Will I encounter snow? New snow survey tool

In early-December an alumnus, Rud Platt, shared with me a project that he’d been working on, snowEvaluator. Its chief function is creating snow coverage maps, i.e. where there is snow, or where there was snow on specific dates in the past.

If you’ve ever been uncertain about whether you’ll encounter snow on an upcoming backpacking or camping trip, this tool will provide a fairly definitive yes or no. At least for this purpose, it’s a better predictor than other resources I’ve mentioned in the past, such as SNOTEL and the National Snow Analyses.

snowEvaluator is more robust than just a static map of current conditions. Features include a:

  • Calendar view, so that you can easily look up historical snow coverage (by 1-week increments); and,
  • Snow Index (screenshot below), which plots snow coverage over time and reveals exactly when an area become covered (or uncovered) by snow.

snowEvaluator creates the maps using high-resolution Sentinel satellite data, which is also used by CalTopo and GaiaGPS, as I discussed here. For comparison, below are screenshots of snowEvaluator and then CalTopo, depicting Yosemite in June 2019.


SnowEvaluator has several advantages over CalTopo:

  • Its false color image better distinguishes snow from clouds. Snow is represented in an unmistakable red color.
  • It automatically displays the least cloudy images, if multiple images were taken of the same area in a given week. This is especially helpful at high latitudes where there is a lot of overlap between neighboring image paths.
  • It’s free forever, whereas CalTopo and Gaia may make it part of their premium subscriptions.

I wouldn’t expect an abandonment of CalTopo, however. In that platform, the snow coverage data can be helpfully overlaid atop topographic layers like USGS and USFS quads. And, in general, it’s a much more robust trip planning system.

Questions about snowEvaluator? Leave a comment.

Posted in on December 27, 2019


  1. Postholer on December 28, 2019 at 11:10 pm

    Coverage only data such as MODIS or Sentinel are a poor predictor as you’ll never know how close conditions are to zero snow until it reaches zero. Your snow index can’t predict that.

    SNODAS modeled snow data is invaluable for giving a very good idea of what quantity of frozen water is sitting on the ground. Where SNODAS struggles is when snow thins out, such as the spring around the edges of the snow pack.

    Coverage data has it benefits as does the modeled data. The ideal solution is the combination of both. The MODIS/Sentinel data layer on the bottom with the SNODAS data layer on top. When the SNODAS shows nothing, the coverage data will show (if snow exists).

  2. Matt Gundersen on December 29, 2019 at 5:07 am

    The map always defaults to Jun 9 – June 16, 2019.

    It would be nice if on initial load, the map defaulted to the most recent data.

  3. Jonathan Doman on December 30, 2019 at 3:44 pm

    Looks like a useful tool. But FYI CalTopo does also provide false color Sentinel imagery.

  4. Postholer on January 5, 2020 at 10:08 pm

    The postholer google maps now has overlays using Sentinel-2, 20m snow cover. The advantage is the trail alignment(s) and 1000’s of way points, not just for the sierra. It’s free and works great on your phone or desktop.

  5. Ellery on March 8, 2020 at 4:37 pm

    It’s worth noting that CalTopo recently made the high res weekly satellite layer a subscription only feature. I am now relying primarily on SnowEvaluator and it seems to be doing the job.

  6. Jon on May 26, 2020 at 11:01 pm

    slight correction – gaiagps does not support sentinel 2 imagery. the post you referenced relates to the snow depth layer in gaiagps, which is significantly lower res than the sentinel 2 images. here’s to hoping gaia integrates it soon.

    caltopo appears to be by far the best source of sentinel 2 imagery for general hiking as it integrates with all their other hiker friendly layers/features. 2nd best imo is https://apps.sentinel-hub.com/eo-browser/ since it has an OSM baselayer where you can see trails w/names if you zoom in far enough.

    • Andrew Skurka on June 4, 2020 at 12:02 pm

      Thanks for pointing that out. I must have misunderstood the original Reddit thread.

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