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Tropical Storm Leslie Forecast Discussion Number 14

2018-09-29 22:33:53| National Hurricane Center (Atlantic)

Issued at 500 PM AST Sat Sep 29 2018 106 WTNT43 KNHC 292033 TCDAT3 Tropical Storm Leslie Discussion Number 14 NWS National Hurricane Center Miami FL AL132018 500 PM AST Sat Sep 29 2018 Leslie has continued to take on more tropical characteristics over the past few hours. Anti-cyclonic outflow is now present to the northeast and southeast of Leslie and its primary convective band. Late-arriving AMSU data from 1316 UTC also indicated that Leslie has developed a deep-layer warm core structure. Based on these factors, Leslie has been designated as a tropical storm. The initial intensity is set at 45 kt, based on a blend of recent satellite intensity estimates from TAFB and the UW-CIMSS SATCON. Leslie continues to move slowly southwestward with an initial motion of 225/5 kt. The guidance is in very good agreement that this general motion will continue for another 24 h or so, before the cyclone becomes nearly stationary by early next week. An approaching mid-latitude trough is still expected to eventually cause Leslie to turn toward the north or northeast, perhaps over its own track, in 4 or 5 days. Only minor adjustments were made to the NHC track forecast, which remains close to the various track consensus aids. The NHC intensity forecast has not been changed in any significant way and remains near the intensity consensus, but slightly favors the regional hurricane models. Leslie is currently located over fairly cool SSTs, and its slow motion could cause further ocean cooling. This could particularly be an issue in a few days when the tropical storm is expected to become nearly stationary. The ocean-coupled regional models (HWRF, HMON, COAMPS-TC) continue to indicate that Leslie will struggle to intensify much over the next few days, despite a fairly favorable upper-level environment. On the other hand, some of the uncoupled global models and the SHIPS and LGEM models suggest that more intensification will occur and that Leslie could become a hurricane in a few days. Highlighting the potential importance of SSTs in this case, the SHIPS and LGEM models began using daily SST fields at 18Z, which show nearly 2 deg C cooler SSTs along the track of Leslie, and those models now show much less intensification than they did before. Large swells generated by Leslie when it was a strong extratropical low will affect Bermuda, the east coast of the United States, and most of the Greater and Lesser Antilles through the weekend. These swells could cause life-threatening surf and rip currents. FORECAST POSITIONS AND MAX WINDS INIT 29/2100Z 34.1N 51.3W 45 KT 50 MPH 12H 30/0600Z 33.6N 52.0W 45 KT 50 MPH 24H 30/1800Z 33.1N 53.0W 50 KT 60 MPH 36H 01/0600Z 32.8N 53.5W 50 KT 60 MPH 48H 01/1800Z 32.5N 53.9W 55 KT 65 MPH 72H 02/1800Z 31.5N 54.7W 55 KT 65 MPH 96H 03/1800Z 31.5N 54.5W 60 KT 70 MPH 120H 04/1800Z 34.0N 54.0W 65 KT 75 MPH $$ Forecaster Zelinsky

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Tropical Storm Sergio Forecast Discussion Number 1

2018-09-29 17:35:51| National Hurricane Center (East Pacific)

Issued at 1030 AM CDT Sat Sep 29 2018 354 WTPZ41 KNHC 291535 TCDEP1 Tropical Storm Sergio Special Discussion Number 1 NWS National Hurricane Center Miami FL EP212018 1030 AM CDT Sat Sep 29 2018 Satellite imagery indicates that convection associated with the low pressure area south-southwest of Acapulco has continued to become better organized, with the low-level center embedded under the convective overcast and near a developing complex of bands in the southeastern semicircle. Based on this and continuity from earlier scatterometer data, advisories are being initiated on Tropical Storm Sergio with 35 kt winds. Sergio is expected to be in an environment of light vertical shear and high humidity, and over warm sea surface temperatures for the next 3 days or so, which should result in steady strengthening. Near the end of the forecast period, the storm may encounter increasing northeasterly shear which could limit intensification. The intensity forecast, which is near the intensity consensus, calls for steady strengthening through 96 h, with Sergio becoming a hurricane by 48 h. The initial motion is a somewhat uncertain 285/11. Sergio should be steered generally westward for the next three days or so by the subtropical ridge over Mexico. After that time, the cyclone should approach a weakness in the ridge caused by a large mid- to upper-level trough over California, and this should lead to a turn toward the northwest. The track guidance is in good agreement with this scenario, and the forecast track lies near the HCCA corrected consensus model. FORECAST POSITIONS AND MAX WINDS INIT 29/1530Z 12.4N 102.1W 35 KT 40 MPH 12H 30/0000Z 12.7N 103.5W 35 KT 40 MPH 24H 30/1200Z 12.9N 105.3W 40 KT 45 MPH 36H 01/0000Z 12.9N 107.3W 50 KT 60 MPH 48H 01/1200Z 12.8N 109.3W 65 KT 75 MPH 72H 02/1200Z 13.0N 113.5W 85 KT 100 MPH 96H 03/1200Z 15.0N 117.0W 95 KT 110 MPH 120H 04/1200Z 16.5N 119.0W 95 KT 110 MPH $$ Forecaster Beven

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Hurricane Rosa Forecast Discussion Number 18

2018-09-29 16:49:23| National Hurricane Center (East Pacific)

Issued at 800 AM PDT Sat Sep 29 2018 521 WTPZ45 KNHC 291449 TCDEP5 Hurricane Rosa Discussion Number 18 NWS National Hurricane Center Miami FL EP202018 800 AM PDT Sat Sep 29 2018 Rosa has become significantly better organized during the last several hours, likely due to the completion of an eyewall replacement cycle before the center moved over cold water. The storm now has a 30-40 n mi wide eye, and the eyewall convective tops have become much colder and more symmetric. The initial intensity has been increased to 90 kt based on a blend of various satellite intensity estimates, and it is possible that this intensity is conservative. The hurricane has good to excellent cirrus outflow in the northeastern semicircle. The current re-intensification was poorly anticipated, and it is unclear how much more strengthening will occur before Rosa reaches the 26C isotherm in about 12 h. Even if the hurricane strengthens a little more, the combination of decreasing sea surface temperatures and increasing shear should cause steady to rapid weakening after 12 h, and the new intensity forecast still calls for the cyclone to weaken below hurricane strength before the center reaches the Baja California peninsula between 48-72 h. After landfall, Rosa is expected to weaken even faster, and the surface circulation is forecast to dissipate just after 72 h in agreement with all of the dynamical models. However, the mid-level circulation and the associated rainfall will continue moving across the southwestern United States after the surface circulation dissipates. The initial motion is now 355/10. During the forecast period, the hurricane will recurve into the westerlies between a deep-layer ridge over northern Mexico and a large mid- to upper-level trough over California and the adjacent Pacific. This should result in a continued northward motion through tonight, followed by a turn toward the north-northeast on Sunday. The new forecast track is similar to, but a little faster than, the previous track and lies near the consensus models. On the forecast track, the center of Rosa will move near or over the central and northern portions the Baja California peninsula on Monday and Monday night, and then move into the southwestern United States on Tuesday. Key Messages: 1. The main hazard expected from Rosa or its remnants is very heavy rainfall in Baja California, northwestern Mexico, and the Desert Southwest. These rains are expected to produce life-threatening flash flooding and debris flows in the deserts, and landslides in mountainous terrain. For more information about potential rainfall in that area, please see products from the Weather Prediction Center and your local NWS forecast office. 2. Rosa could also bring tropical storm conditions to portions of the central and northern Baja California peninsula starting on Monday. Interests in those locations should monitor the progress of Rosa. FORECAST POSITIONS AND MAX WINDS INIT 29/1500Z 20.5N 118.4W 90 KT 105 MPH 12H 30/0000Z 22.0N 118.6W 90 KT 105 MPH 24H 30/1200Z 23.9N 118.4W 75 KT 85 MPH 36H 01/0000Z 25.9N 117.6W 60 KT 70 MPH 48H 01/1200Z 27.7N 116.7W 45 KT 50 MPH 72H 02/1200Z 33.0N 114.0W 25 KT 30 MPH...INLAND 96H 03/1200Z...DISSIPATED $$ Forecaster Beven

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Subtropical Storm Leslie Forecast Discussion Number 13

2018-09-29 16:46:22| National Hurricane Center (Atlantic)

Issued at 1100 AM AST Sat Sep 29 2018 245 WTNT43 KNHC 291446 TCDAT3 Subtropical Storm Leslie Discussion Number 13 NWS National Hurricane Center Miami FL AL132018 1100 AM AST Sat Sep 29 2018 Leslie's cloud pattern has taken on a more tropical appearance since last night. However, the cyclone is still co-located with an upper-level low, so its status as a subtropical cyclone is maintained for this advisory. The initial intensity remains 40 kt, based on a subtropical classification of 35-40 kt from TAFB. The global models forecast that Leslie will separate from the upper-level low over the course of the next day or so, which should complete its transition to a tropical cyclone. While this could result in a short-term increase in shear over Leslie, by early next week the cyclone will likely be located within a fairly low shear environment. However, Leslie is expected to move very slowly by that time, and ocean upwelling could limit or prevent further strengthening. There is a large amount of spread in the intensity guidance by day 5, but it is worth noting that the models most capable of properly representing the effect of ocean cooling on intensity, COAMPS-TC, HWRF, and HMON, all keep Leslie below hurricane strength for the next 5 days. The HWRF in particular forecasts nearly 5 deg C of ocean cooling beneath the cyclone. The NHC intensity forecast is generally a little lower than the previous advisory, but is still near the IVCN and HCCA consensus aids at all forecast hours. If Leslie moves more than expected, it would not be surprising if it strengthened more than currently forecast. Little change was made to the official track forecast. Leslie is still expected to move slowly southwestward for the next couple of days, before coming to a near halt early next week. By the middle of the week, an approaching mid-latitude trough could steer Leslie toward the north or northeast and allow the cyclone to start gaining speed, however, there is a lot of spread among the global models as to how quickly this will occur, and confidence in this portion of the forecast is fairly low. Large swells generated by Leslie when it was a strong extratropical low will affect Bermuda, the east coast of the United States, and most of the Greater and Lesser Antilles through the weekend. These swells could cause life-threatening surf and rip currents. FORECAST POSITIONS AND MAX WINDS INIT 29/1500Z 34.4N 50.8W 40 KT 45 MPH 12H 30/0000Z 33.8N 51.7W 45 KT 50 MPH 24H 30/1200Z 33.3N 52.7W 45 KT 50 MPH 36H 01/0000Z 33.1N 53.5W 50 KT 60 MPH...TROPICAL CYCLONE 48H 01/1200Z 32.8N 54.0W 50 KT 60 MPH 72H 02/1200Z 31.6N 55.0W 55 KT 65 MPH 96H 03/1200Z 31.0N 55.5W 60 KT 70 MPH 120H 04/1200Z 33.0N 55.0W 65 KT 75 MPH $$ Forecaster Zelinsky

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Hurricane Rosa Forecast Discussion Number 17

2018-09-29 10:40:20| National Hurricane Center (East Pacific)

Issued at 200 AM PDT Sat Sep 29 2018 651 WTPZ45 KNHC 290840 TCDEP5 Hurricane Rosa Discussion Number 17 NWS National Hurricane Center Miami FL EP202018 200 AM PDT Sat Sep 29 2018 Rosa's satellite appearance has degraded significantly since the previous advisory with an eye no longer evident in infrared imagery. However, a well-defined low-level eye is evident in recent passive microwave imagery. A comparison of the two data sources indicate that the upper-level circulation is tilted to the east of the low-level eye by 12-18 nmi due to westerly shear of about 15 kt as assessed by UW-CIMSS. The initial intensity of 85 kt is based on a blend of satellite intensity estimates form TAFB, SAB, and UW-CIMSS ADT and SATCON. Rosa is moving northward now and the initial motion estimate is 350/08 kt. During the next 72 hours, the hurricane is forecast to move around the western periphery of a deep-layer ridge that extends from the Gulf of Mexico westward to Baja California, resulting in a northward motion today, and a turn toward the north-northeast at a faster forward speed on Monday and Tuesday as a mid-latitude trough approaches from the west. The latest NHC model guidance is tightly packed about the previous forecast track, so no significant changes were required. The new NHC track forecast lies close to a blend of the consensus models HCCA, FSSE, and IVCN. Rosa's intensity has decreased 40 kt during the past 24 hours, with the bulk of the weakening having occurred during the past 18 hours, and additional weakening is expected due to the cyclone moving over cooler water and into a regime of increasing southwesterly vertical wind shear of more than 30 kt by 48 hours. Although Rosa is currently located over 28 deg C SSTs, the depth of the warm water is quite shallow, so cold upwelling occurring beneath the hurricane will act to hasten the weakening process today. Rosa is forecast to approach Baja California as a tropical storm, and then quickly degenerate into a tropical and a remnant low as the cyclone moves across the mountainous terrain of northern Baja California and northwestern mainland Mexico. The official intensity forecast closely follows the intensity consensus models IVCN and HCCA. Key Messages: 1. The main hazard expected from Rosa or its remnants is very heavy rainfall in Baja California, northwestern Mexico, and the Desert Southwest. These rains are expected to produce life-threatening flash flooding and debris flows in the deserts, and landslides in mountainous terrain. For more information about potential rainfall in that area, please see products from the Weather Prediction Center and your local NWS forecast office. 2. Rosa could also bring tropical storm conditions to portions of the central and northern Baja California peninsula in a couple of days. Interests in those locations should monitor the progress of Rosa. FORECAST POSITIONS AND MAX WINDS INIT 29/0900Z 19.2N 118.2W 85 KT 100 MPH 12H 29/1800Z 20.5N 118.4W 75 KT 85 MPH 24H 30/0600Z 22.5N 118.5W 65 KT 75 MPH 36H 30/1800Z 24.4N 118.1W 55 KT 65 MPH 48H 01/0600Z 26.2N 117.3W 45 KT 50 MPH 72H 02/0600Z 30.9N 115.0W 30 KT 35 MPH...INLAND NRN BAJA CALIF 96H 03/0600Z 38.0N 111.0W 15 KT 15 MPH...POST-TROP/REMNT LOW 120H 04/0600Z...DISSIPATED INLAND $$ Forecaster Stewart

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