Tornadoes are violently rotating columns of air that extend from a thunderstorm down to the ground. While the Great Plains states are commonly associated with this phenomenon, Pennsylvania also experiences significant severe weather activity. Understanding the local risk of tornadoes is important for residents, as these storms can form rapidly and cause substantial damage.
Historical Frequency and Statistical Context
Pennsylvania is not part of the region known as “Tornado Alley,” but it confirms a notable number of tornadoes each year, averaging approximately 16 to 17 annually. This figure is based on data collected over recent decades, with official records from the National Oceanic and Atmospheric Administration (NOAA) beginning in 1950.
Since 1950, Pennsylvania has recorded over 700 confirmed tornado reports, demonstrating a consistent, moderate threat. The state’s frequency is considerably lower than the most active states, which can see hundreds of tornadoes annually. The yearly count varies widely; for example, 1998 was a particularly active year, recording 62 tornadoes in the state.
Geographic and Seasonal Distribution
Tornado activity in Pennsylvania is not evenly distributed. The greatest concentration of occurrences is typically found in the southwestern and southeastern portions of the commonwealth. Counties in these regions, such as Westmoreland and Lancaster, have historically recorded some of the highest numbers of tornadoes.
The peak months for tornado occurrence are generally late spring through mid-summer: May, June, and July. July often records the highest number of events, consistent with the timing of the state’s most frequent and intense thunderstorm activity. Most tornadoes in the state form in the late afternoon and early evening, with activity peaking around 5 p.m.
Understanding Tornado Intensity in Pennsylvania
Tornado intensity is measured using the Enhanced Fujita (EF) Scale, which assigns a rating from EF0 to EF5 based on the damage caused and uses estimated wind speeds correlated with specific damage indicators. The majority of tornadoes that occur in Pennsylvania are classified as EF0 or EF1.
An EF0 tornado causes light damage, such as peeling the surface off roofs or breaking branches, with estimated wind speeds between 65 and 85 miles per hour. An EF1 tornado can cause moderate damage, including overturning mobile homes or removing roof coverings, with winds between 86 and 110 miles per hour. Higher-end tornadoes, such as those rated EF3 or greater, are rare in Pennsylvania, though a few violent storms have occurred, including a rare F5 (the scale used before the EF Scale) in 1985.
Localized Warning Systems and Safety Measures
The National Weather Service (NWS) is the primary source for localized severe weather alerts in Pennsylvania, delivered through two main types: a Tornado Watch and a Tornado Warning. A Tornado Watch indicates that atmospheric conditions are favorable for tornado development, meaning a tornado is possible.
A Tornado Warning means a tornado has been sighted or indicated by radar, requiring immediate action. This alert covers a smaller, specific area where the threat is imminent. Alerts are delivered through NOAA Weather Radio, Wireless Emergency Alerts (WEA) on mobile devices, and local media broadcasts.
When a Tornado Warning is issued, seek shelter immediately in a basement or storm cellar. If a basement is unavailable, move to an interior room or hallway on the lowest floor, away from windows. Cover your head to protect against flying debris, which causes most tornado-related injuries. Mobile homes offer little protection, and occupants should seek a nearby sturdy building.