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000
WTNT42 KNHC 242034
TCDAT2

TROPICAL STORM GASTON DISCUSSION NUMBER 9
NWS NATIONAL HURRICANE CENTER MIAMI FL AL072016
500 PM AST WED AUG 24 2016

Gaston is being affected by southwesterly vertical shear associated with a strong mid- to upper-level trough and cut-off low seen in water vapor imagery near 26n 51w. The shear has caused the low-level center to become partially exposed while much of the deep convection has been shunted to eastern half of the circulation. In spite of the degraded satellite presentation, dropsonde data from the unmanned NASA Global Hawk aircraft investigating Gaston support keeping the intensity at 60 kt. In fact, additional observations from the ongoing mission might reveal that the system is even a little stronger than this estimate.

Gaston is in for a prolonged period of strong southwesterly shear for the next 36 to 48 hours, with the shear possibly peaking around 30 kt during that time. The considerable shear is expected to induce weakening, and it is possible that more weakening could occur than indicated in the forecast despite the cyclone's moving over warmer waters. Once Gaston's interaction with the trough lessens in about 2 days, the cyclone should reach 29 deg C water when the shear diminishes. This should give Gaston an opportunity to re-intensify for at least a couple of days before the models indicate an increase in shear at the end of the forecast period. The intensity forecast is reduced relative to the previous one and is a little below the multi-model consensus through 48 hours, but then reverses and is above the consensus aids from 72-120 hours.

Gaston's heading is now definitively northwestward, and the initial motion estimate is 315/14. A continued northwestward motion is likely for the next few days as Gaston moves between one cell of the subtropical ridge centered over the eastern Atlantic and the cut-off low to the northwest. After 72 hours, Gaston should approach a more significant weakness in the Atlantic subtropical ridge around 60w and slow while turning north-northwestward to northward. There are substantial model differences again this cycle, with the ECMWF indicating a weaker subtropical ridge. The weaker ridge makes Gaston more vulnerable to the mid-latitude westerly flow over the North Atlantic, which results in an earlier recurvature. However, the bulk of the guidance has a stronger ridge, and thus have solutions that go much farther west. No major changes have been made to the previous forecast in the short term but the track has been adjusted much farther to the left after 72 hours, in the direction of but not as far left as the multi-model consensus.

FORECAST POSITIONS AND MAX WINDS

INIT 24/2100Z 17.4N 40.6W 60 KT 70 MPH 12H 25/0600Z 18.8N 42.3W 55 KT 65 MPH 24H 25/1800Z 21.0N 44.5W 45 KT 50 MPH 36H 26/0600Z 23.5N 46.6W 45 KT 50 MPH 48H 26/1800Z 25.7N 49.2W 50 KT 60 MPH 72H 27/1800Z 28.3N 54.6W 65 KT 75 MPH 96H 28/1800Z 30.1N 57.4W 80 KT 90 MPH 120H 29/1800Z 31.6N 58.2W 85 KT 100 MPH

$$ Forecaster Kimberlain/Franklin



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