Weather radar normally shows you where the rain is right now. I wanted to see where it has been. Radar Trail takes the national NEXRAD radar frames for any stretch of time, from the last hour to the last year, and stacks them into a single heatmap of how much rain fell, and how often it rained, across Erie, Colorado and the Front Range around it.
Everything below is live. Pick how far back to look, how big an area to cover and how see-through the overlay should be, then hit Build heatmap. Click anywhere on the map to read the estimated rainfall at that spot.
What you’re looking at
Rainfall total
An estimate of how much rain fell, in inches or millimeters, turned into color. The legend shows how much of the area landed in each color band.
How often it rained
This is the see-through overlay idea. If every radar frame were drawn at low opacity, the spots that show up most would glow brightest. Counting the frames gives the same picture without the colors muddying each other.
Peak intensity
The strongest radar echo at each spot, in the usual radar colors.
Stacked frames and Frame by frame
For windows up to two weeks you can literally stack every frame at low opacity, or step through them one at a time like a weather loop.
Spot check
Click the map to see the estimate at that point, plus a bar chart of radar strength over time there.
How it works
The radar frames come from the Iowa Environmental Mesonet, which archives the national radar mosaic every five minutes back to 2010. Your browser downloads the map tiles for the area you chose, turns their colors back into radar reflectivity, and converts reflectivity to a rain rate with the standard National Weather Service formula. Every frame in the window gets added up, so a week of storms becomes one picture. For windows longer than two weeks it keeps running totals instead of every frame, which is how it can look back a full year.
There is no server behind it. The page is a static site on Cloudflare and all of the math happens in your browser.
Take it with a grain of salt
These are radar estimates, not rain gauge readings. The mountains west of Boulder block the beam, hail reads as heavier rain than it is, and the foothills have ground clutter that shows up as rain that never fell. For windows longer than two weeks the page skips November through March, because radar reads snow as rain, and hides spots that show echoes more than a quarter of the time. The grey areas on a long-range map are those hidden spots.
Looking back a full year pulls thousands of map tiles from the Iowa Mesonet, a university server, so the page asks “Are you sure?” first. Please be kind to it.
