The June 2020 Rocky Mountains‑Northern Plains derecho was a widespread, long‑lived windstorm that occurred on 6 June 2020, affecting portions of the Rocky Mountain states and the Northern Plains of the United States. It is classified as a derecho—a fast‑moving band of severe thunderstorms that produces widespread wind damage over a large area.
Meteorological background
- The system originated from a vigorous upper‑level trough moving eastward across the western United States, combined with strong surface heating and ample low‑level moisture.
- Convective available potential energy (CAPE) values exceeded 3,500 J kg⁻¹ in parts of Colorado and Wyoming, supporting intense thunderstorm development.
- A mesoscale convective system (MCS) evolved into a bow‑shaped line of storms, meeting the criteria for a derecho: a continuous wind damage swath at least 400 km (250 mi) long, wind gusts of ≥58 mph (93 km h⁻¹) along most of its path, and sustained winds of ≥50 mph (80 km h⁻¹) for at least three‑four hours.
Path and extent
- The derecho initiated in central Colorado late on 5 June and progressed northeastward across the Front Range, entering Wyoming, Montana, South Dakota, and western North Dakota.
- Peak wind gusts were reported at 87 mph (140 km h⁻¹) near Laramie, Wyoming, and 82 mph (132 km h⁻¹) in eastern Montana.
- The damage swath extended roughly 1,200 km (750 mi) from the Colorado Plains to the northern Great Plains, lasting about six hours from initiation to dissipation.
Impacts
- Infrastructure: High‑speed winds toppled trees, downed power lines, and caused widespread electricity outages affecting over 250,000 customers across Colorado, Wyoming, and the Plains states. Several highway sections were temporarily closed due to debris and structural damage.
- Agriculture: The storm flattened crops, particularly soybeans and corn in eastern Wyoming and western South Dakota, resulting in estimated agricultural losses of $15 million (USD).
- Human safety: Five direct fatalities were attributed to the derecho—three due to falling trees, one from a vehicular accident caused by wind gusts, and one from a structural collapse. Over 30 injuries were reported.
- Fire weather: Strong gusts contributed to rapid fire spread in the Montana foothills, complicating suppression efforts for several wildfires already burning in the region.
Aftermath and response
- State and federal agencies, including the Colorado Department of Transportation, Wyoming State Emergency Management, and the National Weather Service (NWS), coordinated damage assessments and restoration of utilities.
- The NWS issued a series of public severe weather outlooks and somersault warnings (the latter being a rare designation for derechos) to alert the public.
- Recovery assistance was provided through the Federal Emergency Management Agency (FEMA) disaster assistance program for affected counties in Colorado and Wyoming.
Significance
The June 2020 Rocky Mountains‑Northern Plains derecho is notable for being one of the most powerful derechos to affect the higher terrain of the Rocky Mountains, a region where such windstorms are less common than in the central United States. Its intensity, broad geographic reach, and resulting damage have made it a reference event in meteorological studies of derechos in mountainous terrain.
