A summertime pattern has developed over the Mid-South this week as high pressure is overridding unseasonably warm and moist air near the surface. Upper-level disturbances rotating through the region are helping to set off scattered thunderstorms during the peak heating hours of the day. On Monday, these storms fired over the greater Memphis area and brought reports of hail, some high wind gusts, and torrential rain that led to flash flooding in many locations. A repeat is possible this afternoon.
These storms are sometimes referred to as "pulse" storms and often times have very little motion, which is the reason for the locally high rainfall amounts and flooding potential. Pulse storms, or airmass storms, are frequently associated with hot, humid airmasses (most often in the southeastern and south-central U.S.) that move very little due to weak winds at upper levels. Typically, wind at about 15,000-25,000 feet drives the direction of motion of a thunderstorm. When the wind at those levels is weak or non-existent (as when high pressure exists over the region), the storms move very little. Instead, the entire life-cycle of the storm plays out over a small land area below - from cumulus cloud development through the growth phase, into maturity, and finally the decay phase.
Pulse storms can go through this development cycle multiple times, or more likely produce "child" cells nearby that grow and decay and then produce additional storms. The child cells are typically initiated by outflows from their parent cells which occur as the parent cell begins to collapse, producing a downdraft that hits the ground and spreads out as a gust front or outflow. The downdraft phase is typically when straight-line wind damage is most likely at the surface as wind speeds in these downdrafts can exceed severe wind criteria (58 mph). Flash flooding becomes likely when the movement of these storms, and their child cells, are nearly stationary and the life cycle plays out over a small area, potentially bringing a couple of inches of rain in an hour or two.
For more information, check out NWS Jetstream's pages on the life cycle of a thunderstorm and downbursts and damaging wind.