monsoon moisture does not produce thunderstorms every day, but has a pattern of "bursts" and "breaks." A "burst" is a movement of a weak trough into the upper level westerly winds. It spreads upper level cold air into the region. Meanwhile, the lower levels of the atmosphere experience strong surface heating and transport of moisture from the southwest. These two conditions create a very unstable atmosphere, leading to widespread thunderstorm outbreaks.  A "break" occurs when an enhanced ridge of the Pacific subtropical high pressure moves inland and cuts off the moisture flow, thus stabilizing the atmosphere.
          The monsoons are welcomed because they offer a break from accumulating hot days in June, but they also result in deadly flash floods. As I write this (3:00 PDT) flooding is leading to evacuations in Denver, I-225, I-270, I-70 and I-25 all are experiencing flooding and traffic jams, CU Boulder is closed today and tomorrow, and 5 dams are overtopping or burst. A life-threatening flood emergency has been in effect in Boulder and northern Jefferson counties since last midnight. Three people have been killed due to flooding, and lengthy flash flood warnings and watches are in place.