Remembering the Nashville Tornado Event: One Year Later

Time really flies doesn’t it? It seems just like yesterday that I was waking up to the news of the horrible events that took place during an interesting weather setup in West & Central Tennessee. I immediately reached out to my friends in the Nashville area to check on them. Thankfully, they were all ok. One, who loves weather like me, described a helpless feeling as the tornado literally tracked just a few miles south of his house, as he watched it on radar. It’s stories like these that remind you how real things get in a hurry in these supercell situations.

All said though, this was an event that kicked off a horrible 2020. And I’m obviously not just talking about the weather. This event came just a week and a half shy of the COVID-19 pandemic really taking off. It is crazy to think how it has been almost a year of “lockdown” but it occurred to me this morning how quickly time has flown. The tornado hit Nashville at 12:32 AM CST on March 3rd, 2020. It has been a year, and it seems like it was just yesterday.

This will be a TWO post blog event because there is just so much to go over in regards to this localized outbreak.

In today’s post, I’ll go through the SPC forecasts for that day and we’ll focus on the microscale parameters that allowed one lone supercell to form and become a prolific tornado producer.
In future posts, we will dive into discussions and the paths of the two major tornadoes that day, the Nashville EF3 and the Cookeville/Putnam County EF4.

The Forecasts

The first indication that there could be some severe weather came on the Storm Prediction Center’s Day 3 Convective Outlook, which was issued on Saturday, February 29th. At this point, it was just a Marginal Risk that sat west of the Nashville Metro Area and included West Tennessee, Most of Eastern Arkansas, the far northwest corner of Mississippi, the Missouri Boot Heel, a very small area of Southern Illinois and extreme West Kentucky. The probabilities for severe weather were set at 5%. We have learned over the years, especially as modeling becomes more efficient with analogs, that a marginal on Day 3 is still nothing to scoff at. Sometimes these marginals blow up into Moderate and High Risks as we move towards Day 1. In addition, different microscale features that are not known until the day of a possible event sometimes won’t even appear until hours (possibly even minutes) before storms begin to fire. Sometimes we’re in the middle of the event and one storm just goes haywire (like during this event). At any rate, this convective outlook was issued for the system that eventually did produce the tornado outbreak of March 2-3, 2020. The forecast hinted at the moisture advection that would bring up the humid air behind a warm front, temps would increase and then the clash with the front would occur as the low pressure system moves northeastward. At any rate, timing and sagging of the front become an issue, especially if the storms were to occur overnight (which is when they did, and they still had lots of instability to play with).

By Day 2, the wording got a bit more built up, but a Marginal Risk remained, shifting a bit to the east and now including the Nashville Metro area. The timing was pinpointed as being Monday afternoon into early Tuesday Morning, with isolated severe storms possible and would likely be ongoing to start the day. This was due to a low-amplitude progressive shortwave trough along with an isentropic ascent and theta-e advection that ran along a southwesterly low-level jet. As the warmer air would get drawn up into this area, it would clash with the cold air and touch off some storms Still, some parameters remained unknown, leading to the Marginal Risk (with 2% tornado probabilities and 5% for wind & hail). Instability wasn’t forecast to be the best, but we have learned in these early (and even late) season outbreaks that instability can be low but a powerful outbreak can still occur.

By the time we got to Day 1, a Slight Risk was introduced. However, the tornado threat and wind threat remained the same, and up the upgrade was due to a 15% hail probability. The big issue here was that there was significant differences in the models, specifically pertaining to the cap and how it would erode. (DEFINITION: A “cap” is the layer of warm air aloft, several thousand feet above the ground, that hinders the development of thunderstorms. It separates the warm air at the surface from the cooler above. When it erodes or completely bursts, thunderstorms can go up really quickly as the heat rises and builds towering cumulus clouds). The thought was that the cap in the area would erode slowly, but that supercells would still be able to form, with hail being the most likely, but SPC noted that “locally damaging winds and even a couple of tornadoes would be possible should capping erode to the degree depicted in some guidance”.

In later Day 1 Outlooks (as Day 1’s get updated throughout the day), not much had changed. The tornado threat was eventually increased to 5%, but remained West of the Nashville Metro area (where they were still within the 2%) and the wind increased to 15%. This was just a highly difficult event to forecast, especially since one lone supercell literally went beserk over West Tennessee. Even so, a 2% risk is still a risk that should be noted. They’re just general probabilities within 25 miles of a point. That 2% can change to 100% real quick if microscale features come together (and, obviously, its 100% because the tornado is occurring).

THE EVENT BEGINS

At 3:54 PM on Monday, March 2nd, the Storm Prediction Center put out a Mesoscale Discussion for parts of Northern Arkansas, Southern Missouri, Western Kentucky, Extreme South Illinois and far Northwestern Tennessee. They were coordinating with local National Weather Service offices about potentially putting out a watch. The probability of watch issuance was listed at 80%. Towering cumulus clouds had begun to go up due to surface convergence near the boundary that was lying across the area. These cumulus clouds, the precursor to developing thunderstorms, sat in a field which had steep mid-level lapse rates, strong shear and fast flow aloft. This could lead to the possible development of strong updrafts, with the possibility of large hail being the main threat.

At 5:20 PM, a tornado watch was issued for the area, in effect until 1 AM the next day. The expectation was that thunderstorms would begin to develop and then increase in intensity in the area as the evening moved on. The primary threats included a couple tornadoes being possible, scattered large hail and isolated very large hail to 2.5 inches in diameter likely, as well as isolated damaging wind gusts to 70 mph possible. The higest probability was for hail (70% chance of 10 or more severe hail events (exceeding 1 inch in diameter) and 60% chance of one or more hailstones being larger than 2 inches). Wind sat relatively low, with a 20% chance of 10 or more severe wind events (exceeding 58 mph) and less than 5% of one or more wind events exceeding 74 mph. The tornado threat sat at a 30% chance of 2 or more tornadoes (rather moderate honestly) and a 20% chance of 1 or more strong tornadoes (EF2-EF5). The probability of 6 or more combined severe hail/wind events sat at 80%.

Now, how did this watch box verify? Honestly, fairly well. The hail reports were abundant in these areas of Missouri, Kentucky, Illinois & Arkansas. Some of these reports were rather large, including 2.25 inch hail (roughly tennis ball sized) reported just east of Eminence in Shannon County, Missouri. The first tornado of the event also touched down around 8 PM CST in this watchbox. This tornado, rated an EF1, touched down just south of Crofton and crossed U.S. Route 41. Four farm structures and two garages faced damage while homes sustained roof damage as well as damage to siding and windows. Several trees were uprooted. The tornado lifted 5.51 miles and 6 minutes later just northwest of the community of Fruit Hill. At its widest, this tornado was 400 yards wide.

As you can see by the storm report map above, the watch did verify. In addition, the large hail events that occurred proved the forecast was correct. However, as the night moved on, things began to change to the south really quick, and the violent tornadoes would begin to occur.

The only other Mesoscale Discussion issued for this day was a 20% “Watch Unlikely” MD for portions of Mississippi & Alabama. This was because warm and moist air advection was riding up into the region and it was expected to touch off some isolated severe storms. As you can see by the map above, that didn’t necessarily verify during this days events, but it did lead to a late morning tornado watch downstream that did feature some early morning storms across parts of Alabama (as this entire system continued to sag to the south).

Focus Shifting Towards West Tennessee & Nashville

The earlier convection had weakened considerably as it moved into an area where there wasn’t as much moisture to work with. The surface low, at this time, was moving through Missouri and continuing towards Kentucky. A bit of outflow began to sag to the south, which would later help amplify the major supercell of the day. In addition, the northern jet stream also amplified into this area, combining itself with strong wind shear and low instability. This is where the forecast gets really complex as it is not entirely known why this one supercell was able to be so prolific. There are numerous ideas on what could’ve caused it and radar data continues to be poured through. We still don’t know why some storms produce tornadoes and others don’t let alone how one in this environment can all of a sudden become a monster. We just have to rely on conditions that are similar to other events of the past and weather radar to be able to tell what is going on right then and there.

At any rate, this profile of low instability and high shear did expect to lead to some rather robust severe thunderstorm development, especially for supercells. The Storm Prediction Center had shifted focus to the areas near the Mississippi River, specifically West Tennessee and towards the Nashville area. A tornado watch was going to be issued.

However, as all of this was going on, the supercell in question had already formed. It had already produced it’s first tornado more than an hour before the localized tornado watch for the Nashville area was issued (issued around 11:20 PM CST). This supercell continued to move into more favorable air and from here and out, would only grow in strength as it pretty much isolated itself and was able to create it’s own weather (so to speak) to be able to continue to sustain itself.

This supercell went on to produce 10 different tornadoes over the span of four and a half hours. The storm continued on, however, and didn’t dissipate for a little while still, causing wind damage in Eastern Tennessee as it began to weaken. Out of these 10 tornadoes, one would be rated a high-end EF3 and another an EF4. This lone supercell was responsible for all 25 deaths that occurred that night.

Tomorrow, we will dive in to the supercells path, how it potentially could’ve grown to be this strong, how it isolated itself and kept going, and, of course, the two major tornadoes…the EF3 that hit Nashville and the EF4 that hit Baxter & Cookeville

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Brett
www.universeofadventures.com