Cold Air Pooling in Wisconsin
Unseasonably cold temperatures were found in the early morning hours of 20 January 2026. This map of observed temperatures from 0800 UTC (2:00 AM CST) shows that a tongue of cold air has plunged southward with subzero temperatures found even in the southern portions of Minnesota and Wisconsin while single-digit temperatures were common throughout the Midwest.
Of particular interest, however, are the isolated temperatures between -11 and -18 in southwestern Wisconsin. Are these temperatures observational outliers with no physical basis, or are they really representative of the conditions on the ground? Let’s use satellites to see if we can learn more about what’s going on.
We’ll start with a regional view of Band 13 from GOES-19. This is the standard 10.3 micron infrared band used to identify the positions of clouds at all hours of the day. However, interpreting this band during abnormally cold conditions can be a challenge, because the color scale used to identify higher-altitude clouds paints the cloud-free surface in the same colors because the temperatures are so low. This animation shows this quite well: there is clearly a lake effect band on the western shore of Michigan’s lower peninsula, and yet it has the same cyan and blue colorization as the immobile parts of Minnesota and Wisconsin.
The infrared-observed surface temperatures of the upper midwest are clearly heterogeneous. But what is contributing to all of that variability? To analyze this, let’s adjust the color scale of the infrared image. This is a still frame from the 0800 UTC image above, merely adjusted to cnhance the dynamic range over the observed surface temperatures. Light is warmer than dark.
Compare the patterns in the infrared image above to this terrain map from the Wisconsin Department of Natural Resources. Note how the colder (darker) regions on the infrared map in southwestern Wisconsin correspond to the valleys in the terrain map, with that pair of -11 F temperatures confined to the Wisconsin River valley. The -18 F temperature is found in the valley of the La Crosse River. Both of these rivers flow through the Driftless Area, the rugged heart of the upper midwest that avoided being flattened by glaciers in the most recent ice age.
We commonly associate increasing altitude with decreasing temperatures. So why are the valleys colder than the tops of the ridges? This is a result of cold air pooling and katabatic flow. The night was clear and calm; see how the surface winds in the earlier figure were between 0 and 5 knots everywhere. In the absence of synoptic scale forcing, smaller scale forcing can take effect. In this case, the air at the top of the terrain experiences strong radiative cooling. After all, there were no clouds above to absorb and emit outgoing infrared radiation. Cold air is also dense air, and so these radiative-cooled air masses flowed downhill into the valleys, where they pooled. This downward flow of radiatively-cooled air is known as a katabatic wind. On the scale of the relatively shallow terrain of the Driftless the flow would be difficult to measure directly, but in mountainous regions they can be quite significant.
Based on all of this, it is likely that the anomalously low temperatures in southwest Wisconsin aren’t false readings, but actually representative of their local microclimate.