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The figures below provide verification examples, over multiple or single cases, illustrating typical IFS model characteristics, be they strengths or weaknesses.  Key points for the forecast user are highlighted in the captions.

Fig8.1.16.3-1: Reliability diagram of tropical cyclone strike probabilities, at day 10, for TCs in existence at T+0, for year July to June (years indicated by colours) showing reasonably good reliability (plots are near the diagonal), but a tendency towards over- confidence (plots to right of diagonal).

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Fig8.1.16.3-3: IFS model forecast location error of tropical cyclones (HRESand HRES and CTRL) - blue; Ensemble Mean (EM) - Yellow)  and ENS spread (Red dots).  The diagram indicates that in 2016 the average location error and spread at:

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• The earlier forecast has a 30% threat to coastal central eastern Japan and Tokyo, 10%-20% threat to the islands south of Kyushu, but little threat to the Sea of Japan.
• The later forecast has <10% threat to eastern Japan and Tokyo but 30%-40% threat to the Sea of Japan (and 40%-50% in the south), and 70%-80% threat to the islands south of Kyushu.

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Extra-tropical Transition

Extratropical transition Extra-tropical transition of tropical storms is, in general, difficult for NWP models to deal with.  NWP models often show large run-to-run variability in forecast track, movement and depth of a tropical depression as it moves into the mid-latitudes.  Small differences between the analyses and background fields in sequential runs, or small perturbations developed by NWP models in the circulating flow, can have a major impact on the forecast evolution.  This is especially so with energetic systems, such as well developed tropical storms, and can make some forecast aspects unsafe later in the forecast period (say beyond Day5).

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Fig8.1.16.3-8: Strike probabilities for tropical cyclone17W up to 10 August 2017 (T+240) based on ENS and HRES forecasts, data time 12UTC 1 August 2017.  It is notable that the ENS mean track (dotted line) and the tracks of the majority of ENS members veer eastwards towards mid-Pacific as extratropical extra-tropical transition occurs while HRES (solid line) and a few ENS members curve the low pressure centre westwards towards Japan.    

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Fig8.1.16.3-9: The same case as in Fig8.1.16.3-7.  Top: Probability from ENS members for tropical cyclone 17W to fall into each of the 5 tropical cyclone intensity categories shown at 6hr intervals to 10 days.  Centre and Bottom:  Lagrangian meteograms of distribution of the ENS for the 10m wind (kt) and MSLP (hPa) at tropical cyclone 17W centre.  Tropical cyclone 17W is considered to have become extratropical extra-tropical by Day9 and central pressure and wind information is discontinued at that time.  HRES central pressure and winds diverge from the ENS beyond Day5 and forecasts cease as winds fall below the threshold for a tropical cyclone to exist and the depression effectively transitions to become extratropical extra-tropical (or even non-existent) sooner than it appears to in most (~80%, see top row) of the ENS members.

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Although often very useful it should be remembered that these are an experimental product based upon identification of the warm-core circulation during the forecast period.  The technique can mis-identify misidentify 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.

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A limit is placed on the roughness length scale parameter in order to avoid the effect of too much drag from the sea surface in the lower atmosphere and enable more realistic (stronger) winds to be forecast in the vicinity of relatively intense tropical cyclones.  It should also be noted that tropical cyclone development with strengthened winds has only a limited effect upon the size and character of waves developed by the Wave model.  The model change to limit the roughness length scale at very high speeds was introduced in cycle 47r1 in June 2020.   See Section on Waves near tropical storms.

RSMC official forecasts of tropical cyclones take precedence

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