You probably noticed the big wind gusts and heavy rain that occurred across the Birmingham metro area this morning around 8:00 am. The wind gusted to 46 mph at the BHM airport, 47 mph at the NWS office in Calera, and 52 mph at the Bessemer airport. Trees and/or power lines were blown down in Tuscaloosa, Bessemer, Pelham, and Alabaster, and at least one barn was destroyed. The radar reflectivity (rainfall intensity) picture above shows what appears to be a line of thunderstorms moving through with gusty winds. But where was the thunder (or lightning that causes it)? And why didn’t the temperature cool down like it normally does when a thunderstorm passes by (at BHM it only dropped 1 degree).
It appears that some type of atmospheric wave moved through this morning instead. This wave of high pressure, in this case a type of hydraulic jump called a “bore”, caused a sudden, drastic pressure rise in a very short period of time. The atmosphere couldn’t get in balance with such a rapid increase, so the high pressure just blew the air along. One tell-tale sign of a wave or bore is in the Doppler velocity data from the same time as the picture above.
In the picture above, one can see strong winds blowing toward the radar (from the west) near the area of heaviest rain. But note behind the area of inbound winds, just 1 or 2 miles west, there is a strip of outbound velocity, indicating winds from the east. Then another couple of miles west, the winds go back to inbound again. This periodic change in winds is typical of a packet of atmospheric waves.
The atmosphere was also set up for waves, with cooler, more dense air near the surface than above it (not what you expect in thunderstorms). See the National Weather Service weather balloon data from 6 am CDT this morning in Calera.
In the charts below, 5-minute resolution data from the BHM airport shows the pressure, wind, and wind gusts vs. time. Note the wind gusts to 46 mph as the huge pressure increase moves in. This convergence of air is what created the upward motion and heavy rainband, too. Similar rapid increases in pressure were noted in dozens of official and unofficial (home) weather stations that report to the internet around central Alabama.
Waves are running all over the atmosphere most of the time, but 99% of them don’t cause any noticeable effects (maybe some streaks in clouds, bands of rain, etc.). But this one did. Research that I started at UAH and has been continued by others has shown that when atmospheric waves interact with rotating storms, they can sometimes cause tornadoes. The published paper on that is here: https://journals.ametsoc.org/doi/pdf/10.1175/2008MWR2391.1 . In addition, atmospheric waves can alter the lower atmosphere and make it conducive to late night storms in the Summer (see paper here: https://journals.ametsoc.org/doi/pdf/10.1175/2010MWR3376.1 , create dramatic cloud formations (see paper here: https://journals.ametsoc.org/doi/pdf/10.1175/2010JTECHA1472.1), and can cause wind damage like today (see paper here: https://journals.ametsoc.org/doi/pdf/10.1175/2009WAF2222248.1).
Fascinating weather event, and luckily I have not heard of any injuries from these storms. But there were 2 tornadoes in south Alabama this morning, and those are to be discussed another time.
Dr. Timothy A. Coleman
Coleman and Knupp, LLC