Snowpack Explorer
Daily snowpack percentiles for the lower 48 states and their watersheds, from
NOAA SNODAS, with per-watershed SWE and
HypsomeSWE plots. Built by the Montana Climate Office.
Live at https://snow.climate.umt.edu. Pushing main deploys it (GitHub
Pages from the repository root), so a push is a release.
What it shows
Each day's snow water equivalent (SWE) is ranked against the same calendar day
in past years and colored with the U.S. Drought Monitor categories (D4 to W4).
- Gridded: the percentile of every 1 km SNODAS cell.
- Zonal: the percentile of each watershed's mean SWE, for USGS Watershed
Boundary Dataset units at HUC2, HUC4, HUC6 and HUC8.
- Normals: the past years are turned into a range of normal snow in one of
three ways, which the page can switch between: Smoothed (
zig_smooth, the
default), Fitted (zig) and Empirical (ecdf). The method is in
normals/README.md
beside the data.
- Watershed plots: in the zonal view, clicking a watershed opens its SWE or
HypsomeSWE plot for the date and normals shown. The plots are rendered by the
pipeline, not the page.
Where the data comes from
Everything is read from https://data2.climate.umt.edu/snodas/, written by the
mco-snodas pipeline (a
private repository; its s3-docs/ are published as READMEs in the bucket).
The page has no backend.
| What | Path under snodas/ |
|---|
| Newest published date | latest/manifest.json (date) |
| Gridded percentiles (COG, 0–100) | normals/gridded/{flavor}/{date}/{date}_{flavor}_swe_ptile.tif |
| Normals provenance | normals/gridded/{flavor}/{date}/{date}_{flavor}_provenance.json |
| Watershed percentiles and normals (Parquet) | normals/zonal/wbd/{flavor}/{date}/{date}_{flavor}_swe_normals.parquet |
| Elevation-band table (Parquet) | normals/zonal/wbd/{flavor}/{date}/{date}_{flavor}_hypsomeswe_normals.parquet |
| Watershed boundaries (FlatGeobuf) | cache/data/fgb/huc{2,4,6,8}.fgb |
| Watershed plots (PNG) | plots/{date}/{flavor}/{swe,hypsomeswe}/usdm/{huc}.png |
| A date's plots are complete | plots/{date}/index.json |
Percentiles start on 2025-09-01, the first day of the first snow year the
normals pipeline has built. Plots are not published for every date yet; the
page says so and offers the nearest date that has them.
URL parameters
The URL is the page's state, so a copied link reopens the same view. Defaults
are left out of the URL.
| Parameter | Values | Default |
|---|
date | YYYY-MM-DD | the newest published date (always written) |
view | gridded, zonal | gridded |
huc | 8, 6, 4, 2 (zonal view only) | 8 |
flavor | zig_smooth, zig, ecdf | zig_smooth |
basin | a HUC code, e.g. 1002; opens its plot dialog | none |
plot | swe, hypsomeswe (with basin) | swe |
lng, lat, zoom | the camera | the lower 48 |
theme | dark, light, high-contrast | the system setting |
export | light, dark (true also works): download the map as a PNG on load | none |
kbd | off: turn off the / shortcut to the search box | on |
An old ?method=zig or ?method=ecdf link still opens on that flavor.
Files
index.html: the page, its CSP, and app-only CSS.app.js: everything the page does. A classic script.vendor/parquet.min.js: hyparquet and fzstd, bundled and pinned by
scripts/vendor.sh (bump a version there and re-run it).data/snodas-domain.geojson: the edge of the SNODAS grid, traced from its
NoData mask (NOHRSC publishes no boundary file), outlined in the gridded
view. scripts/build-domain.sh rebuilds it.data/places/: what the search box finds besides watersheds. areas.json
(states and counties, with their bounding boxes), places.json (cities,
towns and Census-designated places) and zips.json (ZIP code areas), each
as compact arrays, lower 48 and DC only. Built from the US Census 2026
Gazetteer and 2025 cartographic boundaries (public domain) by
scripts/build-places.sh. Watersheds come from the boundary files above.verify.config.mjs: scenarios and probes for the house verify harness.
The look, the controls and most behaviour come from the Montana Climate
Office's shared design system,
mco-web-style, pinned by
version with SRI hashes in index.html and app.js. Change shared behaviour
there, not here.
Run it locally
Any static server works:
python3 -m http.server 8000
# then open http://localhost:8000/
Check it
The house verify harness lives in mco-web-style. With that repository checked
out beside this one and its tooling installed (see its
tools/verify/README.md):
cd ../mco-web-style
node tools/verify/head.mjs --root ../mco-snowpack-explorer
node tools/conformance.mjs ../mco-snowpack-explorer
node tools/verify/lint-css.mjs --root ../mco-snowpack-explorer
node tools/verify/axe-matrix.mjs --config ../mco-snowpack-explorer/verify.config.mjs --root ../mco-snowpack-explorer
node tools/verify/keyboard.mjs --config ../mco-snowpack-explorer/verify.config.mjs --root ../mco-snowpack-explorer
The browser checks run against live data on data2, so they need a network
connection.
License
MIT. Copyright (c) 2025–present Montana Climate Office.
