WX BRIEF

Study of indices gives solid forecast

Severe weather coming offers telltale clues.

By Karsten Shein
Comm-Inst, Climate Scientist


Widespread cumuli over Mississippi on a July evening correspond to minor convective potential foretold by atmospheric soundings earlier in the day. The severe weather indices were, however, all below severe thresholds.

Pilots are no strangers to weather maps. Most of us fam­iliarize ourselves with at least 1 or 2 maps prior to any flight and some of us will pore over a half dozen or more before we even call the briefer. Why? The weather is a ubiquitous component of our jobs and the last thing we want is to be surprised by some unexpected ad­verse weather that turns a routine flight into a knucklebiter.

But in reality, most of these products only give us a general overview of potential weather conditions across large areas of airspace. They show us fronts and areas of IFR, temp­eratures and regions of potential convection.

It is difficult, however, from a general weather map or prog chart, to determine the actual likelihood of severe weather, and what is plotted in general terms across a map might lead you to choose a less than optimal routing with significant deviation.

Fortunately, pilots do have at their disposal some powerful tools that will help them better determine the likelihood that the atmosphere through which they are flying will turn against them with severe storms and all the adverse conditions that accompany them. These tools are severe weather indices.

Starting in the 1930s, as more and more upper air weather balloon soundings became available for research, meteorologists developed a broader understanding of atmospheric dynamics and were able to quantify the properties of the air column under varying stability. In particular, meteorologists were able to identify certain atmospheric properties that preceded outbreaks of thunderstorm activity and, in particular, severe convection.

Severe weather researchers soon realized that many of these numbers had thresholds above which severe weather was far more likely than when the values remained below them. These thresholds were refined into a suite of severe weather indices that now allow meteorologists—or anyone else who takes the time to understand them and their meanings—to determine quickly the likelihood of severe weather over a given area.

Soundings

For the most part, severe weather indices are based on the atmosphere’s ability to move energy vertically. This requires knowledge of the state of the atmosphere from the surface up to the top of the troposphere. This knowledge is obtained by instruments sent aloft by the several hundred weather balloons sent aloft once or twice a day around the world.

As a radio­sonde (the instrument bundle) ascends beneath the weather balloon, it sends back observ­ations of ambient air and dew point temperatures. These are subsequently plotted on a sounding chart. The sounding chart is a simple plot of the air and dew point temperatures plotted relative to atmospheric pressure.

Winds aloft, also relative to atmospheric pressure levels, are plotted along the edge of the chart. But it is the temperature and dew point that are the critical component of most severe weather indices. On the typical sounding chart, the first thing you’ll notice is that the horizontal and vertical axes are not simply linear.

Instead, in order to fit the rapidly decreasing temperature and pressure on a single sheet of paper, the vertical pressure axis is logarithmic, and the horizontal temperature axis is skewed—a line of constant temperature drawn from the axis will veer off at a 45° angle to the right side of the page.

The second thing you’ll notice is that there are a whole lot more lines than just temperature and pressure on the chart. Each source for sounding charts may color the lines differently, but for explanation, the following colors correspond to what can be found on the sounding charts available from Unisys Weather at weather.unisys.com.

The grid lines for temperature and pressure extending from each axis are blue. Next are the solid green lines. These are the dry adiabatic grid. For any given surface temperature, they represent a decrease of 3°C/1000 ft that would be experienced if a blob of air were to rise unsaturated from the surface.

The saturation adiabats are light blue dashed lines. These change at a rate of about 2°C/1000 ft, which is the rate of cooling were that blob of air to be saturated. The final set of grid lines on the Unisys sounding chart are the dashed yellow lines.

These are saturation mixing ratio lines (referenced to the yellow numbers along the horizontal axis). Saturation mixing ratio is the amount of water vapor, at a given air temperature and pressure, that the aforementioned blob of air is capable of holding—in other words, if that blob cools or rises further, excess moisture must be released.

Atop these various grid lines, the air temperature (right solid white line) and dew point temperature (left solid white line) are plotted. An additional solid yellow line is drawn on the chart. This is the temperature that the theoretical blob of rising air would have were it to be forced to rise, as would occur in thunderstorm development.

You will notice that, near the surface, the temperature decrease follows the dry adiabatic grid, but as soon as the blob of air reaches the lifted condensation level (LCL), it is saturated and subsequently follows the saturated adiabatic grid. The relationship between the 3 temperature traces is what defines many of the indicators of severe weather potential.

Cape

Nexrad radar mosaic of the US, showing several lines of strong afternoon storms moving across the middle of the country, western New York and spots in the Southeast. Although many hours old, many of the 12Z soundings in these regions already hint at the potential for strong convection.

One of the most straightforward of the severe weather indices is Cape, which stands for convective available potential energy. This is simply a measure of how much energy a rising blob of air would have if it were lifted via free convection.

In other words, it is a rough measure of the buoyancy of the air parcel, and therefore gives an indication whether it will rise gradually or explosively. Cape is found by calculating the area between the yellow parcel line to the right and the temperature line to the left. The area begins at the level where the yellow line becomes warmer than the white line, and ends where it once again becomes cooler.

The larger the area, the greater the Cape number, and the more likely that any convection may give way to severe weather. In general, when Cape is below 1000, thunderstorms are unlikely, and any that may form will likely be extremely weak. Moderate Cape values (1000–2500) increase the like­lihood that any thunderstorms which may form will be at least reasonably strong.

When Cape exceeds 2500, the potential for severe thunderstorms increases dramatically, and if Cape is over 3000 it is highly likely that thunderstorms which may form will be very strong. Despite the connection between Cape and thunderstorm strength, Cape does not actually predict the atmosphere’s ability to generate thunderstorms.

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