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Tropical Storm Sebastien Forecast Discussion Number 20
2019-11-24 09:47:24| National Hurricane Center (Atlantic)
Issued at 500 AM AST Sun Nov 24 2019 000 WTNT45 KNHC 240847 TCDAT5 Tropical Storm Sebastien Discussion Number 20 NWS National Hurricane Center Miami FL AL202019 500 AM AST Sun Nov 24 2019 Sebastien is on the slow boat to becoming fully extratropical. Just within the past few hours, the cyclone has taken on an appearance more resembling a subtropical cyclone, with all of its deep convection located in a band to the north of the center and it being co-located with an upper-level shortwave trough. Based on Sebastien's evolving structure, TAFB provided a subtropical classification of ST3.5. Combining this estimate with the most recent ADT and SATCON estimates, the initial intensity remains 55 kt. Since Sebastien's extratropical transition has been so drawn out, it's difficult to pinpoint exactly when the process will be complete. Phase-space diagrams from the GFS and ECMWF models indicate that the cyclone will be deep cold core within the next 6-12 hours. However, these models keep the low-level vortex separate from a second upstream upper-level trough for the next 30 hours, and they also keep deep convection going (in simulated satellite imagery) near but to the north of the center for that same amount of time. Because of deep-layer shear around 40 kt and sea surface temperatures near 20C, the official forecast continues to show extratropical transition complete by 24 hours, but that forecast should be considered with a margin of error of plus or minus 12 hours. Regardless of its status, Sebastien is forecast to only gradually lose strength over the next 48 hours and is likely to bring gusty winds and heavy rains to portions of the Azores later today and tonight. Please see products issued by the Portuguese Institute for the Sea and the Atmosphere (IPMA) for more information. The northeastward acceleration continues, with Sebastian moving 050/29 kt. Additional acceleration is expected during the next 24 hours, with the new NHC track forecast a little faster than the previous iteration and generally a blend of the TVCN and HCCA consensus aids. No forecast points are provided starting at 72 hours since by that time Sebastien is expected to have merged with another weather system southwest of Ireland and the United Kingdom. This larger complex system could bring gusty winds and heavy rains to portions of western Europe within the next few days. FORECAST POSITIONS AND MAX WINDS INIT 24/0900Z 36.0N 40.0W 55 KT 65 MPH 12H 24/1800Z 38.5N 34.8W 55 KT 65 MPH 24H 25/0600Z 42.1N 26.2W 50 KT 60 MPH...POST-TROP/EXTRATROP 36H 25/1800Z 46.2N 17.5W 45 KT 50 MPH...POST-TROP/EXTRATROP 48H 26/0600Z 50.0N 11.4W 45 KT 50 MPH...POST-TROP/EXTRATROP 72H 27/0600Z...MERGED WITH ANOTHER WEATHER SYSTEM $$ Forecaster Berg
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Tropical Storm Sebastien Forecast Discussion Number 19
2019-11-24 03:55:54| National Hurricane Center (Atlantic)
Issued at 1100 PM AST Sat Nov 23 2019 000 WTNT45 KNHC 240255 TCDAT5 Tropical Storm Sebastien Discussion Number 19 NWS National Hurricane Center Miami FL AL202019 1100 PM AST Sat Nov 23 2019 Over the large scale, Sebastien appears to be undergoing extratropical transition, with a large cirrus shield on the northern side of the storm and unimpressive convection overall. Near the center, however, the storm still has a small inner core, with what appears to be a faint eye feature forming during the past few hours in conventional satellite imagery. This feature also shows up in recent microwave data, suggesting that Sebastien still has more of its tropical character than a cursory look would indicate. Thus, Sebastien is held as a tropical cyclone and, since Dvorak and SATCON estimates remain at 55 kt, the initial wind speed is held at that value. Unfortunately all 3 scatterometer satellites missed the cyclone this evening. Sebastien is quickly moving up on my list of most annoying storms during the 2019 season, and hopefully this isn't another night of it making my forecast look silly. I think (hope?) the intensity outlook is getting easier since the storm will be moving over sub-20C waters by tomorrow morning. There are no signs of significant upper-level cooling at that time, so Sebastien will probably be struggling to produce any organized deep convection. Since baroclinic forcing also appears to be on the wane, a slow weakening is anticipated, and a more complete extratropical transition is forecast by this time tomorrow. This is also indicated by cyclone phase space diagrams derived from the GFS and ECMWF models. While I might have a little more confidence in this prediction than a couple of days ago, I also wouldn't be surprised if Sebastien had another trick up its sleeve. The storm is accelerating northeastward, now estimated at 045/27 kt. The evening model guidance continues the trend noted by my predecessor of a faster motion, so the new NHC prediction is shifted in that direction, and somewhat to the north. Sebastien or its extratropical remnants are expected to bring gusty winds and rain to the Azores beginning on Sunday. Please see products issued by the Portuguese Institute for the Sea and the Atmosphere (IPMA) for more information. FORECAST POSITIONS AND MAX WINDS INIT 24/0300Z 34.4N 43.1W 55 KT 65 MPH 12H 24/1200Z 37.0N 38.7W 55 KT 65 MPH 24H 25/0000Z 40.1N 31.1W 50 KT 60 MPH...POST-TROP/EXTRATROP 36H 25/1200Z 43.9N 22.3W 50 KT 60 MPH...POST-TROP/EXTRATROP 48H 26/0000Z 48.0N 14.5W 45 KT 50 MPH...POST-TROP/EXTRATROP 72H 27/0000Z...DISSIPATED $$ Forecaster Blake
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Tropical Storm Sebastien Forecast Discussion Number 18
2019-11-23 21:38:19| National Hurricane Center (Atlantic)
Issued at 500 PM AST Sat Nov 23 2019 000 WTNT45 KNHC 232038 TCDAT5 Tropical Storm Sebastien Discussion Number 18 NWS National Hurricane Center Miami FL AL202019 500 PM AST Sat Nov 23 2019 Sebastien's cloud pattern is beginning to resemble that of an extratropical cyclone, with a shield of moderately cold cloud tops extending well to the north of the cyclone's surface center. That said, Sebastien is still producing enough convection near its center to be considered a tropical cyclone for now. ASCAT-B data that arrived shortly after the last advisory was issued had support for 55 kt and that is the basis for the initial intensity. The ASCAT data also showed that the surface wind circulation of Sebastien is elongated but still closed. Sebastien is accelerating northeastward, with an initial motion of 045/26 kt. Most of the 12Z models show a significantly faster northeastward track for the tropical storm and the official forecast has been adjusted accordingly. However, the NHC forecast is slower than the model consensus and additional large changes may be required in the next advisory. Confidence in the track forecast remains low due to the continued lack of run-to-run consistency amongst the models. With regard to intensity, a majority of the guidance calls for little change in strength since it seems that baroclinic forcing will at least offset the negative influences of decreasing SSTs and high shear during the next day or two. The NHC intensity forecast is very near the consensus. Cyclone phase-space diagrams derived from the GFS and ECMWF indicate that extratropical transition could occur by Sunday afternoon, but the NHC forecast is slightly more conservative. The reality is that Sebastien could become extratropical or open into a trough at nearly any time between this evening and early next week. Sebastien or its remnants are expected to bring gusty winds and rain to the Azores beginning on Sunday. Please see products issued by the Portuguese Institute for the Sea and the Atmosphere (IPMA) for more information. FORECAST POSITIONS AND MAX WINDS INIT 23/2100Z 32.4N 45.4W 55 KT 65 MPH 12H 24/0600Z 35.0N 41.5W 55 KT 65 MPH 24H 24/1800Z 37.8N 35.3W 55 KT 65 MPH 36H 25/0600Z 40.9N 27.6W 55 KT 65 MPH...POST-TROP/EXTRATROP 48H 25/1800Z 44.8N 19.3W 50 KT 60 MPH...POST-TROP/EXTRATROP 72H 26/1800Z...DISSIPATED $$ Forecaster Zelinsky
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Tropical Storm Sebastien Forecast Discussion Number 17
2019-11-23 15:54:09| National Hurricane Center (Atlantic)
Issued at 1100 AM AST Sat Nov 23 2019 000 WTNT45 KNHC 231454 TCDAT5 Tropical Storm Sebastien Discussion Number 17 NWS National Hurricane Center Miami FL AL202019 1100 AM AST Sat Nov 23 2019 First-light visible imagery and a few late-arriving microwave overpasses revealed that Sebastien's elongated center is located well northeast of previous estimates. It isn't clear if the center reformed or if it simply accelerated more than expected overnight, but the initial motion estimate is now 055/23 kt. A significant change to the track forecast was made based on the updated initial position, and the NHC forecast now shows a much faster motion for Sebastien. The cyclone's cloud pattern is somewhat ragged and does not look entirely tropical. That said, Sebastien is still producing enough deep convection to be classified as a tropical storm. Unfortunately, no ASCAT data was available this morning to help assess the strength of the winds or how well-defined the circulation is at the surface. The initial intensity is conservatively maintained at 55 kt based primarily on continuity from previous advisories, but this could be generous since Dvorak intensity estimates are lower. Every aspect of the forecast remains highly uncertain. The models have been extremely inconsistent in forecasting Sebastien, with the guidance varying drastically from run to run and with unusually large spread in each forecast. The latest indications are that Sebastien will move quickly northeastward with little change in strength for the next few days before it eventually opens into a trough around the time it nears the Azores in a couple of days. Simulated satellite imagery suggests that the cyclone will still produce enough deep convection to be classified as a tropical storm through Sunday, though it could become post-tropical before it reaches the Azores. Given the elongated nature of Sebastien's circulation and its fast forward speed, it could also open into a trough and dissipate at any time. It is not currently clear if Sebastien will reach the Azores as a tropical storm. Regardless of its status, Sebastien or its remnants could bring gusty winds and rain to the Azores beginning Sunday. Please see products issued by the Portuguese Institute for the Sea and the Atmosphere (IPMA) for more information. FORECAST POSITIONS AND MAX WINDS INIT 23/1500Z 30.4N 47.7W 55 KT 65 MPH 12H 24/0000Z 32.1N 44.5W 55 KT 65 MPH 24H 24/1200Z 34.4N 40.0W 55 KT 65 MPH 36H 25/0000Z 36.8N 34.0W 55 KT 65 MPH 48H 25/1200Z 39.3N 26.7W 50 KT 60 MPH...POST-TROP/EXTRATROP 72H 26/1200Z...DISSIPATED $$ Forecaster Zelinsky
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Tropical Storm Sebastien Forecast Discussion Number 16
2019-11-23 09:33:04| National Hurricane Center (Atlantic)
Issued at 500 AM AST Sat Nov 23 2019 606 WTNT45 KNHC 230833 TCDAT5 Tropical Storm Sebastien Discussion Number 16 NWS National Hurricane Center Miami FL AL202019 500 AM AST Sat Nov 23 2019 The cloud pattern of Sebastien has changed significantly overnight. Deep convection has decreased substantially during the past several hours and the remaining thunderstorms are organized around a mid-level eye feature seen in satellite images. Model vertical cross sections indicate that Sebastien is extremely titled with the low-level center, although I can't see it, likely well displaced to the southwest of the mid-level eye. The initial intensity is held at 55 kt based on the ASCAT data that came in a little after 00 Z, but it looks like Sebastien is on a weakening trend. Strong southwesterly vertically wind shear of about 40 kt is responsible for the titled structure of the cyclone. Since the shear is not expected to let up and because Sebastien is headed for steadily cooler waters, it seems reasonable to assume that weakening is likely. Although the models agree that Sebastien should slowly lose its strength, it remains unclear if Sebastien will transition to a post-tropical cyclone before it opens up into a trough during the next few days. The NHC forecast continues to show dissipation by day 3, but it is certainty possible that it dissipates before then. The tropical storm is moving east-northeastward at 15 kt ahead of a cold front and trough. This motion with an increase in forward speed is expected during the next few days as Sebastien becomes more embedded in the faster mid-latitude flow. The models are in relatively good agreement, and only small changes were made to the previous NHC forecast. The initial wind radii were modified based on the aforementioned ASCAT passes, and the 12-ft sea radii were adjusted based on ship and altimeter data. FORECAST POSITIONS AND MAX WINDS INIT 23/0900Z 27.8N 50.6W 55 KT 65 MPH 12H 23/1800Z 29.4N 48.2W 50 KT 60 MPH 24H 24/0600Z 31.6N 44.2W 45 KT 50 MPH 36H 24/1800Z 34.0N 38.8W 40 KT 45 MPH 48H 25/0600Z 37.0N 31.6W 35 KT 40 MPH 72H 26/0600Z...DISSIPATED $$ Forecaster Cangialosi
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