Let's just say that 24 F is a big problem for temperate trees left exposed to it. The problem with the coming freeze is that it is deep and will last from the early evening well into Saturday. Temperature that low and for that long will freeze the soil and roots of most bonsai (even big ones) through and will allow temps inside the pots to drop precipitously and rapidly. If you understand the mechanics of how cold affects roots, forecasts of 25 F or below are concerning. It has to do with lower temps freezing water inside root cells causing root damage. Water can act as an insulator in shallow freezes, since water around roots has to freeze first, then water between cells in the roots, then water inside cells. The first two don't really do anything to the tree's health. The last does.
That's why watering bonsai thoroughly before a shallow hard freeze can provide adequate protection for a night or two. But with this deep cold, things will freeze rather quickly and thoroughly.