There will be times in the EZWxBrief progressive web app where you may see icing probability (and severity) depicted below the freezing level in the EZRoute Profile or Meteogram. That usually means there's a temperature inversion present near the surface. Notice below on this route from the Cumberland Municipal Airport (KUBE) in Cumberland, Wisconsin to Charles B. Wheeler Downtown Airport (KMKC) in Kansas City, Missouri that the freezing level (0°C isotherm) varies from the surface near KUBE (on the left) to nearly 10,000 feet MSL at the destination (on the right). Most of the middle part of the route, however, the freezing level is shown to be around 5,000 feet MSL. But you likely also notice that there's icing probability depicted immediately below the freezing level through most of the latter part of the route. What's going on?
First, this is a clear indication that multiple freezing levels exist. The EZWxBrief app only shows the highest freezing level in most cases. For example, looking approximately 100 nm along this route, the 2-hour forecast sounding from the Austin Municipal Airport (KAUM) in Austin, Minnesota clearly shows two freezing levels with the highest freezing level near 5,000 feet MSL that is shown in the EZRoute Profile view above.
Essentially, given this temperature profile in the forecast sounding, there is likely icing potential below the lowest freezing level at 4,000 feet MSL. At this point in time the EZWxBrief progressive web app only shows a single contour for the 0°C isotherm. This is primarily done to avoid the depicted contours of temperature to potentially become very complex when these kinds of conditions exist. This is especially problematic when the environmental temperature is isothermal near 0°C. In those cases, there may be several freezing levels in a span of 2,000 feet. A future version of the application will present areas where multiple freezing levels are forecast to exist to avoid any confusion.
Interested in learning more about the Skew-T? You can order your copy of The Skew-T log (p) and Me book today in soft cover or eBook format.
Most pilots are weatherwise, but some are otherwise™
Dr. Scott Dennstaedt
Weather Systems Engineer
CFI & former NWS meteorologist