In 2017 there were two small tropical cyclones (TC) that impacted communities but were not expected to develop. Both reached category 2 intensity but would likely not been detected as TCs in the absence of surface observations, radar and microwave imagery. The unnamed TC (22U in March) passed over Port Hedland surprising residents during the middle of the day who received only hours warning while Cempaka (November) in Indonesian waters was the first TC impact to impact Java in known history causing damage in Yogyakarta.
The unnamed TC was confirmed A network of four offshore automatic weather stations measuring winds, waves and pressures supported by a strong radar signature confirmed the size (radius of gales of 50 km) and intensity (category 2) of the unnamed TC. The Yogyakarta doppler radar supported by a strong microwave signature confirmed Cempaka's intensity.
Both cases were underestimated by traditional Dvorak technique and were not designated as TCs by the Joint Typhoon Warning Centre (JTWC). Since 2001 there have been five tropical cyclones in the Australian region that were not named operationally but were upgraded to tropical cyclones upon reanalysis. These short-lived 'unnamed' TCs are typically close to land where they are more likely to be in range of radar and/or observational sites.
There are likely to have been other undocumented events in the historical record making the TC count an underestimate and the absence of a database of tropical lows makes it difficult to reanalyse potential events.
These short-lived events are a subset of the broader set of small TCs that are difficult cyclogenesis cases often developing more rapidly than the accepted satellite (Dvorak) rate of development and are under forecast by NWP especially lower resolution global models. Â Their forecast challenge forecasters make such cases a higher risk to communities and offshore industry.