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Icing - the importance of understanding the big picture

Is it possible that you can accrete airframe ice at a temperature of -30°C at the end of February in southern Idaho? Although it is not typical, it is still possible, especially if the icing is convective. Yes, convective. Let's say you were planning to make a flight at 17,000 feet on February 27th in the late afternoon through southern Idaho. You took a peek at the Forecast Icing Product (FIP) to choose the best altitude. It would be easy to conclude that using this 6-hr forecast for icing severity (below) that the risk of icing at 17,000 over a very large area is very low. In fact, if you consulted FIP at15,000 feet and 16,000 feet as well as all altitudes above 17,000 feet, they all looked pretty similar. Seems pretty much like there's no risk of icing and that 17,000 feet would make a good cruise altitude through this area. Well, that would be a bad assumption.

Turns out the forecast was not entirely correct. In fact, a pilot flew through this area at 17,000 feet and accreted ice at a temperature approaching -30°C. It's not likely that most pilots would pick up on this, but understanding the risk is all rooted in the big picture. Let's take a look at the big picture to understand why.

First, the official forecast for icing from the G-AIRMETs issued at 2045Z on February 27th was that widespread moderate icing would extend as high as 14,000 feet MSL in the southern Idaho area. That would likely be the case for a majority of the region, but the icing at altitudes above 14,000 feet could still be very spotty (not widespread).