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Tropical Storm Sebastien Forecast Discussion Number 5
2019-11-20 15:47:17| National Hurricane Center (Atlantic)
Issued at 1100 AM AST Wed Nov 20 2019 000 WTNT45 KNHC 201447 TCDAT5 Tropical Storm Sebastien Discussion Number 5 NWS National Hurricane Center Miami FL AL202019 1100 AM AST Wed Nov 20 2019 Sebastien continues to struggle with dry air and northwesterly shear, with satellite imagery showing an exposed low-level center to the west of the deep convection. The current structure of the storm should prevent significant intensification during the next 12 hours. After that time, Sebastien will begin to interact with a mid- to upper-level trough that will produce upper-level divergence over the cyclone. This will provide a window for Sebastien to intensify before making the transition to an extratropical cyclone in about 48 hours. All available guidance intensifies the storm, and the dynamical models continue to strengthen it more than the statistical models. In fact, the mesoscale models unanimously make Sebastian a hurricane within 36 hours. However, these models are likely intensifying the cyclone too quickly over the next 12 hours, and therefore may have a slight high bias. The official forecast takes this into account and keeps the storm just below hurricane strength before extratropical transition by 48 hours, which is a blend of the ECMWF and GFS solutions. Sebastien appears to be approaching its westernmost point and a turn to the north should begin later today, followed by acceleration to the northeast starting tonight due to the influence of the approaching trough. The track guidance is in agreement on this overall scenario. The NHC forecast track is just slightly slower than the previous one in the first 24 hours, and is close to the model consensus. FORECAST POSITIONS AND MAX WINDS INIT 20/1500Z 21.1N 61.7W 45 KT 50 MPH 12H 21/0000Z 22.0N 61.7W 45 KT 50 MPH 24H 21/1200Z 23.7N 60.2W 55 KT 65 MPH 36H 22/0000Z 26.3N 57.2W 60 KT 70 MPH 48H 22/1200Z 29.8N 52.5W 60 KT 70 MPH...POST-TROP/EXTRATROP 72H 23/1200Z...DISSIPATED $$ Forecaster Latto/Mello
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Tropical Storm Sebastien Forecast Discussion Number 4
2019-11-20 09:37:13| National Hurricane Center (Atlantic)
Issued at 500 AM AST Wed Nov 20 2019 000 WTNT45 KNHC 200837 TCDAT5 Tropical Storm Sebastien Discussion Number 4 NWS National Hurricane Center Miami FL AL202019 500 AM AST Wed Nov 20 2019 Deep convection developed closer to the center of Sebastien around the time of the previous advisory, but northwesterly shear and dry air have caused it to wane within the past hour or two. A late arriving ASCAT-C overpass revealed peak winds a little above 40 kt. Given the typical undersampling of that instrument, the initial intensity has been raised to 45 kt. The shear and dry air are expected to keep Sebastien's intensity in check today, but by tonight the upper-level divergence is forecast to increase while the storm motion and shear vector line up. This is expected to allow for some strengthening and nearly all of the intensity guidance and global models predict deepening. The new intensity forecast is again similar to the previous advisory and lies between the lower statistical guidance and the more aggressive dynamical models. Sebastien appears to have turned northwestward with an initial motion of 305/7 kt. A turn to the north should occur later today, and then a northeastward motion is predicted to begin by Wednesday night as the storm is picked up by a deep layer trough moving eastward across the western Atlantic. The track guidance is in agreement on this overall scenario but there are some significant differences in how quickly Sebastien accelerates northeastward. The GFS, HWRF, and HMON show the storm recurving quickly ahead of the trough while the ECMWF is quite a bit slower. The NHC track is a little slower than the previous forecast, but remains close to the various consensus aids and the latest UKMET ensemble mean. FORECAST POSITIONS AND MAX WINDS INIT 20/0900Z 21.0N 61.0W 45 KT 50 MPH 12H 20/1800Z 21.7N 61.5W 45 KT 50 MPH 24H 21/0600Z 23.2N 60.8W 50 KT 60 MPH 36H 21/1800Z 25.3N 58.6W 55 KT 65 MPH 48H 22/0600Z 28.7N 54.4W 55 KT 65 MPH...POST-TROP/EXTRATROP 72H 23/0600Z...DISSIPATED $$ Forecaster Brown
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Tropical Storm Sebastien Forecast Discussion Number 3
2019-11-20 03:37:12| National Hurricane Center (Atlantic)
Issued at 1100 PM AST Tue Nov 19 2019 000 WTNT45 KNHC 200237 TCDAT5 Tropical Storm Sebastien Discussion Number 3 NWS National Hurricane Center Miami FL AL202019 1100 PM AST Tue Nov 19 2019 There hasn't much of a change with Sebastien for most of tonight, with the center still exposed to the northwest of a large area of deep convection. Recently, the low-level center appears to be slowing down and gaining some latitude as convection re-fires near the center. The initial wind speed will stay 40 kt, pending receipt of ASCAT data later this evening. The initial motion has been more to the west during the past few hours, but a longer-term motion is 290/8. Sebastien should move generally northwestward on Wednesday, northward on Wednesday night and much faster to the northeast on Thursday as the storm moves along the southwestern periphery of the subtropical ridge and then ahead of an approaching mid-latitude trough. The forecast has been adjusted westward in the short-term due mostly to the initial motion, and then eastward in the longer term due to models suggesting the storm may stay a little more ahead of the cold front than before. These forecast changes, however, are fairly typical for a disorganized system. Sebastien has some chance to intensify by late tomorrow as the shear vector and the storm motion vector line up around the same time as the upper-level divergence is forecast to increase. This should cause the cyclone to strengthen, and the new forecast is closely aligned with the previous one. Around 48 hours, Sebastien is expected to become an extratropical cyclone as it is overtaken by a cold front, and then become absorbed by the larger front on Friday. The latest NHC wind speed forecast is close to the model consensus, but below the regional hurricane models. FORECAST POSITIONS AND MAX WINDS INIT 20/0300Z 20.7N 60.6W 40 KT 45 MPH 12H 20/1200Z 21.5N 61.5W 40 KT 45 MPH 24H 21/0000Z 22.9N 61.5W 45 KT 50 MPH 36H 21/1200Z 24.4N 59.8W 50 KT 60 MPH 48H 22/0000Z 27.2N 56.5W 55 KT 65 MPH...POST-TROP/EXTRATROP 72H 23/0000Z...DISSIPATED $$ Forecaster Blake
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Tropical Storm Sebastien Forecast Discussion Number 2
2019-11-19 21:36:03| National Hurricane Center (Atlantic)
Issued at 500 PM AST Tue Nov 19 2019 000 WTNT45 KNHC 192035 TCDAT5 Tropical Storm Sebastien Discussion Number 2 NWS National Hurricane Center Miami FL AL202019 500 PM AST Tue Nov 19 2019 The low-level center of Sebastien has been exposed over the past several hours, as northwesterly shear of 20-30 kt has been pushing any deep convection off to the southeastern portion of the cyclone. The initial advisory intensity is being held at 40 kt and is based on an earlier scatterometer pass that sampled maximum winds of 39 kt. The initial motion is 305/10 kt. Sebastien is forecast to be steered to the northwest around a deep-layer ridge over the central Atlantic through tonight. By Wednesday, the cyclone should turn to the north, and then accelerate northeastward Wednesday night as the cyclone gets caught up in the flow between the retreating ridge and ahead of an approaching cold front. The model guidance remains in good agreement on this scenario, and the official forecast track keeps the cyclone over open waters for the duration of its existence. The shear is not expected to decrease much over the next couple days while the system remains a tropical cyclone. There may be an opportunity for some strengthening Wednesday and Wednesday night as Sebastien turns to the north and northeast and resides under some favorable upper-level diffluence. By 48 hours, Sebastien is expected to transition to an extratropical cyclone just ahead of an approaching cold front. Therefore, additional strengthening indicated beyond 36 hours should be due to baroclinic processes. The cyclone should then become absorbed by the cold front by late this week. The official forecast was adjusted slightly higher from the previous one at 36 and 48 hours to reflect the anticipated intensification of the system as it begins its extratropical transition. FORECAST POSITIONS AND MAX WINDS INIT 19/2100Z 20.6N 59.7W 40 KT 45 MPH 12H 20/0600Z 21.3N 60.7W 40 KT 45 MPH 24H 20/1800Z 22.6N 61.9W 45 KT 50 MPH 36H 21/0600Z 24.2N 61.0W 50 KT 60 MPH 48H 21/1800Z 26.0N 58.9W 55 KT 65 MPH...POST-TROP/EXTRATROP 72H 22/1800Z...DISSIPATED $$ Forecaster Latto
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Tropical Storm Sebastien Forecast Discussion Number 1
2019-11-19 16:10:45| National Hurricane Center (Atlantic)
Issued at 1100 AM AST Tue Nov 19 2019 000 WTNT45 KNHC 191501 TCDAT5 Tropical Storm Sebastien Discussion Number 1 NWS National Hurricane Center Miami FL AL202019 1100 AM AST Tue Nov 19 2019 Satellite imagery indicates that the overall cloud pattern of the disturbance has improved since yesterday, and that the low has become well-defined. In addition, a late-arriving ASCAT-B scatterometer pass showed 35-38 kt winds extending 90 n mi from the center in the northeastern quadrant. This pass also revealed that the low was nearly closed at the surface, and since the scatterometer may not have resolved the small scale of the low-level center it is likely that the surface low is indeed closed. Based on these data, advisories are being initiated on Tropical Storm Sebastian with an initial advisory intensity of 40 kt. Sebastien will not be in an ideal environment for significant intensification, as it will be battling dry air to its west and about 20 kt of northwesterly shear for the next couple of days. However, due to the presence of an upper trough to the west of the system, a diffluent environment aloft may aid in some slight strengthening over the next day or so. After that time, the storm will begin to interact with an approaching cold front, and some additional intensification may occur due to baroclinic processes. The cyclone is then expected to become absorbed by the front in about 48 hours. The various intensity guidance solutions are in decent agreement, and the official forecast is near the mean of these forecasts. There are some timing variations among the models on when the cyclone will become absorbed by the front, and it is possible that the storm could be absorbed sooner than indicated. The initial motion is 330/07 kt. Sebastien will be steered to the northwest in the near term around a deep-layer ridge over the central Atlantic. By Wednesday, the cyclone should turn north, and then should accelerate northeastward by Wednesday night as the cyclone gets caught up in the flow between the retreating ridge and ahead of the approaching cold front. The model guidance is in good agreement on this scenario, and on the official forecast track the cyclone will remain over open waters for the duration of its existence. FORECAST POSITIONS AND MAX WINDS INIT 19/1500Z 20.1N 58.7W 40 KT 45 MPH 12H 20/0000Z 21.0N 59.2W 40 KT 45 MPH 24H 20/1200Z 22.2N 60.0W 45 KT 50 MPH 36H 21/0000Z 23.5N 60.0W 45 KT 50 MPH 48H 21/1200Z 25.5N 58.3W 45 KT 50 MPH...POST-TROP/EXTRATROP 72H 22/1200Z...DISSIPATED $$ Forecaster Latto
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