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Another tropical depression has formed, to the east of Tropical Depression 16W, to the southwest of Wake Island. The depression has a broad LLCC but a fair amount of central convection and good banding features. Steering currents are weak, so little motion is expected over the next day or so, followed by a faster motion to the northwest. Strengthening will be slow at first due to nearby troughing suppressing outflow, but like with 16W, the environment is likely to become very conducive in a day or so, allowing potentially rapid strengthening at this time.

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Atsani also looks cool in 3D by HWRF data :)

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17W became Tropical Storm Atsani yesterday, and has since strengthened to 45kts. The storm is quite broad, unlike the smaller TS Goni to the west. Nevertheless, rapid strengthening is still expected once Atsani establishes an inner core and the trough causing subsidence eases away.

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Atsani has become a typhoon according to JTWC, with winds of 65kts. The system has developed good, central convection and has strong banding features.

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Atsani continues to strengthen, with winds of 85kts according to JTWC, a category 2 on the SS scale. The typhoon has a large eye, with slightly thinner convection on the northern side due to troughing still causing a little subsidence in this quadrant of the system. As the system turns to the west-northwest, this should become less of an issue.

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Atsani is now a category 3 with winds of 105kts. Outflow has improved in the northern quadrant (conversely it is now a problem for Typhoon Goni to the west), allowing the intensification. Further intensification is expected as Atsani travels to the northwest. JTWC expect a peak of 140kts before shear rises in a few days.

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Atsani continues to intensify, with winds now at 120kts, cat 4. As the environment for the next couple days is forecast to remain favourable, further strengthening is expected, and as SB says, it's another cat 5 forecast!

Track wise, a fairly slow northwesterly heading is expected to persist, followed by a gentle curve to the north.

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Atsani has become the season's sixth Super Typhoon, with winds now at 130kts. Further strengthening is forecast, with Atsani forecast to hecome a category 5 on the SS scale (if this occurs it'll be the fifth of the season).

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Atsani became a category 5 with winds of 140kts this afternoon, but has since weakened to 135kts, cat 4. The very large eye remains well defined, but the convection is a little thin on the northern side. Atsani is expected to reach category 5 status again before weakening as shear increases in a couple days.

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Atsani has weakened to 115kts. The typhoon retains a huge eye, 49 nautical miles wide. The convection on the northern side of the eye has weakened however. Shear is set to increase and waters cool soon along track, promoting further weakening.

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Dry air is filtering into the circulation of Typhoon Atsani. Winds are down to 90kts. Shear is low and waters are warm for now, so the dry air is forecast to produce only slow weakening over the next day or so, followed by faster weakening as Atsani eventually accelerates northeastwards into cooler waters and higher shear.

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Atsani has only very slowly weakened. Winds are down to 80kts. Shear is moderate, but outflow good, particularly in an equatorward direction. Dry air continues to filter in from the west however. The result is a well structured typhoon with a well defined eye, but shallow convection surrounding it, with the deepest convection in the outer bands. The environment described above is expected to persist for the next 36hrs, which should allow Atsani to roughly maintain strength through this period. Beyond 36hrs, shear is set to increase and waters cool along track, initiating extratropical transition. It's interesting to note that whilst Atsani is extratropical it is expected to absorb Central Pacific Tropical Storm Loke which will approach from the south.

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