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Tropical Storm GASTON Forecast Discussion Number 9

2016-08-24 22:35:01| National Hurricane Center (Atlantic)

Issued at 500 PM AST WED AUG 24 2016 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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Tropical Storm GASTON Forecast Discussion Number 8

2016-08-24 16:57:59| National Hurricane Center (Atlantic)

Issued at 1100 AM AST WED AUG 24 2016 000 WTNT42 KNHC 241457 TCDAT2 TROPICAL STORM GASTON DISCUSSION NUMBER 8...CORRECTED NWS NATIONAL HURRICANE CENTER MIAMI FL AL072016 1100 AM AST WED AUG 24 2016 Corrected for initial intensity Gaston's overall cloud pattern has not changed much in organization since the last advisory. The low-level center is located underneath a small central dense overcast (CDO), with hints of a prototype eye within the convective mass. A solid band, attached to the CDO, wraps around the eastern half of the circulation. A 1234 UTC ASCAT pass supports keeping the initial wind estimate at 60 kt. Gaston could still reach hurricane strength today. However, the shear associated with a potent mid- to upper-level trough along 52W is forecast to greatly increase over Gaston and peak in 36 to 48 hours. Even though the cyclone will gradually be moving over warmer waters at that time, the shear should be enough to arrest its development or perhaps even result in weakening. By 72 hours, Gaston's involvement with the trough should lessen and the shear should decrease. Much lower westerly shear is forecast toward the end of the forecast period, which should give Gaston an opportunity to re-strengthen, and perhaps significantly, as it moves over waters of around 29 deg C. The new NHC intensity forecast is reduced some compared to the previous one through 48 hours, but shows slightly greater intensification by 120 hours. The official forecast is in generally in good agreement with the multi-model consensus. The latest fixes indicate that Gaston's heading has a much greater northerly component, and the long-term initial motion estimate is 305/14. Gaston is expected to move northwestward between a subtropical ridge over the eastern Atlantic and the aforementioned trough to the west during the next 3 days. After 72 hours, Gaston should gradually turn north-northwestward and northward with a considerable decrease in forward speed, when it encounters a more substantial break in the Atlantic subtropical ridge along 60W. The ECMWF shows less ridging and makes Gaston vulnerable to the mid- latitude westerlies earlier, with the model showing recurvature before 120 hours. The bulk of the guidance, however, lies farther west, with these models showing more ridging north of Gaston through day 5. The new track forecast is a little right of the previous one through 72 hours in response to the cyclone's abrupt change in heading since the last advisory and is close to a blend of the ECMWF and GFS solutions. The track beyond 72 hours was adjusted westward and remains close to a ECMWF/GFS model blend. FORECAST POSITIONS AND MAX WINDS INIT 24/1500Z 16.1N 39.4W 60 KT 70 MPH 12H 25/0000Z 17.5N 41.2W 65 KT 75 MPH 24H 25/1200Z 19.6N 43.5W 60 KT 70 MPH 36H 26/0000Z 21.9N 45.6W 55 KT 65 MPH 48H 26/1200Z 24.3N 47.9W 55 KT 65 MPH 72H 27/1200Z 27.8N 53.1W 65 KT 75 MPH 96H 28/1200Z 30.5N 55.9W 80 KT 90 MPH 120H 29/1200Z 32.2N 56.5W 90 KT 105 MPH $$ Forecaster Kimberlain

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Tropical Storm GASTON Forecast Discussion Number 7

2016-08-24 10:36:17| National Hurricane Center (Atlantic)

Issued at 500 AM AST WED AUG 24 2016 000 WTNT42 KNHC 240835 TCDAT2 TROPICAL STORM GASTON DISCUSSION NUMBER 7 NWS NATIONAL HURRICANE CENTER MIAMI FL AL072016 500 AM AST WED AUG 24 2016 A 0414 UTC AMSR2 microwave image revealed that Gaston's structure has improved with the development of a well-defined low-level cloud ring and a mid-level microwave eye. However, the mid-level center is displaced about 25 n mi to the northeast of the low-level center. Dvorak intensity estimates from TAFB and SAB are both T4.0/65 kt, but objective numbers are still between 55-60 kt. Given the tilted structure of the cyclone, the initial intensity is only raised to 60 kt on this advisory. University of Wisconsin CIMSS shear analyses indicate that 10 kt of southwesterly shear is affecting Gaston, which could explain the cyclone's tilted structure. The shear is expected to remain low enough during the next 24 hours to allow Gaston to strengthen to a hurricane later today. However, the intensification trend should be interrupted after 24 hours, continuing through day 3, when Gaston moves into a higher shear zone to the east of a mid-/upper-level low. Some strengthening is then probable on days 4 and 5 when Gaston moves north of the upper low into a lower-shear environment. The intensity models are in very good agreement on this general scenario, and the official NHC forecast was only lowered a bit at 36 and 48 hours to be more in line with the IVCN consensus. Tracking the low-level center observed in microwave data yields a motion of 290/15 kt. Gaston is approaching a break in the subtropical ridge caused by the aforementioned mid-/upper-level low, and the steering currents should cause the motion to become northwestward later today and continue along that heading for the next four days. By day 5, Gaston is expected to slow down and turn northward to the west of a mid-level high. There is lower-than- normal spread among the track guidance for the entire five-day forecast period, and the NHC forecast continues to closely follow the TVCN multi-model consensus. FORECAST POSITIONS AND MAX WINDS INIT 24/0900Z 14.9N 38.6W 60 KT 70 MPH 12H 24/1800Z 16.2N 40.6W 65 KT 75 MPH 24H 25/0600Z 18.2N 42.9W 70 KT 80 MPH 36H 25/1800Z 20.4N 45.1W 70 KT 80 MPH 48H 26/0600Z 22.9N 47.2W 70 KT 80 MPH 72H 27/0600Z 26.9N 52.3W 70 KT 80 MPH 96H 28/0600Z 29.5N 55.5W 80 KT 90 MPH 120H 29/0600Z 31.5N 55.5W 85 KT 100 MPH $$ Forecaster Berg

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Tropical Storm GASTON Forecast Discussion Number 6

2016-08-24 04:34:57| National Hurricane Center (Atlantic)

Issued at 1100 PM AST TUE AUG 23 2016 000 WTNT42 KNHC 240234 TCDAT2 TROPICAL STORM GASTON DISCUSSION NUMBER 6 NWS NATIONAL HURRICANE CENTER MIAMI FL AL072016 1100 PM AST TUE AUG 23 2016 Gaston's cloud pattern has a figure-nine appearance, with an apparent dry slot over the southwest quadrant, and lacks a well-defined CDO at this time. The intensity is held at 55 kt in agreement with subjective and objective Dvorak estimates. The system has fairly well-defined upper-level outflow and is expected to remain in a low-shear environment for the next day or so. Therefore, the cyclone is likely to intensify into a hurricane in 12 to 24 hours. In about 36 hours, however, Gaston should encounter increasing southwesterly shear associated with an upper-tropospheric trough near 50-55W. This should at least temporarily halt the intensification process, and probably cause a little weakening. The global models indicate that Gaston will pass by the trough in 72 hours, and reintensification should commence around that time. The official intensity forecast is near the high end of the guidance models through 72 hours, and close to the model consensus thereafter. The motion continues toward the west-northwest, or 290/15 kt. Gaston should gradually turn toward the northwest in response to a break in the subtropical ridge near 60W. The official track forecast, which is very similar to the previous one, follows the latest dynamical model consensus, TVCN. FORECAST POSITIONS AND MAX WINDS INIT 24/0300Z 14.7N 37.1W 55 KT 65 MPH 12H 24/1200Z 15.8N 39.2W 65 KT 75 MPH 24H 25/0000Z 17.5N 41.7W 70 KT 80 MPH 36H 25/1200Z 19.5N 44.1W 75 KT 85 MPH 48H 26/0000Z 22.0N 46.2W 75 KT 85 MPH 72H 27/0000Z 26.5N 51.0W 70 KT 80 MPH 96H 28/0000Z 29.5N 54.5W 75 KT 85 MPH 120H 29/0000Z 31.5N 55.0W 85 KT 100 MPH $$ Forecaster Pasch

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Tropical Storm GASTON Forecast Discussion Number 5

2016-08-23 22:36:27| National Hurricane Center (Atlantic)

Issued at 500 PM AST TUE AUG 23 2016 000 WTNT42 KNHC 232036 TCDAT2 TROPICAL STORM GASTON DISCUSSION NUMBER 5 NWS NATIONAL HURRICANE CENTER MIAMI FL AL072016 500 PM AST TUE AUG 23 2016 The cloud pattern of Gaston has not changed much during the past several hours, although perhaps the convection has become more symmetric. Satellite classifications are about the same as before, so the initial wind speed remains 55 kt. Gaston has about 24-36 hours to intensify before southwesterly vertical wind shear associated with a mid-tropospheric trough is forecast to increase. This shear should result in some weakening of the cyclone in the 2 or 3 day time frame. Thereafter, Gaston moves away from the trough, and since the cyclone should be over warm waters near 29C, some restrengthing is likely. Considering the complexity of the scenario, the guidance is in relatively good agreement on this pattern. The official forecast follows the trend of the previous forecast and the intensity consensus. The initial motion estimate is 290/16. Gaston should gradually turn northwestward during the next couple of days as it moves around the subtropical ridge. Guidance is in very good agreement on the forecast during that time, and little change is made to the previous NHC track in the shorter term. Beyond 3 days, a break in the subtropical ridge is forecast, although the models are not in great agreement on exactly what longitude Gaston reaches before its likely recurvature. Overall, the model guidance has shifted a bit westward, and the latest NHC track forecast is shifted in that direction. FORECAST POSITIONS AND MAX WINDS INIT 23/2100Z 14.2N 35.8W 55 KT 65 MPH 12H 24/0600Z 15.2N 38.1W 65 KT 75 MPH 24H 24/1800Z 16.8N 40.7W 75 KT 85 MPH 36H 25/0600Z 18.7N 43.2W 80 KT 90 MPH 48H 25/1800Z 20.8N 45.4W 80 KT 90 MPH 72H 26/1800Z 25.5N 49.6W 70 KT 80 MPH 96H 27/1800Z 28.5N 53.5W 75 KT 85 MPH 120H 28/1800Z 31.0N 54.5W 85 KT 100 MPH $$ Forecaster Blake

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