O hushed October morning mild,
Thy leaves have ripened to the fall;
To-morrow's wind, if it be wild,
Should waste them all.

"October" by Robert Frost

Who else but a person named "Frost" should write a poem about October. In the month of October, we look forward to vibrant fall colors and those crisp, sparkling days, but October is not always tranquil. On occasion, we have to deal with stormy weather. We've gotten off pretty easy this fall in terms of how few hurricanes and tropical storms have effected the US, and this past spring, the number of tornadoes was way off as well. However, during the last 4 weeks there have been an unusual number of twisters, in tornado alleys (places where tornadoes regularly occur) and in places where twisters are seldom seen. Fortunately, few of the twisters have resulted in fatalities, but a powerful killer storm occurred on the doorstep of out Nation's Capitol a month ago (September 24).

In this storm, a family of tornadoes cut a destructive path from northern Virginia into central Maryland. One of the tornadoes ripped through the University of Maryland campus at College Park, tragically, killing two sisters who were students at the University of Maryland. At least one eyewitness reported that their car was picked up and hurled over a 10 story dorm! The girls had just visited their father at the Maryland Firefighters Training Institute on the university campus. About 50 people were injured in this tornado, and damages totalled in the millions of dollars to the university, to the National Agricultural Research Center in Beltsville, to several schools and churches and to hundreds of homes. This tornado was a F3 on the 5-level Fujita Scale, and it was the most destructive tornado to hit central Maryland in 75 years. An even more powerful F4 tornado, about 40 miles west of College Park in rural Virginia, touched down briefly about the same time as the College Park tornado.

In the past 4 weeks, twisters have struck in Alabama, Arkansas, Florida, Louisiana, Maryland, Mississippi, Missouri, Nebraska, Oklahoma, Texas and Virginia. Since the National Weather Service began tracking tornadoes in 1950, the average number of tornadoes in October is 29. As of now, the number of confirmed tornadoes for the month of October is nearing 70, and in just a 5 day period earlier this month, more than double the average number of October tornadoes occurred. The previous record for the first half of any October was set in 1998, when 47 confirmed tornadoes hit various parts of the nation, and the most tornadoes for any October occurred in 1997, with 100 reported. The second highest number was 86, set in October 1998.

Even though this has been a big month for tornadoes, and this past spring the tornado count was down, the number of twisters observed the past few weeks pales in comparison to the number observed for any 4 week period last spring. For instance, more than 200 twisters were reported in both May and June of 2001 compared to the 70 plus seen since the 20th of September.

In a normal year, approximately 1,200 tornadoes are sighted, and they're sometimes observed in every state. With our burgeoning population and with more people moving into places that were previously less densely populated, and bringing camcorders with them, it's not surprising that more twisters are being seen and reported now than in prior years.

As was mentioned in the science question about tornadoes last spring (May 3, 2000), 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 month 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 we've come to associate with 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. Whenever and wherever strong wind shear occurs, the potential for tornadic development exists.

A cold front propagating across the Mid West, for example, 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. Wind shear is always present in the vicinity of strong cold fronts. In the spring, the above conditions are more likely to be occur than at other time of the year. Because during the autumn, as in the spring, jet streams are vigorous, cold fronts energetic and strong temperature contrasts are often observed, it's reasonable to assume that there may be as many tornadoes in the "ber" months as in the single syllable months.

However, there are some differences between the autumn and spring that make tornado development less likely in October than in May. For one thing, during the autumn, anticyclones (surface high pressure systems), which often produce days of fair weather, are more stable than their counterparts in the spring. With the longer nights of autumn, morning temperatures are typically cooler than in the spring. This surface cooling frequently results in atmospheric inversions, where the temperature at the surface is cooler than it is aloft. This increases the stability of an air mass - stable air is a strong deterrent to storm formation. A dome or ridge of high pressure air may becomes entrenched from the surface through the troposphere, and it's often stubborn enough to prevent other weather systems from moving it out of the way.

A high pressure system that had been in place over much of the Mid West and Atlantic Seaboard earlier this week gave us several days of fair skies and warm "Indian Summer" conditions. This stable air mass finally broke down, enabling the first cold wave of the season to take its place. Yesterday was a wild weather day in the Ohio Valley area. There were 421 reports of damaging winds, 99 reports of hail and 20 tornado sightings - the most amazing October weather day for this part of the country in years. Twisters were observed in Missouri, Tennessee, Illinois, Michigan, Indiana, and Ohio. So far this month, 16 states have been host to these unwelcomed visitors. By months end, it's possible that more tornadoes will have touched down this October than for any other October on record.

It looks like the stormy weather will soon arrive here in the east. So try to enjoy the fall colors today, before the stiff breezes call and the air is riled, for "To-morrow's wind, if it be wild, Should waste them all."

For more about this see the Tornado Project - http://www.tornadoproject.com/faq/faq.htm#top
also see the Earth Science Picture of the Day for October 3, 2001.
http://epod.usra.edu/archive/epodviewer.php3?oid=72192
In addition, see the science question for May 3, 2001.