Hurricane Waves

In the days before satellites and telegraphs and radar, people had to rely on weather signs to tell them what the forecast was. There were signs of an approaching hurricane. A brick red sky in the evening. Mare’s tails in the sky (cirrus clouds.) Animals behaving peculiarly. But one sure sign that trouble was imminent was a change in the sing song pattern of ocean waves breaking on the beaches. Normally, a wave arrives about every six seconds. But as a major hurricane approaches, the period, or time between waves increased dramatically and the power of the waves rises, pounding the beaches with explosive force. Why does this happen?

Wind transfers energy to water. This energy takes the form of waves. The size of the waves generated is dependent on the wind speed, the length of time the wind blows over the water and the fetch, or distance of water over which the wind blows. The higher the wind, the higher the waves that can be generated. These waves move out away from the storm. As they get further away, they decay, or lose height, and become more rounded. Their period, or the amount of time it takes for a full wave to pass also increases.

Since these waves can travel large distances, as Dean gets into the Gulf of Mexico, coastslines far away will begin to see the impact of the storm in the surf.

The problem is that the weather may be very nice at a coastal location as a hurricane passes by several hundred miles offshore heading to a distant location. But the wind field produces these powerful, long period waves that reach the shore where the weather is fine. The swells and waves can be awe inspiring. But many fatalities have occurred where unsuspecting people frolic in the waves under fine weather conditions and get caught in powerful battering ram waves and dangerous rip currents.

From the National Weather Service Mobile…

SWELLS GENERATED BY DEAN WILL BEGIN MOVING NORTH TOWARD
THE MIDDLE GULF COAST REACHING INTO THE COASTAL WATERS WITHIN 36
HOURS OF DEAN PASSING SOUTH OF THE YUCATAN CHANNEL. SEAS WILL BUILD
CONSIDERABLY ABOVE WHAT WOULD BE CAUSED BY WIND ALONE. WIND WAVES
COMBINING WITH SWELL GENERATED BY DEAN WERE FORECAST TO REACH 6 FEET
OR SO BY TUESDAY AND 8 FEET…LIKELY MORE…BY WEDNESDAY THEN SLOWLY
SUBSIDING BY THURSDAY. THE 8 FOOT FIGURE MAY TURN OUT TO BE SOMEWHAT
CONSERVATIVE. RAN SOME CALCULATIONS ON A 30 FOOT 12 SECOND WIND WAVE
AT THE GENERATION POINT IN THE YUCATAN CHANNEL. FOR A SWELL
PROGRESSIVELY DECAYING BY 1/R WHERE R=50 MILES WE GOT 8 FEET….AND
THAT WAS A CONVERGENT ANSWER FOR SMALLER INTERVALS. CURRENT WAVE
DECAY COMPUTATION TOOL INDICATES 18 FEET 15 SECONDS. DOUBT SERIOUSLY
IT WOULD BE THAT HIGH AT THE BEACH DUE TO BOTTOM HYDROGRAPHY. IN ANY
CASE…BEACH EROSION…RIP CURRENTS…AND OTHER DANGEROUS
CONDITIONS…SEAWARD AS WELL AS NEAR THE BEACH…WILL BE ADDRESSED
STARTING AS EARLY NEXT WEEK WHEN A BETTER ASSESSMENT CAN BE HAD.

It will be a week on the Gulf Coast to sit out the waves as Dean likely passes to the south.

Here is a webpage about wave forecasting that I found interesting.

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