Thunderstorms are the nemesis of general aviation. We can’t penetrate them nor fly over them. Fortunately, we have several sophisticated tools at our disposal to help in the preflight planning to make that all important go/no go decision before we head into the murk. Their sophistication is remarkable, and we need to choose those that best satisfy our needs. We’ll review some of the lesser known, less understood ones that you may consider adding to your bag of tricks to help with flight planning when convective activity is a real probability.
One of the first charts to look at is the Radar Summary charts. You probably remember these studying for your private pilot written exam. They were issued hourly and displayed areas of precipitation as well as information about type, intensity, configuration, coverage, echo top, and cell movement of precipitation. They also showed the position and type of fronts, freezing level, and areas of turbulence.
With so many other tools available the Radar Summary chart has been simplified. It now is limited to intensity, coverage, echo top, and cell movement of precipitation. It’s now updated far more frequently giving a more accurate picture of current conditions. It’s also in full color using standard NEXRAD color coding.
Severe thunderstorm and tornado watch and warning areas are depicted when they are in effect. These charts will also depict hail, Mesocyclones (MESO) and Tornado Vortex Signature (TVS). A MESO is storm-scale region of rotation, typically around two to six miles in diameter and often found in the right rear flank of a supercell or on the eastern, or front, flank of a high precipitation storm. A TVS signifies an intense concentrated rotation. This is indicative of the potential for a tornado and is usually found within a MESO.
Some cautions when using these charts. They don’t display cloud coverage so don’t assume the absence of echoes means clear weather. Additionally, these charts reflect precipitation so actual cloud tops will be higher than the tops of the precipitation echoes shown. If a thunderstorm is depicted, turbulence must be assumed and isn’t plotted. This chart should be used during preflight to identify general areas and movement of precipitation and/or thunderstorms. Even though they are updated frequently, the data is aged so they are like cockpit NEXRAD and good for strategic planning—which is what you should be doing being on the ground. Tactical planning on the ground isn’t a wise approach.
Convective Outlook charts help with both short- and long-range planning. These are provided by the National Oceanic Atmospheric Administration’s Storm Prediction Center. Their color-coding can be a little confusing in that they show general thunderstorm activity as light green as opposed to the magenta used with NexRad presentations. Dark green, yellow, orange, red and magenta show increasing levels of severe thunderstorm risk. Remember that severe thunderstorms are those that may spawn a tornado, have gusts of at least 58 mph, and/or have hail at least one inch in diameter. These Outlooks are good to show areas you probably want to avoid altogether or at the least do more detailed research if you need to transit an area depicted.
The Convective Outlook covers a period from six hours to eight days into the future. Between four to eight days ahead, a threshold of 15-percent and 30-percent probability is used to depict severe weather. When there is less than a 15-percent chance of severe weather you will see Predictability Too Low or Potential Too Low. There is an important distinction between these two classifications. Predictability Too Low means that some models have indicated there could be a potentially severe storm in the region but the occurrence or location of the storm is too much in doubt to make a proper prediction. In other words, proceed with caution. Potential Too Low is used when there is no likelihood of any 15-percent chance or higher severe weather events in the region during that period.
The Lifted Index
The Lifted Index is used to assess a low-level parcel of air with regard to its stability and is a simple index to derive. It considers a parcel of air near the surface that is then lifted to 500 mb (18,000 feet). As the air is “lifted” it is considered to cool at the dry adiabatic lapse rate of three-degrees Celsius per 1000 feet, due to expansion. The temperature the parcel is calculated to have at 500 mb is then subtracted from the actual measured 500 mb temperature. This difference is then the lifted index, which can be positive, negative, or zero and indicates the stability of the parcel of air.
Lifted Index Charts can be found is a variety of formats with some color coded similar to a NEXRAD presentation with the indexes displayed on the chart— and some are even animated. Lifted Index values are naturally highest (most stable) in the morning due to nighttime cooling and are lowest (most unstable) in the afternoon due to daytime heating. These values can change rapidly due to moving fronts, dry lines and outflow boundaries. Dry lines are imaginary lines that separate moist from dry air and extend across a continent. In the U.S. this occurs primarily in the central states region separating western dry air from moist air emanating from the Gulf of Mexico. Outflow boundaries are more commonly known as a gust front that emanates from a thunderstorm. These act much like a cold front.
Negative Lifted Index values only signify the potential for a thunderstorm. Lifted Index should only be used to determine stability for warm season convection. Also, the Lifted Index only assesses the instability in one level of the troposphere, what is termed the Planetary Boundary Layer (PBL), which is generally around 1000 meters and varies between day (higher)and night (lower). The PBL is the lowest layer of the troposphere where the wind is influenced by friction.
Convective Available Potential Energy (CAPE) is another index that is considered better at assessing the instability of the troposphere as a whole. It’s a measure of the amount of energy available for convection—sort of like a speedometer that tells the maximum potential vertical speed within an updraft based on its strength measured in joules/kg. A high CAPE value is indicative of a storm that will build vertically very rapidly. Larger CAPE values also indicate the potential for hail.
Inversions can cause issues in evaluating the CAPE values. An inversion will result in lower CAPE values and will typically prevent the air parcels from pushing higher up. But should that air under that warm layer heat up enough to penetrate the inversion, it can cause an explosive development of severe storms. Otherwise, the inversion can limit the strength of the storms, regardless of how much potential energy is waiting to be released. CAPE values are plotted geographically for ease of regional forecasting and can change dramatically over time and region.
The K Index
The K Index is normally used by meteorologists, but is really what pilots need to determine the potential for thunderstorms and is also an indicator for heavy rain. The K Index is computed using a variety of environmental observations and is not based on a parcel of air being lifted and compared to environmental factors. K Index is based on the vertical temperature lapse rate, moisture content, and vertical extent of moisture layer.
It’s derived from the temperature difference between 850 and 500 mb and incorporates moisture parameters by taking the difference between the 850 mb dewpoint and 700 mb dewpoint depression and adding this to the temperature differential. Dewpoint depression is the difference between the temperature and dewpoint. The K Index is useful to provide an idea of the extent of a potential storm. It’s not to be used to determine the severity of a storm. Like CAPE, an inversion can lead to a misleading index. The K Index is best suited for low to moderate elevations and doesn’t provide useful values in high elevations. The values are also location and season dependent.
The Skew-T Log-P diagrams have been a favorite of glider pilots for some time. These diagrams are good at assessing the stability of the atmosphere but they take a bit of learning to use them effectively. Glider pilots use these to determine when lift will potentially occur during the course of the day and to what altitude. They represent the pressure, density, temperature, and water vapor present at different altitudes (pressure in mb). Meteorologists use these to derive many elements of the atmosphere and are instrumental in generating the indexes discussed previously. They are produced twice a day at 0000 and 1200 UTC through the use of weather balloons so the data doesn’t give a true representation of the vertical dimension due to the drifting of the balloon and the time it takes to reach altitude.
Being skilled in the interpretation of Skew-T diagrams will enhance your knowledge of the current weather environment and what to expect over the course of the day. There is a wealth of information contained in these somewhat foreboding diagrams once you learn how to use them.
The Weather Depiction chart is probably another distant memory from when you were learning to fly but they are valuable tools when convection is around. They are issued every three hours starting at 0100 UTC. They present the most significant weather illustrated along with visibility, clouds, ceiling and present weather. They typically display major fronts or areas of high and low pressure as well. Anything in red is IFR (ceiling <1000 feet and/or visibility <three miles) and blue indicates regions of MVFR (ceiling 10003000 feet and/or visibility three to five miles). These are derived from METAR and other surface observations so are very accurate representations of the weather at the time of publication. Like the radar Summary Chart they provide the current weather picture.
Tools are only as good as the person wielding them and it takes time to fully understand and appreciate their use. Pilots are typically not meteorologists, but we certainly can take advantage of some of their tools. And no one tool is ever sufficient. You need a tool bag full to safely achieve your objective.
Richard Lanning Ph.D. is a graduate of the U.S. Naval Academy and a pilot for more than 30 years. He is an ATP, CFII and a recent convert to the gyroplane.
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