NWS Lincoln, IL -- SWOP event summary

July 13, 2015

A potentially volatile weather event on Monday, July 13th never materialized.  So what happened?  Why didn't the storms develop as expected?  First...let's review the basic ingredients for severe weather, then we'll assess whether or not we had those ingredients sufficiently in place.  In order for severe thunderstorms to develop, we need moisture, instability, wind shear, and a triggering mechanism.  

1) MOISTURE  

A very moist airmass was in place across central Illinois on Monday.  Due to all the recent rainfall, wet soil conditions, and evapotranspiration from maturing corn and soybean crops, surface dewpoints climbed to unusually high levels in the upper 70s to around 80 degrees.  An 80-degree dewpoint is relatively rare in Illinois: however, that number was achieved or topped at several local airport sites.  Here is a listing of the peak afternoon dewpoints around the area:

2) INSTABILITY

Instability is needed in order to generate severe thunderstorms.  An unstable airmass exists when a parcel of air remains warmer (and more buoyant) than its surrounding environment and is thus able to continue rising on its own much like a hot air balloon.  One of the primary tools for assessing instability is Convective Available Potential Energy (CAPE).  The upper air balloon launch conducted at National Weather Service Lincoln early Monday evening showed CAPE values of around 6600J/kg, which represents extreme instability that is not often seen.  The image below shows a trace of the temperature (solid red line on the right) and the dewpoint (solid red line on the left) through the entire depth of the atmosphere as measured by our balloon.  The dashed red line represents a parcel of air lifted from the surface.  As long as the parcel is warmer (or stays to the right of the temperature curve), it is unstable and will continue rising on its own.  Note the very large area of CAPE between the dashed line and the temperature plot through much of the sounding highlighted in yellow.  This is the potential energy available to any thunderstorms that develop.  The larger the area, the greater the CAPE, and the more energy available for convective growth.  For more information on CAPE, please refer to our week one Severe Weather Topic:

https://www.weather.gov/ilx/swop-severetopics-CAPE

Also note the small area just off the surface where the dashed line is actually to the left of the temperature plot.  Here the parcel would be cooler than its environment and would thus be stable.  This is what is referred to as a "cap" as it tends to suppress convective development.

3) Wind Shear

Wind shear acts to tilt a storm's updraft such that it is able to maintain itself for a long period of time.  It also enhances rotation within the storm, which can in turn lead to the development of a tornado.  The combination of high instability and strong low-level wind shear is a key indicator for a potential severe weather outbreak.  As can be seen from the sounding above, winds at the surface were from the southwest at around 10kt, but then veered to the west/northwest and increased to around 50kt at about 15000ft aloft.  This represented strong low-level wind shear within the environment.  For more information about wind shear, please refer to our week two Severe Weather topic: https://www.weather.gov/ilx/swop-springtopics

4) Forcing Mechanism

The final ingredient for thunderstorms is a forcing mechanism.  In other words, something to get the air pushed upward.  This is often a frontal boundary, an outflow boundary from previous convection, or simply a thermal of warm air on a spring or summer afternoon.  On Monday, the expected forcing mechanism for the afternoon and evening severe weather outbreak was an outflow boundary left behind by a large complex of storms that dropped through northern Illinois into Indiana during the morning.  The 11 AM surface map below shows the southernmost position the outflow was able to reach...basically from just north of Galesburg...to Lincoln...to south of Paris.  Note the prevailing southwesterly winds ahead of the outflow, while winds behind the boundary had switched to the east-northeast. 

As the day progressed, the outflow boundary quickly mixed back northward.  By 3 PM, it was analyzed across northern Illinois from near Sterling to just south of Kankakee.  This would be the primary location where thunderstorms initiated between 4 PM and 8 PM, with the storms then tracking southeastward along the boundary into Indiana. 

Further south, very little convection occurred across central or southeast Illinois.  A few storms developed ahead of a weak upper-level disturbance across Missouri during the evening, then tracked southeastward and brought scattered wind damage to portions of Scott, Morgan, and Sangamon counties.  Another small cluster of storms formed near Decatur around midnight, then dropped southeastward producing sporadic hail and wind damage reports from Moultrie County to Crawford County.  Other than that, the remainder of the area remained storm-free.  Below is a map of all the storm reports from Monday, July 13th.  Note that most of the severe weather occurred across northern Illinois near the old outflow boundary, then spilled southeastward into Indiana and northeast Kentucky.

In review...while the atmosphere was extremely unstable, strongly sheared, and contained plenty of moisture, there was no significant forcing mechanism available.  In addition, the cap of warm air that developed during the afternoon prevented any developing updrafts from tapping into the large amounts of CAPE aloft.  As a result, central Illinois was spared a major severe weather outbreak.