A potent severe weather event devastated parts of Mississippi and Alabama on St. Patrick’s Day leaving widespread tornado damage in its wake. The severe weather wasn’t over however as the Carolinas had to deal with their own bout of severe weather including tornadoes on the 18th. After the 18th, the following 24-36 hours brought about strong subsidence on the backside of the trough feature that brought about all of the active weather and a drier, quieter weather pattern ensued. This blog post will take a look at what was forecasted to transpire in the 48 hours after the tornado outbreak across the Gulf coast and what dynamically was present to allow for the active weather pattern to continue for another day and what led to conditions drastically improving across the South
During the day Friday, March 18th the upper-level low was still present over the Southeast (over eastern KY) with a weaker jet streak present at 250mb over Eastern GA into the Carolinas (See figure below from 18z Friday the 18th). This implied that upper-level divergence was ongoing and based on mass continuity this evacuation of air in the upper levels causes upward vertical motions as air from the mid/lower levels must rise and replace the air at the upper levels. This resulting loss of air at the surface will keep the surface cyclone strong through the short term. The jet streak is positioned in such a way that the left-jet exit region resides over the Carolinas. This is a region
where upper-level ageostrophic divergence is maximized which will translate downward to the surface and be one of the causes for strong convection to develop during the afternoon hours. The jet streak at 250mb looked to influence the mid-tropospheric trough at 500mb in such a way that it remained amplified and with a negative tilt as it progressed eastward. This orientation of the trough will serve two important purposes for future convective development: Cold air advection on the backside of the cyclone which will tighten the temperature gradient and increase low-level wind shear, and increased vorticity in the mid-levels due to the curvature of the trough. The figure below is forecasted 500mb vorticity at 18z Friday the 18th and shows relatively strong positive vorticity advection occurring over the Southeast. Positive vorticity advection will help to
increase upward vertical motions as well as shear in the mid-levels. This feature along with the 250mb jet are working in tandem to increase overall upward vertical motions and shear which eventually translates down to the lower levels.
Looking at the lower levels, specifically at 850mb shows how southerly flow off of the GOMEX was forecasted to tighten the temperature gradient and contribute to low-level wind shear. The figure below shows tight thickness contours with strong southerly flow over the Southeast. Thickness lines that are packed tightly indicates a strengthening thermal wind. Thermal wind is what will not only increase warm air
advection, but also increase low-level wind shear. The strong southerly flow of air near the surface via the thermal wind can be classified as a low-level jet. This is a feature that is a result of upper-level divergence due to the present jet streak translating down to the lower levels in the form of faster air converging near the surface and the strong temperature gradient only enhances this feature even more. Winds are noticeably veering with height which means a clockwise rotation of the wind barbs as you observe each level of the troposphere which not only is an indication of warm air advection but also directional shear which is an important factor in the development of severe weather. All of the different features present in the various levels of the troposphere will likely translate to rigorous convective development at the surface and another round of severe weather for the Southeast.
Beyond the 18th and over the next 24-48 hours there will be a significant change in the weather pattern across the Southeast. As the trough feature begins to move off the coast a much drier and quieter environment will take shape. The backside of troughs is where you can find areas of subsidence which means sinking air and calm conditions. The overall air flow becomes more northerly and cooler, drier air filters into
the region as a result. As opposed to upper-level divergence like with the jet streak, upper-level convergence will be occurring now which will cause downward vertical motions little to no surface convection. An anticyclone is beginning to form on the backside of the trough which will help to maintain the flow of cool, dry air from the north into our region, drying out the troposphere. At the mid-levels, the forecast shows
negative vorticity advection due to geostrophic flow advecting smaller values of vorticity towards larger values of vorticity, however, there does appear to be some vorticity signatures sticking around our area. This could hint at some possible residual instability and perhaps some enhanced cloud cover or light precipitation over the next 24-48 hours. However, as the ridge begins to build into the region and the anticyclone (high pressure) strengthens, an overall calm and quiet weather pattern appears to follow the active weather pattern that proceeded it.