Just got word back from OpenSummit app folks. They do have off-line information. Plus, I just tested it by putting my phone into airplane mode w/o wifi to see how the app starts up. Here is their verbage with some improvements coming...
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Thanks for reaching out!
You can view forecasts offline on the OpenSummit app currently.
We are planning to combine OpenSummit and OpenSnow into one app this summer, all under OpenSnow.
Our offline forecast functionality is weaker in OpenSnow at the moment but we are planning to make improvements here so that you can view the last available forecast (from when you had service) while you are out on the trail.
Thank you so much for your support and please let us know how else we can help!
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Two Dogs
Derek Koonce