SO JUST WHEN IS “TORNADO SEASON”?

The darkest shading indicates that, historically, there is only a one-tenth of one percent chance of a significant tornado occurring within 25 miles of a given point.  NOAA/SPC/NSSL graphic

Weather-savvy folks are aware that North Atlantic hurricanes have an officially-defined season: June 1 through November 30.  Most, but not all, hurricanes and tropical storms whirl to life within that 6-month period.

There is, however, no counterpart for tornadoes. Twisters can and do spin up at any time of year with the threat peaking in the spring.

One of the reasons there is no single, cast-in-concrete tornado season is that in order to galvanize public awareness there would have to be multiple, geographically-focused seasons.

For instance, consider the threat of “significant” tornadoes, those classified as EF-2 or stronger.  These are the twisters that cause, at a minimum, “considerable” damage (the subjective descriptions range up to “incredible” damage for EF-5s) and not infrequently morph into Grim Reapers.

Throughout the month of January, the threat of significant tornadoes is relatively minimal.  Polar air rules the Lower 48: the bears are sleeping, the ground is hard with frost and snow, and warmth and moisture over the Gulf of Mexico lie largely dormant. The threat of twisters is pretty much confined to the southern U. S. with a low-end bulls-eye over Mississippi and Alabama (see graphic).

In February, the risk area expands and increases (in terms of probability) as Gulf air masses grow restless.  Still, the tornado threat remains focused on the South, especially Mississippi and Alabama, as well as adjacent portions of Arkansas and Tennessee.  A significant twister in the the lower Midwest or southern Plains would be rare but not unheard of.

The area under the gun for nasty storms, as well as the risk, explodes in March and April.  Warmth and moisture from the Gulf make increasingly determined and more frequent raids northward, warring with Old Man Winter hunkered down over the northern U. S.   Thus, we could perhaps think of early March as being the real kickoff for tornado season . . . if there were such a thing.

By early April, most of country east of the Rockies is under at least a low-end threat as the clashes between the growing warmth of spring and the remnants of winter are fully joined.  The prime battleground, a sort of early-season “tornado alley,” extends from eastern Oklahoma and northeast Texas through Tennessee and the Deep South into northwest Georgia.

By the end of April, the climatological tornado bulls-eye has shifted to Oklahoma. But no place from the Great Plains through the Midwest and Deep South is immune to saber rattling by the monster thunderstorms that spawn twisters, supercells.

And speaking of supercells, my novel, Supercell (nice segway, huh?), is set mostly in Oklahoma in late April and early May.  Why?  Because that’s time of year the risk of unusually violent tornadoes is at an historical maximum, especially over the southern Plains.

So, while there is no calendar-constrained tornado season, the greatest menace of the most violent storms on earth looms over much of the U. S. from mid-April through mid-May.

Typically, most of us can rest easy for the next month or so.  Keep in mind, however, that any single year can bring significant variations to the timing and gravity of tornado threats.