If the storm is definitely coming to the area and we're talking about a small business than the employee is free to go. If it's a a place like an hospital, than you have to stay + this places usually have shelters. Your workplace business insurance usually covers you for any injuries you might have had during the storm, so don't worry about that.

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Hi Durel, I'll attempt to provide an answer with the caveat that I'm not a lawyer; my answer is guided by my experience in the business continuity field; and this is not a one-size-fits-all kind of question.  "How long" is completely dependent on the type of storm and warning, and in many cases the location.  A storm watch (and I'll define that as the posibility of a storm developing) would generally not be grounds to change work hours.  This could change depending on the type of storm and area of the country (tornado vs. Winter storm, Michigan vs. Kentucky, etc.).

For storm warnings (my definition is an immediate threat) my experience has been that we may shelter in place during the warning, and then release employees as the situation dictates.  Again, variability between type of storm - for hurricane regions we may actually close certain locations 24 hours or more before the storm (especially for mandatory evacuations), while for winter storm warnings it all depends on the forecast and ability of road crews to manage snowfall/ice.

There are "employers" that do not have the option of closing - hospitals, some government agencies, etc.  They may be able to shift some level of operations to other locations, but that varies from your question about how long an employer can keep a person at work.  

I would also offer that I have worked for employers that used a tiering system, and would allow less critical employees to leave earlier to minimize risk.  Another option is to let each employee determine when they are not cnfortable and let them leave at their discretion, though that works better in some companies than others.