A number of unforecast weather episodes relating to flights into major Australian airports have led to unforeseen diversions, holding, and in some cases, landing below published safe limits. For example, on 18 June 2013, two flights encountered unforecast weather en route to Adelaide, South Australia, leading to a diversion to Mildura Airport, Victoria. Upon arrival, both encountered weather unsuitable for landing. Aerodrome weather forecasts allow pilots and operators to develop a contingency plan during flight planning and en route (such as carrying additional fuel for holding or diversion) when there are indications of conditions potentially unsuitable for landing at the intended destination. Weather unsuitable for landing mostly involves thunderstorms, a low cloud base and/or low visibility, and to a lesser extent, strong winds. This is the first report in a series covering Australian airports supporting regular passenger transport operations. The results will assist aircraft operators to focus on the highest risk seasons and times of day for weather reliability, facilitating better flight planning and support for pilots. They will also allow for more informed prioritisation of investment decisions about aircraft and aerodrome navigational equipment. This report focuses on Adelaide and Mildura Airports.