North Carolina averages fewer than 20 tornadoes per year, many of them produced by hurricanes or tropical storms along the coastal plain. Tornadoes from thunderstorms are a risk, especially in the eastern part of the state. The western Piedmont is often protected by the mountains, which tend to break up storms as they try to cross over; the storms will often re-form farther east. A phenomenon known as "cold-air damming" often occurs in the northwestern part of the state, which can weaken storms but can also lead to major ice events in winter.