A local emergency quickly became national news today: record-setting rainfall caused overnight flash flooding at multiple locations in and around Boulder, CO today. Some homes were swept away and reports list three people killed so far. 6.8 inches of rain fell in 24 hours by early morning today after rain fell for more than two previous days along the Front Range. The previous rains saturated the ground, which set the stage for today’s life-threatening emergency.
From Monday through 11:30A today, eight to nine inches of rain fell over Boulder County. This shattered the previous September record of 4.8″ of rain set in 1919! These values are simply stunning in magnitude. The rain continued to fall through the day today. Additional rain is forecasted for tonight through Sunday, which means this disaster may not be over for days yet.
The flash floods and ongoing rainfall hampered rescue efforts since roads were washed out by creeks and helicopters were grounded. Previous years’ wildfires left mountainsides outside of Boulder devoid of plant life, which allowed the majority of water that fell to flow directly into streams that are not equipped for these rainfall rates. The cities of Lyons and Longmont are currently cut off from surrounding areas by flooding waters. The effects spread this afternoon into north and east Denver suburbs.
These conditions were in stark contrast to different record-setting weather just last week. Three daily high temperatures were set or tied at DIA: 97 on the 5th and 6th, 95 on the 7th. Two record high low temperatures (nighttime) were also set: 68F on the 3rd and 69F on the 4th. Daily average temperatures were 12 and 13 degrees warmer than normal during this late-season heat spell. These were caused by a ridge of high pressure that previously slid west over the Front Range.
The same ridge of high pressure moved back to the east earlier in the week. This allowed a cool front from Canada to slide south over the Denver metro area Monday. It also allowed subtropical monsoon moisture flowing north to move over the Front Range. The cool front brought moist low-level air in from the east and forced it up against the mountains. The combination of low-level and high-level moisture with decent instability generated rains that started on Monday and haven’t really stopped very long since then. The only good news is these rains will lessen the severity of the ongoing drought over the region.
This event harkens back to the Big Thompson River Flood of 1976. More than 100 people died in that event, but it also occurred overnight.