From Rochford v. G.K. Development, Inc. (Iowa Ct. App. Feb. 5, 2014), quoting earlier precedent:

The authorities are in substantial accord in support of the rule that a business establishment, landlord, carrier, or other inviter, in the absence of unusual circumstances, is permitted to await the end of the storm and a reasonable time thereafter to remove ice and snow from an outdoor entrance walk, platform, or steps. The general controlling principle is that changing conditions due to the pending storm render it inexpedient and impracticable to take earlier effective action, and that ordinary care does not require it.

What’s noteworthy, of course, is that this is a legal rule, not just a jury argument — not removing ice and snow during a “continuing storm” is considered non-negligent as a matter of law.