The onslaught of severe thunderstorms will continue for a fourth day in parts of the Upper Midwest Monday as the parade of mid-level disturbances and mid-latitude cyclones heightens its display of force. An active severe weather event is expected, as conditions will be more than favorable for the development of a long bowing segment of severe thunderstorms. Widespread wind damage will be the predominant threat, but large hail and perhaps a few brief tornadoes will also be possible.
Since Friday, the atmospheric pattern has gradually evolved to favor daily and nightly severe thunderstorms from the northern Plains to Lake Michigan. A broad region of high pressure centered over the Southeast US has been intensifying and expanding throughout the weekend. This dome of high pressure has been pumping Gulf of Mexico heat and moisture northward into the Upper Midwest for the past several days. Meanwhile, a broad upper-level trough of low pressure ejected from Canada has settled over the West. The antithesis of the ridge, the trough has been directing cool, dry air to largely eliminate the feeling of summer from Oregon to northwestern Minnesota. Caught between the two air masses is the Upper Midwest , from southern South Dakota to northern Michigan. When such a stark contrast exists between colliding air masses, explosive thunderstorms are the only means of restoring balance.
The cool, dry air pulled from Canada by the upper-level trough has been riding the slopes of the Rocky Mountains for the last several days. Ejected aloft over the Plains, this elevated layer of dry air, known as the elevated mixed layer, will prevent any thunderstorms from developing until a force powerful enough breaks the “cap” on convection. Analogous to steam building in a covered pot of boiling water, extreme quantities of instability will build until the cap is broken. This force will arrive late in the afternoon as a surface cyclone drags a strong, southeastward moving cold front accompanied by a vigorous jet streak from the mid-level shortwave trough. All that brewing instability will be released. Air will rush upwards and quickly spark severe thunderstorms.
Ample wind shear will accompany the mid-level shortwave as its jet streak helps to break the cap on convection. Wind shear is necessary to keep storm updrafts, which supply warm air and moisture, from the rain and evaporation in storm downdrafts. So much shear will be present, in fact, that storms will quickly organize into an evolving squall line ahead of the cold front. Down drafts and rear inflow of air will push the center of the line outward, curving the system into a bow shape known as a bow echo. These systems are ubiquitous with widespread, powerful wind gusts and the trail of damage they leave behind.
If the bow-echo survives long enough and leaves a swath of damage wide enough, it obtain the infamous “derecho” designation. Derechos are long-lived, straight-line wind storms composed of fast-moving thunderstorms. They can produce hurricane-force winds, brief tornadoes, and flash flooding. In order to be deemed a derecho, the bow echo must produce wind gusts of at least 60 mph across a time span of at least six hours and leave a swath of wind damage of 250 miles or more.
Just ahead of the squall line in Minnesota, northern Iowa, and western Wisconsin, a few discrete thunderstorms could develop, prompted by storm outflow or daytime heating just barely piercing the lid on convection. With such extreme instability and rapid decrease in environmental temperature with height, very large hail of one to two inches in diameter is possible alongside strong downward bursts of wind with any discrete supercells that form ahead of the linear system.
The highest concentration of severe storms will occur near the intersection of the warm and cold fronts just east of the surface cyclone. Ascent will be strongest in this area as the mid-level jet streak left exit region will overlap forcing from the fronts. The cyclone is expected to trek northeast, from the western Minnesota and Iowa border toward the Upper Peninsula of Michigan. As storms intensify, winds will increase in the mid-level jet streak, which will evacuate more air from the surface, engaging the thunderstorms in a positive feedback loop that could sustain the potential bow-echo long enough to become a derecho.
Several major population centers are at risk for severe storms Monday afternoon and evening. Most at risk are areas which the surface cyclone will pass through with the accompanied low and mid-level jets, namely southeast Minnesota, northern and central Wisconsin, and the western Peninsula of Michigan . Minneapolis, St. Paul, Rochester; Mason City, and Eau Claire are at risk for both supercell thunderstorms and the intense squall line. Multiple rounds of severe storms are possible here between late afternoon and early evening. North and west of this zone, supercells will develop before the squall line. Hence, the risk of thunderstorms in places like Sioux Falls and Duluth will be lower.
Further east, Appleton and Greenbay, WI and Marquette, MI will be more likely to experience the wrath of a potential bow echo. Storms will arrive here late in the evening and will be quick hitting, Tropical storm to hurricane force wind gusts could arrive with the storms, packing a quick but powerful punch.
As the night wears on, the squall line or bow echo will slowly decay. There is a smaller chance that the storms produce strong wind gusts in southern Wisconsin, the eastern Michigan Upper Peninsula, and western Michigan lower peninsula. Cities like Milwaukee and Grand Rapids should expect overnight thunderstorms, but damage will be minimal.
The line of thunderstorms could make it all the way to Detroit before dissipating between late Tuesday morning and early Tuesday afternoon. By then, thunderstorms will have developed across the same frontal boundary that prompted Monday’s storms, but along an axis further southeast. Some areas in central Wisconsin and western Michigan will have less than 24 hours before confronting severe storms all over again.