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Somerset Squall

Tropical Storm Nepartak

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Tropical Depression 21W has formed around 16N, 143E. The depression has formed from invest 92W which has shown some rapid development over the last 24hrs. The LLC is well defined, and convection is wrapping well along the eastern and northern quadrants. Conditions are favourable for further development, with low shear, warm sea temperatures and good outflow. 21W is therefore expected to become a minimal typhoon on the northward track along the western side of a ridge to the east. On this track, 21W will reach cooler waters and higher shear in about 4 days time which will induce extratropical transition. 21W is not likely to effect land as at this point the system will be pulled northeastwards away from Japan in the mid-lattitude westerlies.

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21W has become a little better organised and has built some convection near the centre overnight, and has been upgraded to Tropical Storm Nepartak with intensity now at 35kts. JMA and JTWC disagree with what will now happen to Nepartak. JTWC predict hardly any more strengthening and accelerates Nepartak quickly into the westerlies (strong shear). JMA favours a slower track, keeping Nepartak over warmer waters for longer which allows more intensification. Chances have significantly reduced for Nepartak to become a typhoon but just how strong it will become is open to question. JMA and JTWC seem to disagree on just how influential the ridge is to the east on Nepartak which accounts for the different track speeds. As usual, we will have to wait and see!

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Nepartak is moving very slowly northwards whilst maintaining an intensity of 35kts. The steering influence of the ridge to the east remains quite weak. Nepartak will eventually turn to the northeast as it rounds the periphery of the ridge, and at this point it will begin weakening over cooler waters and higher shear. However, JTWC and JMA are still in disagreement about track speeds with JMA being more aggressive on intensification as it shows Nepartak reaching the cooler waters later than JTWC. Nepartak currently is maintaining some deep convection near the LLC, but lacks significant banding which does not indicate a strengthening tropical cyclone. There is still a lot of uncertainty with Nepartak but at least it seems like it won't affect land.

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Nepartak has strengthened over the last 24hrs with intensity now at 55kts. Nepartak's small size has allowed this significant intensification to occur. Well done to JMA who has consistantly forecast this strengthening. Nepartak has another 12-24hrs to strengthen and it could become a minimal typhoon before moving northeastwards into unfavourable levels of shear. Nepartak is rounding the ridge to the east and has begun the northeastward turn. On this track, waters will cool too which will aid significant weakening beyond 36hrs.

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Nepartak has been racing northeastwards at around 30kts over the last 24hrs. As a consequence, the storm has traversed much colder waters and excessive shear. Nepartak has responded to the deteriorating environment by becoming extratropical, as evidenced by frontal features forming and the transition to cold core.

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