Snow Today

NSIDC’s Daily Snow Viewer provides daily maps and time series plots of snow cover, including percent, albedo, snow-covered days, and radiative forcing. (See the Snow Today glossary of terms in the About Snow Today page for definitions.) The Daily Snow Viewer also provides data overlays for snow water equivalent (SWE), including changes in SWE, and percentage of median SWE.

Using the Daily Snow Viewer does not require any special software or training. All you need is a web browser. The viewer works best on a desktop computer; using the viewer with a smaller screen may limit the display to a single column of maps and/or plots.

Each map displays the most recent full day of data, and each time series ends on the most recent full day of data. The most recent full day of data is generally from the previous day, but the exact date is listed as a subtitle on the maps and plots. The Daily Snow Viewer is usually updated at 8 a.m. Mountain Time, although occasional processing delays may occur.

Opening the Daily Snow Viewer

Snow Today’s opening screen may require you to select the desired region. As of early March 2024, the only available region is the western United States. More regions will be added with future Daily Snow Viewer development. Click the button for the desired region to begin using the viewer.

Using the controls

Once you have selected a region, buttons across the top of the Daily Snow Viewer let you change the display, and controls can be used independently of each other. You can use these controls to change the display as often as you like. Clicking the Reset button on the far right of the top menu returns you to the Daily Snow Viewer opening screen, in which you select a region, and restores all default settings.

Select a Region

Clicking this button opens a dropdown control panel to filter by region.

As of March 2024, the only region covered by the Daily Snow Viewer is the western United States, specifically these states: Arizona, California, Colorado, Idaho, Montana, Nebraska, New Mexico, Nevada, Oregon, South Dakota, Utah, Washington, and Wyoming. A red border outlines the states included when this region is selected. As additional regions become available, they will also be outlined with red borders for clarity.

Within the western United States, you can focus on a smaller region, filtered by state or Hydrologic Unit Code (HUC). If you zoom in on a region, and if you have opted to display a corresponding plot, the plot will update to reflect data for just the selected region, with some exceptions (see note below).

Note about HUC plot availability: The daily Snow Viewer offers regional filtering for two kinds of HUCs: two-digit (HUC2) and four-digit (HUC4). Two-digit HUCs encompass larger regions, e.g., HUC 14: Upper Colorado. Four-digit HUCs are subsets of two-digit HUC regions, e.g., HUC 1402: Gunnison.

The display will update immediately as you select new regions in this control panel. Once you have chosen the desired region, click anywhere outside the Select a Region dropdown panel to close it. To resume displaying maps and plots for the entire western United States region, click the radio button for None.

Configure Variables

Clicking this button opens a dropdown control panel that allows you to choose the layout of the map and time series graph tiles, as well as the variables shown in each. The minimum option is one tile in a single row and column, and the maximum option is eight tiles in four rows and two columns. The default display is two tiles in one row and two columns, showing the snow cover percent map and total snow cover area time series.

You can select the variables for the maps and plots independently of each other, but each map has a logical variable match for a plot. These are the intended map-plot correlations:

All map and plot variables include information about the data source: sensor, platform, and algorithm. This information is appended to the variable name, enclosed in parentheses and separated by slashes, e.g., (MODIS/Terra/SPIRES), which signifies data from the MODIS sensor on NASA’s Terra satellite using the SPIRES algorithm. Algorithms are described in detail on the About page.

The default display places the map tile on the left and the plot tile on the right, but you can choose any configuration of tiles you like by updating the Tile type dropdown.

The display will update immediately as you select new items in this control panel. Once you have chosen the desired rows, columns, maps, plots, and variables, click anywhere outside the Configure Variables dropdown panel to close it.

Select a Basemap

Clicking this button opens a dropdown control panel to select the basemap that displays behind the snow map. The default is the USGS Topographic map, but you can choose from several options, including maps provided by the US Geological Survey (USGS) or ArcGIS (Esri 2022). Basemaps are described on the About page.

Once you have chosen the desired basemap, the display will update immediately and the dropdown will automatically close.

Select SWE overlay

Clicking this button opens a dropdown control panel to add an overlay. Options include Snow Water Equivalent, Change in Snow Water Equivalent, and Percentage of Median Snow Water Equivalent.

Note: This option adds an overlay to all the maps currently displayed.

Display not-processed area

Checking this box adds an overlay showing where data have not been processed. Within the western United States, portions of Montana, South Dakota, and Colorado are not processed for the Daily Snow Viewer.

Raster opacity

Moving this slider to the left adds transparency to the data on the maps chosen via Configure Variables. The more transparent those raster maps are, the more of the basemap shows through. The default is 100 percent opacity for the raster maps.

Reset

Clicking this button restores the default display.

Working with the maps

Map zoom controls

You can zoom in and out of any map using the + and - buttons in the top left corner of the map, or by using the scroll wheel on your mouse.

Snow Water Equivalent overlays

These overlays, which appear when you select an option under the Select SWE Overlay dropdown, bring up specific data points where on-the-ground (in situ) SWE measurements have been taken. Clicking on any data point will bring up a small data display showing latitude and longitude, elevation, and value. Click the X to close this display. The units for the SWE overlay values are:

• Snow Water Equivalent: cm
• Change in Snow Water Equivalent: cm
• Percentage of Median Snow Water Equivalent: %

Note: When you click on a SWE data point near the left edge of the map, the resulting popup box may display off the left edge of the map. In such a case, you can pan the map leftwards to see the popup box contents.

Interpreting the maps

Each snow cover map is selected through the Configure Variables dropdown, and uses a sequential color palette, as follows:

• Snow Cover Percent: lowest percent is dark blue, highest percent is light green-yellow.
• Snow Cover Duration: lowest number of days is white, highest number of days is dark purple.
• Snow Albedo: lowest albedo is dark blue, highest albedo is white.
• Snow Radiative Forcing: lowest forcing is yellow, highest forcing is dark red-brown.

Each overlay selected through the Select SWE Overlay dropdown also uses a color palette, either sequential or diverging, depending on the variable shown:

• Snow Water Equivalent: sequential; lowest is white and highest is dark blue.
• Change in Snow Water Equivalent: diverging; decrease in red-orange and increase in blue.
• Percentage of Median Snow Water Equivalent: diverging; below 100 percent in red-orange and above 100 percent in blue.

Interpreting the plots

The time series plot on the right side of the Daily Snow Viewer shows the “snow year.” In the Northern Hemisphere, the snow year starts in October and runs through the following September. Assuming the maps and plots displayed correlate logically with each other (see Configure Variables above), the plot shows snow variables specific to the region designated in the corresponding map. For the duration of the current snow season, each plot displays all of the following:

• The start date for each region appears on the x-axis and may vary with the region selected. The water year in the western United States, for example, uses a start date of October 1.
• Year to date (solid blue line): Current snow-year values up to the most recently observed (generally yesterday’s data)
• Median (dashed gray line): Middle value of observed values over the 2001 to 2023 baseline
• Maximum (charcoal dashed-dotted line): Highest of all observed values over the 2001 to 2023 baseline
• Minimum (charcoal dotted line): Lowest of all observed values over the 2001 to 2023 baseline
• Interquartile range (light gray band): Innermost 50 percent of all observed values over the 2001 to 2023 baseline

Moving the mouse over the plot brings up a display for the corresponding date in the snow year. Each value is highlighted on its plot line with a specific symbol shown in the legend.

Known data issues

This section will be updated as issues are identified.

Known Daily Snow Viewer issues

As of March 18, 2024, the Daily Snow Viewer application has some bugs:

1. Some regions do not have SWE data. The application still allows you to select a SWE variable for those regions, but when selected, the application does not respond.
2. When you click on a SWE data point near the left edge of the map, the resulting popup box may display off the left edge of the map.