About the FTB

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.
  • Flood Threat Outlook – This product is an outlook of the flood threat and precipitation amount/probabilities in the state over the next 15 days. It is presented in an event-based format that focuses on periods of potentially wet weather, and a precipitation map is included when the forecast precipitation exceeds 0.5 inches for any area during an event duration. The Outlook is updated on Mondays and Thursdays by 3PM.
  • State Precipitation Map – An overview of recent precipitation and flooding conditions across the state, using MetStorm Live data.

What do the Flood Threat definitions mean?

On a daily basis, users may see the following flood threats posted on the map.

Threat Levels

What goes into determining a threat level?

On a daily basis, our meteorologists look at six primary factors when deciding upon a flood threat.

  1. Intensity of precipitation.
  2. Duration of precipitation: considered durations range from anywhere between 30 minutes to as long as 48 hours.
  3. Probability of precipitation: a higher probability of heavy rainfall translates into a higher threat of flooding.
  4. 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.
  5. 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.
  6. 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:

  1. 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
  2. Prolonged moderate/heavy rainfall: 3+ inches/6 hours, 4+ inches/12 hours, 5+ inches/24 hours.
  3. Rain on snow, which can even occur given light, but long duration rainfall.
  4. Snowmelt induced flooding, which can occur even in the absence of any rainfall.
  5. Riverine flooding, which can occur due to spring/early summer snow melt in the high country, or as a combination of snow melt and heavy rainfall.

What else do we forecast?

Our meteorologists also consider fire weather and burn scars. While this is not an official product that we issue, it is important to highlight where there may be danger from very dry/windy conditions. Recently burned areas are especially susceptible to flash flooding and mud flows due to the lack of vegetation and hydrophobic soil. These variables cause drastically higher runoff, and over a recent burn scar, it can take as little as 0.25 inches of rain to trigger flooding! Thus, forecasting flood potential over burn scars is part of our effort to go above and beyond to make sure our products are as informative as possible. We include vulnerable burn scars on our flood threat maps for reference, and we will provide specific burn scar forecasts when necessary.

Forecast Zones