Michael N. Chard, Director, Boulder Office of Emergency Management. By now some of you may have seen the information from the NWS and the flash flood watch. I spent a good deal of time talking with Mike at the weather service about the current weather models used to generate this forecast.
This is not September 11, 2013, but there is a risk of flash flooding. Right now there are two plumes of moisture pumping moisture into our state which is uncommon causing heavy moisture contents in the atmosphere with slow-moving storms.
We have a high pressure ridge to the North which is keeping the moisture locked in and a mild low pressure system is setting up in the morning just south of Denver and by 6 p.m. it will be near Walsenburg.
This low pressure will shift east of Denver ( it was south of Denver during the Sept 11 event) as we move into the deeper evening hours and start a cyclonic rotation of storm action. This system will cover from Cheyenne to New Mexico so it is a State-Wide threat area. The intensity of rainfall will increase to 1-2 or more inches per hour, with the storm’s focus on the eastern edge of the low, and also Larimer and Weld Counties.
BOULDER will be on the northerly flow side of the low; thus we will get max intensities of ¼ to ½ inch per hour rainfall amounts and should come through this with a lower flash flooding risk compared to Larimer and Weld Counties. By morning the system will weaken and by the afternoon it will have dissipated mostly and the threat long passed.
This is what the models are showing and inaccuracies can occur. If the low drops to the south of Denver then we would be the center of the bulls-eye like we were on Sept. 11. This does not mean we would experience flooding at the levels of the 11th, but we would be the center point of intense rainfall.
#1: Flash flooding—if the low were to drift, then we would be exposed to a higher risk of flash flooding due to the duration and intense rainfall totals for the threat period.
#2: Landslides and Debris Flows—this could be a 18-24 hour rain event and landslide / debris flow risks increase as the ground becomes saturated and rainfall intense events occur.
#3: Road and bridges could become impacted due to high runoff or flash flood events, especially in areas where temporary roads exist in mountain canyons.
#4: Overtopping of rivers and creeks—Since the runoff there has been erosion and deposition of sediment along areas that have structures such as culverts, bridges, and walkways or at flat spots where there is little elevation difference due to capacity reduction in channel ways, due to sediment deposits.
#5: Public Warning—This event will happen at night, which means residents may not have accurate situational awareness of the hazard. It will be harder to notify and residents may have a less than normal ability to be prepared adequately for sudden protective actions.
#1: Individuals should stay aware of developing issues and identify predetermined information sources to maintain situational awareness. Do not overreact to general information that is provided on cell phones. Always look for the hazard, location impacted, and follow directions such as evacuate or climb to safety. Also provide for family preparedness while at work to ensure safety and security for friends and family members.
#2: Monitor internal city / county employee warning systems to obtain direction from City and County leadership.
#3: The EOC will initiate the severe weather protocol at first indication of any weather threats and activate the EOC as a functional level activation, which places OEM staff in position, radar, contact with UDFCD & NWS, web site / social media postings, and stream and river gauge monitoring.
#4: Upon threat identification first responders will be notified to implement the Boulder Flood Annex. First responders please watch East facing and Southern slopes for landslide signs and areas of potential flooding.
#5: Public information and warnings shall be issued based on need.