Weather Wise: Severe WX Basics

After 15 years contributing to IFR magazine, I’ve traded in my old desk for a new one here at AvBrief. For those who followed my work there, welcome back and it’s good to see you again! Unlike digital stockpiles of content reruns found elsewhere, my work here at Instrument Aviator brings fresh new content.

For those discovering me for the first time here at AvBrief, my resume is worth a summary for background on my work here. For many years I was a U.S. Air Force aviation meteorologist, working various roles as an observer, forecaster, analyst, and flight weather briefer. The very beginning of my career brought me headfirst into the events of Desert Storm, where I produced the weather chart packages that were used by senior forecasters to send F-117A Stealth Fighter sorties into Baghdad.  I was stationed at bases across the southern and western states, as well as in Korea, England (RAF Fairford, home of RIAT, the Royal International Air Tattoo), and even Africa. Throughout these assignments I saw a wide-ranging assortment of flight conditions and patterns and worked with F-117, B-1B, C-130, C-5, and T-38 crews, as well as Army and Navy support.

After the Air Force I continued on in the private sector, developing meteorology programs and textbooks. I was even a Texas and Oklahoma storm chaser at one point, but “retired” from that once things got crowded (and crazy). My wife, Shannon Key, worked for the National Severe Storms Laboratory and National Weather Service in Norman, Oklahoma. So you can say we’re a meteorology household. Currently I have a YouTube channel where you can catch a forecast briefing a couple of times a week. 

My intent is that by giving you a bit of experience at the aviation forecaster’s desk, you can take some of that experience and have a sixth sense about what’s going on when severe weather is bearing down on your flying mission—and you won’t need a meteorology degree. On to the lesson.

Severe Season Is Here

Currently we’re getting into the depths of severe weather season. At press time in early March 2026, Michigan experienced a surprise tornado outbreak, followed by an outbreak of 31 tornadoes across the St. Louis and Chicago area. On its heels we saw a rare Moderate Risk of severe weather covering Washington, D.C., and parts of Virginia and North Carolina. And, with Phoenix expecting an unprecedented 107° in March, it’s pretty clear winter has hastily packed its bags and spring has been thrust upon us. There’s a lot to learn.

Moreover, with it still being only March, it’s pretty clear that we’re looking at an increased tempo of severe weather that may last a couple of months before settling down a bit. With this kind of dangerous weather on the charts, it’s worth talking about how all these ingredients come together. 

The key driver of thunderstorms and severe weather is instability. This is as simple as cold air in the upper troposphere (in a typical situation, centered on around 30,000 feet) and warm air in the lower troposphere (generally the surface up to 5,000 feet). When there is heating in the bottom of this layer, the result is the release of kinetic energy in the form of convective currents that ascend upward.

Sounds simple, right?  A rough unit of measure for this kind of instability is the difference in temperature per unit of vertical distance, often given in C° deg/km. This gives you the lapse rate, something glider pilots are immediately familiar with.

But when you stop and think about it, we can get some impressive lapse rates across the Desert Southwest during the summer, even with clear blue skies. So why don’t thunderstorms form? The reason is we’re lacking moisture. With strong lapse rates and no moisture, we get dry convection, which manifests itself as gusty winds on the METAR reports, thermals, low-level turbulence, and the occasional dust devil. Given a strong enough pressure gradient we can generate a dust storm. But that’s about the extent of the problems.

Adding Moisture

By adding rich amounts of moisture to the base of the convective layer, we can release latent heat into the thermals and updrafts. This is heat that is stored within the gaseous water molecules. When saturation occurs, cloud material develops as the molecules change to liquid. This added heat increases the buoyancy of the rising thermals and updrafts.

What kind of moisture is important for instability? A common mistake is using relative humidity, as this is the most common type of moisture value given on websites and books for the general public. This is just a ratio of how close the air is to saturation, and it has a strong diurnal variation. This is not useful for general severe weather forecasting, though high relative humidity does help release latent heat at a lower altitude.

What we need is a physical measure of the total amount of moisture present. Absolute humidity and specific humidity are great indicators, as they measure the mass of moisture that’s present. But a more relatable indicator is available to you right there on the METAR report: dew point. This is measured in degrees Celsius or Fahrenheit.

Dew point is the temperature we must cool the air to in order to achieve saturation. In fact to measure it on ASOS, we take this principle literally: Dew point sensors use a chilled mirror technique, where a mirror is cooled until condensation occurs. The greater the mass of moisture in a given mass of air, the higher the dew point. If you’re unfamiliar with this, values of about 20° to 40° F are common in arid regions during the summertime, while oceanic tropical regions see dew points of about 70° to 80° F. Values over 55° to 60° F are associated with severe weather during all seasons.

And really, that’s it. Almost all the important instability indicators are derived from this. You may have heard about CAPE, convective availability of potential energy. This is measured in joules per kilogram for a hypothetical air parcel. This is a favorite measure for forecasters as it skips past lapse rate, dew point, and so forth, and simply solves buoyancy as closely as possible. In fact CAPE bears a direct relation to updraft velocity, so when CAPE is high (e.g., over 1,000) you really need to give updraft towers a wide berth. You can find CAPE on all kinds of websites with charts, such as Pivotal Weather and the Storm Prediction Center (SPC) Experimental Mesoanalysis.

Shear

Shear is another important process because this helps convert buoyancy into rotation at many different scales— from the entire thunderstorm down to the scale of a tornado. Processes involving shear are also vital for storm longevity and storm organization. If you don’t have shear, you have a typical environment in Arizona or Georgia in July, and very rarely do those thunderstorms produce anything besides gusty winds.

There are two key types of shear we deal with in severe weather forecasting: bulk shear and directional shear. Bulk shear is easy to understand as it’s just the difference between low-level or surface winds and upper-level winds. Typically the 0-6 km AGL layer is used, in other words, between the surface and 20,000 feet. This is directly proportional to storm organization and the generalized severe weather risk, especially when paired with CAPE.

Directional shear is a bit more complicated as this examines the layer of air ingested by the storm. So we look at a sub-cloud layer, from roughly about 0 to 3,000 ft AGL. When there is substantial turning of the wind vector through this layer, such as the winds shifting from strong easterly at the surface to strong southerly at 3,000 feet, this is a strong indicator of directional shear. This is readily converted into tornadic rotation when it is ingested by the storm.

Forecasters solve this by looking at hodograph charts, which takes some skill and experience. But you can get a sense for this by looking at charts giving storm relative helicity (SRH) or simply by being more alert when you see backing (i.e., a counterclockwise change with time) of moderate to strong surface winds, e.g., a METAR wind direction shifting from 17016KT to 12014KT.  If this area is already forecast to have severe weather, this points to an increased tornado threat. This rule is important ahead of existing storms, but is not useful within or behind storms, since we can’t be certain that air will be ingested into an updraft.

Putting It All Together

And really, that is it! Those are the key principles that are in use at forecast offices in the 2020s. Even the acclaimed Supercell Composite Parameter developed at SPC simply combines buoyancy (CAPE), bulk shear, and directional shear. So without being a weather expert, you too can keep all of these principles in mind whenever you pull up a weather chart, and just by looking at the winds, temperatures, and dew points you’ll have better insight into how each one is affecting the forecast.

In meteorology there are some other measures we use, such as DCAPE (downdraft CAPE). This looks for layers of dry air (dry dew points) at roughly 15,000 feet and estimates the negative buoyancy we might see in downbursts and storm outflow. High DCAPE days are quite significant in aviation, giving us for example Delta Flight 191—an
L-1011 that went down at DFW Airport in 1985. These kinds of days are difficult to assess with typical briefing charts. However, SKEW-T diagrams (which we’ll cover in another article) will show layers of this dry mid-level air. On certain websites you might find plots of DCAPE.

In short, it can get quite complicated, so when in doubt, leave things to the professionals. Days with high downburst potential will be covered in the SPC convective outlook, specifically in the wind product. Whenever this is painted in yellows, reds, or worse, then you need to be extra cautious whenever operating around thunderstorms and definitely take that alternate when your destination is overrun by storms. A lot of thought goes into these products, so you’ll definitely want to refer to them to look for problem areas along your route.