A midlevel dry slot is a tongue of dry air in the middle troposphere, visible as a dark region on water vapor satellite imagery, that most often wraps into a mature cyclone or extends downstream of a shortwave trough. It is the classic dark stripe on a WV loop that looks like it is being carved into a comma-cloud head from the southwest.
What the satellite is actually seeing
Water vapor channels measure infrared radiation at wavelengths where atmospheric moisture strongly absorbs and re-emits. The CIMSS GOES-R Band 9 fact sheet notes that this band samples the middle troposphere: where the layer is moist the sensor sees cool, high-in-the-atmosphere radiation and displays it bright, and where the layer is dry the sensor looks deeper into the atmosphere and reports warmer brightness temperatures, which are displayed as dark.
Those dark regions are the dry slot. When the middle troposphere is very dry, the channel can see almost to the ground. That is why CIMSS training on WV interpretation emphasizes reading loops rather than single frames, because the vertical layer the channel is sensing is not fixed.
How forecasters use it
Forecasters track dry slots as three-dimensional features. In a mature midlatitude cyclone, air descending on the back side of the low wraps a dry slot into the storm from the southwest, producing the familiar comma-cloud head-and-tail signature. That dry slot marks where drying, warming, and often enhanced turbulence are moving into the system.
In severe weather setups over the Plains, a dry slot approaching a moist warm sector from the west steepens 700 to 500 mb lapse rates, sharpens the low-level moisture boundary, and can toggle the cap off the boundary layer as it rotates through. Combined with a jet streak passing overhead, the arrival of a midlevel dry slot is a common trigger for a discrete supercell episode.
Where it misleads
The height the water vapor channel is sensing depends on the moisture profile above it. In a very dry column, the channel responds to features well below the layer it nominally represents, so a dark stripe on WV imagery is not always at the height forecasters assume, and its true impact on a storm updraft needs to be checked against a sounding.
A midlevel dry slot is also not a guarantee of severe weather. Dry air aloft over a marginally unstable boundary layer often just kills convection outright through entrainment. Its usefulness depends on what the low levels look like and whether an updraft is strong enough to survive punching through the dry layer.