On 30 September 2020, weather in the continental United States was fairly uneventful. There was only one area that the Storm Prediction Center (SPC) had their eyes on – the state of Michigan. Throughout the day, convective thunderstorms rolled through the region, bringing occasional bouts of heavy rain due to a warm and cold front sweeping through the area within a few hours of each other (Fig. 1).
Figure 1: WPC surface analysis maps from 0000 UTC 30 September 2020 to 0000 UTC 1 October 2020. Isobars (in brown), frontal boundaries, and areas of high and low pressure can be seen on the map. Source: https://www.wpc.ncep.noaa.gov/archives/web_pages/sfc/sfc_archive_maps.php?arcdate=09/30/2020&selmap=2020093000&maptype=usfntsfc
Despite the convective nature of the storms, there was minimal surface-based CAPE present (Fig. 2). CAPE, or Convective Available Potential Energy, is a metric derived from atmospheric soundings to quantify the instability present in the atmosphere. Areas of less than 1000 J/kg of CAPE, seen throughout most of Michigan in Fig. 2, represent areas of weak instability.
Figure 2: SPC Mesoanalysis of surface based CAPE and CIN overlayed onto radar and wind barbs from 1300 UTC on 30 September 2020 to 0000 UTC on 1 October 2020. CAPE is in red contours while CIN is shaded in blue. Source: https://www.spc.noaa.gov/exper/mesoanalysis/new/archiveviewer.php?sector=16&parm=thea&underlay=1#
However, minimal CAPE does not always mean there is no convective activity present. In the atmospheric soundings in Detroit, Michigan between 1200 UTC on 30 September 2020 (Fig. 3 left panel) and 0000 UTC on 1 October 2020 (Fig. 3 right panel), the temperature path almost perfectly follows the moist adiabat. This means that any amount of lifting (such as a frontal passage) would cause an air parcel to rise, resulting in convective thunderstorm activity.
Figure 3: Atmospheric sounding for Detroit, Michigan at 1200 UTC on 30 September 2020 (left) and 0000 UTC on 1 October 2020 (right). Temperature is in red, while dewpoint is in green. Source: https://www.spc.noaa.gov/exper/soundings/20100100_OBS/
Both warm and cold frontal passages can cause precipitation by forcing warm, moist air upward, where it cools and condenses. The right panel of Fig. 4 shows how a warm front (such as the one passing through Michigan around 1500 UTC) could cause precipitation, while the left panel of Fig. 4 shows how a cold front (such as the one passing through Michigan around 1800 UTC) could cause precipitation.
Figure 4: Conceptual model of a cold frontal passage (left) and warm frontal passage (right). Source: https://laulima.hawaii.edu/access/content/group/dbd544e4-dcdd-4631-b8ad-3304985e1be2/book/chapter_6/fronts.htm
Another way to measure the potential for convective thunderstorm activity in this situation is to look at PVA, or positive vorticity advection. Areas of PVA, as can be seen ahead of thunderstorms in Fig. 5, represent areas of upper level divergence that typically leads to rising air at the surface. Just like in the case of a front, this rising air cools and condenses, resulting in precipitation.
Figure 5: Vorticity, positive vorticity advection (PVA), and negative vorticity advection (NVA) overlayed onto radar from 1300 UTC on 30 September 2020 to 0000 UTC on 1 October 2020. Vorticity is shaded, PVA is contoured in blue, and NVA is contoured in red. Source: https://www.spc.noaa.gov/exper/mesoanalysis/new/archiveviewer.php?sector=16&parm=thea&underlay=1#
Even though the frontal passage on the 30 September 2020 did not result in significant rainfall totals in Michigan, the atmospheric setup that resulted in convective activity in the region remained, bringing more rain to the area in the days that followed.