This app shows a map of snow cover duration anomaly in the Western USA since January 01 until today in comparison with the average over the same period of the year between 2000-2020. The app also shows the evolution of the snow cover area since January 01 (in fraction of the entire domain). The snow cover area was derived from Nasa's Terra/MODIS daily observations. The app is automatically updated when new observations are available.
- The daily NDSI is extracted from the MOD10A1.006 collection (~500 m spatial resolution). Missing values are filled using a linear interpolation in the time dimension .
- The resulting series of daily gap-free NDSI is converted to a series of binary snow maps (snow / no-snow) using the threshold NDSI>0.2.
- The snow duration is the sum of the snow maps along the time axis.
- The snow maps over 2000-2020 were pre-computed and stored as an asset to reduce the app loading time (but it's still slow, blame google, not me).
Earth Engine users can get the source code in my Apps repository:
git clone https://earthengine.googlesource.com/users/sgascoin/apps
 Missing values account for about 40% of the pixel.days, mostly due to cloud cover. Although more sophisticated algorithms exist to fill gaps in MODIS snow cover maps, the linear interpolation in the time dimension is a good trade-off between efficiency and accuracy.