For example, in recent years the metropolitan Washington and Baltimore area has taken a beating by tornadoes. In 1998, a powerful F4 tornado struck near the town of Frostburg in western MD. Last spring a F1 twister hit the town of Olney, MD, and last summer a F1 tornado touched down near Waldorf, MD. This past fall, a F3/F4 twister ripped through College Park, MD, killing 2 students, and less than 2 weeks ago, the terrible F4 tornado devastated LaPlata, MD, killing 5 people. If this wasn't enough, just last week, a F2 twister damaged a number of homes in Calvert, MD, (northeast of Baltimore). What the heck is going on?

If these twisters occurred a few days apart and were related to a particular circulation system, it might be easier to comprehend why a location in the Middle Atlantic states, not usually accustomed to tornadoes, would fall victim to these dangerous storms. But that's not the case. So why has the Washington/Baltimore area suddenly become the "tornado alley" of the east?

The LaPlata tornado was a product of a supercell thunderstorm that formed ahead of a cold front pushing east across the Mid West. An energetic cold front can act as a triggering mechanism for tornado formation if a large area of moisture is surging northward from the Gulf of Mexico and if a vigorous jet stream is aloft.

Before the cold front arrived on Sunday the 28th, the MD/VA area was soaked with between 1 to 3 inches of well-needed rains. Sunday the 28th was warm and very humid. Dew points across the region were in the 70s, and temperatures rose into the upper 70s and lower 80s. Thus, plenty of fuel was available to ignite the storms. Moreover, the jet stream, positioned over the Middle Atlantic states, provided the upper air support (vorticity or spin) to favor development of severe thunderstorms. Because of the very unstable atmosphere, the National Weather Service issued severe thunderstorm warning and tornado watches well before the first funnel cloud was sighted in the late afternoon near Near Market, VA (in the Shenandoah Valley).

The supercell that gave birth to the funnels moved directly eastward at a healthy clip of 50 miles per hour (80 km per hour). A twister descended from a shelf cloud along the southwest flank of the storm, their preferred location, but did not remain in contact with the ground until it crossed the Potomac River into southern MD. Over the flat terrain of this coastal plain, the tornado slammed to the ground and remained there for approximately 24 miles (38 km), roaring like a train on a 1/4 mile (0.4 km) wide track. Initially, its strength was measured as a F2, but it soon powered up to a F3, and before tearing apart LaPlata, it exploded into a monster F4, with winds up to about 260 miles per hour (116 m per second).

The LaPlata tornado was likely the strongest to ever hit the state of Maryland. The towns of La Plata and Prince Frederick, Maryland, both county seats, were hit hard, and parts of La Plata were leveled. Ironically, Maryland's worst tornado disaster also occurred near La Plata 76 years ago when a schoolhouse was demolished and more than a dozen children were killed. Over southern Maryland, - baseball size hail was observed in the vicinity of La Plata. After destroying LaPlata, the twister crashed into Prince Frederick, MD, killing 2 people, and then hopscotched across the Chesapeake Bay onto MD's Eastern Shore. In essence, the storm cell associated with the deadly LaPlata tornado persisted all the way from the Appalachian Mountains to the Atlantic Ocean. In addition to the 5 lives claimed by the tornado, more than 100 people were injured and damage estimates may be near $120 million.

On perhaps 2 dozen occasions over the past 5 years, the atmospheric conditions that lead to tornadic development have been prevalent over Maryland . That's pretty impressive for a pint-sized state like Maryland. However, while MD has been battered by a number of unwelcomed twisters in recent years, most of the rest to the country has dodged them. In fact for the last 2 years, and thus far this year, there have been a below average (based on the last 30 years) number of tornadoes, nationwide. In a normal year, approximately 1,200 tornadoes are sighted, and they're sometimes observed in every state.

In 2000, according to the Storm Prediction Center, there were 1,071 reported tornadoes, the fewest since 1989. For any other country in the world this would have been considered a terrible year for twisters, but in the US it was regarded as a rather quiet year. Most tornadoes typically occur in May (231), but the number of twisters in May and April the past couple of years has been down, whereas the number sighted in other seasons is up. In October of 2001, there were more twisters reported in the US than in any other October.

Although the tornado count has been down the past couple of years, 1999 was a big tornado year - 1,343 twisters. The record year for tornadoes was in 1998 when 1,472 tornadoes racked the nation, killing 130 people - not close to the record of over 1,000 (1884). Actually, with an ever growing population and with more people spreading into places that were previously uninhabited, and bringing camcorders with them, it makes sense that more tornadoes are being seen and reported now than in prior decades.

As was mentioned in the science question on May 3, 2001 because there's no theory on tornado formation agreed to by the entire severe storm community, it's hard to know exactly why more or less tornadoes occur in a given season or year than in another season or year. Severe weather is almost always associated with strong differences in temperature and moisture, but for tornadoes to form, the contrasts have to be strong at small spatial and temporal scales. There must be extreme atmospheric instability and low-level boundary conditions present - clashes between air having differing temperatures and moisture conditions.

The storm system that hit Virginia and Maryland last fall demonstrated that thunderstorms need not penetrate the troposphere in order to be capable of producing a funnel. There can be substantial spin in the air without the presence of towering clouds that are more likely to produce severe thunderstorms and tornadoes. Wind shear is associated with "spin" and is an essential ingredient for tornado development. For rotation to begin in a funnel, winds need to be moving in different directions and or at different speeds at different heights in the atmosphere (wind shear). Whenever and wherever strong wind shear occurs, the potential for tornadic development exists.

Typically, a tornado-producing thunderstorm has a lifetime of less than about 3 hours, often forming only one funnel. Remember, a funnel cloud is only designated as a tornado when it reaches the ground. Supercells are more rare, and more dangerous. These huge rotating storm systems are like giant heat engines capable of spawning families of twisters over the course of their lifetime, which may be several hours or longer.

It's interesting that while shear is the bane of hurricanes (wind shear aloft deters hurricane development), it's an essential ingredient for tornadoes. One of the reasons that no hurricanes hit the Atlantic Seaboard the past 2 years is that there was sufficient wind shear over the tropical Atlantic waters to discourage the strengthening of tropical storms into hurricanes.

Even though the weather has been more severe in the metropolitan Washington/Baltimore area in recent years, in least in terms of tornadoes, climate change is not solely to blame. After all, Maryland's little neighbor to the east, Delaware, hasn't suffered from an abnormal number of strong twisters. Maryland has a lot going for it, including the Chesapeake Bay, Atlantic beaches, wooded mountains, and the national NCAA basketball champion Maryland Terps. Hurricanes are infrequent visitors, there aren't any volcanoes or destructive earthquakes, and until recently, the tornadoes that touched down in the "Free State" were most always F0s or F1s.

It seems that Maryland's twisters are a result of natural climate variability. It's just by chance that the cast of nasty characters that must appear on the stage at the same time in order to produce the fiercest winds on Earth has done so with some regularity during the past couple of years. Maryland, nor any other state in the Middle Atlantic or New England regions, needn't worry about becoming a new "tornado alley." As a Maryland resident, I'm not digging a storm shelter just yet, but I've got a shovel handy. As one of the axioms of Murphy's Law states, if it was bad, it'll be back.

For more about this see the Earth Science Picture of the Day for May 1 and May 10 (http://epod.usra.edu/) as well the Earth Observatory New Features (http://earthobservatory.nasa.gov/) and also a NOAA storm site (http://www.erh.noaa.gov/er/lwx/publicpix/index.htm)