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Surviving Sudden Thunderstorms

2009 September 1
We've all experienced weather conditions different than what had been predicted. A number of factors are responsible: alterations in the speed of weather systems, interactions between separate systems, upper-air influences and locally developed disturbances that are not seen on the large-scale weather charts. These all are important considerations to be taken into account when planning a departure, and emphasize several key elements of offshore preparation.

"We can't control the weather we encounter at sea, but we'd better control the weather in which we leave for sea." This is a refrain my offshore students hear frequently, and it points out the fact that, once at sea, we have to be able to cope with whatever cards nature chooses to deal us, including unexpected heavy weather conditions.

Downloaded weather charts are important for gaining perspective on the large-scale developments to expect. These must be supplemented by a separate weather log (Figure 1) on which local conditions are logged and studied. These two sources of information can then be combined to formulate more accurate forecasts on the local level.

During a recent spring passage, Voyager, our 46-foot Beneteau, was en route from St. Thomas USVI to Bermuda. Our crew of five and myself were sailing in light to moderate breezes that favored the light end of it. Several low-pressure systems had been parading across the Atlantic from the Northeastern states toward the east, and the latest had diminished the trade winds by drawing air to the north; hence our light conditions. There was no cold front extending southward from any low, and yet a curious cloud bank was building to the southwest (see photo above).

Its characteristics were much like those of an approaching cold frontal boundary. It stretched out for a good distance on the horizon, had the sharply demarcated lower portion with unstable, billowing clouds that extended much higher. Lightning began to streak across the clouds and to the water in rapid succession, indicating that tremendous electromagnetic potential was being created between the burgeoning disturbance and the earth.

This was an air mass thunderstorm, and it was going to be more ferocious than they are normally. The air mass thunderstorm is usually a development of moderate intensity that forms away from frontal systems or other large-scale disturbances. They form where moist and unstable conditions exist in the atmosphere, and warm, moist air rises. The resulting thunderstorms are usually mild because they form in areas with little upper-air wind shear. This results in short-lived, mild thunderstorms. When winds aloft drive the upper sections of the system horizontally, so that the ascending winds are apart from the descending rain and cool air drafts, the thunderstorms may produce brief high winds or hail, which develop because of high instability.

Although several storm cells can develop within these systems, each individual cell lasts less than an hour, and has a life cycle involving three stages. Close observation of these stages helps us in determining the severity of the thunderstorm, and ultimately how to prepare the boat before it strikes.

Stage 1: Early development (cumulus) phase
" Observable updrafts of warm, moist air forms ever-expanding cumulus clouds.
" Precipitation begins to form in the clouds, producing a gray tone compared with the brilliant white of the cloud itself.
" As the cloud tops ascend, super-cooled water droplets are carried far above the freezing level.
" Watching the upper portions of the clouds, a boiling appearance signifies instability and a greater likelihood of storm activity.
" With greater ascending turbulence, air outside of the cloud is mixed into the updraft. This mixing signifies the development of a more severe thunderstorm.

Stage 2: Mature stage
" This is the active storm portion of the cycle. Cloud tops can reach tropopause height (above 30,000 feet), and can contain both water droplets and ice.
" Lightning and thunder may be present.
" Cold precipitation falls from the high elevations and carries cold air along with it. This produces the cold downdrafts of air associated with thunderstorms. Microbursts are very strong gusts of this cold air propelled by the fast-moving water and ice. Although violent, microbursts typically last less than five minutes.
" The storm is fueled by warm updrafts and emits cold downdrafts. The storm duration and character are determined by the presence of wind shear aloft, which can separate these opposing forces, thus intensifying and extending the n Low layers of dark gray/green clouds indicate severe bursts of rain and wind. Another indication of impending severity is the presence of white, billowing, turbulent clouds amidst the dark lower sections. These arise due to cool air being aspirated into the system from outside.
" If the system is downwind from the prevailing wind direction, the surface wind will die or become very light as the thunderstorm's descending outflow counteracts it.
" Watch the sea surface in the distance toward the storm; it will begin to bubble when the rain drives downward into the waves, and will actually flatten the existing wave patterns.
" Preparation of the boat must begin when the cumulonimbus clouds are upwind and heading toward the vessel. This is high time to prepare for rain and possibly high winds with microburst gusts.

Stage 3: Dissipating stage
" Opposition of updraft vs. downdraft winds; rising air is shut off as the system becomes dominated by the downdraft winds and rain.
" Outflow winds diminish, and precipitation intensity at the ground weakens.

The air mass thunderstorm we encountered had all of these characteristics, and we took note and prepared for it. The choice in this situation is to either run with such a storm or heave-to. I usually choose the latter because it's much safer. We flew a double-reefed mainsail with the 135-percent jib rolled in 90 percent. Voyager was hove-to when the first burst hit, and we all sat together, safely under the dodger, and watched the show.

This system of squalls had several different cells. Fortunately, they were easy to track on the radar, and were seen as separate, circumscribed systems within the generalized mass of clouds. The strong cells appear circular, with outer bands that surround an inner core of activity. Our black and white radar shows darkened outer portions surrounding a lighter inner section. Color radars would demonstrate blue outer portions with yellow or orange colored-cores of rain activity and high winds.

The winds gusted more than 50 knots, with horizontal rain driven hard enough to sting the skin. Existing sea swells were dampened and the previous wind and wave system was completely diminished by the onslaught of rain.

What allowed us to get through this powerful thunderstorm system without loss, damage or injury? The fact that we observed the formation and watched the progression of this air mass allowed us to prepare adequately. We had an idea of the severity it would reach, so the instruments were unplugged and the crew all donned rain gear as we waited for the show. My student crew now understood why I'm such a stickler for monitoring local weather developments rather than depending solely on NOAA charts, and that even nasty disturbances like this can be easy to handle. Huddled together beneath the safety and security of the dodger and bimini, we watched as rain diminished visibility to less than a boat length, winds buffeted and screamed through the rigging, and Voyager lay hove-to and withstood the onslaught with ease.

Winds following the passage of these systems very often go light, but eventually freshen in about the same direction as the previous prevailing winds. Make sure no other developments exist to windward before resuming course and shaking out reefs. The other piece of advice I can give, as an air mass thunderstorm develops and then approaches your boat, is to make sure your camera is handy, as the pictures are usually dramatic.

Quick Tips

" Be prepared for unexpected weather before leaving the dock.
" Supplement weather charts and forecasts with a weather log updated regularly, such as after each watch.
" Observe developing storms for clues to their severity and prepare boat and crew appropriately.
" Unplug instruments to protect them from lightning damage.
" Decide whether to run before the storm or heave-to, and match the sailplan to the conditions.
" Have a camera handy to capture Mother Nature's unexpected fury.