From Climate Central's Andrew Freedman: The Boulder, Colo. area is reeling after being inundated by record rainfall, with more than half a year’s worth of rain falling over the past three days. During those three days, 24-hour rainfall totals of between 8 and 10 inches across much of the Boulder area were enough to qualify this storm as a 1 in 1,000 year event, meaning that it has a 0.1 percent chance of occurring in a given year.
At least four people have been confirmed dead so far, and thousands have been evacuated from their homes and businesses. All along Colorado’s Front Range from Denver northward to Boulder and in nearby areas, small creeks have been transformed into raging rivers, and surges of water, mud, and debris have blasted their way through canyons, at one point trapping a firefighter on a treetop before being rescued. Numerous longstanding records have been smashed, including the all-time 24-hour rainfall record in Boulder.
“This is clearly going to be a historic event," National Weather Service Director Louis Uccellini said in an interview. “The true magnitude is really just becoming obvious now.”
Uccellini said the Weather Service has initiated a review of its performance leading up to and during the event. Although the potential for heavy rainfall was in the agency's forecasts a week in advance, he said, “Clearly the magnitude of the rainfall and the repetivieness of it in some critical areas was not pinpointed” well ahead of time. Uccellini said that this event will be the new historical high water mark for many affected rivers and streams. In a technical discussion on Thursday, the NWS described the rainfall amounts as "biblical."
On average, Boulder gets about 1.7 inches of rain during September, based on the 1981-2010 average. So far this month, Boulder has received 12.3 inches of rain. This smashes the record for the wettest month ever in Boulder, which was set in May 1995 when 9.59 inches of precipitation fell — and September isn’t even half over! Not only that, but the average yearly rainfall in Boulder is 20.68 inches. This means that Boulder picked up well over half its annual precipitation in just a couple of days.
This comes on the heels of a summer when Boulder experienced a moderate drought according to the U.S. Drought Monitor. This summer also featured the Colorado's most destructive wildfire on record.
During the past couple of weeks, the weather across the West has featured both an active Southwest Monsoon and a broad area of low pressure at upper levels of the atmosphere, which has been pinned by other weather systems and prevented from moving out of the region. It was this persistent low pressure area that helped pull the moisture out of the tropics and into Colorado. Signs point to the tropical Pacific being the source of the abundant moisture according to the University of Wisconsin’s Cooperative Institute for Meteorological Satellite Studies. From there, the moisture plume was transported northeastward, over Mexico and into Texas, and then northward by upper level winds.
This tropical air mass, which is more typical of the Gulf Coast than the Rocky Mountains, has been forced to move slowly up and over the Front Range by light southeasterly winds. This lifting process, known as orographic lift, allowed the atmosphere to wring out this unusually bountiful stream of moist air, dumping torrents of rain on the Boulder area for days on end. There was also broader-scale lifting of the air that resulted in heavy rain in areas of eastern Colorado and western Kansas as well.
According to meteorologist Jeff Masters of Weather Underground, the amount of moisture in the atmosphere, or "precipitable water," as measured by a weather balloon over Denver on Sep. 12, reached record values for the month of September..........