Now that convective SIGMETs have morphed into icing G-AIRMETs, it's important to understand that the icing forecast in EZWxBrief is a calibrated probability that ranges from 0% through 85% as shown in the legend below. Icing forecasts in EZWxBrief rendered in the meteogram and in the EZRoute Profile carry a lead time as far out as 18 hours in the future. So what is a calibrated probability and how should it be interpreted?
Essentially, a calibrated probability is one that as the forecast lead time increases, the probability of icing will naturally decrease due to uncertainty in the forecast. For example, in the meteogram below depicting the icing probability forecast at Lewis University (KLOT), you will notice that the probability of icing with the most recent time (closest to the current time on the left of the meteogram) is 74% at 5,000 feet MSL. However, it may appear to the unwary eye as if the icing chances are lessening over time at this particular airport given the probability drops to 34% at 5,000 feet MSL about 15 hours later. While the risk of icing may indeed be less 15 hours later, the most likely reason for the decrease in probability is uncertainty in the forecast. In other words, with a model-based solution used by EZWxBrief, how sure can you be for an icing event that’s nearly 15 hours in the making with any certainty?
If you are looking at your route early in the morning, it might be tempting to think, "I'll just wait until later in the afternoon to make the trip." The important takeaway is that you shouldn't assume that lower probabilities of an icing event that is nine or more hours in the future means less of a risk of encountering airframe ice at those times. Even at 12 hours in the future, it is rare to see icing probabilities exceeding 50% even when the the signature of supercooled liquid water are quite strong. Nevertheless, the higher the probability, the more certainty there is of the presence of supercooled liquid water.
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