Dicky (Lee) Armstrong and Yuh-Lang Lin
North Carolina A&T State University
Tropical cyclones (TCs) are powerful low-pressure systems that often bring disastrous high winds, dangerous storm surge, and inland flooding. In addition, occasionally they may induce tornado outbreaks if the atmospheric environment of a landfalling tropical cyclone becomes favorable for tornadogenesis.
We found that tornado outbreaks associated with landfalling TCs on the Gulf Coast and East Coast of the United States can be broken down into 3 main categories depending on the presence and type of frontal boundary found near the tropical or post tropical cyclone circulation. These categories are: (1) Type A: it involves an interaction with, track along, or total absorption by a cold frontal boundary, (2) Type B: an interaction or merger with a stalled frontal boundary, and (3) Type C: it encounters no frontal boundaries.
Analyses were done based on outbreak Types using the ERA5 dataset along with data from the Storm Prediction Center, Weather Prediction Center, and the National Hurricane Center. These analyses consist of examining the storm environment for vorticity and vertical velocity, 0-1 km shear, 0-3 km helicity, low and mid-level relative humidity differences, CAPE, and Potential Instability. The environmental vorticity was then broken down into different terms to examine the vorticity tendency.
Based on the preliminary analysis, we found that Type A outbreaks tend to possess both the most common and the largest outbreak in both tornado production and size, such as Hurricane Michael (2018). This case study reveals that the increased tornado production occurred because the frontal boundary acts like a strong low-level convergence zone such that the flow around the tropical cyclone interacts with the front and increases in the stretching term of the vorticity equation.
Type B outbreaks appear to follow a similar, albeit weaker pattern to Type A with an increase in the vorticity noted in convection located in the quadrant nearest the frontal boundary such as Hurricane Florence (2018). A strong convective band developed in the area between the tropical cyclone and the stalled frontal boundary to the north and was associated with an increased area of convergency and convection.
Type C outbreaks appear to be more dependent on the surrounding environment with most Type C outbreaks occurring in systems landfalling in Mexico to Louisiana and moving into antecedent conditions that are favorable for tornadogenesis, such as Hurricane Allen (1980) which moved into an area with increased shear and CAPE during its landfall in Texas.
The results of this research can be used to help improve forecasting tornado outbreaks associated with landfalling TCs in the United States and expand the understanding of vorticity production associated with the interaction of a frontal system and a TC.
- Indicates paper has been withdrawn from meeting
- Indicates an Award Winner