There has been a highly amplified upper-air pattern over the continental US over the past few days. Here's yesterday morning's 500mb analysis from the HOOT site:
The green colors in that analysis image show relative humidity at the 500mb level. There has been a steady stream of moisture being pulled up from the south around this low over the high plains and eastern Rockies. This moisture can also be monitored in real-time using satellite-derived water-vapor imagery, like this image from last night:
at their post here. Water vapor imagery often isn't very good for showing us the low-level moisture; what you're seeing above is mostly moisture aloft. Checking last night's sounding from Denver we can see that there was far more moisture in the lower levels of the atmosphere:
NWS Rapid City page:
Another thing you'll note on the sounding above is that near-surface winds are out of the southeast. This promotes upslope flow--easterly winds forced to rise when they meet the mountains. This constant rising motion in very moist air produces continuous condensation, clouds and rain. You can see how steady the rainfall was last night in looking at this timeseries of observations from the NCAR Foothills lab on the northeast side of Boulder.
The rain gauge reset itself to zero this morning (hence the big drop in precip down to zero again), but you can see that over just last evening over 6 in. of rain had fallen. In one evening. The rainfall was also quite steady over this entire period--no big spurts or jumps in the precipitation that you'd expect if this was due to strong convection. In fact, not much lightning was reported with this rain (though there was some). There have been several reports of particular locations in the foothills getting over 8" of rain so far. This also shows an interesting feature of this event. Remember our precipitable water above? It was only 1.3 inches. If that was the total precipitable water, how could we be getting 6-8 inches of rain? Remember that there is that stream of moisture constantly being brought up on the south--we saw it on the water vapor imagery. This keeps replenishing the water vapor in the atmosphere, bringing in more and more moisture as the current moisture rains out. Still--too get the much rain with these precipitable water values, these clouds have been very efficient at their rain production.
The rainfall overnight consisted of broad areas of stratiform precipitation in some locations, but also (particularly near Boulder) a series of more intense precipitation cells, probably somewhat convective, that repeatedly formed east of town and then moved west over the foothills. Here's an example radar image from last night:
stream gauge data for Boulder Creek at Broadway in downtown Boulder:
an infamous flood in 1976--which had distinctly different meteorological origin, being due to a parked convective storm as opposed to widespread, more stratiform rain).
Unfortunately it doesn't look like it's time to take a breath just yet. The rain died down a little overnight, but it's forecast to pick up again today. We're already seeing that throughout the region. Here's the latest High-Resolution Rapid Refresh model's forecast for total accumulated precipitation through tonight.