Storm & Flood Damage Recovery in Nashua, NH

That is the annual pattern here rather than an anomaly. Soil goes into the freeze already wet from a heavy October, dense till holds that water through the winter, and then the snowpack melts onto ground that is still frozen below the surface. All of that water travels at or near the surface and arrives at foundations. It is predictable to within a few weeks, which at least makes it something you can prepare for.

It matters more than the age of the house. The older core sits low near the river confluence and deals with river and drainage backup; the post-war suburbs sit on till and deal with slow drainage and thaw saturation. Those need different remedies, and advice that circulates locally often comes from one setting and gets applied in the other. Knowing which of the two you are in resolves most of the confusion.

The lower level, particularly. Below-grade space in a mill building was built as working space, with thick masonry set into wet ground and no drainage layer, and it was expected to be damp. A conversion that added finishes without adding drainage traps moisture behind materials that cannot handle it. If you can find out whether the conversion included a drainage system, that answers most of what you need to know about the risk.