About
The map ranks each day's snowpack against the same calendar day in past years. A percentile
of 50 is a typical amount of snow for that date. A percentile of 10 means only one past year
in ten had less.
The snow data is NOAA's SNODAS
model of snow water equivalent (SWE): the depth of water the snowpack would make if it melted.
It covers the lower 48 states on a 1 km grid, once a day. Percentiles start on
September 1, 2025.
What the colors mean
The colors and category names follow the
U.S. Drought Monitor.
- D0 to D4 (yellow to dark red) mark less snow than usual for the date. D4 is
below the 2nd percentile.
- Near normal (white) is the 30th to 70th percentile.
- W0 to W4 (light to dark blue) mark more snow than usual. W4 is above the
98th percentile.
A place with no snow on a date when it is always bare reads 50, near normal.
In the gridded view, a dashed line marks the edge of the SNODAS data: the lower 48 states
and the Canadian part of the Columbia River basin. The map is blank outside it.
Using the explorer
- Search. Type a state, county, watershed, city or ZIP code
to go there. Press / to jump to the search box (add
?kbd=off to the address to turn
that key off). A city or ZIP code is marked with a dot, and a watershed is outlined.
- Date. Pick a day, or step one day at a time with ▲ and ▼.
- View. Gridded shows every 1 km cell. Zonal averages over watersheds, from
the smallest (HUC8 subbasins, the default) to the largest (HUC2 regions); choose the size with Level.
- Normals. The past years each day is measured against. Smoothed is the default;
the three choices are explained below.
- Click or tap the map. In the gridded view this reads out the percentile at that
point. In the zonal view it opens that watershed's plots. With a keyboard, the watersheds
are also a table after the map: press Enter on a row to open its plots.
- Zoom and pan. Scroll or pinch to zoom and drag to pan. The buttons at top right
zoom in, zoom out, and return to the whole country.
- Share. Copies a link to exactly this view, open plots included.
- Export. Downloads the map as a PNG with its legend.
- Theme. Switches between dark, light and high contrast.
Which normals should I use?
Each choice turns the past years for a date into a range of normal snow in a different way.
- Smoothed (the default) fits one curve through the whole snow year, so each day's
normal leans on the days around it. It is the steadiest from one day to the next.
- Fitted fits a curve to each calendar day on its own.
- Empirical ranks this year directly against the past years for that day, with no
curve. It is the most literal, and the noisiest, and it can't rank snow beyond the most or
least on record.
The three differ most where few past years had snow on the date, such as early and late in the
season and at low elevations.
Watershed plots
In the zonal view, click or tap a watershed for its plots. The date and normals in the plot
window are the map's own: change them there and the map changes too.
- SWE shows the watershed's snow water, in acre-feet, through the whole snow year
(September 1 to August 31). The colored bands are the range of past years; the black line is
this year.
- HypsomeSWE splits the same snow into 100-foot elevation bands. Beside it are each
band's percentile, its share of the median for the date, and its area.
A watershed with no snow on the date and none in its normals has no HypsomeSWE plot. Plots are
not yet published for every date. When a date has none, the window offers the nearest date that
does. Data for this plot, under each plot, lists its numbers.
