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Tropical Cyclone Diagrams - existing TCs

Tropical Cyclone Products

Products are generated for all tropical cyclones that have been officially observed (by a Regional Specialized Meteorological Center (RSMC)) at the initial time of the forecast.  Both the strike probability charts and the Lagrangian meteograms (i.e. following the tropical cyclone forecast positions) are dependent on these official reports from around the world and do not take tropical cyclone genesis into account.   The available charts are:

• Cyclone position:

Once official reports signify the existence of a tropical cyclone, it is automatically tracked.  The tracking algorithm uses HRES output and is based on using the extrapolation of past movement and the mid-tropospheric steering flow to obtain a first-guess position 6 hours into the future.  The actual forecast position is then determined by searching for mean sea level pressure (MSLP) and 850hPa vorticity extremes around the first-guess position.  In some circumstances the thickness maximum, the central MSLP, surface winds, and the orography are also considered in the evaluation.  This process repeats at 6 hour intervals through the forecast until either the tropical cyclone dies or the end of day10 is reached.

When two Tropical Storms are relatively close the possibility arises that the tracking algorithm may jump from one to another - e.g. if one is decaying (see Fig8.1.10.1.5).  This seems to be a very rare occurrence and which is under investigation, but users should be aware of the potential for this error.

• Strike probability charts:

Strike probability is defined as the proportion of members that predict that the tropical cyclone will pass within a 120km radius of a given location at any time during the next five days.  In other words, the time dimension is integrated over the forecast range (see Fig8.1.10.1.2B).  This allows for a quick assessment of high-risk areas, regardless of the exact timing.  A 40% probability at a specific location means that approximately 40% of ensemble members place that particular tropical cyclone centre within a 120km radius of that location at some point during the coming 10 days.  If a tropical cyclone is forecast to loop back on itself, and so pass close to the same location two or more times, as happens occasionally, the probability shown on the plot will be the highest probability encountered in any given 6h time interval during the forecast period.

• Lagrangian meteograms:

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 10m wind speed maximum predicted within a 7ºx7º lat-long box centred on the cyclone and following its motion in each forecast member.   The "box and whisker" symbols are similar to those used on the Ensemble Meteograms.  However, unlike the standard meteograms, the number of ensemble members represented by the boxes and whiskers varies, and can be assessed by reference to the strike probability chart or individual forecast trajectories chart.

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A quick-look graphical guide to interpreting these products is available.

Examples of forecast tropical cyclone products

On web site

The ensemble Forecast Tropical Storm Products for tropical cyclone 07S over the western Indian Ocean in the spring of 2017 are used as an example below. 

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Fig8.1.10.1.3C: Sample CDFs (at the point indicated by the arrow) for the same ensemble forecast as shown in Fig8.1.10.1.3A.  Note the high EFI for 24hr maximum 10m wind gust, which can be very useful, though the user needs to be aware of the innate difficulties of representing the absolute values of winds around tropical cyclones.

On ecCharts

Tropical cyclone activity charts for named or pre-existing tropical cyclones are also available on ecCharts.  The information can be combined with other significant weather parameters (e.g. 10m winds, significant wave height, etc).

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Fig8.1.10.1.5A(Right): Locations of TC Ferdinand from HRES forecast DT 00UTC 23 Feb 2020 (solid line on chart).  At 12UTC 29 Feb 2020 the algorithm incorrectly identifies the location of Ferdinand at the location of TC Esther and subsequently follows the movement and developments of TC Esther.  The forecast values of ensemble mean and ensemble probabilities do not suffer from the same problem.

RSMC official forecasts of tropical cyclones take precedence

Note: IFS products on these pages regarding tropical cyclones are generated automatically without any editing by forecast experts.   RSMCs (Regional Specialized Meteorological Centres) have ultimate responsibility for official forecasts of tropical cyclones within their respective regions (ECMWF is one of a number of centres that provide data to them).  Up-to-date information is available by direct access to official RSMC forecasts through the WMO Severe Weather Information Centre.  For up-to-date forecast information for their own local area users should refer to forecasts from their own National Meteorological Service.

Additional Sources of Information

(Note: In older material there may be references to issues that have subsequently been addressed)

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