Areas that don't get heavy rainfall are going to underestimate it, but only once.
My family has a cabin near Rocky mountain NP and the Big Thompson River which experienced disaster-level flooding in 1976 and is still embedded in the local psyche so everybody familiar with the area has grown up hearing about it and knows what to do. The 1976 flood killed 143. They rebuilt, put in signs "in case of flood, climb to safety", and created a system to distribute warnings of impending flash floods. In 2013 another flood surpassed the 1976 high water mark and caused more property and infrastructure damage but "only" killed 9. My parents were at the cabin a few weeks ago and got an automated phone call at 1:30am issuing an evacuation. The cabin is hundreds of feet above the river so they stayed and it worked out.
Both the major floods were associated with nearby areas reporting totals within what is being predicted for this storm. In those areas, these are 100 and 500 year storms. In parts of the eastern half of the country it's a particularly wet Tuesday, but without the topography funneling it down canyons and magnifying the water depth.