Below are some frequently asked questions regarding the Colorado Flood Threat Bulletin website (www.coloradofloodthreat.com).
What is the Colorado Flood Threat Bulletin?
The FTB is a service provided to the residents of the State of Colorado by the Colorado Water Conservation Board. It is meant to provide an overview of the potential flood threat facing the State on a daily basis from May 1 through September 30. It consists of three different products:
- Daily Flood Threat Bulletin – This product is issued daily before 11:00 am and is used to identify areas of the state that are at risk of flooding. Updates can be issued as needed by weather situation.
- Statewide Storm Total Precipitation Map – This map is created by merging the 24-hr Storm Total Precipitation (STP) product observed regional WSR-88D and the MADIS observational database. Issued at 9:30 am every day.
- Flood Threat Outlook – This product is an outlook of the flood threat and precipitation amount and chance in the state over the next 15 days. It is presented in an event-based format that focuses on periods of potentially wet weather.
What do the Flood Threat definitions mean?
On a daily basis, users may see the following flood threats posted on the map.
What goes into determining a threat level?
On a daily basis, our meteorologists look at six primary factors when deciding upon a flood threat.
- Intensity of precipitation -
- Duration of precipitation -
- Probability of precipitation -
- Ground type – When rain falls on hard, non-porous surfaces such as roofs and streets in urban areas, it runs off into drainage systems much faster than rural locations. Similarly, forests that have been recently burned experience significant runoff problems compared to pristine areas.
- Spatial coverage of precipitation – By their very nature, isolated thunderstorms potentially producing heavy rain are difficult to forecast, while large, slow moving general rain systems can be more accurately forecast.
- Antecedent moisture – When the ground is already saturated with moisture from previous rainfall, it is much easier for flooding conditions to be initiated.
What are common scenarios that can result in a flood threat?
Due to the varying topography of the State of Colorado, we are generally concerned with four types of scenarios that can result in a flood threat:
- Short-term very heavy (i.e. thunderstorm) rainfall: 1 inch/hour over mountainous terrain, 2 inches/hour over the Plains, 0.75 inches/30 minutes over urban areas
- Prolonged moderate/heavy rainfall: 3+ inches/6 hours, 4+ inches/12 hours, 5+ inches/24 hours.
- Rain on snow, which can occur given even light, but long duration rainfall.
- Snowmelt induced flooding, which can occur even in the absence of any rainfall.