High values of CAPE = clear skies?

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 and your flight instructor never knew about it either.

I know there's a ton of confusion as to what these indices mean from a convective potential. I see that 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 puzzling since high values of CAPE (and very negative LIs) may actually mean great flying weather.

First, let's talk pie. To make an apple pie, you need a few ingredients. You need flour, sugar, salt, nutmeg, cinnamon and pie crust. Oh yeah, you also need apples...can't have an "apple" pie without apples, right? CAPE is like an apple pie without apples.

Deep, moist convection (or thunderstorms) need ingredients to form as well. You need instability, moisture and a form of outside energy contribution (a.k.a. lift) to make a thunderstorm pie. But CAPE only tells you about two of the three ingredients, namely, instability and moisture...not if there's enough outside energy contribution to get the air moving in the vertical and tap into those high CAPE values. Here's an example of a case with 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 there are areas in the central Plains with CAPE values exceeding 4,000 J/kg over a very large area and even some areas exceeding 5,000 J/kg. Most of these are under a pretty large subtropical ridge as shown below for this 500 mb chart also valid at 00Z.

While there is a cold front draped through the central Plains shown below on this prog chart valid at 00Z, it is relatively dry where the area of high CAPE values are located. This is due to the subsidence or sinking air that often occurs under a ridge.

The TAF issuances shown below from the EZWxBrief progressive web app for Chanute Martin Johnson Airport (KCNU) located in southeastern Kansas shows good flying weather through the entire forecast period.

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. It's the big picture that is more important here. The real story is the subsidence 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) 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 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 northeast Missouri.

Want to learn more about the Skew-T? You can order your copy of The Skew-T log (p) and Me eBook today.

Most pilots are weatherwise, but some are otherwise

Dr. Scott Dennstaedt

Weather Systems Engineer

Founder, EZWxBrief™

CFI & former NWS meteorologist

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