Enroute Icing

### Produced by The COMET® Program

### Case Study: Overview

In this case study, you’ll examine a weather situation in the U.S. Midwest, concentrating on the service area covered by the Chicago CWSU on 26 to 27 February 2011. The Chicago CWSU area includes southern Wisconsin, southwestern Michigan, northern Indiana, Illinois, and eastern Iowa, as shown below. To see how the Chicago CWSU fits into a larger CONUS context, access the map on an Aviation Weather Center webpage.

In forecasting the risk for icing, we will use the following procedure.

- Diagnose the locations and future evolution of icing: Use observational data and NWP analyses to determine areas and levels of icing across your forecast area, and make a “first forecast” of icing, including severity.
- Adjust forecast of future icing locations, using NWP icing forecast tools to adjust the "first forecast" as needed.

Recall that icing occurs in the presence of supercooled liquid water - in clouds and/or precipitation in regions of sub-freezing temperatures often associated with upward motion near low pressure systems. These layers appear on atmospheric soundings as moist layers with temperatures in the 0°C to -22°C range. While the -22°C is often used as the lower temperature limit for supercooled cloud water, it can be observed at lower temperatures, especially in mountain wave and convective clouds.

NOTE: In this lesson, we will use "flight level" or "FL" for all elevations. In some jurisdictions, including the United States, the standard notation for levels below FL180 is 000'-179' or zero feet to 17900 feet.