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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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