Weather Can Elevate Rip Current Risk
Angie Enyedi, National Weather Service Jacksonville
Rip currents are the leading surf hazard for beachgoers, especially for weak swimmers. Rip currents cause about 100 drowning every year in the United States, and more than 80% of water resuces on beaches are due to rip currents.
A rip current is a horizontal current. Rip currents do not pull people under the water–-they push people away from shore. Drowning deaths occur when people pushed offshore are unable to keep themselves afloat and cannot swim back to shore. Prohibitive factors may include any combination of fear, panic, exhaustion, and/or poor swimming skills.
Rip Current Formation
A storm system that is producing strong winds over ocean waters usually causes a rise in sea heights close to the center of the storm. These are called wind-induced waves. As these waves propagate away from their generation area close to the storm, the waves become swells. Figure 1 shows a tropical storm over the Atlantic Ocean and the direction and approximate height of swells that are traveling away from the storm. Swells typically have a more regular and longer period than wind-induced waves.
As the swells travel away from a storm center and impact a coastline, they enhance the amount of water (or surf) that accumulates along the coast. As the surf begins to subside seaward, the greater weight of water from the swells can carve out trenches in sandbars within surf zone. A large mass of water retreats seaward in these channels or ‘rips’ and creates a strong seaward retreating force or current of water. These channels are rip currents. Figure 2 illustrates the development of a classic rip current.
If the prevailing low level wind is onshore, it can also force a greater than usual amount of water on the coastline, which consequently increases the likelihood of rip current development.
The seaward pull of rip currents varies; sometimes the rip current ends just beyond the line of breaking waves, but sometimes rip currents continue to push hundreds of yards offshore. Rip current speeds are typically 1-2 feet per second. However, speeds as high as 8 feet per second have been measured--this is faster than an Olympic swimmer can sprint! Thus, rip currents can sweep even the strongest swimmer out to sea.
Rip Current Evasion
If a beachgoer is forced out to sea in a strong current of water, it is likely a rip current. The key is to not panic, then to start swimming perpendicular to the current until you do not feel the force of the water pushing against you. Rip currents are relatively narrow channels, so swimming perpendicular to the channel will allow you to escape the force of water pushing you farther offshore (as shown in Figure 3).
Rip Current Risk Forecast
The National Weather Service Jacksonville issues a daily Surf Zone Forecast, and within this product we headline the daily rip current risk. There are several factors we consider when determining the daily rip current risk including the surface wind speed, swell heights, swell periods, swell direction and the time of low tide during the day. If all of the above are high values and the time of low tide (retreat of water seaward) is between 10a-4p, then there would likely be a high risk of rip currents.
The rip current risk level ranges from a low risk, to moderate risk to high risk. The risk level implies that rip currents will likely be stronger (more forceful) and more numerous on moderate and high risk days compared to low risk days. Figure 4 gives an example of our rip current risk graphic when a high risk of rip currents was expected for the Florida coast with a moderate risk in effect for the Georgia coast.
For those that are inexperienced swimmers, referring to the daily surf zone forecast before going to the beach may be a prudent safety measure. Please visit the National Weather Service Jacksonville Rip Current Information webpage for more detailed local surf forecast information.