Understanding how convective weather works can help you plan your flight with a higher likelihood of success, as well as safety. You may decide the airplane is better off left in the hangar, unless, of course, the tornado finds your hangar instead.
A low-pressure “bomb cyclone” was straddling the Midwest. A dry line extended through central Kansas into Oklahoma. Ahead of the dry line, southerly winds were howling at 22 gusting to 45 kts. By early afternoon, a line of thunderstorms had formed along the dry line and were racing eastward at around 40 mph. By late afternoon, near-gale force winds were howling and torrential rain resulted in severe flooding along the Missouri River valley.
Ahhh…Kansas in springtime. Tornado Alley – which includes a wide swath of Texas, Oklahoma and Kansas – is the place where so much nasty convection is found in the spring and summer. Having lived in the Midwest my whole life, I’ve witnessed my share of crazy weather. When traveling and the question comes up regarding where I live (the answer is Kansas), the inevitable next question is: “How many tornados have you seen?” The answer is, “Thankfully none, although one did destroy my airplane in 2017.”
As a pilot, timing a flight near an area of active weather takes careful analysis of situation using multiple sources of information and using all the tools in your cockpit toolbox. Understanding how convective weather works can help you plan your flight with a higher likelihood of success, as well as safety. Furthermore, it may help you decide whether the airplane is better off left in the hangar.
One of my favorite weather books is written by Tom Horne, an aviation author, called “Flying America’s Weather.” Tom wrote the book because he believed if pilots understood the larger climatic forces that affect a particular region, they would be able to interpret and even anticipate the weather along their intended route of flight.
So why we do get so many strong thunderstorms in the Midwest? One big reason, according to Horne’s book, is because the leeside of the Rockies tends to serve as the breeding ground for low-pressure systems. As they move east, they get stronger. The second reason is the low-level jet streams often shoot northward out of Texas carrying warm, moist air from the Gulf of Mexico. This low-level jet is most evident in advance of cold fronts common during the transition from winter to summer.
Third, the high-altitude jet stream that cycles around large upper level troughs can impart lifting and destabilizing forces to the air beneath them. In other words, the high-level jet delivers cold air over the warm, moist, Gulf-fed air masses in the lower levels, creating an environment ripe for convection.
Lastly, within the jet’s core of strongest winds is something called ageostrophic flows. Normally, air moves with the isobars. As you probably guessed, ageostrophic flow is air that flows across isobars toward low pressure. As a result, the low deepens. At the surface, they are contributing factor to squall lines and fast-moving cold fronts.
If you take a look at the 500 mb (high altitude) and surface maps during the period leading up to a severe weather outbreak, you’ll notice that the surface low is most likely located in the southeast corner of the trough aloft. If this scenario develops, you can look for the surface low and front to intensify below the leading edges of the trough aloft.
One term you may see in aviation weather products is CAPE, or convective available potential energy. CAPE is a measure of the positive buoyancy of a rising parcel of air, calculated from the temperature and moisture structure of the atmosphere. Basically, a type of stability index. Measured in joules/kilogram, CAPE is typically 2,000 to 5,000 on severe storm days, however anything over 1,000 is significant. The bigger the CAPE number, the greater the instability present, and the greater likelihood there will be strong thunderstorms.
Another term you might see is storm-relative helicity, which gives a measure of the rotational potential of a thunderstorm updraft. This gives forecasters an indication of an environment that is favorable for supporting the development of thunderstorms with rotating updrafts, a precursor to super-cell thunderstorms and tornado development. For mesocyclone development, storm-relative winds typically have speeds greater than 20 knots and turn clockwise with height by at least 90 degrees in the lowest three kilometers of the atmosphere. Values of helicity greater than +150 are considered significant, although there is no “magic” value that indicates whether a rotational thunderstorm will develop.
Since we’re talking about Tornado Alley, here are some fascinating facts about this destructive weather event. (Courtesy of Horne’s “Flying America’s Weather”) Tornados tend to form between 4 and 8 p.m. and most form when surface temperatures are between 65 and 84 degrees Fahrenheit. The 700-plus observed tornadoes each year in the United States last on average of about a half-hour and their average ground track is six miles.
Tornadoes also tend to form in groups, and some of them can be very large. Once they’re on the ground, they tend to move along at 25 to 45 mph southwest to northeast. They have an average width of 400 yards, although its width can be much less at the surface.
And finally, more than half of all observed tornadoes occur in – you guessed it – Tornado Alley. Although, as Dorothy says, there’s no place like home, I am good with sustaining my “no tornado witnessed” record.