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User Guide to ECMWF Forecast Products > Derived products from the EPS > 
Plans Cyclone track maps  

Tropical cyclone diagrams

Ensemble mean and spread charts
Wave EPSgrams
The Extreme Forecast Index (EFI)
Tropical cyclone diagrams
Cyclone track maps

The ECMWF tropical cyclone forecast products are designed to provide both deterministic and probabilistic information on the movement and intensity of individual tropical cyclones.

The perturbation calculations are always conducted for the Caribbean area, irrespective of whether a tropical cyclone exists, since it is an important “breeding ground” for extra-tropical cyclones affecting the North Atlantic and Europe.

a)      Cyclone position: Once official reports signify the existence of a tropical cyclone, it is automatically tracked both in the deterministic and the EPS forecasts. The tracking algorithm is based on the extrapolation of past movement and of the mid-tropospheric steering flow to obtain a first-guess position. The actual position is determined by searching for mean sea level pressure (MSLP) and 850 hPa vorticity extremes around the first-guess position.  In some circumstances the thickness maximum, the central MSL pressure and the orography are also considered in the evaluation.

b)      Strike probability charts: Strike probability is defined as the proportion of EPS members that predict that the tropical cyclone will pass within a 120 km radius of a given location at any time during the next five days. In other words, the time dimension is eliminated (see Figure 55). This allows for a quick assessment of high-risk areas, regardless of the exact timing. A 40% probability at a specific location means that, within a circular area of 120 km, 20 members have a tropical cyclone centre during the coming five days.


Figure 55: Strike probability map for tropical storm Danielle from the 28 August 2010, 00 UTC.

c)      Lagrangian meteograms are a convenient way of evaluating the forecast for a specific tropical cyclone. They contain time series of the central pressure and of the 10 m wind speed maximum predicted within a 7˚ x 7˚ lat-long box, centred on the cyclone and following its motion in each forecast member. The symbols are similar to those used on the EPSgrams. The number of EPS members which contain the tropical cyclone is also presented in the diagram at the top of Lagrangian meteograms; the other parameters need to be interpreted with this number in mind (see Figure 56).


Figure 56: Corresponding Lagrangian meteogram for Danielle, 28 August 2010. Note the decreasing number of EPS members which forecast any tropical cyclone at the end of the ten-day period. Although there is no marked difference between the EPS and the operational system’s wind speed forecast (blue line), the latter forecasts deeper central pressure. The green line indicates the Control forecast.

Both the strike probability charts and the Lagrangian meteograms are dependent on observations from various tropical cyclone centres around the world and do not take TC genesis into account.

A parallel system shows the potential tropical cyclone activity at different time ranges during the forecast. It includes both tropical cyclones that are present at analysis time and those which may develop during the forecast but have not yet come into existence. The maps show the "strike probability", based on the number of EPS members that predict a tropical cyclone, each member having equal weight. To be counted, the tropical cyclone centre must track within a 300 km radius of the location within a time window of 48 hours (see Figure 57).


Figure 57: The Tropical Storm Strike Probability chart from Saturday 23 July 2011, indicating the probability of the passage of two not yet developed storms, within a 300 km radius, between 5 and 7 days ahead, i.e. during the 48 hours between Thursday and Saturday 28-30 July 00 UTC. The western system developed into tropical storm NOCK-TEN and briefly reached typhoon status with maximum sustained winds of 75 mph late on Tuesday evening.The eastern system developed into a tropical depression on the 25th, a tropical storm, named MUIFA, on the 28th and a typhoon on the 30 July.

This product 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 (wind speeds 8-16 m/s), tropical storms (17-31 m/s) and hurricanes/typhoons (> 32m/s). Wind assignation is tuned on the 7° × 7° latitude/longitude box maximum, as represented on Lagrangian meteograms.

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