Thunderstorm Ingredients
In addition to a source of lift mentioned earlier, cooking up a severe thunderstorm requires a few additional ingredients.
Warm Moist Air
Severe thunderstorms need an ample of supply of warm and moist air in the lower levels of the atmosphere. On Saturday, generally sunny skies and southerly winds sent temperatures across southern Minnesota and central Wisconsin into the mid to upper 80s (click here). In addition to warm temperatures, surface dewpoints (Figure 10) surged into the upper 60s and low 70s across the region as moisture from the Gulf of Mexico flowed north.
While conditions near the surface were warm and moist across the region of Enhanced risk, severe thunderstorms require that the layer of moist air extend at least a few thousand feet above the surface. The analysis of Deep Layer Moisture from the SPC Mesoanalysis page (click here) indicated moisture in the lowest 5,000 feet was relatively high across southern Minnesota and Wisconsin. Plenty of warm moist air would be available for any thunderstorms that developed.
Instability
Thunderstorms require an environment that is unstable, or in other words, an atmosphere that is supportive of upward motion through a deep layer. Meteorologists use many parameters for assessing instability. While technically a measurement of the energy available for a thunderstorm updraft, Convective Available Potential Energy (CAPE) is commonly used as a proxy for instability through a deep layer of the atmosphere. CAPE forecasts are readily available on many commonly used forecast apps.
CAPE can be calculated in several ways, but I tend to rely on Mixed Layer CAPE (MLCAPE). The methodology for calculating MLCAPE tends to avoid unrealistically high CAPE values that often occur using other methods. The MLCAPE analysis at 19 UTC / 2:00 pm on July 23rd (Figure 11) showed a broad area of 4,000 to 5,000 J/kg of CAPE across southern Wisconsin. These values were quite high, indicating a very unstable environment supportive of speedy thunderstorm updrafts was in place upstream of Lake Michigan.
Wind Shear
Wind shear is the change in wind speed and/or wind direction with increasing altitude. For example, southerly at 10 knots at the surface, and westerly at 50 knots much higher in the atmosphere. Thunderstorms suffer when cool air, rain, and hail in the downdraft interact with the storm’s relatively warm and moist updraft. Too much interaction between the downdraft and updraft can bring a strong thunderstorm to a quick end.
Strong wind shear through a deep layer of the atmosphere promotes storm organization and limits the interaction between the downdraft and updraft. Wind shear, particularly shear in the lowest layer of the atmosphere, also provides a source of spin which may result in rotating updrafts and the potential for tornadoes. Squall lines and long-lived severe thunderstorms tend to thrive in environments characterized by strong wind shear (greater than 30 knots).
The analysis of Surface to 6 km Shear valid at 2:00 pm CDT (1900 UTC) on July 23rd (Figure 12) indicated values of 40 to 50 knots across southern Minnesota and Wisconsin. A separate analysis valid at the same time indicated that the magnitude of wind shear had increased upstream of Lake Michigan over the previous three hours (click here). With pre-existing robust values of wind shear increasing across the area, any thunderstorms that developed were likely to be well-organized and long-lived. There was also the potential for tornadoes.
Help From Above – Shortwave Troughs
Weather features residing at 20,000 to 30,000 feet above the surface play an important role in severe thunderstorm development and intensification. Shortwave troughs can increase instability by cooling the mid-levels of the atmosphere. The approach of a shortwave trough may also increase wind shear, thereby improving storm organization and extending a storm’s lifespan. The discussion accompanying the SPC Convective Outlook mentioned that “an upper-level trough … will move east-southeastward across the northern Plains today. As the trough approaches the northern Plains this morning, scattered convective initiation should take place”. Click here to view the approach of the shortwave trough.
Help From Above – Jet Streaks
In the upper reaches of the atmosphere, pockets of faster moving air – jet streaks – travel through the jet stream. Areas of divergence occur downstream of jet streaks which act to increase upward motion in the air column. This upward motion strengthens a thunderstorm’s updraft by enhancing lift in the storm’s environment. The jet stream analysis valid at 2:00 pm CDT (1900 UTC) on July 23rd (Figure 13) shows a broad area of upper-level divergence approaching Wisconsin (click here for an annotated image).
All the ingredients for a severe thunderstorm outbreak were in place across southern Wisconsin. Let’s examine how the day unfolded.
