I’ve had the question a few times this weekend: what happened Friday to cause such an outbreak?
Thunderstorms, including the super-cell, or rotating storms that can spawn tornadoes, have some basic requirements: moisture, instability, and some sort of lift, or trigger mechanism.
Moisture first streamed northward Friday with a warm front. A good way to gauge the amount of moisture in the atmosphere is the the dew point temperature. The dew point in Louisville jumped almost 20% – from 51 to 60 – between 10am and noon Friday.
Instability is what lets the air in the low levels of the atmosphere rise into the upper levels. Without that the atmosphere can’t support big storms. Instability can be increased through daytime heating, and Louisville’s weather station reached 74 before the storms hit. There are many trigger mechanisms, such as what’s called “low-level convergence,” Convergence is air streams flowing into one another or stronger wind moving into weaker wind. When air piles together in the low levels of the atmosphere it has no place to go but up. That air was piling into a deep surface low pressure that moved out of southwest Illinois to the northeast through the day.
The low’s associated warm front, and (eventually) a cold front both went through the affected area Friday. Surface winds turned from an easterly direction at 5am to southeasterly in the morning as the warm front approached. South/southwest winds then prevailed until the late-afternoon passage of the cold front switched the wind direction to west.
An active jet stream is an important component to super-cell enhancement, with speed shear created by super speedy wind flow, and directional shear created by winds roaring in from a different direction compared to the surface. A potent wave, or trough of lower pressure in the upper atmosphere was approaching Missouri at this time, providing an extra “tilt” for the thunderstorms. Jet stream winds were exceeding 160 mph.
This kind of outbreak is unusual for the its northward extent so early in the season, but it’s always worth noting that although tornadoes are most likely “in-season,” they can occur at any time, given the proper conditions. In fact, the last deadliest tornado event in Indiana came in November 2005, when 25 lost their lives in southwest Indiana.
Never miss me! Subscribe for free. My Huge Radar has real-time weather tracking, current temperatures, and severe weather watches and warnings. Get detailed Indiana conditions by clicking here. Click here to see my central Indiana 7-Day Forecast. Follow these links to get my forecasts for Lafayette, Muncie, Hendricks County, and Hamilton County. Need a second opinion? Click here for central Indiana National Weather Service forecasts. (Some charts via WeatherBELL.)