Severe Weather Headed Our Way

http://philadelphia.cbslocal.com/2013/04/19/severe-weather-headed-our-way/

PHILADELPHIA (CBS) – A strong cold front has been wreaking havoc on the weather across the US this week, and now it’s our turn!
This system separates warm, moist air to the east from much cooler, drier air to the west, and that clash of air masses is producing a volatile situation as the front marches eastward tonight.
Thunderstorms are moving from southwest to northeast and there is a good amount of shear in the atmosphere. For tornadoes to form, you need pretty specific conditions, and wind shear plays a large part in that.

There are two types: speed shear (wind speed increasing as you move up through the atmosphere), and directional shear (wind direction changing as you move upward through the atmosphere). You must have both for a tornado to form.