More Out with the Old, In with the New
Earlier this week I was speaking to a pilot about a Private Pilot checkride that he had recently completed. He indicated that things went positively on the oral examination though the weather portion had been a struggle. The struggle was not because the applicant didn’t have a knowledge of current weather products, but because the examiner had some favorite weather products that he always liked to test on examinations. Products, by the way, that are no longer available from the National Weather Service.
When the applicant asked the examiner for more information, the examiner pulled out a worn and out of date copy of Advisory Circular (AC) 00-45, Aviation Weather Services. AC 00-45 is a great reference but even the latest edition as of this writing is not up to date with recent weather product changes. An older version of the book should only be used for a history lesson, not current flight instruction or examinations. By the way, I do not know this examiner nor is it someone that our flight school uses, but it sounds like it is time for him to update his Plan of Action and reference materials.
Back in February, 2015, I wrote a post about the demise of the old Radar Summary Chart and TWEB products. This month I’ll cover the Lifted Index & K-Index chart and its replacements.
Lifted Index / K-Index Chart
I first encountered the Lifted Index Analysis / K-Index Analysis chart (L/K) as one of the four charts contained on a Composite Moisture Stability Chart (CMSC). The CMSC disappeared a couple of years before the L/K but was a tool for determining the stability, moisture content, and freezing levels of the atmosphere below 18,000 feet. It included the L/K, a Precipitable Water Analysis, an Average Relative Humidity Analysis, and a Freezing Level Analysis. Using the four charts together you could get a rough idea about the possibility of thunderstorms, the likelihood of their severity, and how much moisture was available to feed the storm.
The L/K was a big piece of this analysis. These indexes measured moisture and stability (or instability); key ingredients in the formation of thunderstorms.
A Lifted Index (LI) is purely a measure of stability. It is the difference between the theoretical temperature of a parcel of air lifted from the surface to around 18,000 feet (500mb) and the actual temperature at that level. A positive LI means that the atmosphere is essentially stable below 18,000 feet and a negative LI means it is unstable.
The K-Index (KI) measures both moisture and stability at certain levels of the atmosphere and is a bit more complicated to calculate. The formula is KI = (850mb temp – 500mb temp) + (850mb dew point) – (700mb temp/dew point spread). 850mb is around 5000 feet, 700mb is around 10,000 feet, and of course 500mb is around 18,000 feet.
Values for KI range from high positive values to low negative values. High positive KI (greater than +20) implies moist and unstable air. Low or negative KI (less than +20) implies dry and stable air.
When used together, the LI & KI values told you the likelihood of thunderstorms and their severity if they happened. See the charts below for their use.
If you knew how to use this chart, it gave a quick overview of thunderstorm potential. If you wanted to know about the instability of the lower atmosphere, which is important to many pilots, the LI portion was great for a glance.
On the negative side, most pilots did not have any real idea how to use the chart. It wasn’t intuitive and the key wasn’t included on the chart. It also only came out twice per day and it was based on historical temperatures and dew points determined during recent soundings. It wasn’t useful for fast moving storms and the further you got from the time of the soundings the less reliable the data became for determining thunderstorm likelihood.
It appears that the National Weather Service issued their last L/K chart on March 16th, 2009. As of today, I could only find a couple of sources with a current L/K chart and one may be gone by the time this is published. The one that may be gone was DTC DUAT and they lost their recent bid with the FAA to remain a DUAT provider. Depending on the outcome of their dispute with the FAA and internal decisions, their L/K may be a thing of the past. Lockheed-Martin FSS was the other source with a true L/K chart. (CSC DUAT has a variant with some significant differences.)
So What Can I Use?
There are numerous products currently produced by the National Weather Service to replace the intended use of the L/K chart. Some of my personal favorites can be found on https://aviationweather.gov/convection. This includes several charts related to thunderstorms and is known as the ADDS Convection page. You need to click into the chart shown on the page to get the full details.
The Current Convective SIGMETs chart shows a graphical representation of Convective SIGMETs and outlooks for convective activity. This shows where things are currently bad and could become bad with regard to thunderstorms.
The Experimental Collaborative Decision Making (CDM) Convective Forecast Planning Guidance is known as the CCFP. These graphics are produced every 2 hours and are valid for 2, 4, 6, and 8 hours after issuance. While the product was designed for the FAA to help make decisions about traffic flow, it can be useful for pilots as well for short term evaluation of thunderstorm forecasts.
The Extended Convective Forecast Product (ECFP) Planning Tool is another graphical representation of the forecast probability of thunderstorms. It is related to the CCFP and uses similar graphics but shows the likelihood of thunderstorms over the next 72 hours instead of 8.
The National Convective Weather Forecast (NCWF) product shows current convective hazards and 1-hour extrapolation forecasts of thunderstorm hazard locations. It looks a bit like a radar screen but it is only showing convective hazards. It updates every 5 minutes. Clicking on a region of the map allows you to zoom in more closely.
The national radar is easily accessed from ADDS Convective page along with a map of current weather watches and warnings issued by the Storm Prediction Center (SPC) in Norman, Oklahoma.
The final product shown on this page is the Convective Outlook also produced by the SPC. This product is produced as Day 1 through Day 8 charts. The shorter range charts tend to be more useful and provide a general expectation about the chances of any thunderstorm but more importantly give an indication about the risk for a severe thunderstorm. Information on the Day 1 Convective Outlook page also shows expectations for tornados, damaging winds, and hail. If you learned about Convective Outlooks more than a year ago, you will need a refresher as categories for the severe thunderstorm outlooks were changed earlier this year.
As you might imagine, the Storm Prediction Center, https://www.spc.noaa.gov/, has quite a few excellent products showing forecasts for storms. Take some time and explore their site before your next flight with convective weather nearby.
For many years, change in aviation was a slow process and it really still is, but change does come and all of us, especially flight instructors and pilot examiners, need to keep up. The L/K chart has essentially become a relic of history but this twice a day product of historical data has been replaced with faster and easier to interpret products. From twice a day to up to 288 times per day. I think there may have been some improvements.
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