Bobbi, CAPE is Convective Available Potential Energy. "Potential" might be the key word here, as you'll see later. The higher the CAPE, the more buoyancy you have. And it serves as an indicator for how unstable the atmosphere is or may become. It is the amount of energy a parcel of air would have if taken and lifted vertically through a certain distance of the atmosphere. But a parcel of air is kind of hard to visualize, so from here on out, I'll refer to this parcel as a bubble. Bubbles are easy to visualize.
OK, so how do we take these bubbles and get them aloft? You need a source of lift. That can be a cold front or more ideally (for chasing anyway) a dryline. Without that source of lift, all the CAPE in the world becomes wasted convective energy. But once you get a warm, moist bubble of air rising, it will continue to rise as long as the atmosphere around it is colder than it is, or until something physically stops it from rising any higher. One of the things that can prevent that from happening is a capped atmosphere, where you have a thin layer of warm air aloft in the mid-levels that acts as a lid and prevents the rising bubbles from going higher. Low to moderate CINH (convective inhibition) can often be overcome through erosion over the course of an afternoon, or broken by strong enough forcing mechanisms. That then leads to the birth of thunderstorms.
Now, there are also various kinds of CAPE. You have surface based CAPE (SBCAPE), mixed layer CAPE (MLCAPE), most unstable CAPE (MUCAPE), etc. Depending on the situation, sometimes one of those forms of CAPE will be more appropriate to use than another. But that discussion would probably be best to focus on as a follow-up. So for now we'll just discuss CAPE in generic terms.
Generally, to have any potential for severe thunderstorms or tornadoes, you need at least some marginal CAPE of 500-1000 j/kg. Many of the winter (as well as early spring and late fall) events will fall into this category. During that time of year you often have very strong dynamics to work with, but only marginal CAPE. Once the measure of CAPE reaches 2,000 or 2,500, you are dealing with much greater buoyancy and a moderately unstable atmosphere. Then there are the days in late Spring where you have enormous CAPE that can reach 3,000....4,000...or even 5,000 j/kg or more. Many would regard this as extreme instability. But often times, you have to deal with the reverse of the situation mentioned previously. In late spring, you often have days characterized by very strong CAPE but with much less favorable dynamics (mid and upper level winds).
As with everything when it comes to weather forecasting, CAPE is just one ingredient in what I like to call "Mulligan Stew". That is where you take everything and throw it in a big pot together. But it is always used in conjuction with or measured against other things, never regarded alone. I have seen days where low level shear would very much support the development of tornadoes be ruined because CAPE was just too marginal to support anything beyond a thin line of low-topped convection that produced little more than rain and some gusty winds. On the flip side, there are days when extreme CAPE is available that you don't even get storms, either because there are no forcing mechanisms to get surface air parcels started on their journey aloft,. or the CAP is simply too strong to overcome. On those days, chasers are often rewarded with a nice sunburn. Ideally what you are looking for is the best of both worlds, where the best combination of dynamics and instability overlap. There are days where you will actually purposefully choose to chase in areas with less CAPE. There will be days when you have extreme CAPE available down in Texas, but the area will be so strongly capped that convection is unlikely to ever initiate. So on a day like that, you would be better off chasing farther N around the triple point that may be somewhere in KS, even if the CAPE is significantly less there. You are constantly having to weigh one thing against something else and you generally shoot for the best compromise.
Here is what the NAM model is forecasting in terms of CAPE for 4:00 PM CDT today (5/27):
[Broken External Image]:
http://www.twisterdata.com/data/mod...27/00/NAM_221_2015052700_F21_CAPE_SURFACE.png
Needless to say, it doesn't appear that CAPE will be lacking in and around Childress, TX!
I'll try to come back later and talk some about the SKEW-T. I have found this is one of toughest things to try to explain in terms that makes sense. But basically it is a snapshot on the atmosphere from the surface to the upper levels that allows you to look at a whole host of things collectively using one diagram. It takes a while to learn to read them and figure out what all of the squiggly lines represent, but once you start to grasp it, it really helps you to speed read the condition of the atmosphere and not have to look at a bunch of individual conditions and parameters in isolation.
I hope my incoherent babbling has been somewhat helpful. Feel free to ask any follow-up questions you may have.
