Longmont Weather -- Boulder County, CO

Longmont, CO -- St Vrain Creek City in Boulder County

Longmont is the largest city in Boulder County, sitting at the northern edge where the plains meet the first foothills. At 5,000 feet, it is the lowest-elevation city in the US-36 and north corridor group, with predominantly flat terrain that stretches east toward Weld County and north toward the Larimer County line. The city spans both Boulder and Weld counties, with most residential development on the Boulder County side. The St Vrain Creek enters Longmont from the west, channeled through the canyon mouth at Lyons before fanning onto the plains through the city center and continuing east to the South Platte.

At this elevation and on open plains, Longmont has the warmest summer temperatures and lowest annual snowfall totals of the Boulder County corridor cities. The flat terrain provides no buffering from the northeast winds that push down from Wyoming during storm cycles, and the same Chinook downslope events that shape weather in the foothills accelerate across the plains here with little interruption. Wind events from the west are notably strong along the Diagonal Highway (US-287) corridor connecting Longmont to Boulder.

The September 2013 regional Front Range flood defined a generation's understanding of the St Vrain Creek. Record rainfall over the watershed sent flows through Lyons and into Longmont that overtook levees, breached the downtown corridor, damaged the wastewater treatment plant, and destroyed multiple bridges. The Federal Emergency Management Agency designated it a major disaster; rebuilding required years and significant flood mitigation infrastructure that continues to shape city planning along the creek.

Fire Risk and History

Longmont carries a low wildfire risk as a plains city. The primary exposure is along the western edge near McIntosh Lake and the agricultural and residential interface approaching the foothills corridor. The 2021 Marshall Fire, which originated near Marshall to the south and burned through Louisville and Superior, demonstrated how quickly grassland fire can move across the plains under extreme downslope wind conditions — Longmont's western neighborhoods share a similar plains-foothills interface. The Colorado State Forest Service Community Wildfire Protection Plan provides county-wide defensible-space guidance for residents near open space boundaries.

Elevation and Microclimate

At 5,000 feet on the open plains, Longmont receives approximately 20 percent stronger UV exposure than sea level. Summer high temperatures regularly exceed those of higher-elevation Boulder and are among the warmest in Boulder County. Annual snowfall averages significantly lower than foothills communities — typically 50–60 inches per year versus 80–100 inches at mountain towns like Nederland or Ward. The flat plains terrain maximizes wind exposure from all quadrants, with northeast storm-track winds and Chinook downslope events both arriving with little terrain obstruction.

Flood Zone Information

Longmont is a FEMA Flood Zone AE community with significant mapped flood hazard along the St Vrain Creek corridor and Left Hand Creek, which joins the St Vrain in the eastern part of the city. The September 2013 flood exceeded 100-year flow levels in the St Vrain channel, overtopping levees and spreading through residential and commercial districts downstream of Lyons. Properties along either creek and their tributaries should verify their specific parcel designation at FEMA's Flood Map Service Center.

Nearby Weather Pages

• Lyons weather -- canyon-mouth headwaters 12 miles west
• Niwot weather -- unincorporated CDP to the south
• Erie weather -- plains community to the southeast