The National Weather Service (NWS) provides both zone area forecasts for entire regions and point forecasts for small geographical areas all over the country, which can be helpful for trip planning. Additionally, Caltopo offers weather forecast overlays, which include temperature, precipitation, and wind.
Zone area and point forecasts
Zone area forecasts
Zone area forecast for the Eastern Sierra Slopes of Inyo County. Unsurprisingly, this gives a general ("average") forecast for the slopes of the Eastern Sierra for the entire north/south range of Inyo County, from the border with Kern County in the south, to the border of Mono County in the north.
Note: one can use the NWS site to create user-defined area forecasts (i.e. custom zone area forecasts) as well. Simply visit the NWS user-defnied area forecast page, scroll around on the map to where you want, and start clicking to create a polygon covering the area for which you desire an "average" forecast, and then click "Get Area Forecast" at the bottom.
Point forecast for Onion Valley. Though the forecast may appear to be for Onion Valley, it's actually for at area nearby, and slightly west, of Onion Valley, and for an elevation of 10,351 ft. When using point forecasts, be sure to look under the map to tee the elevation for which the forecast is given, along with the last update time for the forecast, and how long the forecast is valid.
To get more point forecasts, simply zoom in/out and scroll around on any point forecast map or zone area forecast map, and single click on an area for which you'd like a point forecast. That will load up a page with your forecast.
Additionally, to get an hourly weather forecast graph that corresponds to your point forecast, look under "Additional Resources" on the right below the mini map, and click on "Hourly Weather Forecast". For an example, see the hourly weather forecast graph for 10,351 ft. just west of Onion Valley.
Webcams can help a bit with snow, though the Caltopo Sentinel Weekly overlay is better. But using ski area web cams (e.g. Mammoth Mountain) can sometimes help. There are whole networks of webcams out there, but be sure to check the dates of the photos, as sometimes they're months or years out of date, even when they're supposedly updated daily. Here's a few links to get started:
Caltopo weather overlays
For more in-depth discussion of using Caltopo, see the the Digital mapping article. However, at a high level, simply look at the various weather options and overlays in Caltopo, and select the overlay and time period you wish. These will let you monitor snow cover, forecast highs and lows, wind speeds, etc.
For example, consider the following images showing the forecast 24 hour max wind speed, high, and low predictions:
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Screenshot showing the 24 hour low temperature forecast for just south of Onion Valley. See the upper right for the legend.
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Screenshot with the Sentinel Weekly layer (set to False Color Green) for snow detection, with the Weather Grid option under "Forecasts" ticked.