The Big D Is for Derecho
A storm system to beware of


By Marlin Bree

If sailors on Lake Superior or other northern lakes sometimes feel that the breezes are a little stiff during early thunderstorm gusts, they might like to know that their gut feelings were correct:

Can you say, "derecho?"

Author on foredeck of boat

In his 20-foot wood/epoxy sloop, Persistence, author Marlin Bree sailed through a rare, progressive derecho on July 4, 1999, that also became the "storm of the century" in the BWCAW blowdown. Bree's experience on Lake Superior in the 100-mph. downbursts is included in the NOAA's "About Derechos" pages on its web site.

A derecho is a superstorm that can come up over the horizon without much warning. There'll be the usual dark clouds that'll sweep toward you with remarkable speed, and, unlike many storms that you can take in your stride, this one will be different, as you will quickly find out. If you are lucky, you may get a few minutes' severe weather warning on your boat's VHF, but don't mistake it for another squall that will be over in a few minutes.

The first winds to hit you will make their strength felt on your mast, and, your boat will heel alarmingly, and, if you are so unfortunate as to still have sails up, your main probably will have a chance of either being stretched badly or even ripped. Winds are often initially in the 60 to 80 mph range, which is an insane amount of wind to expect a small sailboat to stand up to.
 
But there's worse: Out of the clouds will come downbursts of cold, heavy winds that will be at even greater speed than the straight-line winds. Thus is a derecho (pronounced Day - Ray -Cho) and it is a Spanish word meaning, straight ahead, as opposed to tornados, which means turning.

However you pronounce it, the Big D derecho is a nasty business - nature's apocalypse -- as I came to know from first-hand experience. I am one of the few boaters ever to have been caught in a progressive derecho, one of the worst kinds that are now in the record books, and, I managed not only to sail through one, but also to write record its effects on my small sloop on Lake Superior. In fact, "my derecho" is detailed in the new NOAA web pages, "About derechos," which tells of my boating experience as well as others caught in the decade's worst derechos.

I learned about the NOAA site when I first heard from Bob Johns, a veteran of NOAA's Storm Prediction Center in Norman, Oklahoma, who had a special interest in the superstorms called derechos and did several studies about them. He invited me to take an advance look at the test site, "About derechos," before NOAA published it officially in July. The web site is found at www.spc.noaa.gov/misc/AbtDerechos. Developed by Johns and fellow forecaster Jeff Evans, the web pages define derechos, tell what causes the superstorms, gives strength and variation of winds, and, presents some information on historic derechos. It includes satellite and radar images.

1999 July 4 Derecho chart from NOAA site:

The July 4, 1999 derecho was a huge superstorm that lasted farther and longer than many news reports have indicated. Beginning on the western border of Minnesota, the storm accelerated to become a derecho in the proximity of the BWCAW and northern Lake Superior. It did not end in the BWCAW but instead blasted eastward in a path of destruction for several days all the way to Maine.

For boaters, it can be fascinating viewing, but you can surmise through the pages that although forecasting is improving, it's still not easy to identify a derecho until it has formed. That means a derecho can pose a particular danger to boaters. As I already found out, a derecho can move so quickly that its fast winds can catch a boater off guard. A derecho has severe wind gusts greater than 57 mph., and, straight-line winds that can travel long distances at 60 to 80 mph., but that's not the worst.

Embodied in a derecho is a family of downburst clusters. The derecho is a "worst first" storm, with the strongest winds coming during the opening minutes of the storm, with a sustained series of downbursts can blast the surface below with cold, heavy air. Downbursts can have speeds of more than 100 mph., and, over open water, speed up. Unlike many common thunderstorms, which can be over in an hour, a derecho can last for ten hours or more as it moves through various areas.

Derechos are a unique danger to boaters because the superstorms can't be predicted in advance, and, they can arise quickly with little or no warning. Boaters may not be able to get to a place of shelter, and, they can be caught out with their boats unprepared for a superstorm. The worst form of derechos, the progressive derecho, often occurs most during prime boating months.

The chief danger is not the derecho's straight-line winds, which can be in the severe wind range, but its many 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.

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

Despite advances in meteorology over the last decade, derechos are still somewhat of a scientific mystery.

An example of one is found in NOAA's Historic Derecho Events, which gives information on significant derecho events causing severe damage and casualties in the last several decades. Among them are Independence Day Derecho Events, and, here you can click on the July 4-5, 1999, "The Boundary Waters - Canadian Derecho," which lasted far longer than news reports indicated and which caused huge damage not only in Minnesota's BWCA but also all the way east into Maine, a track of destruction of about 1,500 miles. This was the derecho I encountered while sailing my 20-foot ultra-lightweight centerboard sloop, Persistence, solo out of Grand Portage, MN., to Thompson Island, located at the mouth of Thunder Bay, and I was overtaken by high winds that set my sailboat on its beam ends, out of control. I was especially fascinated on the NOAA site to click on the Duluth NWS forecast office as well as the American Museum of Natural History links to see radar images of that very derecho forming. With winds estimated in excess of 100 mph., this extraordinary derecho tore down huge amounts of trees in the BWCA, injured 70 people and killed two persons before it ultimately ended in Maine.

In the BWCA storm, the American Museum of Natural History (AMNH) on its EarthBulletin Storm Tracking web pages had an interesting comment that as the storm passed to their north, meteorologists in Duluth saw the wind signature diminish on the screen. Meteorologists could view a bow-echo reflection of raindrops, so they still had a rough idea that the gust front was located, though they could not longer determine actual speeds at ground level. "It was precisely the moment when (the storm) evolved into a full-blow derecho," AMNH reported. It added, "As fate would have it, however, Duluth was the last radar station along the path to northern Minnesota - and the last radar to see the storm before it turned into a derecho."

During that onrushing storm, little advance warning was given boaters, including myself. I heard a Canadian VHF warning only minutes before the derecho swept onto Superior. The Thunder Bay Coast Guard had issued a Mayday for a sailboat that had overturned, with three persons in the water. Part of the difficulty was that the storm had knocked out electricity as it hit the Ontario city, and, one fireman I talked to reported having about the ten minutes warning, "Something big is coming through." They saw a blackness rolling across the Sawtooth Mountains, and, watched the sky turn green. In the fire tower, the rain was blowing so hard it shot through the top door's seals, with water pouring down steps "like somebody was up there with a fire hose."

Though the July 4, 1999 "Green Storm" tore up 477,000 acres in the BWCAW, in what was one of the largest North American forest disturbances in recorded history with its wind gusts "in excess of 100 mph.," that derecho was not the strongest to come through the northern area. On July 4, 1977, a derecho that pounded across northern Wisconsin had measured speeds of 115 mph., and another one, on May 31, 1998, a derecho with wind gusts at 128 mph and gusts estimated at 130 mph., hit Lower Michigan.
Though the derecho has only begun to be understood and recognized - as well as remaining difficult to forecast until it has actually formed - it probably should be in the lexicon of most boaters. It is a rare, but not infrequent storm, sometimes preceded by only minutes of advance warning of heavy weather on the VHF, or identified visually by heavy, black clouds quickly rising on the horizon, with a distinct lowering. Boaters should be aware that a derecho is a large, dangerous storm, and skippers should be wary, exercise caution, put heavy weather sailing techniques into practice, and not be lulled into a false sense of security or caught off guard.

Marlin Bree (web site: www.marlinbree.com) contributed to NOAA's "About Derechos" web site and his nonfiction book, Wake of the Green Storm: A Survivor's Tale, tells of his and other boater's adventures during the July 4, 1999 "Green Storm" record progressive derecho that devastated the BWCA and surrounding areas.