Low Pressure Area
at 20:00 HKT 25 September 2017
21.7 N, 106.0 E (about 850 km west of Hong Kong)|
|Maximum sustained wind near centre:
( Past Positions and Intensities )
- The symbol will be displayed when the tropical cyclone is expected to weaken into a low pressure area or transform into an extratropical low.
- Past track is in black, forecast track is in red.
- The tropical cyclone symbol in different colours are the forecast positions of the tropical cyclone at different times (as shown at the upper left corner of the figure).
- While the forecast positions and track indicate the most likely future path of the tropical cyclone, they may deviate from the actual path taken by the storm. The yellow shaded area on the map indicates the probable area within which the tropical cyclone will fall with a probability of 70% ('Potential Track Area'). Literally, it means that in 10 times of track forecast for similar situations, the tropical cyclone will be centred within the indicated area in about 7 times. The area grows as the forecast hour increases. It will be particularly larger beyond 72 hours and is indicated with lighter shadings on the map. The size in terms of radius of the 'Potential Track Area' corresponding to different forecast hours are determined from the error statistics of the forecasts issued in past years and are as shown in the following table:
|Analysed Position||30 km|
|24-hour forecast position||125 km|
|48-hour forecast position||225 km|
|72-hour forecast position||325 km|
|96-hour forecast position||400 km|
|120-hour forecast position||500 km|
- When an area of low pressure intensifies into a tropical cyclone, the Observatory will issue the information of tropical cyclone track, analysed positions and maximum sustained wind near centre within two hours. It will be updated twice daily at around 10:00 and 22:00 Hong Kong time about two hours after the time of observation. When the tropical cyclone is expected to enter the area bounded by 10N and 30N, 105E and 125E in about 24 hours, the Observatory will also update the tropical cyclone track at 04:00 and 16:00 Hong Kong time (corresponding to the observation time at 02:00 and 14:00 respectively).
- The past tropical cyclone track, analysed positions and maximum sustained wind near centre may be updated when additional data is received.
- Short-term erratic departures of the tropical cyclone from the general direction of movement may occur from time to time. These departures are partly due to uncertainties in locating the centre of the tropical cyclone, and partly due to actual short-term variations in the direction and speed of movement of the tropical cyclone itself.