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Tropical Storm Leslie Forecast Discussion Number 24

2018-10-02 10:52:20| National Hurricane Center (Atlantic)

Issued at 500 AM AST Tue Oct 02 2018 326 WTNT43 KNHC 020852 TCDAT3 Tropical Storm Leslie Discussion Number 24 NWS National Hurricane Center Miami FL AL132018 500 AM AST Tue Oct 02 2018 Infrared and passive microwave satellite imagery indicate that convection has increased and become better organized around the well-defined low-level circulation center. In fact, recent microwave images showed that Leslie has developed a closed, 20- to 25-nmi wide low-level eye. Based on the presence of the distinct low-level eye feature and a Dvorak intensity estimate of T3.5/55 kt from SAB, the cyclone's intensity has been increased to 55 kt. The initial motion is southwestward or 220 degrees at a faster forward speed of 6 kt. For the next 24 h or so, Leslie is forecast to move slowly southwestward to south-southwestward, trapped between a deep-layer ridge to the west and a mid- to upper-level low to the east. After possibly becoming stationary near the 36-h period, a ridge to the southeast and east of Leslie is forecast to become highly amplified, forcing the cyclone northward into the higher latitudes through 72 h. On days 4 and 5, an approaching mid-latitude shortwave trough well to the north of Leslie is forecast to nudge the cyclone eastward, but only slowly at forward speeds of less than 10 kt. The new NHC forecast track has again been adjusted to the left of the previous advisory track in the 36- to 72-h period, and is similar to but slightly east of the various consensus models. Leslie's outflow pattern has become a little more symmetrical during the past 6 h, and further improvement is expected through 72 hours. In addition, lightning data and satellite imagery during the past few hours indicate that convection has been developing in the dry slot to the north and east of the main convective band, suggesting the mid-level environment is finally beginning to moisten. This dry intrusion has been hindering the development of deep eyewall convection and, thus, the lack of intensification of the cyclone. However, given the strong instability that is forecast to develop in the inner-core region region due to very cold air aloft moving over relatively warm waters of near 26C beneath the cyclone, along with increasing outflow and mid-level moisture, slow but steady strengthening seems reasonable for the next 48 hours or so. By 72 h and beyond, gradual weakening is expected as Leslie moves back over cooler oceanic temperatures and dry mid-level air from the mid-latitudes associated with the aforementioned shortwave trough once again gets entrained into the cyclone's circulation. The official intensity forecast is just an update of the previous advisory, and is similar to the weaker IVCN consensus model. Large swells generated by Leslie will continue to affect portions of the southeastern coast of the United States, Bermuda, and the Bahamas. These swells could cause life-threatening surf and rip currents. Although the swells are forecast to abate temporarily in the Bahamas later today, they are expected to increase again on Wednesday and Thursday, and propagate farther southward into the Greater and Lesser Antilles. Please consult products from your local weather office. FORECAST POSITIONS AND MAX WINDS INIT 02/0900Z 31.6N 55.6W 55 KT 65 MPH 12H 02/1800Z 30.6N 56.1W 60 KT 70 MPH 24H 03/0600Z 29.9N 56.5W 65 KT 75 MPH 36H 03/1800Z 30.2N 56.6W 70 KT 80 MPH 48H 04/0600Z 31.3N 56.4W 70 KT 80 MPH 72H 05/0600Z 35.0N 56.3W 65 KT 75 MPH 96H 06/0600Z 37.1N 54.8W 60 KT 70 MPH 120H 07/0600Z 37.1N 51.9W 55 KT 65 MPH $$ Forecaster Stewart

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Tropical Depression Rosa Forecast Discussion Number 29

2018-10-02 10:51:51| National Hurricane Center (East Pacific)

Issued at 300 AM MDT Tue Oct 02 2018 276 WTPZ45 KNHC 020851 TCDEP5 Tropical Depression Rosa Discussion Number 29 NWS National Hurricane Center Miami FL EP202018 300 AM MDT Tue Oct 02 2018 Satellite imagery continues to show a lack of convection near the center of Rosa this morning. The remaining convection is located well northeast of the center, and is spreading inland over northern Mexico and portions of the Desert Southwest. A couple of partial ASCAT passes revealed an area of 25-30 kt winds over the central Gulf of California, and this is the basis for reducing the initial intensity to 30 kt. The data also suggest that the circulation has become elongated and that a new center may be forming near the northeastern coast of the Baja California peninsula. The initial motion estimate is 030/9 kt. Rosa is expected to move northeastward between a deep-layer ridge over northern Mexico and a mid- to upper-level trough over the northeastern Pacific. These steering features should bring the center of Rosa across the northern Gulf of California and into northern Mexico this afternoon. Southwesterly shear and land interaction should cause the cyclone to continue to weaken today, and Rosa is expected to dissipate over northern Mexico or southwestern Arizona by tonight. Although Rosa is forecast to weaken, the system is still expected to bring heavy rainfall and life-threatening flash flooding to portions of northwestern Mexico and the Desert Southwest during the next day or so. Key Messages: 1. The main hazard expected from Rosa or its remnants is very heavy rainfall in Baja California, northwestern Sonora, and the U.S. 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 those areas, please see products from the Weather Prediction Center and your local NWS forecast office. FORECAST POSITIONS AND MAX WINDS INIT 02/0900Z 29.3N 114.9W 30 KT 35 MPH 12H 02/1800Z 31.8N 114.0W 25 KT 30 MPH...POST-TROP/INLAND 24H 03/0600Z...DISSIPATED $$ Forecaster Brown

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Hurricane Sergio Forecast Discussion Number 11

2018-10-02 04:58:17| National Hurricane Center (East Pacific)

Issued at 900 PM MDT Mon Oct 01 2018 797 WTPZ41 KNHC 020258 TCDEP1 Hurricane Sergio Discussion Number 11 NWS National Hurricane Center Miami FL EP212018 900 PM MDT Mon Oct 01 2018 A just-received GMI overpass indicates that Sergio has become better organized with formation of a well-defined eye and eyewall under a convective overcast with cloud tops to -85C. Satellite intensity estimates are in the 55-65 kt range, and based on the improved structure, the intensity will be set at the upper end of that range. Thus, Sergio is upgraded to a hurricane. The intensity forecast is low confidence. The better structure of Sergio is now conducive for rapid intensification. However, the large-scale models forecast northeasterly vertical shear to increase to 20-25 kt by 24 h, which should be strong enough to stop rapid development, although the guidance suggests at least slow strengthening during this time. The intensity forecast follows the guidance trend in calling for gradual strengthening, but there could be a 12-h or so burst of rapid intensification before the shear gets too strong. The shear should subside after about 36 h while Sergio is still over warm water, and the forecast shows the system becoming a major hurricane near the 72 h point in response to the more favorable conditions. After 72 h, a combination of decreasing sea-surface temperatures along the forecast track and entrainment of drier air should lead to gradual weakening. The initial motion is 265/12. As stated in the previous discussion, a westward motion at about the same forward speed is expected during the next day or so while the storm remains on the south side of a mid-level ridge. A turn to the west-northwest is forecast by late Tuesday, followed by a northwestward motion on Wednesday as the western portion of the mid-level ridge erodes due to a large scale mid- to upper-level trough near the southwestern United States. By the end of the forecast period, a ridge is expected to build to the north of Sergio, which should cause a turn back toward the west. The track guidance has changed little since the last advisory, and the new NHC track is close to both the previous forecast track and the consensus models. FORECAST POSITIONS AND MAX WINDS INIT 02/0300Z 10.9N 112.0W 65 KT 75 MPH 12H 02/1200Z 11.2N 113.7W 75 KT 85 MPH 24H 03/0000Z 11.7N 115.6W 80 KT 90 MPH 36H 03/1200Z 12.6N 117.2W 85 KT 100 MPH 48H 04/0000Z 13.7N 118.5W 95 KT 110 MPH 72H 05/0000Z 16.0N 121.0W 100 KT 115 MPH 96H 06/0000Z 17.0N 124.0W 95 KT 110 MPH 120H 07/0000Z 17.5N 127.0W 85 KT 100 MPH $$ Forecaster Beven

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Tropical Storm Rosa Forecast Discussion Number 28

2018-10-02 04:42:45| National Hurricane Center (East Pacific)

Issued at 800 PM PDT Mon Oct 01 2018 201 WTPZ45 KNHC 020242 TCDEP5 Tropical Storm Rosa Discussion Number 28 NWS National Hurricane Center Miami FL EP202018 800 PM PDT Mon Oct 01 2018 Rosa continues to lack deep convection near the center, which is currently over sea surface temperatures of 21C. However, strong convection is occurring to the northeast of the center over the northern Gulf of California and the adjacent land areas. The Mexican automated station at Isla Cedros reported 30 kt winds with gusts near 40 kt a few hours ago, and passive microwave data suggests the strongest winds in the cyclone are currently moving onshore south of Punta Eugenia where no surface observations are available. The initial intensity is kept at 35 kt, and it is possible that this is generous. Rosa should weaken as it crosses Baja California, but it is likely the warm water of the Gulf of California will maintain the system as a tropical cyclone until final landfall in northwestern Mexico. After that landfall, Rosa should quickly degenerate into a remnant low over the southwestern United States on Tuesday. The initial motion is 035/9. Rosa should move generally northeastward between a deep-layer ridge over northern Mexico and a mid- to upper-level trough over the northeastern Pacific until the cyclone dissipates. The dynamical models suggests the possibility that the center of Rosa will reform over the Gulf of California in the next 12 h, which may lead to some erratic changes in the forward speed. Key Messages: 1. The main hazard expected from Rosa or its remnants is very heavy rainfall in Baja California, northwestern Sonora, and the U.S. 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. Tropical storm conditions are affecting portions of the central and northern Baja California peninsula, especially over higher elevations. These conditions could spread into the northern Gulf of California later tonight. Interests in those locations should monitor the progress of Rosa. FORECAST POSITIONS AND MAX WINDS INIT 02/0300Z 28.9N 115.3W 35 KT 40 MPH 12H 02/1200Z 30.8N 114.5W 25 KT 30 MPH 24H 03/0000Z 33.5N 113.5W 15 KT 15 MPH...POST-TROP/INLAND 36H 03/1200Z...DISSIPATED $$ Forecaster Beven

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Tropical Storm Leslie Forecast Discussion Number 23

2018-10-02 04:42:16| National Hurricane Center (Atlantic)

Issued at 1100 PM AST Mon Oct 01 2018 075 WTNT43 KNHC 020242 TCDAT3 Tropical Storm Leslie Discussion Number 23 NWS National Hurricane Center Miami FL AL132018 1100 PM AST Mon Oct 01 2018 Satellite imagery shows that the convective organization of Leslie has changed little during the past several hours. The eye-like feature, however, has become a little more distinct and symmetric. The subjective and objective intensity estimates haven't changed though, and the initial intensity remains at 50 kt for this advisory. The initial motion is estimated to be a southwestward drift, or 220/4 kt. Leslie is forecast to continue drifting generally southwestward through the 36-hour period in relatively weak mid-tropospheric steering flow produced by a ridge extending eastward from the southeast United States to near Bermuda. Afterwards, an amplifying mid-latitude shortwave trough approaching the cyclone from the Canadian Maritimes should induce a north-northeastward to northeastward motion at an increasing forward speed. The NHC forecast is adjusted just to the left of the previous forecast track beyond 36 hours to align more with the TVCN consensus guidance. The intensity forecast philosophy remains basically the same, this evening. Strengthening is still expected during the next 36 hours and Leslie is likely to become a hurricane Tuesday tonight, as indicated by the statistical and multi-model intensity guidance. Near the 72-hour period, Leslie should begin a gradual weakening trend as the cyclone moves back over cooler oceanic temperatures and some invading drier mid-level air, associated with the aforementioned shortwave trough which stabilizes the surrounding environment. The official intensity forecast is an update of the previous one and is close to the IVCN and NOAA-HCCA consensus aids. The forecast wind radii have been adjusted based on the RVCN (GFS/ECMWF/HWRF) multi-model consensus. Large swells generated by Leslie will continue to affect portions of the southeastern coast of the United States, Bermuda, and the Bahamas. These swells could cause life-threatening surf and rip currents. Although the swells are forecast to abate temporarily in the Bahamas Tuesday, they are expected to increase again Wednesday and Thursday and propagate farther southward into the Greater and Lesser Antilles. Please consult products from your local weather office. FORECAST POSITIONS AND MAX WINDS INIT 02/0300Z 32.4N 55.0W 50 KT 60 MPH 12H 02/1200Z 31.3N 55.7W 55 KT 65 MPH 24H 03/0000Z 30.2N 56.3W 65 KT 75 MPH 36H 03/1200Z 29.9N 56.6W 70 KT 80 MPH 48H 04/0000Z 30.6N 56.4W 70 KT 80 MPH 72H 05/0000Z 34.4N 55.9W 65 KT 75 MPH 96H 06/0000Z 37.0N 55.0W 60 KT 70 MPH 120H 07/0000Z 37.3N 52.5W 55 KT 65 MPH $$ Forecaster Roberts

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