With a strong La Nina in place, a large scale pattern has arisen to some very cold temperatures here on the west coast and very violent and extreme weather in portions of the Midwest and Southeast. What happens is an upper level trough will set up over the west coast or Northwest and that means a ridge builds up over the Midwest or East coast. Fronts and mid latitude cyclones like to develop in between the trough and ridge where short impulses move along this wavelike pattern. Think about a rope that you move up and down. The up wave is the ridge and the down wave is the trough. If your moving the rope up and down but every once in a while jerk the rope, you’ll generate a shorter wave that moves faster than the big ones. It works the same way in the atmosphere in simple terms. These shortwave impulses move along the long-waves and they produce the low pressure systems in between the trough and ridge. Usually that location is just east of the Rockies and in the Midwest.
Another feature of this pattern is that winds tend to follow this large scale pattern higher up in the atmosphere, not near the ground. I don’t want to get into the physics of supercells, tornadoes, and clouds but you need plenty of moisture, wind, and heating for these things to take off. Well with the ridge over the Midwest, it allows a stream of moisture coming up from the Gulf of Mexico to collide with winds coming down off of the Rockies and central plains. In short, it is a breeding ground for these strong storms. It is also the time of year where the sun it getting stronger and the upper atmosphere layers have not quite warmed up. This leads to more buoyant air in lower levels and that air will rise as the cooler air sinks. This is another ingredient for storms called mixing, it helps create those towering cumulonimbus clouds (thunderstorms, also known as anvil clouds).
Recently, since we have been stuck in this pattern, there have been a series of storms producing EF-3 tornadoes (out of a scale of 6). There will certainly be more to come as well. I'll talk about the physics and thermodynamics of these storms in another blog.
