Page 6 - May 2019

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 travel-
ing 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.”
Position Report
by Dianne White
Tornado Alley
Alow-pressure “bomb cyclone” was straddling the Mid- west. 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 flood- ing along the Missouri River valley.
leeside of the Rockies tends to serve as the breeding ground for low-pressure systems. As they move east, they get stron- ger. 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 f lows. Normal- ly, air moves with the isobars. As you probably guessed, ageostrophic flow is air that flows across
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 convec- tive 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 Mid- west? One big reason, according to Horne’s book, is because the
4 • TWIN & TURBINE / May 2019
Understanding how convective weather works can help you plan your flight with a higher
likelihood of success, as well as
safety.Youmaydecidetheairplane isobarstowardlowpressure.Asaresult,thelow
is better off left in the hangar, unless, of course, the tornado finds your hangar instead.
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.