WX BRIEF

Study of indices gives solid forecast

Severe weather coming offers telltale clues.


Forecast discussions are among the most useful bits of expert weather information available, as they provide details of why a forecast was made a certain way. But these discussions, written for other meteorologists, often assume that the reader understands severe weather indices.

A region could have a high Cape but remain cloud-free all day because other factors intervene. One such factor is another index called the Cap—a stable region of the atmosphere that may be present at lower levels and is preventing the heated surface air from rising into the free atmosphere.

Caps are often due to temperature inversions or layers with a significant moisture deficit. On the sounding chart, an inversion is shown as a part of the air temperature trace that veers to the right for a short period, indicating slight warming.

Low-level temperature inversions often form overnight and, if they are strong enough, may persist through the middle of the day.

The Cap value is simply the maximum difference between the blob of air and the air temperature on the Skew-T.

If the maximum temperature difference (air warmer than blob) is small—say, less than 2°C—it will only require a little added energy to overcome the Cap, and therefore the rising parcel will probably not translate into a thunderstorm. On the other hand, if the Cap is over 4°C, it is likely too strong for the air parcel to break through, regardless of whether other conditions are right for thunderstorms to develop.

Dangerous Caps are those between 2 and 4°C—these moderate Caps help hold heated air near the surface until they have gained large quantities of energy and are able to burst through the Cap explosively, transforming into significant thunderstorms. Combined, Cape and Cap can give a pilot a pretty good idea of the potential for thunderstorms and their likely strength.

However, there are several other indices that can help corroborate what Cape and Cap are indicating, and also provide a better idea as to whether you’ll be facing isolated or wide­spread storms.

A variety of flavors

Although there are literally dozens of severe weather indices that forecasters use to evaluate convective pot­ential, some of the more popular and useful indices that you will likely see in weather service forecast discussions or hear thrown about by briefers are the lifted index (LI), K index (KI), Showalter index (SI), total totals (TT) and sweat index (SW).

The lifted index is a measure of instability near the surface relative to the middle troposphere. It is calculated as the difference between the air temperature and the parcel temperature at 500 mb. The more negative the value, the more unstable the lower atmosphere is, and the more rapidly the air parcel will accelerate upward.

Positive LIs indicate stable air, while small negative values indicate only slight instability. At values below –4 the atmosphere is sufficiently unstable to generate storms, and by –8 severe storms may result. Unfortunately, LI has only proven useful in warm weather and only accounts for the lower troposphere, whereas Cape accounts for the entire troposphere.

Like the LI, the K index accounts for temperature differences—but, unlike the LI, the KI compares the air temperature difference between 850 and 500 mb and also accounts for moisture by adding in the 850-mb and 700-mb dew point depression (difference between air and dew point temperatures).

This index therefore looks at how quickly the lower atmosphere is cooling, and whether the air is becoming more or less saturated. KI values below 25 indicate poor convective potential, while values above 40 mean a pretty good chance for convection. Some of the limitations of the KI are that it doesn’t work well for high-elevation or montane locations, and that it doesn’t indicate the potential strength of storms.

KI works best in warm, moist air masses such as those found in the Southeastern US. The Showalter Index looks at stability in yet another part of the lower atmosphere. Convection is likely when the lower atmosphere is unstable relative to the middle troposphere.

The SI puts a number to that instability by comparing the air temperature at 500 mb with the temperature an air parcel would have if it were raised to 500 mb from 850 mb (in­stead of the surface like the LI). SI values are similar to LI values in interpretation—a value below –4 indicates strong instability, while values below –8 are extreme.

A cluster of strong storms plays out over the Baltic Sea off the coast of Germany in July. Nearby overnight soundings indicated the possibility of non-severe convection in the region.

While other indices provide a good idea of the likelihood of storms and severe weather, total totals is a useful index for determining the frequency and potential strength of storms in the vicinity of a sounding.

TT takes into account the temperature decrease between the lower and middle troposphere and adds to that the difference between the 850-mb dew point and the 500-mb temperature.

Values below 44 mean a likely lack of convection, while 44–50 indicates likely thunderstorms, 51–52 isolated severe storms, 53–56 widely scattered severe storms, and above 56 scattered severe storms. TT tends to work best at low to moderate elevations.

Although the sweat index is an acronym for severe weather threat index, you might remember it as sweat values above 300 will make you sweat in the cockpit if you have to fly through the resulting severe weather.

Sweat incorporates several other variables, such as TT to determine the likelihood of severe and even tornadic storms. Unlike the preceding indices, sweat also uses 850-mb and 500-mb windspeed and how much the wind direction veers between those 2 levels.

The differences in speed and directional shear lend the sweat index a good measure of the ability of a severe storm to rotate and thus become tornadic. SI values above 150 indicate the possibility of severe storms. Increasing values above 150 correspond to increased likeliness of severe weather, with tornadoes possible if the SI is above 400.

Overall, each of these indices provides a bit of new information and a lot of overlapping information. As a result, it is best to look at all of them together. It is when they are all in agreement that you’ll have the most confidence in what they are indicating. Severe levels on all counts will most likely translate into experiencing at least some severe weather.

On the other hand, just one benign index value (such as a strong Cap or weak Cape) may be all the information you need to decide that a severe storm is not likely (or one favorable index is all you need to guess that a severe storm is going to happen).

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