Section 8.2.6 Tropical Storm Output

Predicted tropical storm frequency

These parameters are calculated over the same weekly periods as the other monthly forecast products and are averaged over an ocean basin.  The tropical cyclone identifying algorithms are the same as are used for the medium range tropical cyclone activity products.  

Fig8.2.3.1: To view Tropical Storm Frequency.

1. On Charts page, enter Tropical Storm Frequency
2. Click on Tropical Storm Frequency diagram - Extended Range
   
3. As desired select other base times, or Accumulated Cyclone Energy or Tropical Storm Frequency from drop down menu.

Predicted accumulated cyclone energy (ACE)

Accumulated cyclone energy is the sum of the squares of the estimated maximum sustained velocity of every active tropical storm at six-hour intervals.

The "Climate Means" (orange bars) are derived from observations only (for both frequency and ACE).  The "Forecast Means" (green bars) are based on the output of the tracking algorithms.  To derive the height of  the forecast bars a multiplying (normalising) factor is applied first to account for differences between the characteristics of tropical cyclones identified in the re-forecasts and the characteristics of tropical cyclones actually observed during a contiguous period.  This normalisation makes the orange and green bars directly comparable.  Furthermore, for ACE only, a further normalisation is done to set the climatological value to always equal 1; then values above or below 1 for the forecast bar denote above or below average respectively.

Fig8.2.3.2: The Accumulated Cyclone Energy (ACE) product takes into account number, strength, and duration of all the tropical storms forecast by the extended ensemble during the weekly period within each arbitrarily defined ocean basin, and totals their energy.  This value is compared with a similarly derived ACE from the extended range for the same week to determine the significance level for the given basin.  If this is greater than 90% the basin is shaded.  Green bars represent the forecast and orange bars the real climatology - these can be directly compared.

Tropical storm strike probabilities

This product shows a grid point chart of tropical storm strike probability (probability of a tropical storm passing within 300km), calculated over weekly periods.  It is similar to the tropical cyclone activity charts produced for the medium-range forecasts. 

Three product options are available:

• (i) raw strike probabilities,

• (ii) frequencies in the re-forecasts
• (iii) anomaly based on the difference between (i) and (ii).

Users should note that neither (i) nor (ii) incorporate any bias correction to adjust for differences relative to tropical cyclone observations. Therefore both products may suffer from systematic under- or over-estimations.

Fig8.2.3.3: To view Tropical Storm Probabilities.

1. On Charts page, enter Tropical Storm Probabilities.
2. Click on Tropical Storm Probabilities diagram.
3. As desired, select other base times, or other areas, or tropical storm strike information and climate from drop down menu.

Fig8.2.3.4: Weekly mean tropical storm strike probability of the extended range forecast.  Colours give probability in percentages.

Fig8.2.3.5: Weekly mean tropical storm strike probability deduced from the extended range re-forecasts, for the same week as Fig8.2.3.4.  Colours give probability in percentages.

 Fig8.2.6: Weekly mean anomaly of tropical storm strike probability for the same week as Fig8.2.3.4.  Colours give indication of more likely (positive probabilities, red to brown/black), or less likely (negative probabilities, cyan to blue/black) than shown in the extended range re-forecasts.

_{Updated/Amended 24/10/20 - amended chart links to open access.}