What is a derecho?
The NTP definition of a derecho is based on the definition developed by Squitieri et al. (2025) and used at the US Storm Prediction Center. However, minor modifications were needed for the Canadian context (italicized text).
A derecho is a widespread severe windstorm characterized by a family of destructive downbursts containing hurricane-force gusts associated with an extratropical, cold-pool-driven mesoscale convective system (MCS). An MCS produces a derecho when the following criteria are met:
* Widespread severe reports (wind damage or 26+ m/s gusts) comprising the wind swath all occur from the same MCS
* All reports in the wind swath occur in a progressive sequence
* The parent MCS forward speed is greater than the mean wind speed
* No more than one hour may elapse between reports or areas of damage in the wind swath
* Spatial gaps between reports or areas of damage in the wind swath may not exceed 200 km
* The wind swath must be at least 400 km long
* At least five 33+ m/s gust reports (measured or estimated) or five areas of EF0+ damage, separated by 80 km, are required in the wind swath
* At least three measured 33+ m/s gust reports or three areas of EF1+ damage, separated by 80 km, are required in the wind swath
* The wind gusts and areas of damage considered must not be due to tornadoes or tropical cyclones.
Note that any combination of radar, lightning, satellite, surface station data and public reports (with times) can be used to help assess the presence and evolution of the MCS. The mean wind speed may also be diagnosed using NWP model output.
Reference
Squitieri, B. J., A. R. Wade and I. L. Jirak, 2025: On a Modified Definition of a Derecho. Part I: Construction of the Definition and Quantitative Criteria for Identifying Future Derechos over the Contiguous United States. Bull. Amer. Meteorol. Assoc., DOI: 10.1175/BAMS-D-24-0015.1.
