Grey Zone Uncertainty

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What does the grey uncertainty area on the forecast track map represent?

The Tropical Cyclone forecast track map contains the grey uncertainty area described as the range of likely tracks of the cyclone centre for the next 72 hours as shown below.

Historical data indicates that the entire 72 hour track of the cyclone centre will remain within the uncertainty area about 85 per cent of occasions.

The grey uncertainty area is determined by using the historical forecast skill at the 70th percentile over the past five years, and the uncertainty of the analysed position which varies from one time to the next. The forecaster will sometimes modify the area to reflect the confidence for that particular forecast. For example when there is greater uncertainty than normal reflected in a large spread of computer model tracks, the area will be made larger, or conversely when there is high confidence the area can be made smaller.

Typically the most likely track lies in the middle of the uncertainty area. The shape of the uncertainty area can vary depending on the motion of the cyclone. A fast moving cyclone will have typically have a long skinny uncertainty area while a slow moving system will smaller but 'fatter' area.

Sample forecast track map (TC Ita) showing the grey uncertainty area.

Sample forecast track map (TC Ita) showing the grey uncertainty area.

Forecast track map issued for a developing low (pre-Olwyn) showing a very asymmetric uncertainty area about the most likely track. In this case the forecaster considered a track further to the west was possible even though the most likely track was further east as shown.

Forecast track map issued for a developing low  (pre-Olwyn) showing a very asymmetric uncertainty area about the most likely  track.