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Tropical Depression TEN Forecast Discussion Number 1

2015-09-18 16:32:27| National Hurricane Center (Atlantic)

Issued at 1100 AM AST FRI SEP 18 2015 000 WTNT45 KNHC 181432 TCDAT5 TROPICAL DEPRESSION TEN DISCUSSION NUMBER 1 NWS NATIONAL HURRICANE CENTER MIAMI FL AL102015 1100 AM AST FRI SEP 18 2015 Satellite data indicate that the area of low pressure over the tropical eastern Atlantic has acquired sufficient organization to be classified as a tropical depression. The low-level center appears embedded beneath a relatively small cluster of deep convection, but convective banding is also increasing to the west and north. The initial intensity is set at 25 kt based on Dvorak estimates from TAFB and SAB and recent ASCAT data. The depression is located near the southwestern edge of a mid-level anticyclone, giving the cyclone an initial motion of 285/8 kt. The anticyclone is expected to move westward in tandem with the depression over the next couple of days, keeping it on a general west-northwestward track for much of the forecast period. By days 4 and 5, however, there is significant uncertainty in the track forecast. The GFS and GFDL show the cyclone feeling the influence of an amplifying deep-layer trough over the eastern Atlantic, which turns the depression northward. The ECMWF and UKMET, on the other hand, keep the ridge as the dominant influence and continue a west-northwestward motion. Due to the large spread in the guidance, the official forecast shows the cyclone slowing down considerably by days 4 and 5, and is a little west of the model consensus aids at those times. The environment looks conducive for at least gradual strengthening during the forecast period. Sea surface temperatures are warm, vertical shear should remain generally low, and environmental moisture patterns appear favorable for at least the next two days. Beyond 48 hours, most of the global models show the cyclone threading the needle beneath a narrow upper-level ridge, with mid-level dry air possibly getting into the western part of the cyclone's circulation. This pattern makes the intensity forecast complicated because hostile upper-level winds won't be too far away, and the ultimate strength of the cyclone will depend heavily on its eventual track. For now, the NHC official forecast shows only gradual strengthening through day 4, more or less in line with the SHIPS and LGEM guidance. FORECAST POSITIONS AND MAX WINDS INIT 18/1500Z 13.1N 36.7W 25 KT 30 MPH 12H 19/0000Z 13.4N 37.6W 30 KT 35 MPH 24H 19/1200Z 14.2N 39.0W 30 KT 35 MPH 36H 20/0000Z 15.2N 40.8W 35 KT 40 MPH 48H 20/1200Z 16.4N 43.0W 40 KT 45 MPH 72H 21/1200Z 18.3N 46.4W 45 KT 50 MPH 96H 22/1200Z 19.5N 48.0W 50 KT 60 MPH 120H 23/1200Z 20.0N 49.0W 50 KT 60 MPH $$ Forecaster Berg

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Tropical Depression NINE Forecast Discussion Number 8

2015-09-18 10:42:45| National Hurricane Center (Atlantic)

Issued at 500 AM AST FRI SEP 18 2015 000 WTNT44 KNHC 180842 TCDAT4 TROPICAL DEPRESSION NINE DISCUSSION NUMBER 8 NWS NATIONAL HURRICANE CENTER MIAMI FL AL092015 500 AM AST FRI SEP 18 2015 A burst of convection with cloud tops colder than -70C has developed near and northeast of the center of Tropical Depression Nine. Satellite intensity estimates from TAFB and SAB are 25 kt, and that will be the initial intensity. The depression continues to be in an environment of 25-30 kt of westerly vertical wind shear, and water vapor imagery shows mid- to upper-level dry air being advected into the system. The dynamical models suggest these conditions should persist for at least 48 hours, and based on this the depression is forecast to degenerate to a remnant low pressure area in 24 hours or less. The initial motion remains 330/6. A low- to mid-level ridge to the north of the depression is expected to steer it or its remnants generally northwestward for the next several days. The new forecast track is similar to the previous track, with a little nudge toward the south during the first 48 hours due to the initial position. FORECAST POSITIONS AND MAX WINDS INIT 18/0900Z 17.6N 45.7W 25 KT 30 MPH 12H 18/1800Z 18.3N 46.6W 25 KT 30 MPH 24H 19/0600Z 19.2N 48.1W 25 KT 30 MPH...POST-TROP/REMNT LOW 36H 19/1800Z 19.9N 49.5W 20 KT 25 MPH...POST-TROP/REMNT LOW 48H 20/0600Z 20.7N 51.0W 20 KT 25 MPH...POST-TROP/REMNT LOW 72H 21/0600Z 22.5N 54.0W 20 KT 25 MPH...POST-TROP/REMNT LOW 96H 22/0600Z 24.0N 56.0W 20 KT 25 MPH...POST-TROP/REMNT LOW 120H 23/0600Z...DISSIPATED $$ Forecaster Beven

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Tropical Depression NINE Forecast Discussion Number 7

2015-09-18 04:33:54| National Hurricane Center (Atlantic)

Issued at 1100 PM AST THU SEP 17 2015 000 WTNT44 KNHC 180233 TCDAT4 TROPICAL DEPRESSION NINE DISCUSSION NUMBER 7 NWS NATIONAL HURRICANE CENTER MIAMI FL AL092015 1100 PM AST THU SEP 17 2015 The depression consists of a circulation of low clouds with a very small patch of deep convection to the north of the center. Based on continuity and a partial ASCAT pass, the initial intensity is kept at 25 kt. Given that 20 to 25 kt of shear and dry air are expected to continue in the vicinity of the depression, weakening is forecast, and the depression will likely become a remnant low in about 12 hours or so. Satellite fixes show a motion toward the north-northwest or 330 degrees at 6 kt. The depression has become a shallow cyclone, and the subtropical high is forecast to rebuild to the north. Consequently, the depression or the remnant low is forecast to turn toward the northwest and then west-northwest. This turn to the left is the solution provided by most of the track models. FORECAST POSITIONS AND MAX WINDS INIT 18/0300Z 17.3N 45.3W 25 KT 30 MPH 12H 18/1200Z 18.0N 45.7W 25 KT 30 MPH...POST-TROP/REMNT LOW 24H 19/0000Z 19.0N 47.0W 25 KT 30 MPH...POST-TROP/REMNT LOW 36H 19/1200Z 19.7N 48.5W 20 KT 25 MPH...POST-TROP/REMNT LOW 48H 20/0000Z 20.5N 50.0W 20 KT 25 MPH...POST-TROP/REMNT LOW 72H 21/0000Z 22.0N 53.0W 20 KT 25 MPH...POST-TROP/REMNT LOW 96H 22/0000Z 23.0N 55.0W 20 KT 25 MPH...POST-TROP/REMNT LOW 120H 23/0000Z...DISSIPATED $$ Forecaster Avila

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Tropical Depression NINE Forecast Discussion Number 6

2015-09-17 22:34:54| National Hurricane Center (Atlantic)

Issued at 500 PM AST THU SEP 17 2015 000 WTNT44 KNHC 172034 TCDAT4 TROPICAL DEPRESSION NINE DISCUSSION NUMBER 6 NWS NATIONAL HURRICANE CENTER MIAMI FL AL092015 500 PM AST THU SEP 17 2015 The depression remains poorly organized, with the center still exposed to the west of the deep convection. A few bursts of convection have been trying to redevelop closer to the center, but the 20 to 25 kt of shear is keeping the convection more than a degree from the center. The initial intensity has been lowered to 25 kt based on ASCAT data from earlier today. The depression should continue to slowly spin down due to shear and dry air in the mid and upper levels. The cyclone is forecast to become a remnant low in 24 to 36 hours, although timing when deep convection will dissipate is highly uncertain. The low-level circulation is forecast to dissipate in 4 to 5 days, in best agreement with the latest GFS forecast. The motion of the center has been slow and erratic today, with an has initial motion estimate of 005/03 reflecting a jog to the right during the past few hours. All of the guidance, even the shallow BAM, is insistent on a turn back toward the northwest in the next 12 to 24 hours, and that is reflected in the NHC track forecast. Through 24 hours the NHC track is on the right side of the guidance to account for the initial motion. After that time, a gradual bend toward the west-northwest is forecast as the shallow system becomes steered by the low-level flow. Later in the period the NHC track is close to the previous one adjusted for the initial position and motion and is near the shallow BAMS and the GFS. FORECAST POSITIONS AND MAX WINDS INIT 17/2100Z 16.5N 44.8W 25 KT 30 MPH 12H 18/0600Z 17.2N 45.5W 25 KT 30 MPH 24H 18/1800Z 18.2N 46.6W 25 KT 30 MPH 36H 19/0600Z 19.0N 47.9W 25 KT 30 MPH...POST-TROP/REMNT LOW 48H 19/1800Z 19.7N 49.1W 20 KT 25 MPH...POST-TROP/REMNT LOW 72H 20/1800Z 20.7N 51.6W 20 KT 25 MPH...POST-TROP/REMNT LOW 96H 21/1800Z 22.0N 54.0W 20 KT 25 MPH...POST-TROP/REMNT LOW 120H 22/1800Z...DISSIPATED $$ Forecaster Brennan

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Tropical Depression NINE Forecast Discussion Number 5

2015-09-17 16:34:20| National Hurricane Center (Atlantic)

Issued at 1100 AM AST THU SEP 17 2015 000 WTNT44 KNHC 171434 TCDAT4 TROPICAL DEPRESSION NINE DISCUSSION NUMBER 5 NWS NATIONAL HURRICANE CENTER MIAMI FL AL092015 1100 AM AST THU SEP 17 2015 The center of the depression can be seen in visible imagery exposed to the west of what remains of the deep convection, due to about 25 kt of shear over the cyclone as indicated by UW-CIMSS satellite analyses. The initial intensity remains 30 kt based on the latest Dvorak estimates from TAFB and SAB. The environmental conditions become even less favorable with time, with the shear forecast to continue while the cyclone moves into an increasingly dry environment. These factors should result in slow weakening, and the NHC forecast shows the depression becoming a remnant low by 36 hours, with dissipation expected in 4 to 5 days. The initial position of the depression is a little south of that from the previous advisory, as the partially decoupled low-level circulation is moving northwestward at about 5 kt. The track forecast philosophy has not changed, as the weakening cyclone should move generally northwestward for the next 24 to 36 hours and then turn west-northwestward as a shallower system. The new NHC track is in best agreement with the GFS and shallow BAM and lies south of the multi-model consensus. This forecast is similar to the previous one updated for the initial position and motion. There are a couple of alternate scenarios, however. The depression or its remnants could merge with the disturbance to its east as seen in the UKMET solution or survive as its own entity a little longer as indicated by the latest ECMWF forecast. However, the NHC track forecast continues to favor a solution with a weakening cyclone that remains separate from the disturbance to the east and dissipates by 5 days. FORECAST POSITIONS AND MAX WINDS INIT 17/1500Z 16.1N 45.2W 30 KT 35 MPH 12H 18/0000Z 16.8N 45.8W 30 KT 35 MPH 24H 18/1200Z 17.8N 46.8W 30 KT 35 MPH 36H 19/0000Z 18.8N 47.8W 25 KT 30 MPH...POST-TROP/REMNT LOW 48H 19/1200Z 19.5N 49.0W 25 KT 30 MPH...POST-TROP/REMNT LOW 72H 20/1200Z 20.5N 51.5W 20 KT 25 MPH...POST-TROP/REMNT LOW 96H 21/1200Z 22.0N 54.5W 20 KT 25 MPH...POST-TROP/REMNT LOW 120H 22/1200Z...DISSIPATED $$ Forecaster Brennan

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