Flash Flood Guidance Bulletin

Flash Flood Guidance Bulletin

Date: 31-3-2026  

Time of issue: 14:30 MST

Area of Concern (AoC): Nail

Imminent Flash Flood Threat (IFFT)

Imminent Flash Flood Threat (IFFT) is not shown.

Persistent Flash Flood Threat (PFFT)

Persistent Flash Flood Threat (PFFT) is not shown.

06 hours Forecasted Flash Flood Threat (FFFT)

No Flash Flood Threat (FFFT) for the next 06 hours.

24 hours Flash Flood Risk Outlook (FFR)

Flash Flood Risk (FFR) is not shown during the next 24 hours.

Terms and Terminologies

Flash Flood: The flood, occurs as a result of heavy or excessive amounts of rainfall within one day or hours of time, causing water to rise and fall quite rapidly in small rivers and streams.

Merged Mean Areal Precipitation (Merged MAP): The text and images provide 1-hour, 3-hour, 6-hour, and 24-hour totals of the Merged Mean Areal Precipitation for each system sub-basin. This product is derived for each basin based on the best available mean areal precipitation estimates from bias-adjusted MWGHE or bias-adjusted GHE or the gauge interpolations.

Flash Flood Guidance (FFG): The text and images provide 1-hour, 3-hour, and 6-hour Flash Flood Guidance for each sub-basin. The FFG value indicates the total volume of rainfall over the given duration which is just enough to cause bankfull flow at the outlet of the draining stream. Consequently, rainfall volumes of the same duration that are greater than the FFG value indicate a likelihood of overbank flows at the draining stream outlet. Each of the FFG products is updated every hour.

Imminent Flash Flood Threat (IFFT): The text and images provide 1-hour, 3-hour, and 6-hour Imminent Flash Flood Threat (mm) for each sub-basin. The values indicate the difference of the Merged MAP of a given duration and the corresponding past navigation hour FFG of the same duration for a given sub-basin. In this sense, the IFFT value is considered a current “observation”.

Persistence Flash Flood Threat Product (PFFT): The text and images provide 1- hour, 3-hour, and 6-hour Persistence Flash Flood Threat for each sub-basin. The values indicate the difference of recent persisted Merged MAP of the given duration and the corresponding current FFG of the same duration for a given sub-basin. In this sense, the PFFT is considered a “forecast” with persistence used as the rainfall forecast.

Forecast Flash Flood Threat (FFFT): FFFT products include text tables and images of 1-hour, 3-hour, and 6-hour Forecast Flash Flood Threat for each sub-basin. The values indicate the difference of forecasts of mean areal rainfall of the given duration using the MMRWRF FMAP and the corresponding current FFG of the same duration.

Flash Flood Risk (FFR): Flash Flood Risk products include text tables and images of 12-hour, 24-hour, and 36-hour relative frequency of positive flash flood threat for the entire forecast lead time interval that is due to forecast rainfall of 3-, or 6-hour duration from the MMRWRF Model Forecast. This is a measure of the forecast flash flood occurrence frequency.