A devastating blast

On Friday, April 24, six days before the end of the South Pacific cyclone season, my wife Lorraine and I motored Southern Cross, our teak Seawitch ketch, to Apu Bay, a deep cut in the southwest coast of Isle Tahaa, French Polynesia. Tropical Cyclone Alan was bearing down on us from the northwest, and Apu Bay was the only anchorage we could find resembling a “hurricane hole” in the Leeward Islands.

Image Credit: Robbie Coleman
Above, the author’s 36-foot on deck teak Seawitch ketch Southern Cross off Honolulu. The author was voyaging in the Society Islands when, with six days left in the South Pacific cyclone season, they were threatened by Tropical Cyclone Alan.

Deluges dumped on us as we tucked behind a tiny islet named Toapuhi. With protection from all directions but southwest, we carefully laid out our primary anchor and 400 feet of 3/8-inch BBB chain. The downpour was so heavy that Lorraine couldn’t see me from the cockpit when she backed down under full reverse to ensure a proper set.

Laboring on deck was like working under a waterfall, but we managed to prepare Southern Cross for heavy windssecuring our roller-furling drums, readying a storm hook for quick deployment. We had weathered many disturbances and depressions like Alan without rigging our storm hook (a large fisherman type), but the earliest and latest storms during cyclone season are strange. Though Alan was small and forecast to diminish, we were uncommonly jittery.

Not only was Alan steaming toward us from the north, gale warnings, for 35- to 40-knot southeasterlies, had been issued for a huge area south of us, from the Australs to our doorstep in the Society Islands. Southern Cross was trapped. I spent a sleepless, rain-drenched night on anchor watch, wondering when the gusty southeast winds in Apu Bay would shift northerly, heralding Alan’s approach.

On Saturday morning, however, we learned Alan had stalled near 16° south, 152° west50 miles northwest of Bora Bora and 65 miles from us. Sustained winds near the center had reached 45 knots on Friday (in the South Pacific, storms greater than 35 knots are called cyclones). By Saturday, however, Alan’s winds had dropped below 35; the central pressure had risen to 1003 millibars; and the storm was downgraded to a tropical depression. All this was good news.

Heavy rainfalls

Though Alan’s movement had stopped, the soggy misery he’d been sending to the Society Islands continued relentlesslythe storm dropped up to six inches of rain in one hour alone. During the night, unimaginable rivers of rain had carved great swathes of jungle from the lush hillsides; countless homes had been washed away or inundated with mud; and five people had been buried alive on Tahaa and an equal number on Isle Raiatea. The once turquoise and indigo lagoon had turned sewer brown and was full of flotsam. Still the merciless deluge fell. Despite incessant thunder and rain, many boats felt safe enough to leave their “hurricane holes” Saturday morning and return to the open anchorage near the boatyards on the northwest tip of Raiatea. Our instincts told us to stay put.

Lorraine and I watched our French pals, Gerard and Denise, on their 50-foot catamaran, Ilyaka, motor south past the entrance to Apu Bay. They had to meet friends arriving from France. Karen and Peter on their 50-foot aluminum sloop, Piquet, had never left the open anchorage, and when I told Peter on the VHF, “We aren’t moving till Alan does,” he responded, “Well, the fat lady hasn’t sung yet.”

Late Saturday afternoon, our barometer on Southern Cross fell and matched Alan’s central pressure of 1003 millibars. The sun emerged, illuminating the deadly mudslides around Apu Bay, and the wind shifted from southeast to north-northeast, blowing at a comfortable 15 to 20 knots. Lorraine asked me, “Do you think this is Alan’s eye?” I shrugged my shoulders and tried to download a weatherfax, but interference made it illegible. The tropical sun boiled away newly fallen rain and the humidity soared. Unbeknownst to us, a broad northwest mid-level airflow had developed over Alan and had literally blown his top off.

Months later, Tim Craig from the National Weather Service Office in Honolulu gave me a copy of Advisory #A20 issued by the Cyclone Warning Center in Nadi, Fiji, for Saturday afternoon, April 25. Regarding tropical depression Alan, it said, “Convection (the vertical movement of air that forms clouds) has sheared off from the lower level convergence center (LLCC) and lies about two degrees south and southeast. In this strong shear it is unlikely that this system will redevelop. This is the final tropical disturbance advisory on this system.” A satellite photo also obtained from Mr. Craig, taken at 0222 UTC April 26th (1622 Saturday in Polynesia), showed massive convection and building storm clouds just south of Raiatea.

At 2000 Saturday evening (0600 UTC on the 26th), I picked up the American weather service from Honolulu on SSB voice radio. Broadcasting information from Nadi, they gave Tropical Depression Alan’s 0000 UTC position as 16.3°S 152.1° W, moving south-southeast at 12 knots with maximum winds 25 to 35 knots near the center. If Nadi was correct, Alan had already passed us!

Storm’s position unchanged

Forty minutes later, however, we received a report on local VHF from Meteo France, the French weather service in Papeete, that said Alan’s position remained unchanged since morning and maximum winds still blew 34 to 37 knots. Who was right? Meteo France also continued the gale warnings for the area southeast of us, this massive cool airflow from the southeast had stopped Alan in his track, allowing the convection to shear off.

An inter-office memo issued shortly thereafter by the weather service in HonoluluPHNL 26040put Alan’s center, “accurate within 20 miles,” at 16.3° S by 152.3° W, 30 miles west-northwest of Bora Bora. The memo continued, “A powerful cumulonimbus (CB) cell has its northern edge about 60 miles south of the center of low clouds”; i.e., just south of us. We had no access to this information.

By the time that memo was issued, the wind had shifted northwest over all the Leeward Islands, funneling directly into the “powerful CB” This warm, tropical, moisture-laden air reached the updraft in the cloud and soared skyward at speeds close to 75 knots. As it rose, it cooled, passed dew point, and condensed into droplets, releasing immense amounts of heat, adding more power to the growing thunderstorm. Some drops fell back toward Earth, forming a downdraft.

The same mid-level northwesterlies that had sheared off Alan’s convection created shearing in the growing storm cloud, forcing it to tilt southward. Without this vertical shearing, the falling rain might have extinguished the updraft and killed the storm. Because of the tilt, however, the downdraft missed the updraft, and only served to fuel this growing weather machine. The falling rain also created small vortices and eddies in the wind flow behind the towering storm, and these eddies caused the tower to arch backward like a bow.

As the cloud bowed, strong, cool, mid-level southeasterlies rushed in from the rear. The falling rain dragged these titanic winds to the ground, creating a colossal downdraft known as a bow echo. The bow echo whipped around the western edge of the cloud and smashed Bora Bora from the south with winds more than 50 knots. It was a little before 2100.

Meanwhile, 15 miles away in our anchorage on Isle Tahaa, we still had northwesterly winds coming down the mountains at 20 to 25 knots, gusting to more than 35 in short-lived squalls. Southern Cross’ barometer stuck at 1003, and I climbed outside for my hourly “deck check.” Clouds blocked all starlight; no lights shone from the shore; we were utterly alone in blackness.

The fringing reef between Southern Cross and the island was invisible, but I knew it lay barely submerged under the lowest tides seen in Polynesia since April 1983, just before Cyclone Veena crushed Tahiti. It was 0700 UTC on April 26new moon dayand, as Pacific islanders have known for centuries, the nastiest weather in this part of the world occurs on or right after new moon. I peered northwest where the French had said Alan should be and saw faint lightning flashes. No thunder accompanied the light show. I climbed below when Gerard from Ilyaka called on the VHF.

Gerard told me that the open anchorage near the boatyards was “agite, mais pas mal” (rough but not bad) with northwest winds 25 to 35, gusting to 40. Like Tropical Cyclones Osea and Martin way back in November, tropical depression Alan spun clockwise and appeared to be passing us to the west. Completely oblivious to the gigantic storm cloud building just south of us, I told Gerard, “That might explain our northwesterly wind shift; maybe it will be over soon.” Neither of us realized that 15 miles away the bow echo was wreaking havoc in Bora Bora.

The bow shock hits

Several boats washed ashore in the strong southerlies in Bora Bora’s lagoon; an airport ferry sank, and roofs blew off a few houses. Like us, our friends on the wooden sloop, Starlight, had expected northwesterlies, and had anchored near the airport, northwest of the island. The bow echo slammed them from the south, however, creating a long fetch. Starlight started their engine and motored ahead at 2,500 rpm to take strain off their anchor. The skipper estimated the winds at near 80 knots, perhaps accelerated by venturi or “island effect.” Their hook held, but their chain began sawing into the bow in the surging waves.

At 2130, a final squall blew down Apu Bay and I recorded some notes in our little cassette recorder aboard Southern Cross:

· “highest gust, 38 knots from the northwest.”

· “since the squall, the wind has dropped below 10 knots”

· “lightning has increased, still no thunder”

· “maybe this sucker is moving away from us; that would be great”

Unfortunately, Alan hadn’t budged; indeed he just kept feeding the behemoth storm cloud behind us. Warm, moist northwesterlies fueled the storm, rushing to heights of more than 50,000 feet. Releasing heat all the way up, condensing water vapor formed “super-cooled” droplets and eventually ice crystals in temperatures close to 80° Centigrade. The ice crystals grew extremely dense in the “overshooting cloudtop” that towered so high above the surrounding clouds that it cast a shadow off to the east.

The super-cooled “dry air balloon” at the top grew heavier and heavier until the cloud reached critical mass and let go with all its pent-up fury. The cold air plunged earthward, picking up momentum and rain, accelerating all the way down till it struck the ocean and fanned out at speeds up to 150 knotsa classic downburst. The leading edge of this enormous downburst, the “gust front,” struck the Leeward Islands just before 2200. Our world exploded!

A hellish blast of wind from the southwest slammed Southern Cross, knocking her on her beam ends. I grabbed hold, and stared in astonishment while the windspeed numbers on our anemometer skyrocketed beyond hurricane strength. I turned and lunged for the companionway right behind Lorraine. We scrambled out the hatch as the demoniacal wind doubled in velocity (one boat clocked 132 knots). That quadrupled the force on our boat and created enormous waves in Apu Bay that broke over Southern Cross and pushed her toward the fringing reef. Lorraine stuck her head around the dodger and pulled it back instantly. Wide-eyed, she screamed over the wind-roar, “We can’t go out there! We need harnesses! Let’s start the engine.” We jumped into survival mode, but there was nothing we could do to save our boat.

Dismasted by the wind

The crushing winds swung over Raiatea, bashing boats and the island with ruthless fury. Peter and Karen on Piquet had readied their boat for the storm and deployed a second anchor, but nobody could prepare for such an onslaught. Peter watched incredulously as his dinghy and outboard flew over his head into the dark like a child’s paper airplane. Piquet’s staysail unfurled; the windage of the flapping sail broke a mast fitting; and the great rig tumbled with a noise more frightening than the wind-roar.

The broken spar wedged in the coral bottom and kept Piquet from being washed ashore like most other boats in the anchorage. A nearby charter catamaran flipped and sank on her mooring: luckily nobody was aboard. Sixty-nine-ton Danae III broke her 5/8-inch anchor chain in the first massive gust and began slamming her way onto the reef. N’shuma, a CSY 44, had wrapped its chain around a coral head and somehow it held. Our friends on Ilyaka weren’t so lucky.

When the wind died shortly after 2130, Gerard told Denise he thought Alan had passed. Thirty minutes later, the wind from hell struck. Ilyaka’s brand-new anchor chain snapped like a worn-out rubber band and the big catamaran took off. Denise leapt to the nav station, watching awestruck as the anemometer climbed to 98 knots.

Ilyaka smashed into a rusty barge and grated alongside, gouging the entire port ama terribly. Ilyaka then broke loose and blew into a boat owned by a French singlehandler named Irve. Irve’s boat broke from its mooring, bounced a quarter of a mile across the reef, smashed through a wooden dock, and continued bouncing and crashing northeast while Irve struggled to hold on.

Gerard tried to steer his catamaran toward the island. Ilyaka missed a rock breakwater by a scant few feet and mushed bow-first into soft mud that days before had been part of the surrounding mountains. South of Ilyaka’s resting place, boats crashed and fell in the boatyards. Liveaboards were flung about inside their falling boats; diesel and water tanks broke loose; masts crumpled; cradle supports punched right through hulls. One multi-hull took off, flew 30 feet and crashed to Earth upside-down.

Gust fronts are uneven masses of roiling, turbulent, surging air. The “nose” of the gust front often rolls back on itself, rising to heights of more than 3,000 feet above the Earth’s surface. As it curls back, it forms a “head,” and an area of spinning air known as the “wake.” Such winds are deadly to aircraft and strike with incredibly destructive selectivity. They rolled over Raiatea and Tahaa like a series of bowling balls.

Plunging barometers

Most boats in the boatyard were knocked over like bowling pins, while a few, including the prophetically named Inshallah, escaped unharmed. Just before the blast of wind hit, local barometers had dropped nearly 10 millibars for an instant and then began pumping. The horrendous wind battered the islands in bursts that made barometers bounce up and down with every surge.

Metal roofs ripped loose from homes and flew thousands of yards like giant corrugated Frisbees. One hit power lines and tumbled the poles in a shower of sparks that plunged the north end of the island into darkness. For nearly one half hour the raging wind-torrent plowed on, mowing down forests and blowing apart houses.

One Tahitian friend told us later that he was afraid to stay in his ramshackle palm-thatched hut on the south end of Raiatea, but before he could drag his kids out from under the bed and take them next door to a concrete building he saw the cataclysm coming. Sometimes a shelf or arcus cloud precedes the nose of the gust frontperhaps that’s what he saw. Easily visible in the incessant lightning, he described it as massive “smoke on the water” roaring straight toward him from the southwest.

He hit the floor as the windows on the concrete house next door imploded and the roof blew to smithereens. His flimsy house was unscathed. Small wonder Tahitians believe in tupapaugruesome ghosts that haunt Polynesian nights. The tempest moved east toward Huahine leaving more than 300 homes in ruins on Raiatea and a dozen boats bent and broken on reefs in the lagoon, including Southern Cross.

Less than a minute after the gust front belted Southern Cross, Lorraine and I had fired up our big diesel engine, but with myriad lines blown over the leeward side of the boat, the motor was useless. Huge waves crashing over Southern Cross made it impossible to reach the foredeck to deploy our storm hook. Lorraine hopped below to check our position on radar and heard the first sickening crunch of wood against coralperhaps four minutes had passed since the initial explosion of wind.

Foaming waves crashed over the whole boat, inundating everything with broken reef debris and salt water as Southern Cross smashed her way higher and higher onto the reef. The jarring was terrifying; movement below nearly impossible. Our wetsuits hung unreachable in the chain locker, so we struggled into our foul-weather gear and boots in case we had to attempt the impossible, crossing the murderous reef to reach shore.

Incredibly, 20 minutes after we first struck coral, Southern Cross lay still. At 2230, the wind died completely in Apu Bay, and the temperature soared, maybe 10° Centigrade. In shock, I asked Lorraine, “Is that the end or only the beginning?” While we trembled in uncanny calm on the reef in Tahaa, Starlight and the other boats anchored a few miles away in Bora Bora fought to stay put in the ongoing “bow echo.”

Broken anchor chain

The devastating gust front reached the north end of Huahine and swept over that island as an angry arm might sweep dishes off a table. It started westerly and swung through southwest to south before blowing itself out 30 minutes later. The ritzy Hana Iti Hotel was leveled; boats were beached and holed; and more than 400 houses lay in tatters. Somehow, a catamaran named Saveur De Soir sailed upwind over the barrier reef after their anchor chain broke, and they found themselves bobbing safely in the open ocean.

At 2300, the wind vanished in Huahine, and the bow echo finally died in Bora Bora. Starlight shut down their engine. The massive thunderstorm had blown itself out; hot spooky silence engulfed the Leeward Islands. Tahiti, 125 miles to the east, had felt nothing; nor did Maupiti, 45 miles to the west. The rest of the night passed in mind-boggling serenity.

By the following morning, the remnants of the stalled tropical depression dissipated. Alan had never reached hurricane strength (64 knots,) but his progeny, a 120-knot macroburst, caused more damage in 30 minutes than any cyclone that season. By Monday, April 27, gentle trade winds and puffy cumulus clouds returned; the cyclone season was over.

Survivors later asked themselves what, if anything, could have been done to mitigate the disaster. For all, the answer was easy: “Nothing.” Understanding how these systems form and function might help in some instances.

For example, a knowledge of the effects of windshear on thunderstorms may assist a fast ship in taking evasive action, similar to avoiding the “dangerous semicircle” of a hurricane or cyclone. Since this particular storm tower tilted south, the brunt of the gust front was expelled toward the north. (Imagine a fire hose held at an angle and directed at a slab of concrete.) A ship north of this storm might have escaped by steaming at high speed east or west, and a vessel south of the storm would have experienced much less wind.

Each thunderstorm is unique; its “tilt” depends on windshear, which may be discerned by noting the difference in direction of movement between lower- and upper-level clouds. Occasionally, the direction of “tilt” in a thunderstorm can be seen from sea level, but usually such destructive storm towers are imbedded in and hidden by other clouds.

Real-time satellite photos would help if one had the ability to read the pictures and understand the potential danger in phenomenon like overshooting cloudtops. Unfortunately, weather information broadcast to mariners in the Pacific is often six hours old. Thunderstorms often form, explode, and dissipate in less time than that. Hopefully, information like “near real time” infrared rain-rate “photos,” which are currently available to the military, will soon be made available to civilian mariners.

The best a prudent skipper can do is to be aware of conditions that may spawn such devastating tempestsconverging air masses over warm ocean water; windshear through ascending layers of atmosphere; and unstable air evidenced by rapidly growing towering cumulus and cumulonimbus clouds. Fortunately, macrobursts, microbursts, and bow echoes are localized phenomena, and all are rare.

Robby and Lorraine Coleman have been voyaging the Pacific for five years aboard Southern Cross and have just completed a refit in Honolulu.

By Ocean Navigator