What are “ridge rider” thunderstorms?

We typically associate severe thunderstorm outbreaks east of the Rockies with passing low pressure weather systems and frontal boundaries, however, meteorologists forecasting for the southern tier of the U.S. are also very interested in what the upper-level summer subtropical high pressure circulation is up to. Yes, I know that terminology was a mouthful! This atmospheric feature, with clockwise or “anticyclonic” rotation, may also be referred to in weather discussions simply as an “anticyclone circulation”, “high pressure ridge”, “monsoon ridge”, “subtropical ridge”, or just “ridge”.

For the Northern Hemisphere, semi-permanent areas of high pressure aloft are a common feature on weather maps bounded somewhere within the “subtropical belt” latitude range (approximately 20-40°N or roughly between central Mexico and the southern half of the U.S). What’s important for forecasting purposes is tracking where, when, how big, and how strong a subtropical ridge becomes. Focusing on the Great Plains and southeastern U.S. for this discussion, the configuration of high pressure circulations can be a significant player for preventing and directing where severe thunderstorm outbreaks start and ultimately travel to!

As shown in this post’s satellite/radar composite loop focused over the southern Great Plains (7/7/2023), the center of a subtropical ridge happens to be over northern Mexico/western Texas. The clockwise rotation in the upper levels of the atmosphere is denoted by the red wind barbs (they look like an arrow with feathers; the arrow points to where the wind is going). At these higher altitudes (around 20,000 feet MSL) such winds tend to steer deep thunderstorms in a prevailing direction. In this example, by late morning and early afternoon, strong to severe thunderstorms start to erupt along the Front Range of New Mexico and southern Colorado north of where the high pressure circulation is anchored. Multiple severe thunderstorm ingredients were certainly in place reviewing other important weather data. Namely, 1) vertical wind shear (speed and directional), 2) ample low-level moisture from the Gulf of Mexico with surface dewpoints in the 60s flowing westerly under the upper-level ridge and upslope towards the Front Range, and 3) plenty of regional sunshine early in the day to generate strong thermals lifting available moisture and helping to destabilize the lower atmosphere in general. So, we were definitely checking the boxes for severe thunderstorm potential before the outbreak began!

Now watch again how the day evolves in the animation loop. Notice how the thunderstorms quickly became organized into a line soon after leaving the Front Range given the overlapping severe thunderstorm environmental parameters in place. Also, the line of convection is being effectively steered easterly/east-southeasterly into Texax/Oklahoma around the northern periphery of the anticyclonic circulation aloft. This movement pushes the line of thunderstorms deeper into a higher moisture regime (more fuel to be entrained by thunderstorm inflow and updrafts). Finally, I want to point out how the southern portion of the line of thunderstorms trying to propagate south closer to the high pressure center eventually struggles to maintain itself, despite plenty of moisture to work with! The inhibiting factors at this point are convective storms at the southern end of the line starting to encounter both the 1) sinking/warmer air aloft (a region of “subsidence” or capping inversion) and 2) calming of upper-level winds associated with the center of the high pressure (diminishing wind shear). This convectively hostile geographic zone is important to forecast for as its presence may squash any hopes of seeing thunderstorms in the local area.

Putting it all together here, the term “ridge riders” are referring to long-lived organized thunderstorms east of the Rockies forced to rotate with but restricted to the periphery of a high pressure ridge circulation. If the large-scale synoptic weather pattern holds for an extended period then multiple waves of "ridge rider" thunderstorm complexes impacting the region become possible!

Data Source: College of DuPage