Tropical disturbance continues to spreading showers across Caroline Islands; may be next named storm


Invest 99W RGB Enhancement Imagery – 0700 UTC 28 December 2017

What had been tracked as Invest 98W was dropped earlier today by the team at NRL, only to have the convection further west christened as Invest 99W.

99W maintains a very broad circulation–still more of a vigorous trough than a low pressure area.  The next 48 hours will be the most important for the forecast:  The more it develops in this critical window, the more trust we can place in the verification of the forecast guidance.  The consistency of the models (run-to-run across the various models) give some level of confidence that a weak tropical cyclone may develop this weekend, but it remains early to nail down any specifics.

One problem remains is the inconsistencies of how strong said tropical cyclone will be.  GFS remains the most bullish, suggesting 99W could be a severe tropical cyclone or typhoon by the New Year.  I’d take this lightly given that all of the other major models suggest a minor tropical storm at best.  With the consensus, I would lean toward a tropical depression to weak tropical storm at this time, but we cannot let our guard down for two main reasons:  The Philippine Sea tends to fuel tropical cyclones into rapid development and it doesn’t take much of a tropical cyclone to cause problems in the southern Philippines (Reference Vinta).

Again, I have to emphasize that long-term forecast discussions on a system that hasn’t developed into a low introduces potential for significant error.  However, the consensus track does point toward the northern Mindanao and the Visayas–which is very concerning.  If this does verify, the main threat will be heavy rainfall in an area still dealing with Vinta.  Early projections suggestion 100-200 mm can be expected across northeast Mindanao and eastern Visayas during the early half of next week.

All this said, currently 99W is not being discussed in the JTWC outlooks nor the JMA 48-hour forecast.  Doesn’t mean the threat isn’t there, but it’s still too early to get carried away.  Time to remain vigilant.

Mike Adcock
Meteorologist, Western Pacific Weather


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