A Quiet Low-Pressure System (author: Jordan Murdock)

Tuesday November 17th, 2021, proved to be an uneventful day in the atmosphere over North America. There were no severe weather reports of any high winds, hail or tornadoes and no severe precipitation seen. However, though a quiet day for most of the United States, the Great Lakes region did receive some light precipitation at 12Z as can be seen in Figure 1a. Above Minnesota there is a low-pressure system that produced snow and is heading east to the Great Lakes. Already present over the Great Lakes is precipitation following a strong cold front that is attached to a low-pressure system over Lake Michigan seen in Figure 1b. Focusing on this low-pressure system, it is set up by the cold front to strengthen and is what led to the precipitation present. Cold, dense air squeezes its way through the warmer, less-dense air, and lifts the warm air. Because air is lifted instead of being pressed down, the movement of a cold front through the warm front led to the low-pressure system and the precipitation in this area.

Figure 1: a. Pivotal weather map from 12Z 17 November 2021 showing rain (green fill), snow (blue fill) and low-pressure systems. b. WPC surface analysis 12Z 17 November 2021 showing the cold front and low-pressure system over the Great Lakes

 

The 300 mb map in Figure 2 shows a deep level trough over Wyoming at 12Z on November 17th, 2021. At the base of the trough there are light blue fill patterns representing stronger winds that continue northeast along the jet stream towards Wisconsin and the dark purple fill pattern that represents even stronger winds. East of these darker purple fill patterns is a ridge at 300 mb, though it is not as prominent. In between the trough and ridge there are pink contours representing divergence in the atmosphere that is still present even though the ridge is not as pronounced. The strong winds present over the Great Lakes and the Michigan, Wisconsin region along with the upper-level divergence between the trough and the ridge leads to the low-pressure system and the associated cold front.

Figure 2: 300 mb map from 12Z 17 November 2021 showing the jet streak (fill pattern) and divergence (pink contours)

 

As the low-pressure system strengthens, so does the associated cold front. Figure 3 shows the locations at which frontogenesis takes place, represented by the red contours. Frontogenesis is the intensification of a front when the horizontal temperature gradient amplifies by at least an order of magnitude. This process occurs when a warm air mass meets a cold air mass since a temperature difference is essential in the definition of a front because it implies a density difference. Figure 3 shows frontogenesis located over Oklahoma, Missouri and Illinois following the cold front that attaches to the low-pressure system over the Wisconsin and Great Lakes region. This area of frontogenesis is located in between the trough and the ridge which is the same area as divergence. All these components led to the strong cold front and the attached low-pressure system which resulted in the precipitation, no matter how limited, in the region.

Figure 3: Surface frontogenesis map of 12 Z November 17, 2021 where frontogenesis is represented by the red contours