Breaking Down the "High Risk" Severe Weather Outbreak on March 15, 2025

        Last week, the arrival of a powerful, complex storm system into the Midwest and deep south created a widespread 3-day outbreak, with anticipation building that Saturday (March 15) would be the not so pleasant blockbuster day.  A rare "High Risk" severe weather category was issued by the Storm Prediction Center (SPC), indicating the usually high confidence in a large, devastating outbreak of severe weather. Unfortunately, many strong and fatal tornadoes occurred both during the evening on Friday and on Saturday, with well more than 150 tornado reports across the midwest and deep south. While the event was certainly impactful and many lives were lost, the forecasted environmental parameters suggested something even worse. 

SPC Filtered Storm Reports for March 15. Particularly hardest hit was a corridor from southern Mississippi to west-central/central Alabama south of Birmingham

        Before going any further, its important to be clear that this event was not a "bust". Many strong tornadoes did occur and unfortunately, there were many injuries and fatalities. Instead of saying how this event was  a "dud" or "bust", this blog takes a look at what actually helped make this event slightly less severe than forecasts originally called for. Taking a look at the data can provide us with a better sense for how tricky weather forecasting still is. In this blog, I specifically note 3 key things on March 15th that helped prevent this event from being even more devestating: 

• Weaker warm air advection and weaker instability than forecast 
• Poorer mid and upper level lapse rates 
• Mixed/messy storm mode that limited the development of many robust supercells 

Getting Juiced on Some of That Rich Gulf Moisture... 

        To support a robust outbreak, you need lots of thunderstorm "fuel". To get big thunderstorms, you'll need a warm, moist airmass that is more buoyant and "eager" to rise. Warmer, more water laden air is less dense than colder, drier air found in the upper parts of the atmosphere. This density difference is a fundamental part of atmospheric dynamics that drives the development of updrafts and downdrafts. The warmer and more moist the airmass is compared with the upper atmosphere, the more that air wants to rise and condense to create clouds. This "eagerness" to rise is generally described by meteorologists as "instability." 

        

        For Saturday, instability was expected to be moderate to high for this time of year. A measure of this updraft potential energy, Mixed Layer Convective Available Potential Energy (MLCAPE), was forecasted by the SPC to be on the order of 2000-3000 j/kg. However, this MLCAPE never materialized for the northern regions of the risk zone (Central MS and AL) and much of the better instability was shunted farther south. This was down to two factors: lack of better moisture from the Gulf making it northward and a less steep temperature change with height (lapse rates discussed in the next section). Simply put, dewpoints were not nearly as impressive and this gulf moisture didn't make it as far north as anticipated, with Central Alabama still stuck with dewpoints in the 50s by 4 PM. 

SPC Mesoanalysis MLCAPE Values (Red Lines) at 4 PM. Only a pocket of 1000 j/kg was able to make into into the risk area with pockets of higher values well to the south. Thick red line indicates general area where warmer airmass was in place. 

Changing The Temperature... 

        To promote the warmer, water rich air to rise, you'll need the atmosphere above it to be much colder and (somewhat) drier to enhance the density differences between the airmass. The change in temperature per km of altitude in the atmosphere is called the "lapse rate". This is the second ingredient in achieving an "unstable" atmosphere and one that promotes strong thunderstorm development. Initially, the SPC was calling for fairly steep low and mid-level lapse rates on the order of 7-8 degrees C/Km. On the day of the outbreak, however, these lapse rates were not as steep and were "meager." Instead, low level lapse rates only managed to get into the 6.0-6.5 degrees C/KM range in the major risk area with steeper lapse rates to the east. While generally, anything above 6 degrees C/KM is good enough for thunderstorm development, lapse rates during the day of the outbreak were unimpressive compared to what was forecasted. Combined with lower degree of moisture return, this promoted weaker overall instability and an airmass that was less "eager" to overturn. 

SPC Mesoanalysis Low Level Lapse Rates at 4 PM. Notice the lower lapse rates in Mississippi and western Alabama. While Central and Eastern Alabama had more impressive lapse rates, this was offset by lack of moisture. 

When Thunderstorms Take On Other Thunderstorms...

        Even as a severe weather enthusiast, sometimes too many thunderstorms is a bad thing. Strong thunderstorms that manage to avoid major disruptions to its updraft by other nearby storms can mature into powerful rotating supercells that take full advantage of the environment. Although not always the case, many of the most impressive tornado outbreaks occur due to storms maintaining a "discrete" or "semi-discrete" nature in which there are more individual supercells than a large cluster of storms. 

        Oftentimes in southeastern outbreaks, there is usually a mix of different types of storms due to the presence of strong moisture and no way to prevent thunderstorms from forming (i.e. a cap). This results in a very messy, sloppy storm system in which some storms are able to mature and others struggle to get going due to updraft disruptions from a crowding of nearby storms. Furthermore, the highly saturated airmass can fuel cooling, rainfilled downdrafts which can cool the environment and lower instability. 

        

        Initially, the SPC predicted a more robust "discrete" supercell scenario in which fewer storms would form and many would remain undisrupted by other nearby development. During the first part of the afternoon, we saw this storm mode play out with 4-5 discrete supercell thunderstorms erupted out of Louisiana and moved into Mississippi, spawning multiple tornadoes. 

SPC Mesoanalysis Radar Imagery At 3 PM. An arc of supercells (circled in red) was ongoing in Mississippi, dropping multiple tornadoes. Also notice how this area of supercells generally lines up with the corridor of multiple tornado reports in Mississippi on the SPC storm reports figure in the intro. 

        As the day progressed, however, other storms began to fire and disrupted these supercells. In addition, northern portions of the risk area has less instability than the southern portion due to lower dewpoints. These lower dewpoints resulted in storms becoming less surface based and more "elevated" in nature, creating additional, disorganized convective "slop." Has these supercells progressed further into Mississippi and Alabama without these issues, I have no doubts that an even more devastating outbreak could have occurred. 

SPC Mesoanalysis Radar Imagery at 5 PM. Just 2 hours after the arc of supercells wrecked havoc in Mississippi, these supercells grew into a linear convective band of storms. Also notice the distinct 2 linear clusters of storms and how few individual cells are left. 

What Did We Learn Kids? 

        Though March 14-15 outbreak was impactful for many, analyzing the data after the outbreak reveals how weather is still an incredibly finicky thing. After looking over the data, its clear that this outbreak had the potential to be far worse than it actually was. A lack of thermodynamic support relative to forecast and a much more "messy" storm mode helped limit the severe weather outbreak somewhat. If more robust instability had developed across the risk area and supercells were allowed to mature and "run free", given the intense wind environment present, this could have been so much worse.