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Subtropical Storm Leslie Forecast Discussion Number 12

2018-09-29 10:32:51| National Hurricane Center (Atlantic)

Issued at 500 AM AST Sat Sep 29 2018 753 WTNT43 KNHC 290832 TCDAT3 Subtropical Storm Leslie Discussion Number 12 NWS National Hurricane Center Miami FL AL132018 500 AM AST Sat Sep 29 2018 The deep convection has become more concentrated near the center during the past several hours indicating that Leslie is gradually acquiring some tropical characteristics, but the cyclone is still too attached to a complex deep-layer low. On this basis, Leslie is still considered a subtropical storm in this advisory with an initial intensity of 40 kt. Global models indicate that the subtropical cyclone will continue to be embedded within strong northerly shear during the next 2 days or so. After that time, the shear is forecast to relax and with a warm ocean along the cyclone's path, Leslie is forecast to become fully tropical and reach hurricane status by the end of the forecast period. SHIPS model and its derivatives strengthen Leslie significantly, but the NHC forecast is a little less aggressive, and it follows the intensity consensus. Leslie has been moving toward the southwest or 230 degrees at 6 kt, steered by the flow around the western side of the large deep-layer low in which Leslie is embedded. Since this steering pattern is not expected to change, Leslie will probably continue on the same general track for the next 3 to 4 days. After that time, the large low weakens and the cyclone will become steered slowly northward by the flow ahead of an approaching mid-latitude trough. Nevertheless, Leslie is forecast to meander over the north-central Atlantic for several more days. This is the solution provided by most of the global models, and the NHC forecast follows very closely the multi-model consensus aids and the corrected consensus HCCA. Large swells previously generated by Leslie when it was a stronger extratropical low have already reached Bermuda, and will soon reach the Lesser and Greater Antilles. These swells are expected to reach portions of the east coast of the United States by Sunday. These swells could cause life-threatening surf and rip currents. FORECAST POSITIONS AND MAX WINDS INIT 29/0900Z 35.2N 50.3W 40 KT 45 MPH 12H 29/1800Z 34.4N 51.3W 45 KT 50 MPH 24H 30/0600Z 33.8N 52.2W 45 KT 50 MPH 36H 30/1800Z 33.5N 53.3W 50 KT 60 MPH 48H 01/0600Z 33.2N 54.0W 50 KT 60 MPH...TROPICAL CYCLONE 72H 02/0600Z 32.3N 55.0W 60 KT 70 MPH 96H 03/0600Z 31.0N 55.5W 65 KT 75 MPH 120H 04/0600Z 32.0N 56.0W 70 KT 80 MPH $$ Forecaster Avila

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Subtropical Storm Leslie Forecast Discussion Number 11

2018-09-29 04:52:18| National Hurricane Center (Atlantic)

Issued at 1100 PM AST Fri Sep 28 2018 020 WTNT43 KNHC 290252 TCDAT3 Subtropical Storm Leslie Discussion Number 11 NWS National Hurricane Center Miami FL AL132018 1100 PM AST Fri Sep 28 2018 Convective banding has been consolidating to the south of Leslie's center since the afternoon, but recent scatterometer data indicate that the maximum winds have decreased to 40 kt and the area of gale-force winds has decreased in size. The convective structure suggests that Leslie may be taking on some tropical characteristics, but since wave vapor imagery still shows the cyclone embedded within a complex deep-layer low, Leslie is still being designated as subtropical. Leslie is moving west-southwestward, or 255 degrees at 10 kt. Leslie is entrenched between several mid-tropospheric highs located to its east and west, and these features are expected to push Leslie slowly southwestward for the next 3 days. The track guidance is tightly clustered during this period, and the NHC forecast is very similar to the previous one. After day 3, Leslie is likely to meander on days 4 and 5 in weak steering, and the updated official forecast has been adjusted southward and eastward at the end of the forecast period to account for the latest model solutions. Global model fields indicate that Leslie has migrated to the northwest of its parent upper-level low, which is putting it under a regime of moderate north-northeasterly shear. For the next 48 hours, this shear is expected to continue, and phase-space diagrams suggest that Leslie will be straddling the line between shallow and deep warm core. As a result, only modest strengthening is anticipated during this period, and the official forecast maintains Leslie as a subtropical storm through 48 hours. However, the transition to a tropical storm could occur any time during the next day or two. After 48 hours, Leslie should definitely be deep warm core, and more significant strengthening is expected, with the cyclone forecast to reach hurricane intensity by day 4. This scenario is shown by the various intensity models, and the NHC intensity forecast is very close to the HFIP Corrected Consensus aid and the Florida State Superensemble. If the statistical- dynamical models are correct, Leslie could be stronger by the end of the forecast period than is indicated in the NHC forecast. Large swells generated by Leslie when it was a stronger extratropical low have already reached Bermuda, will soon reach the Lesser and Greater Antilles, and should reach portions of the east coast of the United States later this weekend. These swells could cause life-threatening surf and rip currents. FORECAST POSITIONS AND MAX WINDS INIT 29/0300Z 35.7N 49.4W 40 KT 45 MPH 12H 29/1200Z 34.9N 50.8W 45 KT 50 MPH 24H 30/0000Z 34.0N 52.0W 45 KT 50 MPH 36H 30/1200Z 33.5N 53.0W 50 KT 60 MPH 48H 01/0000Z 33.1N 53.9W 50 KT 60 MPH 72H 02/0000Z 32.2N 55.4W 60 KT 70 MPH...TROPICAL CYCLONE 96H 03/0000Z 30.5N 56.0W 65 KT 75 MPH 120H 04/0000Z 31.5N 56.5W 70 KT 80 MPH $$ Forecaster Berg

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Remnants of Kirk Forecast Discussion Number 21

2018-09-29 04:42:47| National Hurricane Center (Atlantic)

Issued at 1100 PM AST Fri Sep 28 2018 900 WTNT42 KNHC 290242 TCDAT2 Remnants Of Kirk Discussion Number 21 NWS National Hurricane Center Miami FL AL122018 1100 PM AST Fri Sep 28 2018 Even though there still appears to be a circulation center in satellite imagery, an Air Force Reserve Hurricane Hunter plane was unable to find westerly winds that would support a closed circulation. Therefore, Kirk has degenerated into an open wave, and this will be the last advisory. The SFMR on the plane still measured winds around 35 kt in the convection to the east of the wave axis, and that will remain the initial intensity. The wave will continue moving westward across the Caribbean Sea during the next few days, with its winds gradually decreasing due to continued westerly shear. Heavy rains are still possible over portions of eastern Puerto Rico and St. Croix during the next day or two while Kirk's remnants pass to the south. Additional information on this system can be found in High Seas Forecasts issued by the National Weather Service, under AWIPS header NFDHSFAT1, WMO header FZNT01 KWBC, and available on the Web at https://ocean.weather.gov/shtml/NFDHSFAT1.shtml. FORECAST POSITIONS AND MAX WINDS INIT 29/0300Z 15.1N 65.8W 35 KT 40 MPH...REMNANTS 12H 29/1200Z...DISSIPATED $$ Forecaster Berg

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Hurricane Rosa Forecast Discussion Number 16

2018-09-29 04:42:17| National Hurricane Center (East Pacific)

Issued at 800 PM PDT Fri Sep 28 2018 777 WTPZ45 KNHC 290242 TCDEP5 Hurricane Rosa Discussion Number 16 NWS National Hurricane Center Miami FL EP202018 800 PM PDT Fri Sep 28 2018 The satellite appearance of Rosa continues to degrade due to an ongoing eyewall cycle with weaker convection noted near the eyewall and an apparent moat region. Dvorak estimates continue to fall, and a blend from all agencies gives 95 kt as an initial wind speed. Continued weakening is in the forecast while Rosa moves across progressively cooler SSTs into a higher shear and drier environment. There is some chance that the weakening could pause if the eyewall cycle finishes, but that's less likely as the cyclone encounters the less favorable environment. Thus Rosa is forecast to continue to gradually weaken until it approaches Baja California as a tropical storm. The intensity guidance is in good agreement on this scenario, and the new NHC prediction is basically an update of the previous one after accounting for the initial winds. Rosa has turned to the north-northwest and accelerated, with an initial motion of 340/8. The cyclone should turn to the north tomorrow and move faster to the north-northeast on Sunday ahead of a mid- to upper-level trough approaching the west coast of the United States. There are no substantial guidance changes to consider, and with the model consensus being quite reliable so far, it makes sense to stay near that aid, which is also very close to the previous forecast. 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 few days. Interests in those locations should monitor the progress of Rosa. FORECAST POSITIONS AND MAX WINDS INIT 29/0300Z 18.4N 118.0W 95 KT 110 MPH 12H 29/1200Z 19.5N 118.4W 85 KT 100 MPH 24H 30/0000Z 21.4N 118.8W 75 KT 85 MPH 36H 30/1200Z 23.3N 118.6W 65 KT 75 MPH 48H 01/0000Z 25.1N 117.9W 55 KT 65 MPH 72H 02/0000Z 28.9N 116.1W 40 KT 45 MPH 96H 03/0000Z 36.0N 112.0W 15 KT 15 MPH...POST-TROP/REMNT LOW 120H 04/0000Z...DISSIPATED $$ Forecaster Blake

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Hurricane Rosa Forecast Discussion Number 15

2018-09-28 22:55:45| National Hurricane Center (East Pacific)

Issued at 200 PM PDT Fri Sep 28 2018 259 WTPZ45 KNHC 282055 TCDEP5 Hurricane Rosa Discussion Number 15 NWS National Hurricane Center Miami FL EP202018 200 PM PDT Fri Sep 28 2018 Rosa's appearance has rapidly deteriorated since this morning. There is still evidence of an ongoing eyewall replacement cycle, and an apparent moat region has occasionally been apparent in conventional satellite data during the past several hours. The slow motion of the hurricane since last night could also be causing upwelling that would lead to further weakening. Satellite intensity estimates from all agencies have dropped substantially accordingly, and the initial intensity has been lowered to 105 kt, based on a blend of Final-T and CI numbers from TAFB and SAB. Rosa is still moving northwestward, with an initial motion of 325/5 kt. The model spread has increased a little, with the GFS and its associated regional models showing a slightly right (southeast) track, compared to earlier cycles, while the ECMWF has shifted an equal amount to the left (northwest). The consensus models have not changed much as a result of these offsetting model trends, so almost no change was made to the NHC track forecast. Rosa is still expected to turn northward, and then north-northeastward ahead of a mid- to upper-level trough approaching from the northwest. The cyclone should then accelerate a little as it approaches the northern coast of the Baja California peninsula early next week. At this point, the structure of Rosa has degraded to the point that substantial restrengthening appears unlikely. Since the hurricane still has about 24-36 hours before it reaches much cooler SSTs, only gradual weakening is forecast, though most of the intensity guidance shows more rapid weakening than currently indicated. After that time, a more rapid rate of weakening could occur due to cooler SSTs and an increase in wind shear associated with an upper-level trough approaching from the west. By early next week, Rosa is forecast to move inland over northern Baja California, and its surface circulation will likely dissipate shortly thereafter. However, moisture associated with Rosa is expected to spread northeastward through parts of the southwest United States. For more information about potential rainfall in that area, please see products from the Weather Prediction Center and your local NWS forecast office. Key Messages: 1. Rosa could bring tropical storm conditions to portions of the central and northern Baja California peninsula in a few days. Interests in those locations should monitor the progress of Rosa. 2. Heavy rainfall associated with Rosa or its remnants is expected to affect parts of the southwest United States by early next week, which could cause flooding in this region. For more information about potential rainfall and flooding, please see products from the Weather Prediction Center and your local NWS forecast office. FORECAST POSITIONS AND MAX WINDS INIT 28/2100Z 17.7N 117.8W 105 KT 120 MPH 12H 29/0600Z 18.6N 118.3W 95 KT 110 MPH 24H 29/1800Z 20.3N 118.8W 85 KT 100 MPH 36H 30/0600Z 22.1N 118.9W 75 KT 85 MPH 48H 30/1800Z 24.0N 118.4W 65 KT 75 MPH 72H 01/1800Z 27.7N 116.8W 45 KT 50 MPH 96H 02/1800Z 32.3N 114.5W 30 KT 35 MPH...INLAND 120H 03/1800Z...DISSIPATED $$ Forecaster Zelinsky

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