Derecho storms: the latest weather threat

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Meteorologists are just starting to unravel the mysteries of this superstorm

Meteorologists are just starting to unravel the mysteries of this superstorm

 

Imagine a dark wall of clouds storming toward you. Suddenly, the sky turns black and raindrops like hail pound down. The wind makes the rigging moan, then pitches to a high shriek. Something wild grips your mast, shaking it and driving your sloop’s spreaders into the sea. You try to head off, but your boat is out of control. Take a deep breath; your day is about to get even worse.

The heavy weather you just encountered may be a derecho (pronounced de-RAY-cho), a not-well-known superstorm with high winds, downbursts and accompanying high seas. And it can pose an immense danger to boaters on big waters, from the Great Lakes to the North Atlantic. I know something about these conditions because I experienced the above scenario. I am one of the relatively few boaters who has been caught directly in a derecho and count myself lucky to have survived.

Anatomy of a derecho

Derechos can surprise. Sneak storms, they arise quickly and typically can’t be forecast until they’ve formed. By that time it’s often too late for boaters to seek shelter in the minutes before they hit.

The word derecho means “straight” in Spanish, and the storm packs straight-line winds, as opposed to “tornado,” which means “turning” and is characterized by circular winds. Derecho winds must meet the National Weather Service criterion for severe wind gusts (greater than 57 mph), although with stronger derechos winds can exceed 100 mph, according to the National Oceanic and Atmospheric Administration.

A derecho by definition is composed of “a family of downburst clusters” and must be at least 240 miles in length. A sustained series of downbursts can scour the sea or land below with blasts of cold, heavy air that can pin keelboats on their beam-ends, overturn small sloops, and cause even well-anchored powerboats to break loose. The strongest winds occur just above the ground behind the gust front, then curl upward and back. The surface winds, according to NOAA, are “apt to be well in excess, perhaps as much as double, the gust-front speed.” Surface winds also speed up over water. Though a common thunderstorm can last about an hour, it’s not unusual for a derecho to last for 10 hours.

The historic derecho I personally know about began as a severe windstorm in the early morning hours of July 4, 1999, roared across northern Minnesota, and turned into a derecho with its bow-shaped echo as it neared the state’s Boundary Waters Canoe Area Wilderness. It continued to roar eastward to end in Maine near the Atlantic coast, with a life- span of almost 24 hours, and left a path of destruction nearly 1,500 miles long. In the BWCAW, old-growth pine and spruce trees were broken like matchsticks, with devastation to 477,000 acres in one of the largest North American forest disturbances in recorded history. The straight-line winds were clocked at 80 mph, and the downbursts, though not recorded, were conservatively estimated to be well in excess of 100 mph. It ceased being a derecho as it moved into Maine but continued as a storm that swept out over the North Atlantic for a thousand or more miles, reversed, then swept back along the East Coast, causing more destruction.

Stronger derechos have been recorded. On July 4, 1977, a derecho swept across northern Wisconsin with winds measured at 115 mph. During a May 31, 1998, derecho that thundered across Wisconsin and Michigan, a wind gust was recorded at 128 mph in eastern Wisconsin, and gusts were estimated at 130 mph in southern Michigan.

Derechos can come out of the sky anywhere over water or land in the northeastern and east-central parts of the United States, the southern regions of Canada, and the central and northern plains regions. About a dozen derechos are identified in the United States each year.

In 2004 an “About Derechos” section became part of NOAA’s Storm Prediction Center Web site, www.spc.noaa.gov (search keyword “derecho”). Developed by Robert H. Johns, a former lead forecaster at the Storm Prediction Center in Norman, Okla., and fellow forecaster Jeffry S. Evans, the pages define derechos, tell what kind of storms cause derechos, give strength and variation of derecho winds, and assess casualty and damage risks. The Web site also cites some historic events, as well as gives satellite and radar images that show derechos forming and evolving.

The resulting body of information is impressive and might come as a surprise to many boaters, because derechos often aren’t properly identified as they form, after they hit, or in news accounts of the damage. Derechos also occur in other parts of the world, though only one such event had been documented prior to June 2004, when a serial derecho occurred over eastern Germany and adjacent portions of other European countries. In Berlin and surrounding areas, eight people were killed and 39 were injured from falling trees.

Nobody knows exactly how many derechos occur worldwide, but it’s thought that some deadly storms known as nor’westers in Bangladesh and adjacent portions of India could be progressive derechos. Much of what is known about the storms has been gleaned only with recent advances in weather science and technology, which permits meteorologists to study them and probe their secrets. They’re tricky to forecast, and a warning often gets out only minutes before one hits. That’s what happened to me.

Surviving a derecho

When nasty clouds appear on the horizon, boaters need to take heavy weather precautions quickly, as there is little time to react to a derecho. This is a storm that is known as the “first worst,” meaning that the initial wind fronts will be severe and with heavy rain, and out of the storm will come downbursts like little weather bombs containing additional heavy and malevolent winds. The derecho that found me in the early afternoon of that Fourth of July was like this.

I was sailing my home-built 20-foot wood veneer-epoxy centerboard sloop, Persistence, on the Canadian waters of Lake Superior, heading to a remote island at the mouth of Thunder Bay, Ontario. The day had begun with bright sun, blue waters and clearing fog on the nearby Sawtooth Mountains to the north. NOAA weather had predicted the “hottest day of the year,” with possible thunderstorms later in the afternoon. Because of the windless conditions, I had my 5-hp Nissan 2-stroke purring along, sails uncovered but furled.

My VHF blurted a storm klaxon, and I heard warnings that in the city of Thunder Bay, about 18 miles north of me, a sailboat had overturned, with three people in the water. I turned my eyes northward and saw off my port quarter that a crown of dark fog was intensifying about the peaks, but I figured I’d make my destination about 20 minutes away. The sky turned black, and I could see that a thick, dark cloud mass was quickly sweeping out over the open waters of Superior toward me. Thunderous raindrops enveloped my boat, and out of the rain came strong winds. I listened as the sound in the shrouds went from a moan to a high-pitched shriek I had never heard before. A wall of wind hit me, literally picking up my ultralight sloop and catapulting it forward.

Persistence’s bow dug in. I was lifted from my seat and managed to duck as the companionway came up suddenly and I slammed head-first into the starboard quarter berth. Water sloshed through the centerboard trunk. I managed to raise my head in time to see that the starboard port light had turned green — it was under water. Persistence teetered on her starboard beam-ends but not for long. There was a distinct howl of wind, and I felt the boat surge as if it were flipping over. Capsize, I thought, and I had visions of being trapped under an overturned hull on my way to the middle of Superior. But the mast came out of the water — I could feel it — saved by the buoyancy I had foamed into the top of the spar.

In the cockpit I found it hard to take breaths, and the temperature seemed very cold, though that was the least of my worries: My boat lay on its side, spreaders nearly down to the water, and not coming back up. We had done a partial pitchpole, and a quick glance showed me that nothing in my boat was broken. Around me the surface water was smashed flat, but was filled with writhing haymow-shaped contrails thousands of feet long.

I did what any reasonable sailor would have done under the circumstances: I threw myself to one side. But putting my weight on the port side as far out as I could wrestle Persistence had no effect, and she remained on her side for several minutes, engine roaring, prop out of the water and out of control. As near as I could figure, we were crabbing sideways downwind.

As the blast lessened, the mast came up, the propeller dug back in, as did the tip of the rudder blade, and we had steering again. Timing the weakening gusts, I turned Persistence around and finally fought my way to the shelter of a small island, where I remained under power until I later made it to the island harbor, shaking with the cold and in some state of hypothermia.

I later learned that news accounts estimated wind gusts of up to 115 mph. The NOAA report conservatively referred to the speed as “well in excess of 100 mph.”

I had survived a rare and extremely dangerous progressive derecho.

Shelter from the storm

Early on, the storm completely took control and it seemed that the best course of action was simply to let the boat find its way and handle the tempest as best as it could. I basically hung on, since nothing I did seemed to be of any use. Heading off wasn’t working, neither was going faster off-wind. With no sail up and in the moments I had, there wasn’t a lot I could do. I was lucky.

A sailboat attempting to enter a breakwater in Thunder Bay was shoved backward out into the bay and overturned. Two people and a dog were rescued. Another sailboat, a 33-foot keelboat, had been peacefully hanging on a 35-pound plow anchor in a sheltered bay, protected by Pie Island, as low-lying clouds suddenly blocked the hills. When the winds swept over the island, Easy Blue’s hook began to drag, despite the engine cranking full revs. The boat twisted 45 degrees under the strain and white spray moved horizontally around the sloop several feet above the waves, which had grown to 6 to 8 feet and were coming from two directions. After the first blasts, the all-male crew wrestled up the hook and ran downwind to the shelter of another, larger island.

From the reports I heard on the VHF — in the shelter of an island harbor — and later from published news accounts, I never knew the storm I survived was a derecho. In July 2004 I was contacted by the NOAA forecaster Johns, who had read newspaper reports of my encounter. He was developing the “About Derechos” Web site, and this was the first time I’d learned that the superstorm that hit me had been anything other than a local phenomenon, though I had done some research on the Internet. Had I known a derecho was off my port quarter, or known about the downburst bombs that were about to hit, my strategy would have been different in the few minutes remaining to get to shelter. I would have headed for the nearest shore or island.

More derecho information

In addition to the derecho pages on NOAA’s Storm Prediction Center Web site, you can find more information on severe weather in Environment Canada’s online Severe Weather Watcher Handbook

(www.msc.ec.gc.ca/education/severe_weather). Take a look at the photographs and descriptions of lowerings, in particular. The lowering is the cloud attached to the underside of a storm, and all storms, including derechos, seem to have a unique one. If you could look at only one storm feature to determine its potential severity, it would be the lowering, according to information on the Web site.

An interesting description of the July 4, 1999, derecho is on the American Museum of Natural History’s Earth Bulletin Web site (earthbulletin.amnh. org/D/2). It gives wind speeds as high as 150 kph (93 mph) and notes that the derecho is a storm so infrequent that only in recent years have meteorologists begun to understand, recognize and forecast it. As a result, the bulletin says, “Derechos often catch people off guard.” A special danger is that many derechos occur on the hottest days of the summer, when people are outdoors. (It cites an Independence Day derecho in 1969 that hit Ohio and killed several boaters whose vessels were overturned by high winds.)

Noting that their rarity, speed and large scale have made them difficult to predict, the Earth Bulletin says that although derechos likely have occurred since “time immemorial,” little was known about them and scientists often called them tornadoes. But with advances in weather science and technology, including Doppler radar and satellite imagery combined with computerized programs, weather forecasters now are gathering volumes of information about derechos.

I also received an interesting report from Tim Marshall, an expert on assessing storm damage and wind strength by the damage caused on land. As one who has surveyed a lot of tree damage, he made some general observations: “Tall pines typically snap or topple in 50- to 80-mph winds, with widespread damage occurring around 100 mph.” Since the area just north of where I had been sailing that Fourth of July had seen about 25 million trees fallen or damaged, it seems to me that his comments were particularly on target.

Marlin Bree, 73, of St. Paul, Minn., is a former magazine editor for the Minneapolis Tribune, a freelance writer, and the author of “Broken Seas” (Marlor Press, St. Paul, Minn., 2005) and “Wake of the Green Storm” (Marlor Press, St. Paul, Minn., 2001), non-fiction books that feature accounts of his experience with the July 4, 1999 derecho (www.marlinbree.com ). The material presented here was compiled with technical help from weather forecaster Robert H. Johns, a former lead forecaster for the NOAA Storm Prediction Center in Norman, Okla.

The sneak, freak storm

• Derechos are a unique danger to boaters because the initiation of these superstorms can’t be predicted very well in advance, and they can arise quickly, with little or no warning.

• Preceding a derecho, the atmosphere may appear dead or still, followed by a quickly rising storm pushed along by 60- to 80-mph winds. Onrushing dark clouds may be confused with a thunderstorm or a tornado.

• Boaters may not be able to find shelter, and, they can be caught out with their boats unprepared for the quick-rising superstorm.

• The worst form of derecho, the progressive derecho, typically occurs most during the prime boating months.

• The derecho’s straight-line winds contain outbreaks of downburst clusters with intense winds of more than 100 mph that rush downward and bowl over anything in their path.

• Derechos last for long periods of time and cover huge areas.

• Despite advances in meteorology, derechos remain somewhat of a scientific mystery.