Kevin Myatt is The Roanoke Times' weather columnist.

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By Roanoke Valley standards, June has been an active severe weather month with Sunday's hailstorm and the small tornado on June 3.

Thursday provided a little more severe weather, with some reports of trees blown down at Roanoke's western edge in a late afternoon storm.

June may go out with another round of thunderstorms this weekend.

Some of those storms will probably be severe. Severe thunderstorms, as defined by the National Weather Service, are those containing winds of 58 mph or greater, hail ¾-inch in diameter or larger, or a tornado.

Four ingredients are generally needed to produce widespread severe weather.

The first is moisture, which is on the increase as southerly winds bring it from the Gulf of Mexico.

The second is instability, or the ability of air to rise by having warmer air near the surface push into cooler air aloft. This can be difficult to come by in the summer when warm layers of air higher in the atmosphere block updrafts from the surface.

As the weekend transpires, a low spinning over the Great Lakes is expected to swing through some pockets of cold air that will move overhead, allowing warm, moist air at the surface to start bubbling upward into thunderstorms.

The third is a source of lift. Daytime heating is one source of lift, and locally, the terrain helps air to rise. To get intense thunderstorms, there usually needs to be a mechanism that shoves air upward with great force. An approaching cold front, acting like a snowplow, should be able to provide lift this weekend.

Finally, there needs to be wind shear, or winds changing with direction and/or speed with height, to maximize storm severity.

This is the hardest ingredient to come by in summer because typically the strongest winds high in the atmosphere retreat to the far northern states and into Canada.

But this weekend, there is a chance that at least marginal wind conditions necessary to cause storms to organize and possibly even rotate will be present. The low spinning over the Great Lakes will force the jet stream farther south, bringing some faster winds high in the atmosphere closer to us. Also, a few other pockets of strong, swirling winds aloft may rotate around the main low, moving over or near our region.

So there will be a broad threat of thunderstorms, some of which could be severe, throughout most of the East.

Pinpointing local areas that will have the greatest risk of strong winds, large hail or even tornadoes will depend on a variety of meteorological minutia that can't be seen with any detail more than a few hours out.

It's easy to say there will be a threat of severe weather over several states, but very difficult to say whether or not quarter-sized hail will fall at your house again.