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Tropical Storm Sebastien Forecast Discussion Number 10

2019-11-21 21:32:13| National Hurricane Center (Atlantic)

Issued at 500 PM AST Thu Nov 21 2019 000 WTNT45 KNHC 212032 TCDAT5 Tropical Storm Sebastien Discussion Number 10 NWS National Hurricane Center Miami FL AL202019 500 PM AST Thu Nov 21 2019 Convection associated with Sebastien has become better organized during the past several hours, as a band has formed near and over the center in the eastern semicircle. Overall, the cloud pattern remains elongated from north-to-south as the storm is interacting with a cold front not far to its northwest. Scatterometer data received just after the last advisory indicated 45-50 kt winds, and based on this and current satellite intensity estimates the initial intensity remains 50 kt. The cyclone has turned northeastward with the initial motion now 050/8. Southwesterly mid-latitude flow should steer Sebastien or its remnants quickly northeastward, and the guidance is in good agreement on the direction of movement during the next several days. However, there is a significant spread in the forward speed, with the new ECMWF run being much slower than the other guidance. The new track forecast will be only slightly slower than the previous forecast, but adjustments to the forecast forward speed may be necessary later. Sebastien is in an apparently unfavorable environment for development due to strong shear, decreasing sea surface temperatures along the forecast track, and an expected extratropical transition as it merges with the front. The intensity guidance is rather divergent during the first 24 h, with the SHIPS and LGEM models showing weakening while the dynamical guidance continues to forecast strengthening despite the environment. After that time, all guidance indicates that Sebastien should weaken. The intensity forecast continues the trend of the previous forecast in showing some strengthening during the first 24 h in agreement with the dynamical models, followed by weakening and extratropical transition by 48 h. Given how close the cold front is to the storm at this time, it is possible that extratropical transition could occur earlier than currently forecast. FORECAST POSITIONS AND MAX WINDS INIT 21/2100Z 23.8N 59.7W 50 KT 60 MPH 12H 22/0600Z 25.5N 57.1W 60 KT 70 MPH 24H 22/1800Z 27.9N 53.2W 65 KT 75 MPH 36H 23/0600Z 30.6N 48.4W 55 KT 65 MPH 48H 23/1800Z 33.5N 43.0W 45 KT 50 MPH...POST-TROP/EXTRATROP 72H 24/1800Z 39.0N 31.0W 35 KT 40 MPH...POST-TROP/EXTRATROP 96H 25/1800Z...DISSIPATED $$ Forecaster Beven

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Tropical Storm Sebastien Forecast Discussion Number 9

2019-11-21 15:49:08| National Hurricane Center (Atlantic)

Issued at 1100 AM AST Thu Nov 21 2019 000 WTNT45 KNHC 211449 TCDAT5 Tropical Storm Sebastien Discussion Number 9 NWS National Hurricane Center Miami FL AL202019 1100 AM AST Thu Nov 21 2019 Sebastien continues to produce a large area of deep convection over the eastern semicircle of the circulation, but the banding features are not very well defined. The intensity estimate remains at 50 kt based on a blend of subjective Dvorak estimates from TAFB and SAB. A diffluent upper-level wind environment and warm waters are expected to bring about some strengthening through tonight, however the window of opportunity for strengthening will probably close tomorrow morning. After that time, drier air and strong shear should cause gradual weakening. The official intensity forecast is a little above the model consensus. In a couple of days, the GFS shows Sebastien becoming embedded in a frontal zone, signifying the transition to an extratropical cyclone as shown in the NHC forecast. The ECMWF model suggests that this event could occur later, but the current close proximity of a front to Sebastian argues for extratropical transition to occur within 48 hours. The circulation is a bit elongated from south to north, and the center remains difficult to locate. This results in a greater than normal uncertainty in the initial motion, and my best guess is 030/7 kt. A turn toward the northeast, with acceleration, is expected during the next couple of days due to the influence of a mid-level trough. There is considerable along-track model spread, with the ECMWF forecast being much slower than the majority of the guidance. The official forecast is somewhat slower than the dynamical model consensus out of respect for that typically reliable model. FORECAST POSITIONS AND MAX WINDS INIT 21/1500Z 23.5N 60.5W 50 KT 60 MPH 12H 22/0000Z 25.0N 58.3W 65 KT 75 MPH 24H 22/1200Z 27.2N 55.0W 70 KT 80 MPH 36H 23/0000Z 30.0N 50.4W 65 KT 75 MPH 48H 23/1200Z 33.2N 45.2W 55 KT 65 MPH...POST-TROP/EXTRATROP 72H 24/1200Z 39.7N 31.5W 35 KT 40 MPH...POST-TROP/EXTRATROP 96H 25/1200Z...DISSIPATED $$ Forecaster Pasch

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Tropical Storm Sebastien Forecast Discussion Number 8

2019-11-21 09:32:05| National Hurricane Center (Atlantic)

Issued at 500 AM AST Thu Nov 21 2019 000 WTNT45 KNHC 210831 TCDAT5 Tropical Storm Sebastien Discussion Number 8 NWS National Hurricane Center Miami FL AL202019 500 AM AST Thu Nov 21 2019 Sebastien continues to produce a large area of showers and thunderstorms over its southeastern quadrant with more disorganized convection to the north and west of the center. Despite the fair amount of convection, the cloud pattern lacks banding features and the center of the storm has been challenging to locate. The initial intensity is held at 50 kt based on ASCAT data from several hours ago. This data also indicate that Sebastien's wind field is quite lopsided, with all of its tropical-storm-force winds confined to the eastern side of the circulation. The initial motion of the storm is tough to assess given that there is more than the usual amount of uncertainty in the current position, but my best guess is 015/7 kt. A deep-layer trough over the western Atlantic is approaching Sebastien, and that feature should cause the storm to accelerate to the northeast during the next few days. Although the models agree on the overall scenario, there are major along-track or speed differences among the models, with the GFS/HWRF/HMON being the fastest solutions and the ECMWF being the slowest. In fact, at 48 hours, the spread between the GFS and ECMWF models is more than 700 n mi. The NHC forecast lies roughly near the middle of the guidance envelope and ends up near the UKMET model, but this prediction is of low confidence. Upper-level diffluence, relatively warm SSTs, and a moist environment should allow for Sebastien to strengthen during the next 24 hours or so. In fact, all of the hurricane regional models show significant or even rapid intensification during that time period. Given the asymmetric structure of Sebastien currently, rapid intensification seems unlikely, and the NHC intensity prediction lies closer to the lower end of the guidance in the short term. Beyond 24 hours, Sebastien will be moving into progressively more hostile conditions of increasing shear, drier air, and cooler waters. These conditions should end the opportunity for strengthening and begin the process of extratropical transition. Sebastien is expected to become fully extratropical in about 48 hours, but this could occur sooner if the GFS is correct or later if the ECMWF is correct. The extratropical low is forecast to slowly weaken and dissipate in about 4 days. FORECAST POSITIONS AND MAX WINDS INIT 21/0900Z 23.0N 61.2W 50 KT 60 MPH 12H 21/1800Z 24.5N 59.3W 60 KT 70 MPH 24H 22/0600Z 26.6N 56.4W 70 KT 80 MPH 36H 22/1800Z 29.4N 52.0W 70 KT 80 MPH 48H 23/0600Z 32.4N 47.1W 60 KT 70 MPH...POST-TROP/EXTRATROP 72H 24/0600Z 38.6N 34.0W 45 KT 50 MPH...POST-TROP/EXTRATROP 96H 25/0600Z...DISSIPATED $$ Forecaster Cangialosi

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Tropical Storm Sebastien Forecast Discussion Number 7

2019-11-21 03:54:03| National Hurricane Center (Atlantic)

Issued at 1100 PM AST Wed Nov 20 2019 378 WTNT45 KNHC 210253 TCDAT5 Tropical Storm Sebastien Discussion Number 7 NWS National Hurricane Center Miami FL AL202019 1100 PM AST Wed Nov 20 2019 Deep convection has intensified near and to the southeast of the center of Sebastien this evening, and a recent partial ASCAT pass suggests the center is still on the edge of the thunderstorms. Satellite intensity estimates have risen slightly, so the initial wind speed is set to 50 kt, on the high side of the recent values. The storm continues to move slowly to the north tonight. Increasing deep-layer flow ahead of a mid-latitude trough should turn Sebastien northeastward overnight and cause the cyclone to further accelerate by late Thursday and Friday. The models aren't in great agreement on how quickly the storm will move to the northeast, however, with the model guidance having a stronger storm generally moving faster to the northeast. Since Sebastien isn't expected to get that intense, it makes physical sense to avoid the fastest solutions. Thus, the new forecast is slower than the last one, closer to the model consensus than the GFS-based guidance. The environment is forecast to become more conducive for strengthening during the next 24 hours as significant upper-level divergence increases near Sebastien. Combined with low or moderate shear and warm waters, these conditions should support further intensification, and the new NHC forecast now shows Sebastien as a hurricane for a short period of time. Notably, this forecast is still on the conservative side of the guidance, with all of the regional hurricane models showing Sebastien becoming a fairly potent hurricane in a day or two. This doesn't seem likely after examining the model initial structure of the HWRF/HMON models, which show a much more vertically aligned cyclone than Sebastien appears to be now, so the NHC forecast is only a bit higher than the last one. In a couple of days, the storm will be moving over cooler waters, with increasing shear and baroclinicity. That should facilitate Sebastien's transition to a non-tropical cyclone by the end of the work week, but note that the extratropical transition has been shifted back about a day due to the stronger-than-forecast cyclone likely staying a little more separate from an incoming cold front. FORECAST POSITIONS AND MAX WINDS INIT 21/0300Z 22.2N 61.5W 50 KT 60 MPH 12H 21/1200Z 23.3N 60.5W 55 KT 65 MPH 24H 22/0000Z 25.5N 57.7W 65 KT 75 MPH 36H 22/1200Z 28.0N 53.9W 65 KT 75 MPH 48H 23/0000Z 31.0N 49.0W 60 KT 70 MPH...POST-TROP/EXTRATROP 72H 24/0000Z 37.0N 38.0W 45 KT 50 MPH...POST-TROP/EXTRATROP 96H 25/0000Z...DISSIPATED $$ Forecaster Blake

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Tropical Storm Sebastien Forecast Discussion Number 6

2019-11-20 21:37:55| National Hurricane Center (Atlantic)

Issued at 500 PM AST Wed Nov 20 2019 000 WTNT45 KNHC 202037 TCDAT5 Tropical Storm Sebastien Discussion Number 6 NWS National Hurricane Center Miami FL AL202019 500 PM AST Wed Nov 20 2019 Sebastien has changed little in organization today, with northwesterly shear displacing the deep convection to the southeast of an exposed low-level center. An ASCAT-C overpass late this morning sampled peak winds of 41 kt, and so the initial advisory intensity will remain 45 kt due to the assumption of undersampling by the scatterometer. Sebastien made its anticipated turn to the north earlier today, and the current motion is now 360/05 kt. The cyclone should turn to the northeast tonight and begin to accelerate as it gets embedded in increasing southwesterly flow ahead of an approaching cold front and associated mid- to upper- level trough. The official forecast track is near the previous one through 24 hours, and a little slower and to the south of it beyond that time due to a shift in the model guidance. The approaching trough should provide a diffluent environment aloft over Sebastien beginning tonight and persisting until the system makes the transition to an extratropical cyclone in a couple of days. This is expected to result in some intensification over the next day or so. Once extratropical, the cyclone should gradually become absorbed into a frontal zone through the end of the week. The official forecast is changed little from the previous one, and is near the IVCN consensus. FORECAST POSITIONS AND MAX WINDS INIT 20/2100Z 21.7N 61.5W 45 KT 50 MPH 12H 21/0600Z 22.7N 61.0W 50 KT 60 MPH 24H 21/1800Z 24.5N 59.0W 60 KT 70 MPH 36H 22/0600Z 27.2N 55.5W 60 KT 70 MPH 48H 22/1800Z 30.0N 51.4W 60 KT 70 MPH...POST-TROP/EXTRATROP 72H 23/1800Z...DISSIPATED $$ Forecaster Latto

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