After a wet, stormy morning, parts of the Midwest will only have an afternoon to dry out before storms return Friday evening. The storms will be less widespread than Friday morning, but they will carry a threat for all modes of severe weather. Damaging winds, serious hail, and a few tornadoes are possible late Friday afternoon.
Torrential downpours have been ongoing across much of the Midwest within a broad swath of convection Friday morning. The heaviest cluster of storms pushed eastward through Iowa, Missouri, and Illinois. These storms will weaken throughout the day as its supporting low-level dynamics wane. Skies will slowly clear in the wake of the morning downpours, allowing extreme instability to build, especially over Missouri, Iowa, Kansas, and Arkansas. The heavy morning rain will keep the airmass wet and heavy, contributing to the instability.
With ample fuel for strong thunderstorms in place, all that is needed is a trigger to force air upward. A Rocky Mountain-instigated shortwave trough of low pressure will serve that role. These systems are ideal for instigating violent storms because they usually carry an elevated mixed layer.
As air blows over the mountains, moisture evaporates out. Water vapor is lighter than air, so air rolling down the mountains lacking water vapor is more dense than it was going up. This dry layer puts a “lid” on convection until broken by daytime heating and large-scale ascent. With the robust lift to the east of the upper-level low, the air being lifted will become unstable as the moist air at the surface cools much slower than the dry air it will encounter aloft due to heat released from condensation. Given wind shear to separate ascending from descending air, this elevated mixed layer will contribute to strong rotating storm updrafts that could support torrential rain, very large hail, and tornadoes. Such conditions are usually in place during the most violent severe storm outbreaks.
Storms will be triggered late in the afternoon beneath the left exit region of the mid-level jet streak over eastern Minnesota and along a warm-frontal boundary in southern Iowa, northern Missouri, and possibly northeastern Kansas. Strong wind gusts are the most likely threat from the storms over Minnesota. Instability and shear will be too weak to support supercells. Thus, storms will be more violent further south,
All modes of severe weather are possible in southern Iowa and northern Missouri. Scattered storms will first develop along a warm front late in the afternoon. Robust low-level shear will contribute to rotating updrafts, especially in northeastern Missouri and southeastern Iowa near the Mississippi River. The combination of strong low-level shear, low cloud bases, and vicinity near the most intense upward motion will contribute to the highest threat for tornado-generating supercells developing over this area. These supercells will also be capable of producing hail of up to two inches in diameter. Such storms could also develop further north and west over central Iowa and western Missouri.
The storms will quickly organize into one prime linear system sliding southeastward across Missouri and Illinois throughout the evening. Isolated wind damage will be the dominant threat with the linear system of storms. This linear system will continue into the early overnight hours, but severe chances will decrease with the loss of daytime heating.
Tornadoes and hail of at least two inches in diameter could threaten Des Moines, IA and Kirksville, MO. Iowa City, Cedar Rapids, and Waterloo could also be impacted by these threats, though chances will be lower. Rochester, MN and Mason City, IA are more likely to experience straight-line wind damage.
Shortwaves will be deterred from spawning more severe weather over the next few days as a dome of high pressure expands to bring tropical heat and humidity to the eastern US. It will take a much broader, deeper trough of low pressure to force the ridge of high pressure eastward. Such a system is expected to cross the western US early next week, and could be the source of the next widespread severe weather event.