Enroute Icing

### Produced by The COMET® Program

### Step 3: Use NWP Guidance to Make Adjustments, If Needed » Case Summary

The severe icing situation that occurred in the Chicago CWSU service area on 26 February 2011 could largely be inferred from the analysis and observations, and predicted from the NWP forecast and FIP guidance.

To show what transpired during this forecast period and beyond, we will use hourly CIPs from 15 through 23 UTC 26 February overlaid with actual icing PIREP locations and severity. These are shown at hourly intervals in the animation below.

As you can see, the Chicago CWSU service area was almost completely covered by at least light icing from 15 UTC through 23 UTC. Icing intensities increased after about 17 UTC and then remained steady through 23 UTC.

Conditions did not improve after 23 UTC. In fact, a severe icing report was issued at 0043 UTC for FL030 15 miles east of KORD. In addition, a ground stop was ordered at KORD because of severe icing on one of ORD’s runways.

The RUC2 analysis sounding from KORD at 01 UTC 27 February shows a deep saturated sub-freezing (-3° to -10°C) layer from the surface to 750 hPa, with dry air above, conditions that are are consistent with severe icing. The red oval in the analysis indicates where it’s fully saturated. In the range of temperatures where snow grows rapidly at the expense of supercooled liquid water (-12° to -19°C), the atmosphere is dry. No ice crystals were expected to reduce the amount of supercooled water. It isn’t surprising that there was serious icing at KORD at this time in the air and on the ground!