WISCONSIN — Strong winds impacted parts of Southeast Wisconsin Sunday morning. The winds caused power outages and downed trees in parts of Milwaukee county. We normally see strong winds on the leading edge of storms, or with severe thunderstorms, but in this case, the wind came after the rain was gone. Why? Due to the 'wake low' that developed. Before discussing the wake low, first, let's look at the weather set up.
Early Sunday morning a Mesoscale Convective Complex (MCC) moved across the Midwest. A MCC is a large, roughly circular area of rain and thunderstorms.
The northern end of the MCC brought some light to moderate rain across parts of Southeast Wisconsin. The rain was not very impressive locally, under a quarter inch for most places. However, the winds that developed behind the rain were impressive!
Here are a few official wind reports recorded Sunday morning:
St. Francis - 55 mph wind
Pleasant Prairie - 46 mph wind
Shorewood - 43 mph wind
Greenfield Fire reported on Twitter that snapped tree branches resulted in in power outages:
We have and continue to respond to wires down, arching and trees smoldering. Please stay well clear to #prevent a bigger emergency. @we_energies working hard to get to them all. pic.twitter.com/r3dMrNgSfO— Greenfield Fire (@greenfieldfire) September 6, 2020
Now time to answer the question - what exactly is a wake low? It's an area of lower pressure that develops behind a complex of rain and storms. It forms due to air sinking on the back side of the area of rain. This sinking air warms and becomes less dense. The wake low then causes a pressure gradient to develop between the low and the rain/storms.
Due to the pressure gradient force, air then accelerates toward the low. That accelerating air can mix down to the surface, causing strong winds.
Signals of the wake low include lowering pressure, strong winds, and sometimes it's possible to see rain dissipating on the back edge of the rain shield as well.
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