README.md

---
title: Seventy-Seven Years of U.S. Tornadoes
emoji: 🌪️
colorFrom: blue
colorTo: red
sdk: static
app_file: index.html
pinned: false
license: cc-by-4.0
---

# Seventy-seven years of U.S. tornadoes

An interactive browser for annual tornado density across the contiguous United States,
1950–2026. Drag the timeline, scroll over the map, or use the arrow keys.

Every tornado in the NOAA/NWS Storm Prediction Center database is placed at the **midpoint
of its track**, binned onto an 80 km Lambert conformal grid, and smoothed with a 120 km
Gaussian kernel — the gridding convention of Brooks, Doswell & Kay (2003, *Wea. Forecasting*
**18**, 626–640). Black dots are individual tornadoes.

## Reading it honestly

- **The color scale is fixed across all years**, so a pale map is a genuinely quiet year
  rather than a rescaled one.
- **Reports are not a fixed yardstick.** How many tornadoes get recorded depends on
  population density, observing technology, and public awareness, none of which has held
  steady since 1950. Early years are undercounted, especially for weak tornadoes.
- **2026 is incomplete.** It covers 1 January – 14 August only, and comes from preliminary
  reports that were deduplicated (DBSCAN, 20 km / 30 min) and rescaled to a final-count
  basis using the 2010–2025 relationship between deduplicated preliminary reports and final
  counts. Its all-time rank is therefore a floor, not a settled figure.

## Data

The gridded density field ships with this Space as a Zarr v2 store:

```
tornado_density_CONUS_80km_1950-2026.zarr
├── density            (year, y, x)  float32   tornadoes (10^4 km^2)^-1
├── year, x, y, lat, lon
├── annual_max, rank_all_time, tornado_count   (year,)
└── lambert_conformal_conic                    grid mapping
```

```python
import xarray as xr
ds = xr.open_zarr("tornado_density_CONUS_80km_1950-2026.zarr")
ds.density.sel(year=2026).plot()
```

Source: NOAA/NWS Storm Prediction Center — 1950–2025 final severe weather database and 2026
preliminary daily storm reports.
Analysis: Prof. Steve Nesbitt, Department of Climate, Meteorology & Atmospheric Sciences,
University of Illinois Urbana-Champaign — [snesbitt.github.io](https://snesbitt.github.io).