Intensifying Storms Over Upper Mississippi Valley Could Be a Sign of Bigger Changes

Growing up in New England as an enthusiastic weather observer and snowstorm lover, I delighted in the nor-easters that frequently brought blizzards to my home in western Massachusetts. I marveled at the way storms seemed to follow the Atlantic coastline, sensing the border where land met water.

Later, in teaching synoptic meteorology courses at Penn State, I called these storms “smart bombs,” using the somewhat colloquial term “bomb” adopted by the National Weather Service to describe a cyclone that deepens at least 24 millibars in 24 hours. I explained the storm formation in terms of formal dynamical arguments, stating that the explosive intensification of low-pressure systems needs a strong temperature gradient, especially when large bodies of relatively warm water (e.g., the Gulf Stream) lie on the warm side of cold continental air. Storms (Figure 2) that seemed to hug the Atlantic coastline were simply following the strongest horizontal temperature gradients–their energy source. As exceptions I would point out the occasional occurrence of rapid cyclone development over the Midwest, emphasizing the infrequence of such storms in the absence of the ocean-land temperature contrast.

So it was with some surprise that, during this past year or two, I began to note the frequent occurrence of cyclone intensification over the upper Mississippi Valley in which low pressure systems reached their maximum intensity (i.e., the lowest central pressure) near the Great Lakes. During April 2023, I noted five such storms, an example of which is shown in Figure 1. Although these did not quite qualify as true bombs, the rapid intensification leading to minimum central pressure over the same small area is worth noting. During the five-month period between December 1, 2022, and April 30, 2023, when a total of 17 such storms were observed.

Figure 1. Surface weather map showing isobars, fronts, station plots and areas of precipitation (green shading) at 1200 GMT April 16, 2023. The low pressure center over Lake Michigan is a typical example of the type of storm system referred to in the text (see Table 1).

Figure 2. Portion of upper Midwest, showing locations of the five April 2023 cyclone centers for storms listed in Table 1 at locations where they reached maximum intensity. April dates are shown in the boxes.

Figure 3. Spatial patterns of the observed May-August changes in maximum temperature (top; Tmax), minimum temperature (middle; Tmin) and precipitation (% relative to 1957-2000) for 2001-2020 versus 1957-2000. (From Eischeid et al, The Journal of Climate, October 2023).

Recently an article was published in the Journal of Climate suggesting that global climate change was having an effect on the weather pattern over the Midwest. In this paper, the authors investigated a phenomenon known as the “Warming Hole.” Specific to the Midwest and particularly to the warm season of the United States, the Warming Hole extends roughly from Canada through the Great Lakes to the Gulf of Mexico (Figure 3). This phenomenon is characterized by a region of near or slightly below temperatures and above-normal rainfall surrounded on the east and west by above normal maximum (Tmax) and minimum (Tmin) temperatures. Known since the 1990s, the Warming Hole was once thought to be a transient pattern resulting from natural fluctuations of the general circulation. In fact, the pattern has persisted to the present, leading the authors of that paper to conclude that the Warming Hole appears to be a new, permanent aspect of global circulation patterns. Based on simulations with a global climate model, the authors concluded that the Warming Hole is a consequence of persistent above-normal sea surface temperatures over the southern tropical Pacific Ocean acting on the downstream atmospheric circulation. The result is a “hole” in the normal global warming pattern over the United States, within which remain cooler and wetter conditions surrounded by above-normal temperatures.

All of the storms that I recorded that came close to qualifying as bomb cyclones over the 5-month period between December 1, 2022, and April 30, 2023, reached their maximum intensity over a small region including the Great Lakes and their surrounding states, principally Minnesota, Illinois, and Michigan. Most formed or began intensification in the lee of the Rocky Mountains between Wyoming and western Texas, and first tracked southeastward and then northeastward across the Great Lakes. A few tracked southeastwards from Western Canada in the form of the so-called Alberta Clippers. Many were accompanied by outbreaks of cold air streaming down from Canada west of the storms’ centers. This widespread array of these cyclone tracks covers the interior of the Warming Hole and may account for the above-normal precipitation shown in Figure 3. Much like the coastal cyclone development, these storms intensify in the presence of a strong gradient of temperature between the cold air flowing down from Canada to the west of these storms and the warmer and moister southerly airstream moving northward from the Gulf of Mexico.

The Warming Hole has been documented for the spring and summer months. However, if it is caused by above-normal temperatures over the Pacific Ocean, then, as the authors of the Journal of Climate paper conclude, it is likely a result of global climate change that persists throughout the entire year, and therefore, the profound changes in the temperature and precipitation patterns over the Midwest caused by the above-normal sea surface temperatures over the Pacific could be related to the increased cyclone activity in this region of the United States. This is merely speculation for the present. In any case, climatologists and other weather observers should be aware that this new pattern may presage important changes in the rainfall and temperature patterns over the eastern half of the United States. Clearly, more investigation is necessary to establish a firm relationship between this type of cyclone behavior, the Warming Hole, and global climate change.

A view of the blizzard conditions in South Dakota during the April 5 cyclone.

Snowfall map for April 3-5 cyclone.

Table 1. Dates, near minimum central pressures of storms shown in Figure 2 and description of significant meteorological events.

Date April 2023	Central pressure (mb)	Severe storm events
1	989	Heavy snow northeastern Wisconsin and Upper Michigan; high winds along front Mid-South.
5	985	Blizzard conditions central South Dakota, eastern North Dakota and Northwest Minnesota.
16	996	High winds in the Upper Great Lakes region.
20	1000	Hail, high winds, and tornado reports from Texas up through the Great Lakes.
30	992	Prolonged period of rain, snow, and high winds 29 April–2 May over Michigan’s Upper Peninsula; snowfall totals of 9–18” inches over the western UP, 24–48” over the north-central; May snowfall records broken in some areas.

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No potential conflict of interest was reported by the author(s).

Additional information

Notes on contributors

Toby Nahum Carlson

TOBY NAHUM CARLSON is a Professor of Meteorology, Emeritus, from Penn State University.