The muzzled gun
Destructive and deadly tornadoes ripped through parts of Mississippi, Alabama and Tennessee on Monday while Southeastern Louisiana experienced virtually no active weather. The quiet outcome occurred despite the fact that areas of the Bayou State actually had an atmosphere just as, if not more primed for strong thunderstorms than hard hit areas near Birmingham, Alabama. How is this possible?
Daily weather balloons launched by the National Weather Service rise and retrieve data allowing forecasters to understand exactly what is happening in the atmosphere above, at least for the short term. These balloons are launched each morning and evening. On days when active weather is anticipated, NWS forecast offices may elect to send out an afternoon balloon to gather more details during the typically most unstable part of the day. Monday, both forecast offices in Slidell, Louisiana and Birmingham, Alabama initiated an afternoon sounding. Examining and comparing afternoon "snapshots" of the atmosphere for Slidell and Birmingham reveals that Southern Louisiana actually had more impressive ingredients available for nasty storms. (By no means does this suggest Birmingham's profile was unimpressive)
In the associated image, look for red marks highlighting the more favorable parameters. Slidell had higher CAPE (convective available potential energy). This is a measure of the total energy available for thunderstorms. In many cases, values over 1,500 are typically sufficient for storms. Slidell's profile returned an incredibly high CAPE value of 4,000. LI (lifted index) is another parameter that assesses instability. Values below -5 denote a very unstable atmosphere. Southern Louisiana checked in around -9 while Birmingham displayed -7. The K-Index is similar to the LI, using moisture as a driver for instability. Values in the 20s are favorable for thunderstorms and Louisiana had a higher reading than Alabama. Mathematical equations based on initial conditions that calculate significant tornado and hail chances all put checkmarks on the board for Louisiana. Shear, or the vertical change in wind speed and direction, showed a more favorable wind flow for tornadoes in Louisiana than Alabama. In Louisiana, the winds changed speed and direction with height while in Alabama it was just a change in speed with height. Finally, the more that air temperatures cool with height (lapse rates), the more explosively air is able to rise. Louisiana had much higher lapse rates.
The character of the sounding showed that the actual temperature lapse rates were much steeper than the rate at which air tends to rise. Along with a good bit of vertical moisture, the resulting profile displayed an image forecasters call the "loaded gun." This is a moniker used to convey a potentially volatile atmosphere. The loaded gun sounding means severe weather WILL happen IF there is something to "pull the trigger."
In Alabama, a vigorous upper-level disturbance served as the trigger to spark super-cell thunderstorm development. In some cases, these storms lead to large, long-lived tornadoes. A strong tornado passed through a southwest suburb of Birmingham but fizzled just before reaching the metro area. In Limestone County, Alabama at least two were killed. Similar saddening stories could be found all across Dixie Alley.
For Louisiana, despite even more "ammunition in the barrel" there was no trigger to send buoyant air pluming into the skies above. Noted by Meteorologist Josh Eachus' on Facebook and Twitter on Monday Afternoon was the lack of lifting mechanisms and the necessity for Southern Louisiana to reach the day's convective temperature to allow air to begin freely rising in the absence of a trigger. Weather balloon information displayed a convective temperature of 87°; Baton Rouge recorded an afternoon high of 86°. As a result, little more than a shower was wrung out for a few Parishes northeast of Baton Rouge. Less than 30% of the viewing area recorded rainfall, and no storms developed, busting the forecast and the storm threat.
The setup changed on Tuesday, however. The gun was loaded; once again the atmosphere was ripe for strong thunderstorms. Most importantly for storm development, there was a trigger, two in fact. First, a cold front was approaching from the west. Another trigger, not uncommon to Southeastern Louisiana, was an outflow boundary approaching from the east resulting from morning storms on the Mississippi and Alabama Gulf Coasts. An outflow boundary is an advancing pocket of colder air along the surface caused when rain cooled air crashes into the ground during a thunderstorm and then spreads out. The process can be likened to smashing a water balloon on the sidewalk with the fluid being dispersed outward in all directions. The surging cooler air crashes into warmer unstable air forcing it upward and leading to thunderstorm development.
Noticing these features interacting with extreme instability, the National Weather Service issued tornado watches for most of the WBRZ viewing area, including Baton Rouge. During the afternoon hours, impressive looking towering cumulus clouds evolved into thunderstorms producing heavy rain, gusty winds and even some hail in Southern Mississippi. Severe thunderstorm warnings had been issued for Northern East Baton Rouge, East Feliciana, St. Helena and Tangipahoa Parishes. These strong storms would continue into Wilkinson, Amite and Pike Counties, though just below severe levels. Overall, about 40% of the viewing area recorded measurable rainfall, verifying the forecast and the storm threat.
You can get forecasts from Meteorologist Josh Eachus weekdays on 2une-In from 5-7am and News 2 at Noon from 12-1pm. Additionally, you can get the fastest and latest forecasts and weather news by checking in with wbrz.com/weather, liking Josh on Facebook and following him on Twitter.