Today | Weather | Wednesday, March 18, 2026

Active Weather Warnings

Red Flag Warning

Weather in Longmont, Colorado

08:58 AM

Now

55° F

Feels Like 55°

Precipitation 0 %

Wind 7 mph

UV Index (0 - 11+) 2

Humidity 23 %

Today

Day 82°

Night 46°

Precipitation 1 %

Wind 10 mph

UV Index (0 - 11+) 5

Longmont Weather Forecast Updated

17 minutes ago

Last updated on March 18, 2026 at 8:40 AM

Weather Forecast History

Last Year's Weather on This Day (March 18, 2025)

Day

63°

Night

32°

Wind

15 mph

UV Index

Precipitation

60 %

Air Quality in Longmont

NOW

32 AQI

Good air quality Dominant pollutant: O₃

Pollutant Concentrations:

PM_2.5 0.71 µg/m³

PM₁₀ 1.19 µg/m³

O₃ 0.035 ppm

NO₂ 0.6 ppb

SO₂ 0.1 ppb

CO 0.08 ppm

NH₃ 0.68 µg/m³

NO 0.29 µg/m³

PM_2.5 – Fine particles <2.5μm

PM₁₀ – Inhalable particles <10μm

O₃ – Ozone

NO₂ – Nitrogen dioxide

SO₂ – Sulfur dioxide

CO – Carbon monoxide

NH₃ – Ammonia

NO – Nitric oxide

Health Recommendations:

Air quality is considered satisfactory, and air pollution poses little or no risk.

Show raw pollutant concentrations (µg/m³)

PM_2.5 – 0.71 µg/m³

PM₁₀ – 1.19 µg/m³

O₃ – 68.16 µg/m³

NO₂ – 1.16 µg/m³

SO₂ – 0.34 µg/m³

CO – 93.67 µg/m³

NH₃ – 0.68 µg/m³

NO – 0.29 µg/m³

View Hourly Air Quality Forecast

Weather Alerts and Warnings for Longmont

Severe Expected Likely

Red Flag Warning

Red Flag Warning issued March 18 at 3:36AM MDT until March 18 at 7:00PM MDT by NWS Denver CO

Mar 18, 11:00 AM → Mar 18, 7:00 PM

* AFFECTED AREA...Fire Weather Zones 239, 240, 243, 244, 245,
248, 249, 250 and 251.

* TIMING...From 11 AM this morning to 7 PM MDT this evening.

* WINDS...Northwest 10 to 20 mph with gusts up to 30 mph.

* RELATIVE HUMIDITY...As low as 8 percent.

IMPACTS: Conditions will be favorable for rapid fire spread.
Avoid outdoor burning and any activity that may produce a
spark and start a wildfire.

+ Read More

A Red Flag Warning means that critical fire weather conditions are either occurring now....or will shortly. A combination of strong winds...low relative humidity...and warm temperatures can contribute to extreme fire behavior.

Updated: Mar 18, 8:40 AM Source: NWS Alert