Posted on May 8, 2014
by Roger Whittier
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When winter weather begins to recede, so does the risk of encountering icing during a cross-country flight in Cirrus.
But now it’s time to think about thunderstorms, which is the other bane of every pilot’s existence when it comes to significant weather threats. Many COPA Pilots have Stormscope, XM based weather and ADS-B based weather products in their planes, which provide timely information on thunderstorm activity while enroute. But that’s only half the picture, because it is not prudent to begin a flight without a comprehensive understanding of what the forecast calls for. The following memo reflects my personal views, based on various experiences and pre-flight planning techniques. Let me incorporate and invoke the usual caveats here, since none of this stuff is to be found in, or condoned by, the POH or FARs.
Let’s say that you’re thinking of flying from White Plains, NY (KHPN) to Nashville, Tennessee (KBNA) tomorrow morning.
So you go to your favorite prog chart and see something like this:
It shows a significant area of thunderstorm activity (the red threat) over Mississippi, Alabama and parts of Arkansas and Tennessee, as of the early morning (6:00 am Central time).
The next chart in the sequence confirms that this early-morning thunderstorm is no fluke, and will be blanketing the area all day:
It shows the storm smack over nearly all of Tennessee as of 6:00 pm, meaning that the storm is going to be lingering around the destination airport virtually all day. The only good news thus far is that its heading east, rather than northeast, so it won’t be an issue for the return flight to New York.
So next you go to the Plymouth Skew-T homepage. It will show the following:
Enter 4-digit ICAO Station ID (e.g. KCON) or Lat,Lon (43.68,-73.88), then select model, type of product, forecast time, latest or previous model run, whether you want lifted parcel temperatures plotted or not, and size of diagram.
Type in KHPN in the ID/LatLon box on the left, and select the forecast timeframe from the Time box. Leave the other choices alone.
The site will then produce a Skew-T forecast (shown below) at HPN for 15Z (10 am eastern time) on Tuesday, March 22. The red line is temperature and the dotted black line is dewpoint. The temperature scale starts on the bottom (in Celsius), with gray lines slanting up to the right. The altitude scale is to the left, in both millibars and meters (multiply by three for a rough approximation of feet, since a meter is 39 inches). I’d also do a Skew-T plot for 27 and 30 hours, to cover my return flight.
The significant graphic element in looking for thunderstorm activity is the yellow line, which represents the theoretical parcel lapse rate (e.g., the change in temperature/stability of a theoretical box of air as it is lifted up through the atmosphere). Go read a textbook on weather if you want a fuller explanation, but for laymen the thing to watch out for is when the yellow line intersects, or is to the right of, the red temperature line. That’s not the case in the chart just above, so this is a classic example of a sunny day, with a nary a cloud in the sky (since the dewpoint and temperature lines don’t intersect either).
But the Skew-T for Memphis, Tennessee (west of our destination on tomorrow’s theoretical flight) tells a very different story:
You’ll immediately notice that there is a significant gap between the red temperature line and the yellow parcel lapse rate line to its right, and it extends from 1,183 meters (roughly 3,000 feet) all the way up to more than 9,142 meters (28,000 feet). So, yes, thunderstorms are forecast for the area at 21Z (4 pm), and up to heights that cannot be climbed over in Cirrus. But to really understand how strong this convective activity might be, and whether it has a strong probability of occurring, consider the seemingly gibberish numbers to the right of the graph: Items like LI, SI, KI, CAPE and its neighbor CINH are stability indices which together answer those questions.
LI is the lifted index at 500 millibars (roughly 15,000 feet). In technical terms, its the difference between the actual and parcel temperature at that atltitude. Less than zero means the possibility of thunderstorms, while –4 or below means severe thunderstorms. In this case the lifted index is –7.3, which means serious convective activity will exist (if it actually happens).
SI is the same thing as LI, but at 850 millibars (roughly 5,000 feet). More than +3 indicates showers and thunderstorms; zero indicates showers; and –3 or below indicates severe convective activity. In this case the SI is –0.7, which again confirms serious convective activity at this lower altitude (if it actually happens).
Incidentally, if both the LI and SI are negative, then the atmosphere from 15,000 feet and down is highly unstable; conversely, if both are positive, then the atmosphere is quite stable. However, if the LI is negative but the SI is positive, then the boundary layer is unstable and some capping exists: so the thunderstorm activity will not be of the towering variety.
KI is the K-Index, which is a more sophisticated index than LI or SI because it assesses convective potential by measuring the temperature and dewpoint spreads at both 500 and 850 millibars. It is, in a way, a combination of the LI and SI indicies. KI of 15-25 means small potential for convective activity; 26-39 means moderate potential; while more than 40 means a high chance of convective activity. In this case the KI is 24, which is right on the borderline of real, potential convective activity.
CAPE is a very powerful index, because it shows the potential energy for convective activity stored in the atmosphere by measuring the area between the temperature curve (the red line) and the parcel laps rate curve (the yellow line). Essentially, it adds up the LI (lifted index) for all altitudes, not just at 500 millibars. A CAPE value of less than 1,000 means weak convective activity; 1,000 to 2,500 is moderate activity; and greater than 2,500 means strong convective activity. In this case the CAPE is 1,958, which means that tomorrow’s convective potential is worthy of a pilot’s respect. But at that level, it does not presage strong convective activity.
Finally, CINH measures convective inhibition: how likely it is that a thunderstorm will develop. It computes the amount of energy that the lifting mechanism must expend to overcome the cap on unchecked convective activity. Essentially, it tells whether there is enough CAPE to unleash extensive convective activity. CINH of more than 200 means the cap is quite strong, so its highly unlikely that thunderstorms can develop. Between 50 and 15 means that strong lines of clouds will develop; between 15-50 means that some strong thunderstorms are likely; and less than 15 means fair weather cumulus clouds. In this case, CINH is 6, which is not very threatening.
The bottom line analysis of the graph above is that thunderstorms are likely, but not towering. So from a decent altitude (10-13,000 feet), it may well be possible to stay above most activity and to veer clear of anything that is particularly tall and threatening.
By comparison, look at the Skew-T for Nashville at 18Z (1 pm eastern time), right around the expected arrival time:
None of the five key indicies (LI, SI, KI, CAPE and CINH) have interesting values: in particular, there just isn’t enough CAPE to light a firecracker. And the graph confirms this at a glance: the yellow line never really crosses to the right of the red line. The only interesting thing to note from this is that the cloud layer will extend virtually from the surface to 15,000 feet or higher, with the freezing level at 2,733 meters (roughly 8,500 feet). So icing is still a consideration, even with thunderstorms not very far away at Memphis.
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