November Turning Stormy?

October was relatively quiet in terms of severe thunderstorms and tornadoes, but November is threatening to become stormy. Computer weather forecast models suggest the possibility of severe thunderstorms and tornadoes on Wednesday and Thursday, November 5-6, then possibly a pair of weather systems that could bring severe thunderstorm threats to the Gulf Coast states starting Sunday night and next week.

While there aren't usually as many days with severe thunderstorms in November as there are in May, there can be some large tornado outbreaks. The diagram below shows that four of the largest 30 tornadoes outbreaks on record -- ones with at least 65 tornadoes -- have occurred in November. Only May, with 12, has had more, while April has also had four.

Most of the largest tornado outbreaks concentrate in the spring and early summer from March through June. July, August, October, and December have not had a tornado outbreak rank among the top 30. September snuck in with three -- all related to tropical cyclones that made landfall.

As I write this on Tuesday afternoon (Nov. 4), the ingredients do not appear to support a top-30 tornado outbreak this week. The map below for the early evening of November 5 shows a typical combination of severe weather ingredients, however. A strong surface low is expected to develop in southeast South Dakota, with cold front trailing south and southwest across east Nebraska into central Oklahoma and southwest Texas. The upper-level jet stream (broad arrow) is strong in a large trough pattern from Idaho to New Mexico to near Omaha, Nebraska. Ahead of the cold front a fairly narrow belt of moisture flows north from the Gulf of Mexico (narrow arrows). Severe thunderstorms and at least a modest-sized outbreak of tornadoes are possible from southeast South Dakota down the moist plume into north TX, where all of these ingredients come together. Limited moisture and instability appear to hold down the magnitude of this event.

So what makes some events big tornado outbreaks big and others minimal? It's how strong the ingredients are and how optimally they come together. The biggest November tornado outbreak, on November 21-23, 1992 (105 tornadoes) had a surface low that was much closer to the Gulf of Mexico, going northeastward from Texas across Arkansas and a jet stream that moved across Texas. This brought the ingredients together into the richest Gulf of Mexico moisture. The large tornado outbreak of November 9-11, 2002 (82 tornadoes) had surface low in a similar position to the one this week, but had a much wider and richer flow of moist, unstable air from the Gulf of Mexico. The large tornado outbreak of November 23-24, 2004 (93 tornadoes) was similar to the one in 1992, while the fourth of the largest November outbreaks, from November 23-24, 2001, had a low-pressure track about halfway between the ones in these cases, and had an ample feed of warm, moist, unstable air.

Stay safe and don't be surprised by stormy November weather!