Kevin Myatt is The Roanoke Times' weather columnist.

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Hurricane season has a tidy definition.

In the Atlantic basin, it starts June 1 and ends Nov. 30. Sure, there are a few storms earlier or later than that now and then, but ocean and atmospheric currents pretty much lock in what period of time to be most alert for hurricane formation in the Atlantic.

Severe weather season in the United States isn't as easy to define. That's because there is often some level of severe weather going on year-round.

First, we should define "severe weather."

While in a generic way, various types of extreme weather can be described as severe, the specific definition of "severe weather" used by the National Weather Service refers to thunderstorms containing one or more of three criteria: winds of at least 58 mph, hail at least three-quarter inch in diameter, or a tornado.

Many weather service offices in the central and western U.S. have adopted 1 inch as the minimum hail size for a storm to be considered severe, and that criterion is likely to be adopted in the East by next year.

March 1 to June 30 would be the best definition of what I would call classic severe weather season nationally, when outbreaks of tornadoes, damaging thunderstorm winds and large hail are more prevalent, especially between the Rockies and the Appalachians.

This is the time of year when cold, dry air and warm, moist air intersect the most often, accompanied by strong winds aloft. A few of these outbreaks cross the mountains and trigger storms east of the Appalachians, but often cooler air is trapped here and storms weaken.

The Gulf Coast often has an active severe weather season through the winter, stretching into early spring. I was surprised to learn that, when looking at the average number of tornado watches issued annually, the bull's-eye isn't somewhere in Oklahoma or Kansas, but is near Mobile, Ala.

I have come to think of June and July as the heart of our local severe weather season. That's when daytime heating reaches a level that allows for storms to build high into the atmosphere quite often.

While we have more wind flow to give storms a good spin in the spring, we're often lacking the heat necessary to push moisture upward very high in the atmosphere.

Virginia's peak tornado month is July, when storms are more prevalent, though the tornadoes tend to be weaker than those that form in the spring, when strong upper-level winds are more prevalent.

Thunderstorms will be possible in Southwest Virginia this afternoon and tonight with a new cold front, but it doesn't appear all the factors will be there for a large severe weather outbreak here, as might happen to our west. Some gusty storms are possible, though.