On 20 March 2000, in flight icing conditions in the Denver International Airport (DIA) terminal area were adverse enough to cause airport operations to come to a standstill for several hours.

The weather and effect on operations will be described. Briefly, low and shallow clouds were present in upslope (north to northeasterly) flow. Surface temperatures in the area steadily dropped during the morning, for example, from 8C at 0600 UTC to -2C at 1600 UTC at DIA. Freezing drizzle and freezing rain were observed at weather reporting stations around northeastern Colorado beginning ~1600. Radar reflectivity was generally <10 dBZ. . Aircraft arrived at DIA heavily iced, causing delays due to the need to remove of this ice prior to turn-around and subsequent takeoff. Pilot reports of severe icing conditions in the area were received throughout the morning, and a SIGMET was issued by the National Weather Service's Aviation Weather Center. Several airlines decided to cease operations at DIA while waiting for these conditions to abate. At ~1900 UTC, a cold surge arrived from the northeast, with deeper and colder clouds and precipitation turning to snow. Apparently this created conditions conducive to a more efficient precipitation process that removed liquid water from the clouds and diminished the icing problem.

The event raises many questions regarding aircraft operations in icing conditions. Forecasts and diagnoses from several sources were basically correct in depicting the location and timing of icing conditions, but did not reflect the severe nature of the hazard. The integrated icing diagnosis algorithm, run at NCAR, indicated high icing potential though the morning, as did an AIRMET issued by the Aviation Weather Center for moderate or greater icing in northeastern Colorado. Ideas for better characterization of the severe icing conditions using remote sensors or model outputs have been proposed and these will be reviewed in the context of an improved operational response to the icing conditions. Training of dispatchers and pilots to better interpret icing information, such as the new graphical icing products both pre-flight and in future cockpit weather displays will also be addressed.