National Weather Service United States Department of Commerce

Tropical System Impacts for the East; Severe Weather and Heavy Rainfall Threats

Heavy rain and dangerous surf conditions up the East Coast are forecast as tropical storm conditions approach late this week and into the weekend. Meanwhile, strong to severe thunderstorms are expected to impact portions of the central U.S. including heavy to excessive rainfall through Saturday. For the Intermountain region, below normal temperatures and some snow for the higher terrain. Read More >

Significant Wave Height

Significant wave height is an average measurement of the largest 33% of waves.  We measure it because in many applications of wave data, larger waves are more "significant" (important) than smaller waves.  For example, the larger waves in a storm cause the most erosion on a beach.

Significant wave height measured by a wave buoy corresponds well to visual estimates of wave height.  Most human observers tend to over estimate the real height of waves.  As the significant wave height is an average of the largest waves over a recording period it should be noted that some individual waves might be much larger than this.

On average, about 15% of waves will equal or exceed the significant wave height. The highest 10% of waves could be 25-30% higher than the significant wave height. And on occasion (about one per hour) one can expect to see a wave nearly twice the significant wave height.

Wind Waves

(1) Waves being locally formed and built up by the wind; SEAS. (2) Loosely, any wave generated by wind.


Wind-generated waves that have traveled out of their generating area. Swell characteristically exhibits a more regular and longer period ( 10 seconds or longer) and has flatter crests than waves within their fetch (SEAS).