Your Average Monthly Tornado Risk

Tornadoes can be a year-round menace in parts of the United States, migrating north and south with the spread of warm, humid air and the jet stream.

Examining average monthly tornado statistics, Dr. Greg Forbes, former severe weather expert for The Weather Channel, put together the maps above to show areas typically susceptible to tornadoes in each month.

Winter

Even in winter, when cold air surrenders its battles in the south and warm air can surge northward to meet the jet stream overhead, tornadoes can occur. These penetrations of warm air occur most often in the South, and this is where most tornadoes occur in January and February.

The February 2008 Super Tuesday outbreak had the largest number of tornadoes for a single event in February with 86 touching down in the two days. Feb. 23-24, 2016, ranks as the second-largest tornado outbreak during February with 75 confirmed, according to NOAA's storm events database.

Winter tornadoes are also routinely quick-moving and not easy to see since they can be rain-wrapped in cold fronts and squall lines. 

Tornadoes can form in March from the central Great Plains into the southern Great Lakes, Ohio Valley and Deep South. Parts of the South, mid-Mississippi Valley and Ohio Valley have the highest likelihood of tornadoes, compared to other regions, on average.

March is also the month when the worst tornado in U.S. history struck. Known as the Tri-State Tornado, it carved a path from Missouri to Illinois and Indiana on March 18, 1925, killing 695 people.

The South then typically sees its greatest threat of tornadoes as we head into March and into the spring season.

Spring

Tornado counts climb quickly in spring. 

With jet stream winds still strong and ample moisture returning north periodically, the threat of tornadoes spreads farther north.

By April, surface dew points in the 60s, or even 70s, penetrate farther north as actual temperatures sometimes climb into the 80s and 90s, supplying fuel for severe thunderstorms. 

The winter jet stream continues to be powerful especially over the southern United States, but it wanders in latitude at times giving quick flicks of atmospheric energy and wind shear every few days or so. 

Summer 

Parts of the Midwest and Plains also see increased tornado activity due to that region's proximity to stronger jet-stream winds that are typically located closer to the Canadian border in summer. Parts of the Mid-Atlantic and Northeast also see increased tornado activity.

Tornadoes can occur pretty much anywhere nationwide during the warmest months. Brief, usually weaker tornadoes can be caused by popcorn sea-breeze thunderstorms and sea breeze collisions closer to the coast, but these thunderstorms typically only last a few minutes. This is most common in Florida and along the Gulf Coast.

Waterspouts in nearshore waters occasionally come ashore even in calmer weather days during the heart of the summer. 

During the late summer and early fall, tropical cyclones can take claim to some tornado activity along the Gulf and Southeast coasts, including Florida. 

About 17 percent of tornadoes during the month of August have been from tropical cyclones (1976-2005), according to Dr. Forbes.

In August, the wet phase of the monsoon season kicks into high gear across the Southwest, leading to a slight uptick in tornado activity and severe thunderstorms in Arizona, which sometimes leads to dust storms. 

A slightly elevated period of tornado activity continues in northern Utah as conditions set up just right between the jet stream overhead and the arriving monsoonal moisture from the south. Terrain also may play a part. 

Autumn

A secondary peak in tornado activity occurs in fall in the Gulf Coast states, but this activity can sometimes spread north to the Ohio Valley and southern Great Lakes.

In November and December, the greatest chance of tornadoes moves back to the South with the warmest air and much of the moisture.

The jet stream also begins to migrate back toward the south for the winter months, but dips in the jet stream farther north behind abnormally warmer weather can spell trouble into the Tennessee Valley and East Coast. 

Late-season tropical cyclones can give a boost to tornado activity along the Gulf Coast. 

While not depicted on the maps above, on rare occasions, weak tornadoes can form on the West Coast in November, extending into the core winter months as well.

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