On the evening of June 4, 2015, a rare and powerful tornado struck along the Boulder-Larimer county line in northern Colorado. Rated an EF3 on the Enhanced Fujita Scale, the tornado brought estimated wind speeds of up to 140 mph and tracked nearly due west—an unusual path for tornadoes in the region. It remains the strongest tornado on record to impact either Boulder or Larimer counties since official tornado ratings began in 1950.
Key Facts:
- Date: June 4, 2015
- EF Rating: EF3
- Estimated Wind Speeds: Up to 140 mph
- Path Length: 6 miles
- Maximum Width: ~0.25 miles
- Homes Damaged: 28 (3 destroyed)
- Injuries: A few minor injuries, no fatalities
Why This Tornado Was Unusual
Most tornadoes in Colorado occur on the Eastern Plains, where flat terrain and ample atmospheric instability create favorable conditions for supercell thunderstorms. Tornadoes west of I-25—especially strong ones—are much less common due to mountainous terrain, which tends to disrupt the dynamics required for tornado formation.
Even more unusual was the tornado’s track direction. While most tornadoes in the U.S. move northeast or east, this one traveled almost due west—toward the foothills—along the Boulder-Larimer county line. That’s a rare path, especially for a tornado of this intensity.
The EF Scale and Tornado Strength
The Enhanced Fujita Scale (EF Scale) is used to rate tornadoes based on the damage they cause, ranging from EF0 (weakest) to EF5 (strongest). An EF3 tornado, like the one on June 4, produces severe damage, including:
- Entire stories of well-constructed houses destroyed
- Large trees debarked or uprooted
- Heavy vehicles lifted off the ground
Tornado Preparedness in the Foothills
While strong tornadoes are rare near the mountains, this event highlights the importance of being weather-aware even in areas not typically considered “tornado-prone.” With increasing development near the foothills, residents should understand local tornado risks and ensure they have:
- Access to weather alerts (via NOAA Weather Radio and the Skyview Weather app)
- A shelter plan, even if a basement is not available
- Emergency kits with essentials in case of prolonged outages or damage
A Close Call with a Positive Outcome
Despite the tornado’s strength, the outcome could have been far worse. Thanks to timely warnings and public awareness, only minor injuries were reported, and no lives were lost. The 2015 EF3 tornado serves as a valuable case study for meteorologists and emergency planners—and a reminder to all Coloradans that severe weather can happen anywhere in the state.