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Posted at 11:00 AM ET, 06/14/2010

Thunderstorms: Fickle & capricious for sure

By Don Lipman

Damaging storms hit NW D.C., Bethesda 21 years ago

* Muggy Monday, then moderation: Full Forecast *

A thunderstorm lights up the Jefferson Memorial on May 9, 2005. By CWG photographer Kevin Ambrose.

Just a little over 21 years after the famous Exxon Valdez oil spill in Prince William Sound, off the Gulf of Alaska, the nation continues to deal with its worst-ever oil spill in another gulf--this time the Gulf of Mexico.

While the earlier event was no doubt an ecological and environmental disaster (although not nearly as catastrophic as the current one), less than 3 months later the Washington area had its own mini-disaster--this one caused by Mother Nature.

It was on June 14, 1989 that one of the most severe thunderstorms to strike the metro area in many years hit a relatively small portion of Northwest Washington and Bethesda, Md. during the late afternoon rush hour. In a short time, wind gusts (probably downdraft microbursts) estimated at more than 80 miles per hour caused an estimated 28 million dollars in damage and the loss of power to 150,000 homes. The rest of our area, however, was virtually unscathed. (I was traveling westbound on that section of the beltway just after the storm wreaked its havoc but, aside from a few small branches on the roadway, noticed nothing out of the ordinary.)

Keep reading for more on summer thunderstorms...

This storm illustrates the extremely fickle nature of summertime squalls. But despite the difficulty in predicting them and the dangers associated with them, thunderstorms (which occur 30-40 days a year here) do have great value.

For one thing, they provide much of our rainfall during the hottest part of the growing season. And by separating atmospheric nitrogen and oxygen, lightning creates vast amounts of nitrogen compounds, or nitrates, which greatly benefit all types of vegetation.

In addition, it is said that lightning helps to restore the earth's electrical balance and help maintain the planet's heat budget. Last but certainly not least, experiments in the 1950s by Nobel Prize winner Harold Urey and Stanley Miller suggested that lightning may have created life itself by combining water, methane, ammonia, and hydrogen. Still other lightning benefits are thought to exist. Can you think of any?

Actually, though we tend to think of thunderstorms as warm season phenomena, they can occur at any time of year, if conditions are right.

Here in the Washington area, some of our worst snowstorms have been accompanied by thunder and lightning, sometimes for extended periods. These "thundersnows," as they're called, are occasionally embedded in an overall snow pattern and are a sign of extreme air mass mixing and convective currents aloft, almost as in summer. Under these conditions, a "heavy" snowfall rate of one inch per hour can turn into a "snowburst" of three to four inches per hour. (In the midst of our current heat and humidity, I hope these cool thoughts will bring your discomfort index down a little--even if you're an ardent snow-hater.)

Aside from "thundersnows," there are three main types of thunderstorms: air mass; cold front; and mountain, or orographic. (Thunderstorms, and even tornadoes, have also been known to form in and around intense forest fires.) The East Coast, including our area, is primarily subject to primarily the first two, discussed below.

Air mass thunderstorms are the typical convective type of storms that form during the late afternoon or evening in the midst of a summer heat wave (we've had a few of those scattered throughout the region over the last several days). Although they bring brief relief, they don't signal a change in the weather. If anything, once they end, air mass storms sometimes leave behind more oppressive conditions than before, due to the evaporation of standing water.

Cold frontal thunderstorms are the most dramatic and usually, but not always, the most violent of all electrical storms and are the ones feared most by pilots. This is because such storms can grow to tremendous heights (10 miles or more) and sometimes form into a nearly continuous line hundreds of miles long. When this happens, aircraft can fly neither over nor around them and must divert to distant locations. (A pilot would almost never risk flying THROUGH even a modest thunderstorm, since extreme updrafts and downdrafts can pose great danger to the integrity of the aircraft.)

Once the cold front causing these types of storms passes, the weather usually does change to a drier, if not cooler, interlude. Strangely enough, the "squall lines" sometimes associated with cold fronts generally precede the actual front by one hundred miles or more. Why this happens is not well understood but it does illustrate, once again, that Mother Nature, as always, plays by her own rules.

By Don Lipman  | June 14, 2010; 11:00 AM ET
Categories:  Lipman, Local Climate, Thunderstorms  
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Next: PM Update: Cold front to bring some relief


I was 9 years old when that storm blew through, but I remember it vividly. It was terrifying. My mother and I were stuck inside a Strosniders hardware when the storm hit, and we could see trees bent over parallel to the ground. When the storm abated enough to drive, we found that we couldn't get home, as there were too many downed trees in our neighborhood. One huge tree in our back yard was badly damaged, and fell down 3 weeks later during my birthday party. Scary stuff!

Posted by: Langway4Eva | June 14, 2010 11:42 AM | Report abuse

"hit a relatively small portion of Northwest Washington and Bethesda, Md."

Not to mention my part of Takoma Park was without power through the following Sunday, for 4.5 days without power.

I had left work about an hour early, so I was home when the storm blew through Takoma Park. I'm glad I did that because I might not have made it home for many hours if I had left on time (I had a 20 minute commute at that time).

Posted by: Murre | June 14, 2010 12:03 PM | Report abuse

I remember the 1989 storm well. My wife was 7.5 months pregnant. I commuted across DC from PG Co. to upper NW, along Missouri Ave, and Military Rd. The fallen trees started just after Rock Creek park. It took me over an hour to get from the Park to Wisconsin Ave. (like 12 blocks). Then our power was out for at least a week, in the height of summer. It ranks right up there with last winter's snow in my DC weather memories.

Posted by: dhb2 | June 14, 2010 12:46 PM | Report abuse

I live upcounty, but I remember seeing all the blasted trees that seemed to block dozens of neighborhood streets. But my question is, over the last 15-20 years, has the number and intensity of severe thunderstorms increased over this region? It seems like there are no more "garden variety" thunderstorms around here anymore. We also seem to be getting more tornadoe watches and warnings than ever before.

Posted by: VikingRider | June 14, 2010 1:22 PM | Report abuse

The statement that we get 30 to 40 thunderstorm days per year disagrees with my isokeraunic chart [from USDA] which shows Washington, D.C. getting only 25 to 30 days. We've had a few years with around 40 days recently, and I think the presence of cool back-door easterly marine winds from the Atlantic [especially in April and May] cuts the local number of thunderstorm days to around 20 or 22 during some years.

The number of thunderstorm days in a given year can be variable. Eau Claire, Wisconsin has an average of 35 thunderstorm days and the extreme years for the period ending in 1960 varied from 20 to 45. Mondovi, Wis., just 15 or so miles south-southwest of Eau Claire, recorded an average of 42 thunderstorm days during roughly the same period, with extreme years varying from 23 days to 60 days. This means that, during their extreme maximum year, Mondovi had as many thunderstorm days as south-central Alabama or southern Georgia would experience in an average year! The study of isokeraunics [thunderstorm frequency] may be a rather sensitive indicator of possible climate change, as thunderstorms are more frequent in warm, moist, continental or mountainous areas.

Posted by: Bombo47jea | June 14, 2010 1:37 PM | Report abuse

could that discrepancy be due to evaluating "d.c." vs. "d.c. area"? surely some storms hit nova or moco w/o hitting dc.

Posted by: walter-in-fallschurch | June 14, 2010 1:59 PM | Report abuse

VikingRider: Speaking for myself, in answer to your question about frequency of severe storms in our area, I don't have any concrete stats at my fingertips but my gut feeling is that the frequency and intensity is no greater but the reporting (via cellphone cameras, etc.) is. Advances in doppler radar and the now recognized tornado signature, usually prompts an immediate National Weather Service warning. I will admit however, that we have had more apparent F3 or greater tornadoes in recent years than before. For many years, the tornado of record for the entire northeastern quadrant of the country (including us) was the Worcester, MA twister of 1953. That storm has now been eclipsed many times over.

Your suspicions, however, are not without considerable support, as global warming is thought by many to provide more energy to the atmosphere and thus, more frequent and intense storms. Time will tell.

Don Lipman, Capital Weather Gang

Posted by: Weatherguy | June 14, 2010 2:08 PM | Report abuse

I too remember that storm of 1989. I was working in DC and living in Takoma Park. I went to a meeting after work. I called my housemate before I left and she said I shouldn't walk home from the metro because it had been a bad storm. I was confused because it wasn't that bad in DC. I soon discovered downed trees and no power! I lost power in this winter's storm -- I would rather have no power in the summer than in the winter.

Posted by: erbele | June 14, 2010 3:27 PM | Report abuse

CWG et al, thanks for your tstorm info. I get my weather info from my trusty Laptop, the radio, and of course, the tv. I understand that the frequency and intensity of tstorms varies depending on location, however, that's climatalogical thing. I've lived in central Md. for about 28 years and it seems to me that the National Weather Service, since they began to widely use Dopplar radar, are a little trigger happy when it comes to issuing tornadoe warches and warning. It also seems to me that somebody needs to update the official averages for this region to reflect what's actually happening and not what the books say is supposed to happen.

Posted by: VikingRider | June 14, 2010 3:42 PM | Report abuse

I was working that afternoon in Bethesda. Police and the fire department had to walk two or three miles into Chevy Chase from East West Highway because Connecticut Ave, Wisconsin, and Western Ave were impassible.

Posted by: MKadyman | June 14, 2010 10:10 PM | Report abuse

Wonder what the numbers are for central Virginia, which must surely be higher, say on the order of 50 days? And higher still if you move to southside along a line from Petersburg to Norfolk?

Posted by: ennepe68 | June 15, 2010 11:46 AM | Report abuse

(QUOTE):Strangely enough, the "squall lines" sometimes associated with cold fronts generally precede the actual front by one hundred miles or more. Why this happens is not well understood (QUOTE)

It fairly well (but not totally)understood. One reason that the most intense squall lines generally happen 100-150 miles ahead of a strong cold front is that jet-stream or other strong winds aloft, blowing with the front or parallel to it, prevent the abrupt lifting of warm, moist air right along the front itself. The warm-moist, and unstable air, helped by "tilted" updrafts, forms a violent line out ahead of the front, sometimes accompanied by tornadoes if there is significant wind shear and/or dry air at mid-levels.

A second reason for pre-frontal squall lines, especially in the TX-OK-KS area, is the presense of what are called "dry lines" ahead of the cold front. This is actually like a pre-cold-frontal "front" separating warm, moist, Gulf air, from warm, dry desert air from the Southwest. The more dense warm dry air forces up the less-dense, warm humid Gulf air ahead of it, and forms a line of storms. behind the warm, dry air (finally) comes the cold front itself.

Posted by: MMCarhelp | June 15, 2010 6:01 PM | Report abuse

(QUOTE)Cold frontal thunderstorms are the most dramatic and usually, but not always, the most violent of all electrical storms and are the ones feared most by pilots. This is because such storms can grow to tremendous heights (10 miles or more) and sometimes form into a nearly continuous line hundreds of miles long(QUOTE)

While pre-frontal squall lines can have some very powerful storms, the worst thunderstorms are usually the classic Midwestern supercells. They can turn into real monsters, because, unlike squall-line cells, they don't continually form, die, and share their energy with other cells next to them in the line. True, Plains-Type supercells usually form ahead of the main squall line, or at some distance separated from it, where, unlike squall-lines, they can draw in heat and moisture from all sides at once and form rotating meso-cyclones. They rotate all the way from top to bottom, and reach enormous heights....(60,000 feet is common, and 70,000 in rare cases), with marked mammatus clouds. Large F4 and F5 "Wedge" tornadoes, with rotating wall clouds, and very large hail to basball and softball-size, are most commonly associated with supercells, rather than conventional squall lines.

American Plains/Midwest Supercell thunderstorms take the cake. Although some others, such as the spring/summer storms in Northern Argentina, Southern Australia and Northern India, are close, no other thundestorms on the planet, for sheer intensity, can compare with the American High Plains supercells.

Posted by: MMCarhelp | June 15, 2010 8:33 PM | Report abuse

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