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The charts show the strike probability based on the number of members that predict a tropical cyclone, each member having equal weight.  To be counted, the tropical cyclone centre must track within a 300km radius of the location within a time window of 48 hours. The charts are also useful during and after extratropical extra-tropical transition of these features as they move into mid-latitudes.  However, the technique can sometimes mis-identify incorrectly identify as a tropical cyclone a high-latitude circulation containing something of a warm core (e.g. a well occluded frontal depression with cooler air encircling some warm, moist air remnants near the centre). The result will be spurious probabilities of tropical storm strikes.  Future improvements to the technique will will aim to remove this problem.

See Tropical Cyclone Products for existing TCs for more detailed products for officially observed (by a Regional Specialized Meteorological Center (RSMC)) tropical cyclones.

Fig81Fig8.1.16.2.A-1: To view tropical cyclone activity:

1. On the charts page, click on latest tropical activity.  Click on the tropical cyclones activity (Including genesis) frame (top left frame).  
2. Colours give strike probability of tropical cyclone etc.  Select point of interest ENSgrams etc..
3. Select time of forecast as desired.
   
4. Display of ENSgrams (including wavegrams),

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1. extreme forecast indices extreme forecast indices (EFI) and cumulative distribution functions (CDF), and vertical profiles for selected point.

These products provide a quick assessment of high-risk areas, allowing for some uncertainty in the exact timing or position.  The strike probabilities can be generated for three storm categories:

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Tropical storms are identified by the existence of a warm core isobaric depression.  The charts show the probability of the passage of storms identified in this way (some possibly not yet developed) within a 300km radius of a given location. 

 Fig818.1.16.2.B:-2  Tropical storm strike probability chart from ensemble data time 12UTC 28 February 2017.  This indicates the probability of the passage of storms (some possibly not yet developed) within a 300km radius, between 9 and 11 days ahead (during the 48hr period between 12UTC 8 March and 12UTC 10 March 2017).   The western storm passed across Madagascar as intense tropical depression ENAWO before weakening by the time of the chart.  The central storm became a tropical depression 07S passing just east of Madagascar.   The north-eastern storm developed a wind circulation near 90E but didn't reach tropical storm status.   These developments are reflected in the probabilities shown on the chart, though of course this will not always be the case.  Probabilities given by colours in the scale.

Fig81Fig8.1.16.2.C-3: Spurious indication of tropical storm strike probability near Iceland.  The technique has mis-identified has incorrectly identified a high-latitude well-occluded frontal depression as being a tropical cyclone (because it has a warm core).

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Strike probability charts for a sequence of forecast lead-times clearly indicates the forecast probabilities of development of a tropical cyclone, storm or hurricane.  The example below (Fig81Fig8.1.16.2.D-4) shows the probability of a tropical system near northeast Australia.  Selection of tropical storms, tropical cyclones or hurricanes from the tab at the top of the display allows investigation of the evolution and forecast intensity of the tropical system.

Fig81Fig8.1.16.2.D-4: Web charts tropical storm strike probability charts from ensemble probability charts data time 12UTC 03 January 2018 verifying at  T+144 (12UTC 09 January 2018) and T+192 (verifying 12UTC on 11 January 2018).   Forecast from ensemble, data time 00UTC 8 January 2018.    The charts show probabilities associated with tropical storm force winds indicated to increase from a weaker pre-existing tropical cyclone near the northwest coast of Australia.

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Tropical cyclone, tropical storm, and hurricane strike probability charts are also available on ecCharts.  These are an experimental product based upon identification of the warm-core circulation during the forecast period.  They are useful during and after extratropical extra-tropical transition of these features as they move into mid-latitudes.  ecCharts have the advantage that boundary layer (or any other) winds can be superimposed.

Fig81Fig8.1.16.2.E-5: ecChart tropical storm strike probability chart from ensemble with 100m winds (m/s) from HRES for T+96 verifying at 00UTC 12 January 2018.  Forecast from ensemble, data time 00UTC 8 January 2018.

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