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. Evaluation of TC tracks for the Extended range ENS is computed at 6hr intervals in the same way as medium range ENS and the outputs are consistent.

Fig8.2.7-1: To view tropical storm frequency.

1. On charts page, click on "Extended Range".
2. On chart selection page, select "Extended" and "Tropical Cyclones".
   
3. Click on tropical storm frequency chart diagram.
4. As desired select other base times or accumulated cyclone energy from drop down menu.

The "Climate Means" of tropical cyclone frequency (orange bars) are derived from observations only.  The "Forecast Means" (green bars) are based on the output of the tracking algorithms.  The height of the forecast bars normalised to account for differences during the contiguous period selected between:

• the characteristics of tropical cyclones identified in the re-forecasts.
• the characteristics of tropical cyclones actually observed .

This normalisation makes the orange and green bars directly comparable.

Fig8.2.7-2: An example chart of monthly forecast tropical storm frequency.  Not much activity is shown over the ocean basins but  there is slightly more activity over tropical waters north of Australia in the Timor Sea and Arafura Sea areas (green bar = 0.4; orange bar = 0.2).

Predicted accumulated cyclone energy (ACE)

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.  

Fig8.2.7-3: To view accumulated cyclone energy.

1. On charts page, click on "Extended Range".
2. On chart selection page, select "Extended" and "Tropical Cyclones".
   
3. Click on tropical storm frequency chart diagram.
4. Select accumulated cyclone energy from drop down menu.
5. As desired select other base times.

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" of tropical cyclone frequency (orange bars) are derived from observations only.  The "Forecast Means" (green bars) are based on the output of the tracking algorithms.  The height of the forecast bars normalised to account for differences during the selected contiguous period between:

• the characteristics of tropical cyclones identified in the re-forecasts.
• the characteristics of tropical cyclones actually observed .

This normalisation makes the orange and green bars directly comparable.

A further normalisation sets the climatological value (orange) equal to 1.   The forecast value (green) indicates above average (value greater than 1) or below average (value less than 1) Accumulated Cyclone Energy respectively.

Fig8.2.7-4: An example of the accumulated cyclone energy (ACE) product.  This 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 climate (ER-M-climate) for the same week to determine the significance level for the given basin.  If significance is greater than 90% the basin is shaded.  Green bars represent the forecast and orange bars the model climatology - these can be directly compared.

Tropical storm strike probabilities

Fig8.2.7-5: To view storm strike probabilities

1. On charts page, click on "Extended Range".
2. On chart selection page, select "Extended" and "Tropical Cyclones".
   
3. Click on tropical storm frequency chart diagram.
4. As desired select other base times.

This product shows a grid point chart of tropical storm strike probability (probability of a tropical storm passing within 300km), calculated over weekly periods.  The charts are based on the number of extended range ensemble members that predict a tropical cyclone, each member having equal weight.  This provides a quick assessment of high-risk areas allowing for some uncertainty in the exact timing or position.  The strike probabilities are generated for three storm categories; tropical depressions (winds >8 m/s), tropical storms (winds >17 m/s) and hurricanes/typhoons (winds >32 m/s).

These charts show the evolution of existing and potential tropical cyclone activity.

• Strike probability chart.  This is the probability (in %) that a tropical cyclone will pass within a 300 km radius from a given location and within the 7-day time window.   The charts are based on the number of extended range ensemble members that predict a tropical cyclone, each member having equal weight.   It is similar to the tropical cyclone activity charts produced for the medium-range forecasts.  
• Probability anomaly chart.  This shows whether the strike probabilities are higher (red shading) or lower (blue shading) than probabilities within a 20-year model climatology within the same 7-day time window.  They are computed by subtracting the model climatological probabilities from the real-time forecast probabilities.  Mean anomalies of probability are shown by the scale.
• Model extended range climate charts (ER-M-climate) show weekly mean strike probabilities (in %) within the same 7-day time window.

Note neither Raw Strike Probability data nor Probability Anomaly data incorporate any bias correction to adjust for differences relative to tropical cyclone observations. Therefore both products may suffer from systematic under- or over-estimations.

Fig8.2.7-6: Weekly mean tropical storm strike probability of the extended range forecast.  Colours give probability in percentages.

Fig8.2.7-7: Weekly mean tropical storm strike probability deduced from the extended range re-forecasts, for the same week as Fig8.2.7-6.  Colours give probability in percentages.

 Fig8.2.7-8: Weekly mean anomaly of tropical storm strike probability for the same week as Fig8.2.7-6.  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.