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Archive | December, 2017

Tropical depression in Philippine Sea remains poorly organized, rainfall threat for Philippines

Happy New Year!  We hope 2018 is good to you and your family.

JMA continues to list the area in the southern Philippine Sea as a tropical depression.  PAGASA and JTWC maintain 99W as a low pressure area, with the latter giving it a medium chance at developing into a tropical cyclone within 24 hours.

20171231 2100 99W METSAT.jpg

As the sun rises on 99W, we see convection remains widely scattered, lacking consolidation over the center of circulation.  Latest microwave pass indicates the low-level circulation remains broad and elongated with a few pockets of winds at 45 km/h.  Given the close proximity to the Philippines, little change in strength is expected over the next couple days.  I wouldn’t be surprised to see PAGASA and JTWC upgrade prior to landfall to a tropical depression–matching JMA’s analysis, but tropical storm status does not seem likely before landfall.  With it now being 2018, a tropical depression declared by PAGASA would name the system as Agaton.

Forecast track continues to move the cyclone across northern Mindanao before emerging in the Sulu Sea on Tuesday.  In the meantime, heavy rain will threaten Mindanao and the Visayas, with 24-hour rainfalls of 50-150 mm.  Flooding and landslides will be possible as 99W passes through.  Once it reaches the Sulu Sea, Palawan and interests across the Spratly Islands will have to monitor the storm as is should being developing at that point.  The consensus forecast has it developing into a tropical storm by mid-week in the South China Sea before threatening coastal Vietnam (likely staying off-shore) toward Thursday and Friday as the storm weakens.

Mike Adcock
Meteorologist, Western Pacific Weather

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Tropical Depression forms in the Philippine Sea; expected to enter PAR tonight

At 0600 UTC (2 pm Philippine Time), JMA–the official Regional Specialized Meteorological Center for the Western Pacific–had upgraded the low pressure area (Invest 99W) into likely the final tropical depression of the 2017 season.

According to JMA, the tropical depression was located near 9°N 136°E or about 225 km northeast of Melekeok, Palau or about 1035 km east-southeast of Surigao City, Philippines.  It was moving toward the west slowly with a central pressure of 1004 hPa.

20171230 0900 99W METSAT

At this time, forecast products are not being issued by PAGASA or JTWC.  The depression should enter the Philippine Area of Responsibility (PAR) overnight.  Once it does and PAGASA recognizes it as a depression, it will gain the local name “Wilma”.  Should it become a tropical storm, the international name would be “Bolaven” which was contributed by Laos to recognize a plateau in Champasak province which holds a significant role in Laotian history.

A microwave imagery pass over 99W earlier today indicated that there was a closed 20171230 0050 99W ASCATcirculation, albeit elongated.  Winds across the depression were generally around 30-35 km/h with pockets up to 45 km/h in thunderstorms displaced well to the northeast of center.  At 0900 UTC, the pressure in Koror, Palau was down to 1007.2 hPa, providing some support to JMA’s analysis of the depression.  Interestingly, the 0600 UTC analysis from JTWC indicated the winds were 30 km/h with a pressure of 1010 hPa–both values too weak when compared to observed data.  None of the major agencies are issues Dvorak analyses at this time.

Forecast guidance is continuing to come in better agreement, strengthening the confidence in the forecast.  All the major global models have backed off on intensity, keeping 99W as a tropical depression or possibly a very weak tropical storm.  Given that the depression is already over Palau, the window of opportunity to strengthen is beginning to close.

No significant deviations to the forecast track as 99W is expected to continue moving toward the west or slightly north of west through landfall, south of the trailed end of a cold front passing through the Philippine Sea.  With this track, CARAGA region is expected to receive the center landfall.  However, with heavy rainfall being the main threat from this system, all interests within the Visayas and Mindanao should continue monitoring the progress of this tropical depression.  Computer projections indicate that the eastern Visayas and northern Mindanao may see rainfall totals of 100-200 mm per day, further hampering recovery efforts from Vinta.

Mike Adcock
Meteorologist, Western Pacific Weather

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

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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