Tornadoes: How they form and how common they are in south-central Pennsylvania
Our Summer Weather Outlook Week continues. The News 8 Storm Team is talking about the various types of severe weather that can impact the Susquehanna Valley.
When you think of tornadoes, you may think of the great plains and deep south, but tornadoes can and do happen in all 50 states.
Tornadoes form when warm, moist air at the surface is heading in one direction, while stronger air aloft is heading in another direction. This is known as wind shear.
Wind shear rolls the air into a horizontal, rotating column of air.
Once the thunderstorm forms, in comes the updraft. The updraft is the rising motion in a thunderstorm created by instability in the atmosphere.
The rising motion causes the rotating column of air to become vertical.
As the storm intensifies, a wall cloud can develop beneath the base of a cumulonimbus cloud. It's an area of the strongest updraft within a thunderstorm.
Wall clouds can be a good warning for possible tornado development. If you do see this cloud, look for rotation. It could mean a tornado is trying to form.
The rotating column of air continues to build downward. If it doesn't reach the ground, it’s a funnel cloud. If it does reach the ground, it's a tornado.
Tornadoes in Pennsylvania
Pennsylvania averages about 16 tornadoes per year, with the majority of them happening in May, June and July. Tornadoes can happen year-round, and they can be strong.
The Susquehanna Valley averages more than two tornadoes a year.
Lancaster County and York County have seen the most tornadoes, with both counties averaging a tornado about once every two years.
Recent tornadoes
On May 27, 2022, we had three tornadoes. There were two EF-0 tornadoes, which hit Cumberland County and Franklin County, and an EF-1 tornado touched down in Kirkwood, Lancaster County.
Our most recent strong tornado hit Campbelltown, Lebanon County, in 2004. The EF-3 tornado had winds around 175 mph and was on the ground for more than seven miles. It destroyed 32 homes and injured 24 people.
How tornadoes are rated
Tornadoes are ranked on a scale based on damage they cause.
The Enhanced Fujita Scale goes from 0 to 5, with zero being the weakest and five being the strongest.
The majority of tornadoes in the Susquehanna Valley are in the EF-0 to EF-2 range.
Even though our tornadoes tend to be weaker, they still can cause considerable damage.
An EF-2 tornado with winds of 125 mph touched down near Whitehorse, Lancaster County, in 2016, causing damage to 50 buildings. The tornado ripped part of a roof off a building where more than 100 people were gathered for an auction.
An EF-2 tornado with winds up to 125 mph hit Saint Thomas Township, Franklin County, in 2019, leaving a three-mile path of damage.
Tornado watches, warnings
Tornado watches are issued by the National Weather Service if conditions are favorable for tornadoes to form.
If a storm is showing signs of rotation on radar or if a tornado has been spotted, a tornado warning is issued.
This is when a News 8 Storm Team meteorologist will go on air to warn those in danger and tell them to take action.
How to stay safe
The safest place in your house during a tornado is the basement.
If there is no basement, go to the innermost room on the lowest part of the house.
Get under a table. A roof or a wall may collapse, and a table is added protection.
Cover your head. Head trauma from flying debris is one of the leading causes of death from tornadoes.
Also, keep away from windows. Even weak spin-up tornadoes can send flying shards of glass through a room that can injure or even kill a person.
