As we begin a new year, we have the opportunity to think about ways we can put our flying machines to good use, be it for business, pleasure or charity flying. It is also the time of year when winter is upon us in full force. That recent blast of Artic air means fantastic climb rates, great runway performance, cool engine temps, and unlimited visibilities. It also means we must pay attention to not-so-great consequences: turbulence and icing. Fortunately, we have a number of great avoidance tools at our fingertips. Referencing the FAA’s Advisory Circular 00-45H, here is a quick review of the strengths and weaknesses of these tools that are found in popular flight planning apps, such as ForeFlight, as well as at aviationweather.gov.
The National Weather Service’s Graphical Turbulence Guidance product solves the mystery of determining the location and intensity of turbulence before you find yourself becoming a PIREP. It includes an analysis and forecast for clear air turbulence as well as turbulence from mountain wave activity. The product is based on an ensemble of turbulence indicators and, therefore, can capture more diverse sources of turbulence to provide a more reliable forecast. The latest version of the GTG product is issued every hour out to a maximum of 18-hour lead-time beginning at 1,000 feet MSL with a vertical resolution of 2,000 feet that extends to FL450.
It’s important to note a couple of limitations of the GTG. First, these are automated forecasts and do not have any human input like you might find with AIRMET Tango, SIGMETs for severe or extreme turbulence and Center Weather Advisories (CWAs). They are dependent on the accuracy of the computer model output used to create them. Second, the GTG does not predict turbulence associated with convective clouds, or small-scale terrain features.
There are two icing tools that should be essential part of every pilot’s flight planning toolbox: the Current Icing Product (CIP) and the Forecast Icing Product (FIP). The CIP combines data from sensors and NWP models to provide an hourly three-dimensional diagnosis of the icing environment. This information is displayed on a suite of graphics and is sometimes referred to as an analysis. But it’s actually a zero-hour forecast.
The FIP gives you the same suite of products as the CIP, but describes the icing environment in the future and is based solely on models. Just like the GTG, the CIP and FIP are produced automatically with no human modifications.
With the CIP and FIP, you get five graphics: Icing probability; icing severity; icing severity-probability less than 25 percent; icing severity-probability greater than 50 percent; and icing severity plus supercooled large drops (SLD). You can cycle through different altitudes and timeframes to analyze and view trends of icing conditions. The threat of SLD is shown as red squares on the map. SLD is defined as supercooled water droplets and include freezing drizzle and/or freezing rain aloft. SLD are outside the icing certification envelopes and are considered particularly hazardous.
If you are a ForeFlight user, keep in mind that unless you are flying with the SiriusXM SXAR1 aviation receiver and subscribe to SiriusXM Pilot for ForeFlight, you will not see the SLD threats on the icing maps.
While all IFR pilots are familiar users of Terminal Area Forecasts, the Model Output Statistics (MOS) provides weather trends for airports without a TAF. The MOS is derived from the output of weather prediction models such as the North American Mesoscale (NAM) and Global Forecast System (GFS).
Another distinct benefit is that the MOS is updated hourly. You’ll get a refreshed forecast at an airport that will tend to track better with the actual weather. Moreover, MOS is highly tailored for each airport. So, if the airport is in a valley or near an ocean, MOS is aware of the nearby terrain features and local effects of the weather for that airport.
MOS is most accurate in the first six hours and, since it is location specific, will usually provide you with more detailed guidance. One thing to note, however, is that MOS should never be used for legal requirements such as determining an alternate on an IFR flight plan.
Finally, one important way to discern the accuracy of a forecast is to check the forecaster’s confidence. The forecast discussion gives you insight into what the meteorologists at the NWS are thinking and what they believe the models are telling or not telling them.
Tomorrow, I am scheduled to fly an Angel Flight mission and the forecast has been a mess. After studying all the resources, I decided to change the patient hand-off point to a different airport. Sometimes these tools don’t give us what we want, but they can give us what we need to stay safe.
Happy New Year and fly safe throughout 2018.
