Microbursts come from powerful thunderstorms that have a sudden down burst of wind. Once the wind hits the ground, it spreads in all directions. Wind speeds in a microburst range anywhere from 60-100mph, damaging roofs, snapping trees and knocking over power poles. In many cases, microbursts can cause more damage than tornadoes.
In the early part of the monsoon, dry microbursts (without rainfall) are common. Read about them here.
Wet microbursts have their own, unique characteristics. Like the dry microburst, wind rushes out of the storm, hits the ground and spreads in all directions. But unlike the dry microburst, a rush of heavy rainfall accompanies the damaging wind.
So, how does a wet microburst form?
First, here's what one looks like.
Figure 1: A wet microburst.
And, here's the damage that can be left behind. Straight-line wind damage will usually uproot trees entirely.
Figure 2: Wet microburst damage to trees on the grounds of Oro Valley Country Club.
It all starts with a typical thunderstorm. So, let's take a look at them.
A thunderstorm develops from rising hot air (the updraft, or inflow) that rises and condenses as rain. The rain then falls back down, creating the downdraft, or outflow. Strong upper-level winds will push the top of the storm along, angling the updraft and downdraft so that the storm can sustain itself as it moves along.
Figure 4: Upper-level wind tilts a thunderstorm, so that the top is not directly over the bottom. This allows the thunderstorm to act like an engine, pulling moist air in, converting it to rain, and pushing rain and wind out.
During the monsoon, especially in late July and August, the steering flow in the upper-level of the atmosphere (see the white arrow in the image) is very weak. That upper-level wind is what tilts the storm, by pushing its top over a bit.
When this upper flow is too weak, the storm will form and then quickly lose its tilt.
Now, it's raining on top of the inflow of the storm. That suffocates the storm, stopping the engine's process, resulting in a quick collapse of wind and rain down to the surface. Voilà: a wet microburst.
Figure 5: Upper level wind is weak, resulting in the thunderstorm's inflow and outflow being on top on one another. In other words, the thunderstorm has lost its tilt. This results in a quick collapse of the storm.