Perhaps you’ve heard news reports about the effects of “wind shear” on aircraft during takeoffs or landings. A similar phenomenon may also occur on the water and effect boats. Caused by thunderstorms, these localized, strong winds are called DOWNBURSTS. Although there are several ways in which these form, they all exhibit certain characteristics: 1) winds may exceed 130 mph - much faster than even the normally gusty thunderstorm winds, 2) the strong winds hit suddenly with little or no warning, and 3) the strongest winds affect a relatively small area.
DOWNBURSTS come from thunderstorms. Therefore, whenever a boater encounters a thunderstorm, a DOWNBURST is possible. This is true if the thunderstorm is all by itself or is part of a series, such as a squall line.
When a DOWNBURST first strikes the surface, it is often concentrated in an area less than 3 miles across. This is generally where the most extreme winds can be found. The term MICROBURST is often used in describing this phase. After striking the ground, the winds begin to spread out, eventually covering an area up to 30 or 40 miles across. However, MICROSBURSTS can still be embedded in the general DOWNBURST.
DOWNBURSTS are usually short-lived high winds lasting only a few minutes. However, one thunderstorm can produce a series of these winds affecting a swath several miles long and lasting an hour or more.
DOWNBURSTS hit so rapidly that few signs may be available to alert the boater as to its presence. Blowing spray under or slightly ahead of a thunderstorm may be the only indicator. However, the best rule is, avoid ALL thunderstorms if possible. If not, expect and prepare for the worst whenever a thunderstorm is encountered.
The hazards of Downbursts are:
Boats caught out on open water under these conditions can encounter a downburst without expecting it. DOWNBURSTS are generally short-lived; lasting less than ten minutes. This fact makes predicting their occurrence almost an impossible task. The sudden loss of the sailing vessel “Pride of Baltimore” in the Atlantic near Puerto Rico in 1986 was attributed to a downburst wind. Witnesses claim in less than two minutes, the ship was blown over, filled with water, and sank. Although this tragedy involved a larger sailing vessel on the open ocean, similar dangers have been experienced by vessels on inland or inshore waters.
- Extreme, sudden winds which can tip a sailboat beyond its range of upright stability.
- Heavy seas that can capsize powerboats too.
- High winds that can blow equipment off the deck and cause persons on board to lose balance and fall overboard.
Thunderstorms can create several downbursts in succession, with varying degrees of intensity. A thunderstorm might even generate a combination of downbursts and tornadoes.
The best source of current weather information is the continuous National Weather Service broadcasts. Seven frequencies between 162.400 and 162.550 MHz have been set aside for this purpose. However, three (162.400, 162.475, and 162.550) are the most common. Taped information is recycled approximately every five minutes with broadcasts updated no less than every 3 to 6 hours. In the event of unusual or severe weather, the programming may be interrupted and broadcasts are done live.
The Coast Guard also broadcasts special marine weather information, including small craft advisories, on VHF Channel 22. Visual warnings are no longer operated by the National Weather Service. However, certain organizations have continued to display lights and flags when rough weather is expected.
- Turn your craft with the bow facing into the wind, and “reef” your sails if you have them. These actions help minimize wind resistance.
- Secure all loose objects and rigging on-deck, and make certain hatches or other openings are covered.
- Wear your personal flotation device, making sure it fits securely. Keep other lifesaving equipment readily accessible, including inflatable rafts and visual distress signals.
Smail Craft Advisory:
Generally associated with sustained winds 18 to 33 knots, or waves hazardous to small boats. These are not issued during the winter months along the Great Lakes.
Sustained winds 34 to 47 knots.
Sustained winds 48 knots or more.
Sustained winds 64 knots or more associated with a hurricane.
Special Marine Warning:
Winds of 35 knots or more lasting generally less than 2 hours. These are usually associated with an individual thunderstorm or an organized series of thunderstorms (squall line, cold front).