In your primary training you may have run across the lifted (or lifting) index (LI). In fact, the FAA has several knowledge test questions about the use of this convective index. But you probably never were taught about the Convective Available Potential Energy index better known as CAPE since it wasn't required to know and your flight instructor never knew about it either.
There's a ton of confusion as to what these indices mean from a convective potential. Some of the heavyweight apps out there have grabbed onto CAPE and now show an alert when when your route has high values of CAPE...this is highly puzzling since high values of CAPE (and very negative LIs) may actually mean great flying weather (e.g., clear sky). CAPE is an energy index measured in Joules per kilogram (J/kg). Let discuss what this means for preflight planning.
Let's break this down in easy terms. Deep, moist convection (or thunderstorms) need three basic ingredients to form. These ingredients include (1) instability; (2) moisture; and (3) a form of outside energy contribution (a.k.a. lift). If one of these is missing in sufficient quantity, thunderstorms are not likely. But CAPE only tells you about two of the three ingredients, namely, instability and moisture...it does NOT tell you if there's enough outside energy contribution to get the air moving in the vertical and tap into those high CAPE values. In other words, CAPE simply says there's sufficient fuel for thunderstorms to develop, but doesn't tell you if there's a spark to ignite that fuel and create the fire. Let's look at an example of a case with relatively high CAPE values (over 4,000 J/kg) and no chance for convection.
In the CAPE forecast valid above valid at 00Z (7 pm CDT), notice in the central Plains CAPE values exceeding 4,000 J/kg over a very large area and even some isolated areas exceeding 5,000 J/kg (pink). Notice in the prog chart below valid at 00Z, there is a cold front draped through the central Plains. It is relatively dry with little or no forecast for precipitation where the area of high CAPE values are located. This is due to the subsidence or sinking air that often occurs under a high pressure ridge.
This is perhaps best seen on the 500 mb constant pressure chart. Once again, most of the high values of CAPE are under a pretty large subtropical ridge (High) as shown below. Such a ridge is dominated by sinking air which tends to heat up and produce a subsidence inversion.
The TAF issuances shown below from the EZWxBrief progressive web app for Chanute Martin Johnson Airport (KCNU) located in southeastern Kansas where CAPE values are approaching 5000 J/kg shows good flying weather through the entire forecast period; no thunderstorms or showers and generally just high cirrus clouds.
The EZWxBrief progressive web app provides the area forecast discussion (AFD) below issued by the National Weather Service in Wichita, Kansas. The forecaster agrees that the chances of convection are limited.
The Skew-T log (p) forecast sounding valid at 00Z is a great tool for exploring what is going on here. The sounding for KCNU shows a most unstable CAPE value of 4,210 J/kg and an LI of -6. If this were truly an indication of convection, there surely would be some strong thunderstorms and perhaps severe thunderstorms. It's the big weather picture that is more important and often most pilots are perplexed on how to extract important information from the big weather picture.
The real story is the subsidence occurring under the ridge creates a fairly strong atmospheric cap. Notice a slight bend in the temperature around 5,000 feet MSL. This little feature creates a significant amount of convective inhibition (CINH) of -107 J/kg (shown by the green hatched region between 4,000 and 10,000 feet MSL) making it difficult for air to easily ascend and tap into all that instability above the level of free convection located at 700 mb or roughly 10,000 feet MSL. In the vicinity of a weak cold front and under a ridge, there's little chance for clouds, much less thunderstorms.
In fact, the radar mosaic below valid at 00Z shows the lack of any precipitation in southeast portion of Kansas and over most of the area that depicted the potential for high CAPE. As the prog charts clearly indicated, this front was largely dry despite the huge amount of fuel for deep, moist convection.
So, yes. It was clear skies over much of this region as shown below in the visible satellite image valid shortly after 00Z. The only exception was a healthy cell on the northern edge of the CAPE gradient in extreme northeast Missouri. All of east and central Kansas remained clear.
If you read the forecast discussions (specifically those written by forecasters at the Storm Prediction Center), you will notice that CAPE is mentioned by forecasters way more often than LI. This is because LI has some important limitations that CAPE does not have. You can learn about those limitations by ordering your copy of The Skew-T log (p) and Me: Primer for Pilots book today.
Most pilots are weatherwise, but some are otherwise™
Dr. Scott Dennstaedt
Weather Systems Engineer
CFI & former NWS meteorologist