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EZTip No. 33 - G-AIRMETs versus Forecast Icing Product

If you look at the Graphical AIRMET (G-AIRMET) for icing in the EZWxBrief static weather imagery, it would seem that a flight through western Texas in the afternoon should not include a threat for moderate airframe icing. However, that would be a bad assumption.

These G-AIRMETs are advisories strictly for nonconvective moderate icing. Convective icing is covered by convective SIGMETs when the area reaches convective SIGMET criteria (after convection has already evolved). However, if you look at the Forecast Icing Product (FIP) in EZWxBrief, you'll notice below that there are some areas of heavy icing expected in western Texas during the afternoon at FL180. This is because FIP includes both convective and nonconvective icing in the forecast (and analysis when using the Current Icing Product). This is also why G-AIRMETs are not utilized in EZWxBrief to evaluate the route against your personal minimums for airframe icing since they only capture nonconvective airframe ice.

The reason for the difference between the G-AIRMET and FIP is because the icing expected in Texas is largely driven by convection as you can see in this 3-hour calibrated probabilistic thunderstorm forecast below from the Short Range Ensemble Forecast (SREF) also available in the EZWxBrief static imagery. Once that convection develops during the afternoon, it is likely a convective SIGMET will be issued for the potential of severe icing.

Much of the airframe icing that exists during the warm season is driven by convection. It is not unusual in the middle of the summer for there to be plenty of airframe icing even though no G-AIRMETs may be issued. As you can see, it's important to understand the limitations of all of the weather guidance you use when making a decision to go or stay.

If you want to learn more about weather and how to interpret the Skew-T log (p) diagram, you can order your copy of The Skew-T log (p) and Me: A Primer for Pilots that is available in both softcover and eBook format.

Most pilots are weatherwise, but some are otherwise™

Dr. Scott Dennstaedt

Weather Systems Engineer

Founder, EZWxBrief™

CFI & former NWS meteorologist

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