“Here we go again.”
That’s the thought that many residents across the Midwest will be thinking Tuesday afternoon, as an intense line of severe storms impact some of the same areas as Monday night. The cold front prompting Tuesday’s severe storms is the same front that hit the Upper Midwest Monday night with 160 reports of severe weather, including golf ball sized hail, widespread hurricane-force wind gusts, and a tornado. Similar dynamics to Monday night will be in play Tuesday afternoon such that all modes of severe weather will again be possible. Fortunately, the focus of Tuesday’s severe weather will be further southeast than Monday.
As of 8:00 am CDT, the thunderstorm complex that produced Monday night’s widespread severe storms was still active over lower Michigan. The explosive development of storms against a stark contrast between air masses generated a vigorous low-level jet streak that fueled the storms all night long by essentially forcing air upwards. Nearly identical conditions will exist Tuesday to support a similar long-lasting line of severe storms.
As heat and humidity intensify under the influence of a dome of high pressure in the Eastern US, waves of low pressure continue to eject into the US from Canada, enforcing a cool, dry regime in the West and the Upper Midwest. Caught between the two air masses are the Great Lakes and parts of the central and upper Mississippi River Valley. Warm, very humid air with dew points in the mid-70s will flow into this region from the Gulf of Mexico at the surface, but cooler, drier air from Canada and the Rocky Mountains will flow aloft. The collision of the slowly southeastward moving cold front into the tropical airmass will again prompt explosive development of thunderstorms, which will quickly organize into a continuous squall line.
Extreme instability will build under a modest elevated mixed layer, a dry layer in the mid-levels of the atmosphere that develops as wind blows over the Rocky Mountains. This dry layer “caps” convection and thunderstorm development until broken by solar heating or strong upward forcing. When air does ascend through the elevated mixed layer, potential instability is released and explosive storm development occurs. This is analogous to steam escaping from a covered pot of boiling water when the lid is removed. The cold front, the series of shortwave troughs, and the vigorous mid-level jet stream will provide sufficient lift to break the cap on convection.
The worst of the severe weather will occur along the axis of the most intense portion of the mid-level jet streak where the temperature gradient is most stark and where the strongest wind shear overlaps the most extreme instability. These conditions will be met along a corridor from northeast Iowa to the western half of Lower Michigan. Hurricane force wind gusts will be the most profound threat here, but large hail and perhaps an isolated tornado will be possible in areas that are impacted by storms before they organize into a squall line.
Discrete storms will initially fire ahead of the cold front Tuesday afternoon in western Iowa and southern Minnesota, gradually organizing into a continuous system as the storms plow eastward. By the tail end of rush hour, a continuous squall line capable of producing destructive wind gusts will exist from eastern Kansas to Upper Michigan. Storms will be quick moving, but hard hitting. A slew of wind damage is likely in the storms’ wake.
Green Bay and Grand Rapids will both be in the region most at risk for these potentially destructive thunderstorms for a second evening in a row. Joining these two cities in the region most at risk for dangerous thunderstorms will be Milwaukee, Madison, and Cedar Rapids. In the latter two cities, hail and tornadoes will be more significant threats, but damaging wind gusts will still be the dominant risk. A slightly lower risk of severe weather will exist further south and east overnight in Kansas City, Des Moines, Chicago, and Detroit.