Mesoscale convective systems crossing the Appalachian Mountains

Forecasting the maintenance of mesoscale convective systems (MCSs) is a unique problem in the eastern United States due to the influence of the Appalachian Mountains. At times these systems are able to traverse the terrain and produce severe weather in the lee, while at other times they instead dissipate upon encountering the mountains. Thus, there exists a need to differentiate between crossing and noncrossing MCS environments. This study examined twenty crossing and twenty noncrossing MCS cases and found that the environment east of the mountains best separated the cases. Crossing cases were unsurprisingly characterized by higher instability in the lee; however, these cases unexpectedly also contained weaker shear and a smaller mean wind. Idealized simulations were used to test the sensitivity of MCSs to the wind profile in a favorable thermodynamic environment. The experiments revealed that the wind profile is indeed an important factor, but does not uniquely determine whether systems have a successful crossing. Furthermore, a hydraulic jump in the cold pool and orographic gravity waves acted to reinvigorate the convection in the lee and produced a crossing MCS in every simulation. However, changes to the mean wind did modulate the magnitude of orographic enhancement and suppression on the windward and lee slopes, while changes to the low-level shear impacted the strength and tilt of the updrafts. Simulations utilizing composite observed crossing and noncrossing wind profiles suggested that the mean wind exerts a stronger influence than the shear. Even so, the presence of ample instability and minimal inhibition appears to be the most important factors to consider when predicting the maintenance of convective systems crossing mountain ridges.