Field notes · Smoke forecasting
How the smoke forecast works, and how to use it
The map starts as a satellite pixel over a burning forest, runs through a 3D weather-and-chemistry model, and ends up as an hourly grid of smoke near the ground.
- HRRR grid spacing
- 3 km
- Forecast horizon
- 48 h
- New model runs
- Hourly
- Smoke near the ground
- PM2.5
01 · Pipeline
From fire to forecast in three steps
- 1Detect
Satellites measure how hard every fire is burning, not just where it is.
- 2Model
A 3D weather-and-chemistry model lofts the smoke and carries it on the forecast winds.
- 3Map
TrailSmoke pulls the hourly grid, adds active fires, and shows it in your local time.
1. Detect: from satellite pixel to fire emission
Two satellite sensor suites do the heavy lifting: VIIRS, on Suomi NPP and NOAA-20, and MODIS, on NASA's Terra and Aqua. Both measure Fire Radiative Power (FRP), the radiant energy coming off burning vegetation. FRP tells the model not just where fires are, but how hard they are burning.
Detections are mapped onto HRRR's 3 km grid, and the model estimates biomass emissions from the FRP signal. A backyard slash pile and a 100,000-acre crown fire do not enter the model the same way.
2. Model: WRF-Chem, plume rise, and 50 vertical levels
HRRR-Smoke is built on WRF-Chem, the Weather Research and Forecasting model coupled with atmospheric chemistry. It was the first U.S. operational weather model to simulate smoke as part of the weather itself. It covers the contiguous U.S. at 3 km with 50 vertical levels up to about 25 km. It runs every hour, and four of those runs (00, 06, 12 and 18 UTC) reach out 48 hours.
Once emissions are set, the model computes plume rise: how high the smoke injects, from the fire's heat output and the stability of the air above it. This is the step that matters most at the ground.
A hot fire in an unstable atmosphere can punch smoke several kilometers up, where upper winds carry it cross-country. A cooler fire under a stable boundary layer keeps smoke trapped near the ground, right where people are breathing.
WRF-Chem then moves PM2.5 through all 50 levels with the model's own 3D winds, turbulence, and boundary-layer mixing. Smoke and weather are two-way coupled: smoke scatters sunlight and cools the surface, which changes the winds that move the next hour of smoke. That feedback is what separates HRRR-Smoke from trajectory tools like HYSPLIT, which answer "where did this plume come from?" rather than "what does the whole smoke field look like tomorrow?"
3. Map: model grid to local time
The raw graphics live on the NOAA HRRR-Smoke page, built for meteorologists: pick a domain, a model cycle, a product, then loop UTC-stamped frames and convert to local time in your head.
TrailSmoke pulls the same forecast, overlays active wildfires from the NIFC feed, and lets you search a town to see when smoke arrives and clears, in local time, on one map.
02 · Layers
Which layer should I use?
Each layer answers a different question. Mixing up surface and column smoke is the most common mistake when reading any smoke map.
NOAA Surface
Will the air be smoky where I'm standing?
HRRR-Smoke PM2.5 about 8 m above the ground. The default layer, and the one to plan a ride or hike around.
FireWork
Does a second model agree?
Environment Canada's smoke model. Coarser (about 10 km), but it covers all of North America and runs out to 72 hours. When it and HRRR agree, trust the forecast more.
NOAA Vertical Column
Will the sky look hazy?
All the smoke in the column overhead. It can be heavy aloft while the ground reads clean, so don't use it to judge breathing air.
AQ Monitors
What is the air like right now?
Current AQI from ground monitors. Observations, not the forecast hour, so use it to check the model against reality.
03 · Colors
Reading the colors
The surface layers are shaded by smoke PM2.5 in micrograms per cubic meter, in bands modeled on the EPA's PM2.5 air quality categories.
- Mostly Clear<3 μg/m³
No significant wildfire smoke forecast at the surface.
- Light Haze3-12 μg/m³
Small amount of smoke detected aloft or at surface. Often noticeable but limited impact.
- Moderate Smoke13-35 μg/m³
Sensitive groups may feel effects. Most people are fine outside.
- Unhealthy for Sensitive Groups36-55 μg/m³
Sensitive individuals should limit outdoor activities.
- Unhealthy56-150 μg/m³
Everyone should limit outdoor activities.
- Very Unhealthy151-250 μg/m³
Health alert. Avoid outdoor activities.
- Hazardous251+ μg/m³
Health emergency. Avoid all outdoor activities.
04 · Planning
Planning a ride around smoke
In valleys, mornings can be the worst
Mountain valleys often trap smoke under a nighttime inversion. It can be at its worst at dawn and clear by early afternoon once the ground heats up and the air mixes. Scrub the timeline to see when that happens for your trailhead.
Check the monitors against the forecast
If the nearest monitors read much worse than the forecast for this hour, the model is behind on a new fire. Lean on the observations until the next run catches up.
Treat day two as a trend
The first several hours are the most reliable, since every hourly run refreshes them. Past a day out, plume rise and wind shifts compound, so read the second day as direction (improving or worsening) rather than an exact number.
Made by
Ryan Weller
Geologist and data scientist in Denver. I built TrailSmoke to plan mountain bike rides with friends through fire season. It's free and ad-free.