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Tropical Storm Sebastien Forecast Discussion Number 15

2019-11-23 03:37:37| National Hurricane Center (Atlantic)

Issued at 1100 PM AST Fri Nov 22 2019 000 WTNT45 KNHC 230237 TCDAT5 Tropical Storm Sebastien Discussion Number 15 NWS National Hurricane Center Miami FL AL202019 1100 PM AST Fri Nov 22 2019 There is not much that I can add that my predecessors have not already addressed except to say that Sebastien does not know that is November near Thanksgiving. One interesting aspect tonight is that although the cloud pattern has not change at all today with the low-level center on the southern edge of the convection due to shear, and with the same Dvorak numbers, ASCAT data that just arrived showed numerous vectors of 50 kt and a few near 55 kt. On this basis, the initial intensity has been adjusted upward to 55 kt in this advisory. Sebastien has to weaken since is heading toward a hostile environment of increasing shear and cooler waters. It might not do it tonight, but the weakening process must begin within the next 24 hours or so. Both the GFS and the ECMWF open up the circulation beyond 48 hours, and with the hostile environment ahead, the NHC forecast calls for gradual weakening and dissipation in 3 days. The cyclone is moving toward the northeast of 055 degrees at 13 kt. Sebastien is embedded within the mid-latitude westerly flow, and the cyclone has nowhere to go but to continue moving northeastward with increasing forward speed. Models vary in speed, but on average they all suggest acceleration. FORECAST POSITIONS AND MAX WINDS INIT 23/0300Z 27.0N 52.2W 55 KT 65 MPH 12H 23/1200Z 28.2N 50.2W 55 KT 65 MPH 24H 24/0000Z 30.5N 46.5W 45 KT 50 MPH 36H 24/1200Z 32.6N 42.5W 40 KT 45 MPH 48H 25/0000Z 35.5N 36.0W 35 KT 40 MPH 72H 26/0000Z...DISSIPATED $$ Forecaster Avila

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Tropical Storm Sebastien Forecast Discussion Number 14

2019-11-22 21:32:58| National Hurricane Center (Atlantic)

Issued at 500 PM AST Fri Nov 22 2019 000 WTNT45 KNHC 222032 TCDAT5 Tropical Storm Sebastien Discussion Number 14 NWS National Hurricane Center Miami FL AL202019 500 PM AST Fri Nov 22 2019 Sebastien's structure has not changed since this morning. ASCAT data that arrived just after the issuance of the previous advisory showed that the system was producing winds slightly above 45 kt, a little higher than the previous estimate. Since the structure of the tropical storm has not changed since the time of the ASCAT, the intensity has been set at 50 kt. It should be noted that this is merely an adjustment of the intensity assessment and is not an indication of strengthening. The agreement between the intensity models is remarkably poor. In just the first 24 h of the forecast, the solutions range from dissipation (HMON), to hurricane-strength (HWRF), to steady-state (most of the global models). Since the shear is high and SSTs ahead of the cyclone will be fairly cold, the solution that makes the most sense to me is that of the global and statistical-dynamical models, which generally show slow weakening during the next couple of days. Dissipation is still expected within 72 h. The Jekyll and Hyde behavior of the models isn't limited to the intensity forecast. While it seems clear that Sebastien will move east-northeastward or northeastward along the southern end of a frontal boundary during the next couple of days, the speed at which it will move is very unclear. The deeper the cyclone remains, the more influence strong upper-level southwesterly winds will have on its track, and the faster it will likely move. The latest track guidance unanimously shows a dramatically faster forward motion of Sebastien. In fact, the consensus now shows a forward speed nearly twice as fast as the previous NHC advisory. The official forecast continues to chase the models but has not been moved nearly as far as HCCA or TVCN. Confidence in the track forecast is quite low. FORECAST POSITIONS AND MAX WINDS INIT 22/2100Z 26.2N 53.7W 50 KT 60 MPH 12H 23/0600Z 27.3N 51.8W 50 KT 60 MPH 24H 23/1800Z 28.9N 49.2W 45 KT 50 MPH 36H 24/0600Z 31.0N 45.8W 45 KT 50 MPH 48H 24/1800Z 33.1N 41.5W 40 KT 45 MPH 72H 25/1800Z...DISSIPATED $$ Forecaster Zelinsky

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Tropical Storm Sebastien Forecast Discussion Number 13

2019-11-22 15:49:52| National Hurricane Center (Atlantic)

Issued at 1100 AM AST Fri Nov 22 2019 000 WTNT45 KNHC 221449 TCDAT5 Tropical Storm Sebastien Discussion Number 13 NWS National Hurricane Center Miami FL AL202019 1100 AM AST Fri Nov 22 2019 Sebastien is strongly sheared with deep convection limited to the northeast quadrant of the cyclone. The intensity has been set at 45 kt, based primarily on the latest TAFB Dvorak classification and recent ADT fixes. A partial ASCAT overpass at 1239 UTC may not have captured the strongest winds, but showed a peak value of only 40 kt. Recent visible imagery and the ASCAT data also indicate that Sebastien's circulation is not quite as well defined as it was yesterday, perhaps due to its close proximity to a nearby frontal boundary. A significant change was made to the intensity forecast earlier this morning, and the latest forecast is in line with that new thinking. Strong shear is expected to prevent Sebastien from getting better organized, so gradual weakening is anticipated. The HWRF is once again a notable outlier, as the 06Z run stubbornly forecasts Sebastien to become a hurricane. While not impossible, that scenario appears unlikely and has been discounted. Aside from the HWRF, the dynamical guidance otherwise dissipates Sebastien within about 3 days, and this is reflected in the official forecast. Given Sebastien's shallow and disorganized structure, it is certainly possible that its wind field may become poorly-defined and the system could dissipate sooner than currently forecast. The tropical storm continues to move at a slower pace and to the right of previous forecasts. The initial motion estimate is 075/13 kt. The latest NHC track forecast is slower and to the right of the previous advisory, closer to the global model consensus. The cyclone is expected to continue generally northeastward or east-northeastward near the southern end of an eastward-moving frontal boundary through the weekend or as long as it remains a tropical cyclone. FORECAST POSITIONS AND MAX WINDS INIT 22/1500Z 25.2N 55.3W 45 KT 50 MPH 12H 23/0000Z 25.8N 53.5W 45 KT 50 MPH 24H 23/1200Z 27.0N 51.3W 45 KT 50 MPH 36H 24/0000Z 28.2N 49.0W 40 KT 45 MPH 48H 24/1200Z 29.3N 46.3W 35 KT 40 MPH 72H 25/1200Z...DISSIPATED $$ Forecaster Zelinsky

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Tropical Storm Sebastien Forecast Discussion Number 12

2019-11-22 09:34:20| National Hurricane Center (Atlantic)

Issued at 500 AM AST Fri Nov 22 2019 000 WTNT45 KNHC 220834 TCDAT5 Tropical Storm Sebastien Discussion Number 12 NWS National Hurricane Center Miami FL AL202019 500 AM AST Fri Nov 22 2019 Sebastien has become less organized this morning. Strong southwesterly vertical wind shear of about 40 kt and dry air have caused the low-level center of the storm to become exposed to the southwest of the main area of deep convection. The initial intensity remains 50 kt, based on the earlier ASCAT data, but this is a little above the latest Dvorak estimates and could be generous. The center of Sebastien has turned more to the right than expected, with the initial motion estimated to be 065/13 kt. The models have changed significantly this cycle, and now show a much slower forward motion to the northeast during the next few days. This change appears to be connected to a more vertically shallow system that moves in the lower-level flow. The NHC track forecast has been adjusted considerably slower and a little to the right of the previous one, but it still is faster than nearly all of the typically reliable models. Future adjustments will likely be made if the model trend continues. It no longer appears that Sebastien will strengthen. In fact, all of the intensity guidance now suggests that the storm will gradually weaken during the next few days as Sebastien remains in strong wind shear conditions and moves over progressively cooler waters. In addition, the model guidance has finally come into agreement that Sebastien will not merge with the nearby cold front. Instead, the cyclone is expected to move ahead of the weakening front and become a non-convective post-tropical cyclone in about 2 days, or less, when it moves over SSTs below 22 C. As mentioned in the previous discussion, NHC and the models have had a challenging time figuring out when Sebastien would lose its tropical characteristics, and we have held the transition timing steady at 48 hours for a while. Although there is still uncertainty, the models are in better agreement in the timing of this transition. This intensity forecast is a little lower than the previous one to come into better agreement with the latest models. FORECAST POSITIONS AND MAX WINDS INIT 22/0900Z 24.8N 57.0W 50 KT 60 MPH 12H 22/1800Z 26.0N 54.3W 50 KT 60 MPH 24H 23/0600Z 27.2N 51.7W 45 KT 50 MPH 36H 23/1800Z 28.7N 49.1W 40 KT 45 MPH 48H 24/0600Z 30.0N 46.2W 35 KT 40 MPH...POST-TROPICAL 72H 25/0600Z 33.3N 38.4W 25 KT 30 MPH...POST-TROP/REMNT LOW 96H 26/0600Z...DISSIPATED $$ Forecaster Cangialosi

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Tropical Storm Sebastien Forecast Discussion Number 11

2019-11-22 03:47:15| National Hurricane Center (Atlantic)

Issued at 1100 PM AST Thu Nov 21 2019 000 WTNT45 KNHC 220247 TCDAT5 Tropical Storm Sebastien Discussion Number 11 NWS National Hurricane Center Miami FL AL202019 1100 PM AST Thu Nov 21 2019 Sebastien continues to produce a central dense overcast and a large band on the southeastern side of the circulation. Recent scatterometer data indicate that the maximum winds are at least 45 kt and, since there has been little overall change in the satellite presentation within the past several hours, the initial wind speed will stay 50 kt. Two notable changes have been made on this forecast. As foreshadowed in the previous forecast, the models are no longer rapidly accelerating the storm to the northeast ahead of a mid-latitude trough. Instead, there is better agreement tonight on a steadier motion to the northeast due to the trough in the short term. Sebastien will then continue moving northeastward embedded in the mid-latitude southwesterly flow over the weekend. The new forecast is substantially slower than the last one, but is still on the faster side of the guidance due to continuity concerns. The second significant change is that Sebastien is no longer expected to become a hurricane. The cyclone will soon be moving over sub-26C waters with strong shear, and it seems unlikely to intensify much in these conditions. Model intensity guidance continues to decrease, and the NHC forecast follows that trend. Weakening should start by the weekend over even cooler waters in a high-shear environment, and these factors are forecast to cause Sebastien to transition into a non-convective low or extratropical cyclone in 36-48 h. Careful watchers of the forecast will note that NHC has predicted extratropical transition at 48 h since Sebastien was born over 48 h ago, and this timing has been pushed back primarily due to the slower-than-expected motion. Thus, this timing can't be considered a particularly confident prediction until the track forecast becomes more consistent. FORECAST POSITIONS AND MAX WINDS INIT 22/0300Z 24.7N 57.8W 50 KT 60 MPH 12H 22/1200Z 26.0N 55.5W 55 KT 65 MPH 24H 23/0000Z 28.0N 51.7W 55 KT 65 MPH 36H 23/1200Z 30.3N 47.7W 45 KT 50 MPH 48H 24/0000Z 32.7N 43.4W 40 KT 45 MPH...POST-TROP/EXTRATROP 72H 25/0000Z 37.5N 34.0W 35 KT 40 MPH...POST-TROP/EXTRATROP 96H 26/0000Z...DISSIPATED $$ Forecaster Blake

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