A series of mid-level disturbances will ride along the flow of an upper-level region of high pressure centered over the Mississippi Valley, triggering several rounds of thunderstorms for the Northern Plains and Upper Midwest through Sunday.
The least active day will be Friday, where Minnesota and Wisconsin will face scattered thunderstorms both in the morning and the evening. Strong wind gusts and isolated large hail will be the main threats with Friday’s storms. The morning storms are not expected to be severe. A moist surface layer will remain in the wake of the morning storms, but clouds and an inversion could prevent most of the area from developing thunderstorm activity until late in the afternoon, when isolated storms may develop in Central Wisconsin and southern and east-central Minnesota where the inversion “capping” convection is expected to clear first.
At the surface, a warm frontal boundary will stall over the Northern Plains and northern Minnesota Friday. South of this boundary, intensifying low-level winds will provide an abundance of moisture and instability, with drier air above it flowing from the Rocky Mountains. The southerly winds will also keep day-time high temperatures in the 90s and dew points in the 70s, conditions that will serve as ample fuel for severe storms provided a proper trigger. These conditions will persist in the Central US until the warm front retreats southward as a cold front over the weekend. Friday’s storms will be less widespread than the weekend storms because there will be very little sources of lift to encourage upward motion and convection for thunderstorm development.
Storms that do pop-up late Friday afternoon have the capacity to become supercellular in the northern halves of Minnesota and Wisconsin. Tornadoes could occur within these storms, but because there will be so few thunderstorms before storms organize into clusters the tornado threat is marginal. Damaging winds will be the most likely threat, followed by the threat of large hail. The most widespread storms will likely occur over central Wisconsin, where outflow from Friday morning’s storms may still be present to initiate convection.
Additional thunderstorm development is likely in eastern Wyoming and western Nebraska where there will be more lift, ahead of a mid-level shortwave but a less unstable boundary layer. The shortwave will travel northeastward across Wyoming this evening, so storms will be greatest in coverage in northeast Wyoming, where scattered strong wind gusts will be likely. Large hail and tornado development is possible, especially along and east of leeward mountain slopes. However, lack of instability near the South Dakota border will limit the spatial extent to which severe storms develop. Severe weather is expected to remain confined to the northeastern corner of Wyoming.
More sources of lift will be present Saturday and Sunday in the Northern Plains and the Upper Midwest, as the frontal boundary slowly shifts southeast and shortwaves of low pressure ride along the boundary aloft. Organized storm clusters Friday night will persist into early Saturday morning, but debris should clear by afternoon. Several shortwaves will ride along the cold front and trigger the development of several clusters of organized storms moving northeastward along the front.
A very dangerous situation may set up over South Dakota, Minnesota, and Wisconsin saturday as a result of these extra sources of lift. Strong wind shear will develop over the region due to intensifying southwesterly flow from an upper-level low to the west and an upper-level high to the east. This wind shear will set up over the profusely unstable airmass at the surface, with Bud’s remnants serving as an ample source of moisture. Scattered evening thunderstorms will become more widespread into the early overnight hours. Wind and large hail are the main threats, but tornado development is possible, being most likely over eastern South Dakota and west-central Minnesota. Areas further east will be spared by the late-night arrival of the storms. The tornado threat will be greater Saturday than Friday.
As the storms become widespread late in the evening, the primary concern will shift to flash flooding due to widespread heavy downpours overnight. Several inches of rain could fall in areas that experience multiple rounds of heavy downpours.
Sunday will largely be a repeat of Saturday, as surface conditions and the upper air-pattern will change little between the two days. The cold front will have sagged roughly 100 miles southeast, shifting the threat zone in suit. Widespread late afternoon and evening thunderstorms will impact many of the same areas as Saturday, plus parts of northeastern Nebraska and northern Iowa. The main limiting factor will be cloud cover that remains from Saturday night’s storms. Severe storms generally will not develop where clouds have lingered most of the day. It is uncertain where and when clouds will clear, but severe storms are likely to develop when this occurs. Regardless of the severity of storms, heavy clusters downpours are still likely to develop with the influx of tropical moisture from Bud’s remnants.
Residents of cities like Rapid City, Sioux Falls, Minneapolis, Duluth, and Omaha should closely monitor the forecast and the sky. Many of the thunderstorms Saturday and Sunday will be dangerous. The limiting factor will be an overabundance of storms, resulting in constricted updrafts and widespread cloud cover during the day for some areas. Regardless of the storms’ intensity, widespread heavy rain is likely, which could result in flash flooding. Flooding of roadways will be difficult to see as most of the storms will occur after daylight hours for the eastern half of the risk zone.
The front will finally pass through the Northern Plains by Monday, stalling over Kansas and bringing the severe threat to the southern Plains and central Mississippi River Valley. Monday is too uncertain to delve into specific details, but with a moisture feed from the Gulf of Mexico, widespread thunderstorms are likely.