MCV may have helped spawn brief tornado, strong thunderstorms

I was glued to the radar and weather maps trying to find clues on what to expect. I had planned on a midnight bed time Saturday night. I was off of work for the weekend for my planned time off. But, you know, weather changes things. 

I woke up at 5 am Sunday to check the radar. Nothing interesting.

I woke up at 6 am. Nothing interesting again. No warnings. Most of the action was to our west. 

I woke up at 7:30 am. Not much in our market. There was a tornado warning just to the east of Panama City. Then the Storm Prediction Center issued a mesoscale discussion that the threat of severe weather was relaxing. But reality will soon change that thought. 

As the minutes ticked by, my NOAA Weather Radio buzzes across the house. A severe thunderstorm warning was issued for parts of the Big Bend and South Georgia – including Tallahassee. 

As I stared at my RadarScope app on the phone, I was watching the cell that was tornado-warned do something that would catch my eye. I then looked at the wind speed and direction from local airports on another app and noticed something else. The wind was mainly out of the south in Tallahassee, south-southeast in Thomasville, Ga., and the north-northeast in Bainbridge, Ga. (two counties to the west of Thomasville). I noticed a pattern at the surface and on the radar of a broader circulation within the band of rain moving through the area. 

At 8:28 am, a tornado warning was issued for Bainbridge and surrounding areas in the southwest corner of Georgia. The rotation was detected within a thunderstorm cell on the radar. 

The velocity mode of the Tallahassee doppler radar shows rotation to the southwest of Bainbridge, Ga. around the time the tornado warning was issued.

 

I sprung out of bed, put on a suit on, grabbed my backpack and raced out the door to head to work. And I got soaked in the process. 

The same tornado-warned cell would continue northeastward and trigger another tornado warning. By 9:30 am, the warning would expire as rotation fizzled out. 

I found out Monday that a brief touchdown did occur. An EF-1 twister traveled under a half mile just a few miles north of Bainbridge near the town of Ausmac. It snapped and uprooted trees, according to the brief National Weather Service report. Maximum winds were estimated to be between 90 and 95 mph with a width of 31 yards. 

Farther east, multiple severe thunderstorm warnings were issued, with plenty of reports of downed trees and power outages. 

The SPC and NWS would soon write in their discussions of an “MCV” moving northeasterly through South Georgia. An MCV, as in a Mesoscale Convective Vortex. 

Let’s break this down. This is what it technically means, per Zhang (1992)* …

“a significant concentration of positive relative vorticity of magnitude at least that of the local Coriolis parameter, eventually leading to the formation of a closed circulation.”

Wut? 

Okay, that didn’t help.

An MCV is essentially a smaller-scale system that forms when you have divergence (think of a small high-pressure cell) at the highest levels of the troposphere. The troposphere is where all of the weather happens and the layer of the Earth’s atmosphere where we, you know, live. There is some relative heating going on below this “high” – near the middle of the troposphere. Below that, convergent flow (think of a small low-pressure cell) develops, with the greatest spin in the mid levels. 

An MCV tends to develop in the lighter rain (stratiform precipitation) behind the main line of thunderstorms with plenty of moisture present (which there was plenty in Tallahassee). As the SPC noted initially, the storms were not as organized before the thunderstorms fired back up. And that’s probably what happened: The MCV helped to reignite the line of showers and thunderstorms that prompted the warnings and caused the tree damage and power outages in the Big Bend and South Georgia. These MCVs do have a history of firing up severe weather, but also flooding events**. 

Without great archived satellite data at my disposal a day after the event, the best way to see it visually is the radar imagery. Radar scans do show a broad circulation moving from the southwest roughly near Liberty County to the northeast near Albany. As the MCV cranks up, reflectivity values with the squall line increase. The line of storms goes from South Georgia to off the West Central Florida coast Sunday morning. 

Radar loop from the Tallahassee radar of the storms rolling through the area.

 

As for the tornado near Bainbridge Sunday morning, the storm was positioned on the southwestern side of the MCV circulation. 

Outside of our area, the SPC noted two reports of possible tornadoes (likely just one based on the proximity of the damage reports) in Pinellas County, Fla. (Tampa Bay). Another possible tornado was reported in Camden County, Ga – just north of Jacksonville – where several trees were uprooted. 

This job is never boring as there is a lot to see and learn. 

References: 

**Lindstrom, S., and S. Bachmeier, (date unknown): Mesoscale Convective Vortices. Accessed 16 April 2018. http://rammb.cira.colostate.edu/training/visit/training_sessions/mesoscale_convective_vortices/video/ 

*Daniel P. Hawblitzel, Fuqing Zhang, Zhiyong Meng, and Christopher A. Davis., 2007: Probabilistic Evaluation of the Dynamics and Predictability of the Mesoscale Convective Vortex of 10–13 June 2003. Monthly Weather Review 135:4, 1544-1563, https://journals.ametsoc.org/doi/pdf/10.1175/MWR3346.1 (*referred by this paper)

Markowski, P., and Y. Richardson, 2010: Mesoscale Meteorology in the Midlatitudes. Wiley-Blackwell, 407 pp. 

Vasquez, T., 2009: Severe Storm Forecasting. Tim Vasquez, 266 pp. 

Zhang, D., 1992: The Formation of a Cooling-induced Mesovortex in the Trailing Stratiform Region of a Midlatitude Squall Line. Mon. Wea. Rev., 120, 2763–2785, https://doi.org/10.1175/1520-0493(1992)120<2763:TFOACI>2.0.CO;2