Earlier this evening, it got quite windy outside, with wind gusts as high as 43 mph at the BHM Airport, and likely higher on ridgetops. These high winds were not caused by a thunderstorm downdraft, nor by more typical atmospheric pressure gradients. They were caused by a phenomenon known as a “wake low”. The pressure at BHM dropped 5 mb (about 0.15″) in only about 30 minutes, then rebounded fairly quickly. The peak winds occurred as the lowest pressure went by. These wake lows sometimes can produce winds over 70 mph, cause downed trees and power lines, and even cause injuries, as one did in Atlanta several years ago.
Sometimes, at the back edge of a convective rain event, something called a rear-inflow jet (RIJ) develops due to thermodynamic imbalances. Air rushes toward the front of the rain mass, in this case at speeds around 40 mph. These RIJ’s are sometimes forced downward, by evaporating rain above them, by being suddenly decelerated in the rain area itself, etc. This downward motion warms the air, and dries up raindrops, causing a sharp back edge to the precipitation, as shown in the radar image from the NWS Birmingham radar below at 2249 GMT (5:49 pm CDT).
This downward motion aloft also caused the air to get compressed, as if putting it into a tire, and warmed. Since warm air is less dense than cool air, low pressure is produced underneath the lighter air at the surface. This warming aloft is what caused the low pressure in the wake of the rain, known as a wake low. Below are cross-sections of reflectivity (rainfall) like you see above, and Doppler radial velocity (green is toward radar, or SE, red is away from radar, or NW). These cross-sections are along the white line in the above radar picture, at that time. You are looking at these images from the side, as if you are looking from the southwest and up to about 10,000 meters (30,000 feet).
Note the RIJ in the bottom panel as a jet of high inbound velocities (blue) descending from about 5000 meters to 3000 m between roughly northern Tuscaloosa County (left side of image) to Shelby County Airport (right side). Near the ground, air is rushing away from the radar (red) toward the low pressure created by the downward motion aloft. This is the wind from the SE that we felt this evening.
If you would like to read more about wake lows, here is an article Kevin Knupp and I wrote in 2016 in the Journal of Operational Meteorology on non-traditional windstorms: https://doi.org/10.15191/nwajom.2016.0415 and here is article we wrote on 3 significant wake lows in the journal Weather and Forecasting: https://journals.ametsoc.org/view/journals/wefo/26/5/waf-d-11-00021_1.xml?rskey=7YL6bR&result=1
Dr. Timothy A. Coleman
Coleman and Knupp, LLC