One important tool for any forecaster is the Skew-T diagram or plot. A Skew-T diagram (Figures 1 and 1A) shows the state of the atmosphere in a specific location at a specific time. Soundings are generated by the information received from a released weather balloon as it rises up in the atmosphere; these balloons are released around 00Z and 12Z Universal Time. The information is then relayed back to the National Weather Service local office and uploaded to create a map of the state of the atmosphere around the United States and southern Canada.
Figure 1. Sounding profile and analysis from Detroit, Michigan from October 1st, 2020 at 12Z. (source: spc.noaa.gov)
Figure 1A. Indexes of the Detroit sounding enlarged for easier reading. (source: spc.noaa.gov)
A lot of information can be gleaned from a Skew-T plot. In particular, Skew-T plots can tell if there is a risk of severe weather. Fortunately for us, computers can do the heavy lifting and we no longer have to do these calculations by hand. The index values are given on the bottom of the Skew-T plot in Figure 1. For severe weather, attention should be paid to the following (which are circled in red in Figures 1 and 1A), Convective Available Potential Energy (CAPE), the Lifted Index (LI), and the K index.
CAPE represents the amount of energy that exists in the atmosphere to generate thunderstorms. Values lower than 1000 J/kg are generally considered a low probability of severe weather, while values over 2500 J/kg indicate a high risk of severe weather (Markowski, 2016 p. 33). In the above example, the highest CAPE is 264, which suggest severe weather is unlikely. The Lifted Index is based on the location of the parcel when lifted from the surface and raised to 500 mb. The parcel temperature is then subtracted from the actual air temperature. If the parcel temperature is higher than the actual air temperature (ie. a negative number), the atmosphere is unstable. The greater the negative value, the greater the risk of severe weather. Severe weather isn’t likely with a LI > -2, and is considered very likely if the LI < -6. In our sounding, the LI is forecast to be -2, which would indicate that severe weather is possible, but not very likely (www.weathertap.com, 2020). Our final index, the K index, is also used to measure the probability of thunderstorms. If you had to calculate the K index, you would take the temperature at 850mb, add the dew-point temperature at 850mb, then subtract both the temperature at 500mb and the dew-point depression (the difference between the temperature and the dew point) at 700 mb (www.weather.gov, 2020 and www.weathertap.com, 2020). The Figure 2 shows the probabilities for thunderstorms and severe weather:
Figure 2. Severe weather and thunderstorm potential based on the Lifted Index and the K index values. (https://www.weathertap.com/guides/aviation/lifted-index-and-k-index-discussion.html)
By looking at the actual weather radar for 12Z, 16Z, and 20Z, rain and a few thunderstorms are seen in the Detroit, Michigan area (Figure 3), but by checking the storm prediction center archives, no severe weather was reported in that region for the day of the sounding (spc.noaa.gov, 2020) verifying that our analysis of the different severe weather indices was accurate.
Figure 3. Radar imagery near Detroit, MI on October 1st, 2020 at 12Z (left), 16Z (center), and 20Z (right). https://www.spc.noaa.gov/exper/mesoanalysis/new/archiveviewer.php?sector=16&parm=pmsl#
References
Markowski Paul and Richardson, Yvette. Mesoscale Meteorology in Midlatitudes, 2016, Wiley-Blackwell.
https://www.spc.noaa.gov/exper/archive/events/
https://www.weathertap.com/guides/aviation/lifted-index-and-k-index-discussion.html
https://www.weather.gov/lmk/indices
https://www.spc.noaa.gov/exper/soundings/
https://www.spc.noaa.gov/exper/mesoanalysis/new/archiveviewer.php?sector=16&parm=pmsl#