2018/01/03 18:00, Tropical Storm BOLAVEN (201801) , Center Location 11.60N 112.20E, Movement: WEST 22KM/HR. Minimum Pressure 998 hpa, Maximum Wind Speed 18m/s, Gust 25m/s, Radius of 15m/s 100km.
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What does typhoon potential track area (hereafter PTA) mean?

PTA represents a domain that a typhoon/tropical storm center will likely moves in at a specific time, which implies track forecast uncertainties. These yellow shaded areas do not refer to the typhoon/tropical storm radius, rather, a circle of 70% probability that the storm center is entering within.

When will the Central Weather Bureau (CWB) issue PTA products?

The CWB will issue PTA products once a typhoon/tropical storm exists over the western North Pacific or the South China Sea. This product will be updated every 6 hours until the typhoon dissipates.

What are the characteristics of the PTA?

The 70% probability circle is obtained based on the CWB track forecast error statistics in the recent years. The radius of PTA circle increases because the track forecast error increases with forecast period. The CWB mean errors of 12-h, 24-h, 48-h, 72-h, 96-h, and 120-h storm track forecast are 61 km, 104 km, 185 km, 298 km, 430 km, and 523 km, respectively.

What are the major factors affecting the size of the PTA?

Size of the PTA circle is primarily related to the forecast period. However, it is also affected by the storm moving speed and direction. The PTA circle is usually larger if a typhoon/tropical storm is expected to move fast or have recurvature.

What are the typhoon/tropical storm 30-kt wind speed probabilities (hereafter WSP) used for?

WSP are calculated based on the CWB track forecast of the targeted typhoon and the error statistics of typhoon center position forecast of recent years. CWB WSP products provide information about probabilities that the storm radius of wind speed of at least 30 kt (scale number seven (7) of the Beaufort Scale) will pass through a specific location in Taiwan area during the 120-hour forecast period.

When will the Central Weather Bureau (CWB) issue WSP products?

Along with the typhoon potential track area product, the CWB will also issue the WSP when a typhoon/tropical storm exists over the western North Pacific or the South China Sea.

What does the probability indicate?

The probability is a measure of how likely it is that a certain event will occur. For example, WSP 80% indicates that 80 times out of 100 forecasts that the storm radius of 30 kt winds passes through a given location.

Why do uncertainties exist in typhoon track forecast?

The uncertainties and errors exist in the typhoon forecasts due to lacking enough observations over the oceans, and also because of the limited knowledge on the physical processes of typhoon movement and structure changes.

How to use this probability product?

Since there are uncertainties and errors in typhoon forecasts, WSP products alone cannot be regarded as an aid for decision making during typhoon period. Due to different risk management considerations, the threshold for different sectors to take a proper action should be determined by the decision maker themselves. Please also note that the WSP product does not represent, infer, or imply the following contents: the probability of typhoon landfall, the scale and probability of typhoon-induced disasters, and the timing of CWB typhoon warning issuance.

Does the higher WSP indicate the stronger winds and heavy rainfall may happen?

As a typhoon approaches Taiwan, the WSP usually will be getting higher, and the typhoon-induced winds and rainfall are also expected to be greater. However, the variation of winds and rainfall may be significantly different with locations due to the topography, which should not be purely determined by the WSP. For those concerned about the local winds and rainfall during typhoon warning periods, please refer to the CWB official specific forecasts.

What do the 6-hourly typhoon/tropical storm 34-kt wind speed probabilities (hereafter WSP-6h) mean?

The WSP-6h are calculated based on the CWB track forecast of the targeted typhoon and the error statistics of typhoon center position forecast of recent years. CWB WSP-6h products indicate probabilities that the storm radius of wind speed of at least 34 kt (scale number seven (7) of the Beaufort Scale) will pass through a specific location in Taiwan area at 6-hour time intervals during the 120-hour forecast period.

When will the Central Weather Bureau (CWB) issue the WSP-6h product?

Along with the typhoon potential track area product, the CWB will also issue the WSP-6h when a typhoon/tropical storm exists over the western North Pacific or the South China Sea.

What does the probability indicate?

The probability is a measure of how likely it is that a certain event will occur. For example, 80% for the WSP-6h indicates that 80 times out of 100 forecasts that the storm radius of 34 kt winds passes through a given location during an individual time period.

Why are there uncertainties in typhoon track forecast?

Uncertainties and errors exist in typhoon forecasts due to lacking enough observations over the oceans, and also limited knowledge on the physical processes of typhoon movement and structural changes.

How do we use this probability product?

Since there are uncertainties and errors in typhoon forecasts, WSP-6h products alone cannot be regarded as an aid for decision making during typhoon period. Due to different risk management considerations, the threshold for different sectors to take a proper action should be determined by the decision maker themselves. Please also note that the WSP-6h product does not represent, infer, or imply the following contents: the probability of typhoon landfall, the scale and probability of typhoon-induced disasters, and the timing of CWB typhoon warning issuance.

Does the higher WSP-6h indicate the stronger winds and heavy rainfall may happen?

As a typhoon approaches Taiwan, the WSP-6h usually will be getting higher, and the typhoon-induced winds and rainfall are also expected to be greater. However, the variation of winds and rainfall may be significantly different with locations due to the topography, which should not be purely determined by the WSP-6h. For those concerned about the local winds and rainfall during typhoon warning periods, please refer to the CWB official specific forecasts.

What is the difference between the WSP-6h and the WSP?

According to the probability theory, the individual 6-hour probabilities will not exceed the 120-hour probabilities, and the cumulative 6-hour probabilities for 120 hours will not be equal to the single 120-hour probabilities; just like that, for example, if the probability of precipitation (POP) for the daytime is 60% and the nighttime 70%, the POP for the whole day can not be 130%.