DENVER - A stalled frontal boundary provided the energy, or lift, needed to create the rain showers that flooded 17 counties along the Front Range.
The monsoon provided the humidity to turn into the intense rainfall. Because the monsoon wanted to rush north, and the frontal boundary wanted to push south, they stopped each other right on top of the Front Range, leading to the "stalled frontal boundary."
Without either weather feature willing to move, the situation allowed for stationary and slow moving individual storms with enough energy and humidity to drop a lot of rain.
Had that cold front been stronger, it would likely have moved farther south before stalling. The opposite is true if the cold front had been weaker -- the stall probably would have been farther north.
In any case, the likelihood for historic rain would have hit someone.
Our terrain contributed too; Boulder County probably received more rainfall than any other county, over 20”, due in part to the up-slope nature of the storms that got stuck there. Up-slope adds lift, or energy, to the storms by forcing the humid air up a mountain side results in very heavy rainfall (just like our up-slope snows). In the case of Boulder County, the Flat Irons and mountains were the perfect, in a negative sense, up-slope addition. The combination of monsoon, stalled frontal boundary, and up-slope "trapped the heaviest rain into a corner" that it couldn't get out of.
That "corner" just happened to be the headwaters for so many rivers and creeks that all conjoin near Greeley, then flow as one river -- Platte River -- through northeastern Colorado. The floodwater is receding now, thankfully, but with trillions of gallons trying to fit down one river it will take a long time to get it out of here; perhaps weeks before all rivers are back to their normal.
-- How do we get rid of so much water? --
Sadly one river, the Platte, will have to handle all of that water that has fallen across the Front Range.
As much as 21-inches of rain fell near Boulder. The top 50 rainfall totals of the week (all more than 10”) coming in from Larimer, Boulder, and Denver Counties.
Sections of Aurora with a foot or more total rain contributed to the flooding of the Platte River.
Boulder County had the most rainfall of any other county and sadly most was all fed into the Boulder Creek, ultimately into the Platte River.
A few weather stations had nearly a foot of rain just west of Fort Collins contributing to the Big Thompson River flowing toward Greeley and the confluence with the Platte River.
Sadly, all drainages that received the rainfall ultimately combine just east of Greeley. Despite only 2-4” of actual rainfall in Greeley, the devastating flood is hitting Weld County the hardest. Trillions of gallons of rainwater falling elsewhere (all along the foothills and Denver metro) now flow through Weld County and then ultimately through Morgan, Logan, and Sedgwick Counties.
Unlike a tropical system, flood waters will affect folks hundreds of miles away from where the storm system hit.
Thanks to our elevation though, our water will flow quickly back to the ocean. Already, a lot of the flood water has begun to recede along and west of I-25.