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Subtropical Storm Leslie Forecast Discussion Number 6

2018-09-24 22:37:39| National Hurricane Center (Atlantic)

Issued at 500 PM AST Mon Sep 24 2018 110 WTNT43 KNHC 242037 TCDAT3 Subtropical Storm Leslie Discussion Number 6 NWS National Hurricane Center Miami FL AL132018 500 PM AST Mon Sep 24 2018 Leslie's cloud pattern has become rather ragged and less organized this afternoon. Visible satellite imagery indicates that the surface circulation has become elongated, from north to south, with multiple cloud swirls rotating within the larger gyre. The initial intensity is held at 35 kt for this advisory and could be generous, based on the earlier partial ASCAT-A wind retrievals. The intensity forecast philosophy remains the same. Leslie is expected to interact with the incoming baroclinic zone approaching from the northwest and begin intensifying as it completes a post-tropical/extratropical transition by mid period. The official forecast is based on the IVCN multi-model intensity consensus through 36 hours, and a blend of the global models after Leslie becomes an extratropical low. Based on a centroid position of the aforementioned multiple surface swirls, Leslie continues to move in an eastward fashion, or 090/6 kt. Leslie should turn toward the east-northeast in 36 hours in response to an approaching mid-latitude trough and associated cold front from the northwest over the central Atlantic. The large-scale models are in good agreement with Leslie completing an extratropical transition at that time. Afterward, the post-tropical cyclone is forecast to become cut off from the mid- to upper-tropospheric westerly steering flow and move cyclonically toward the end of the week between the building Bermuda high to the west and mid-level ridging over the east Atlantic. The NHC forecast now shows Leslie as a strengthening extratropical low through day 5, in agreement with GFS, Canadian, and European models. Subsequently, the wind radii forecast has also been adjusted based on a compromise of the aforementioned large-scale models. FORECAST POSITIONS AND MAX WINDS INIT 24/2100Z 33.0N 47.2W 35 KT 40 MPH 12H 25/0600Z 32.9N 46.2W 35 KT 40 MPH 24H 25/1800Z 32.6N 44.0W 45 KT 50 MPH 36H 26/0600Z 33.4N 41.3W 50 KT 60 MPH...POST-TROP/EXTRATROP 48H 26/1800Z 35.4N 39.9W 55 KT 65 MPH...POST-TROP/EXTRATROP 72H 27/1800Z 36.9N 42.5W 50 KT 60 MPH...POST-TROP/EXTRATROP 96H 28/1800Z 36.7N 46.5W 50 KT 60 MPH...POST-TROP/EXTRATROP 120H 29/1800Z 36.2N 48.5W 45 KT 50 MPH...POST-TROP/EXTRATROP $$ Forecaster Roberts

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Remnants of Kirk Forecast Discussion Number 9

2018-09-24 16:34:42| National Hurricane Center (Atlantic)

Issued at 1100 AM AST Mon Sep 24 2018 137 WTNT42 KNHC 241434 TCDAT2 Remnants Of Kirk Discussion Number 9 NWS National Hurricane Center Miami FL AL122018 1100 AM AST Mon Sep 24 2018 Analysis of the directional ambiguities from a recent ASCAT scatterometer overpass indicated that the system lacks the closed circulation that is a prerequisite for tropical cyclone status. Therefore, advisories are being discontinued at this time. The ASCAT data showed that the system is now a sharp trough accompanied by an area of gale-force winds that will be handled in High Seas Forecasts issued by the National Weather Service. The remnants of Kirk will be monitored during the next few days in case regeneration into a tropical cyclone should occur. Additional information on this system can be found in High Seas Forecasts issued by the National Weather Service...under AWIPS header NFDHSFAT1 and WMO header FZNT01 KWBC. FORECAST POSITIONS AND MAX WINDS INIT 24/1500Z 10.0N 39.5W 35 KT 40 MPH 12H 25/0000Z...DISSIPATED $$ Forecaster Pasch

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Subtropical Storm Leslie Forecast Discussion Number 5

2018-09-24 16:34:42| National Hurricane Center (Atlantic)

Issued at 1100 AM AST Mon Sep 24 2018 138 WTNT43 KNHC 241434 TCDAT3 Subtropical Storm Leslie Discussion Number 5 NWS National Hurricane Center Miami FL AL132018 1100 AM AST Mon Sep 24 2018 Conventional satellite imagery shows little change in Leslie's cloud pattern during the past several hours with the exception of a few new isolated thunderstorms developing near the surface center. Accordingly, the initial intensity for this advisory is kept at 35 kt. The statistical intensity guidance and the large-scale models show Leslie commencing a strengthening trend in 24 hours as a mid-latitude baroclinic system digs southeastward toward the cyclone. Some of the models also indicate that Leslie, after completing an extratropical transition in around 48 hours, will deepen and generate a large area of gale force winds, particularly in the northwest quadrant. The NHC intensity forecast reflects this scenario and is just below the IVCN intensity consensus. The initial motion is estimated to be an eastward drift, or 090/4 kt within the increasing mid-level westerly steering flow. A turn to the east-northeast is forecast as an approaching mid-latitude trough and associated cold front approaches from the northwest over the central Atlantic. The deterministic guidance indicate that the aforementioned baroclinic system will overtake Leslie near the 48 hour period, and influence a transition to a non-tropical low pressure system. Subsequently, the GFS and the European models still show extratropical Leslie as the primary system and deepening with time. For now, the official forecast will indicate a merging scenario, but succeeding advisories may include extratropical low forecast points through day 5. FORECAST POSITIONS AND MAX WINDS INIT 24/1500Z 32.5N 48.0W 35 KT 40 MPH 12H 25/0000Z 32.5N 47.2W 35 KT 40 MPH 24H 25/1200Z 32.4N 45.5W 40 KT 45 MPH 36H 26/0000Z 32.3N 42.8W 45 KT 50 MPH 48H 26/1200Z 33.6N 39.9W 50 KT 60 MPH...POST-TROP/EXTRATROP 72H 27/1200Z...MERGED WITH FRONT AND LARGER LOW $$ Forecaster Roberts

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Tropical Depression Kirk Forecast Discussion Number 8

2018-09-24 10:33:06| National Hurricane Center (Atlantic)

Issued at 500 AM AST Mon Sep 24 2018 632 WTNT42 KNHC 240833 TCDAT2 Tropical Depression Kirk Discussion Number 8 NWS National Hurricane Center Miami FL AL122018 500 AM AST Mon Sep 24 2018 After having a minimal amount of deep convection late yesterday, Kirk is now producing some thunderstorm activity in fragmented bands around the estimated center. However, earlier ASCAT data and low cloud motion suggest that the circulation resembles a sharp trough instead of a closed low that is necessary for a tropical cyclone. Hopefully more visible satellite images and additional scatterometer data will help better assess the circulation and status of Kirk. For now, advisories are being maintained on the system, and the initial intensity is held at 30 kt based on the earlier ASCAT data and a blend of the Dvorak estimates. Kirk is moving very quickly to the west at about 21 kt, which is likely the reason why the system has been struggling. A continued westward motion but at a decreasing forward speed is expected during the next two to three days while the cyclone remains on the south side of a low- to mid-level ridge. After that time, a large deep-layer low pressure system over the central Atlantic should cause Kirk to gain more latitude. The NHC track forecast is largely an update of the previous one, and remains near the southern side of the guidance envelope near the GFS and ECMWF models. This forecast takes Kirk toward the Lesser Antilles in 3 to 4 days and interests there should monitor the progress of this system. Although Kirk will be moving over warmer SSTs and remain in relatively moist conditions, its fast forward speed will likely limit the system from strengthening significantly during the next few days, so only a little intensification is predicted. The global models show a notable increase in westerly shear when Kirk reaches the eastern Caribbean Sea in about 4 days, and that should cause weakening and possibly even dissipation by the end of the forecast period. The NHC intensity forecast lies near the lower side of the guidance envelope, in best agreement with the NOAA HCCA model. FORECAST POSITIONS AND MAX WINDS INIT 24/0900Z 9.5N 37.4W 30 KT 35 MPH 12H 24/1800Z 9.9N 40.5W 30 KT 35 MPH 24H 25/0600Z 10.2N 44.6W 35 KT 40 MPH 36H 25/1800Z 10.5N 48.4W 35 KT 40 MPH 48H 26/0600Z 10.9N 51.7W 40 KT 45 MPH 72H 27/0600Z 12.1N 57.1W 40 KT 45 MPH 96H 28/0600Z 13.2N 61.5W 35 KT 40 MPH 120H 29/0600Z 14.0N 66.0W 30 KT 35 MPH $$ Forecaster Cangialosi

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Subtropical Storm Leslie Forecast Discussion Number 4

2018-09-24 10:32:09| National Hurricane Center (Atlantic)

Issued at 500 AM AST Mon Sep 24 2018 385 WTNT43 KNHC 240832 TCDAT3 Subtropical Storm Leslie Discussion Number 4 NWS National Hurricane Center Miami FL AL132018 500 AM AST Mon Sep 24 2018 Leslie has changed little overnight. The large subtropical cyclone continues to produce patches of deep convection, mostly to the south and east of the center. The initial intensity is again held at 35 kt, which is near the upper end of the satellite intensity estimates. The subtropical storm is moving southward at 5 kt. The steering currents around Leslie are expected to collapse later today and little overall motion is forecast during the next 12-18 hours. After that time, a cold front is expected to approach the system, and that should cause Leslie to move eastward tonight and Tuesday. The front is expected to merge with Leslie by Tuesday night, causing it to become extratropical. While little change in intensity is expected during the next day or so, due to dry air and moderate shear, some strengthening as an extratropical low is likely due to baroclinic forcing. In fact, after becoming extratropical, the models show Leslie merging with another frontal low to its north, leading to the formation of a powerful slow-moving non-tropical low with gale- or storm-force winds over the central Atlantic later this week. FORECAST POSITIONS AND MAX WINDS INIT 24/0900Z 32.6N 48.6W 35 KT 40 MPH 12H 24/1800Z 32.6N 48.3W 35 KT 40 MPH 24H 25/0600Z 32.6N 46.9W 35 KT 40 MPH 36H 25/1800Z 32.5N 44.8W 35 KT 40 MPH 48H 26/0600Z 32.7N 42.0W 45 KT 50 MPH...POST-TROP/EXTRATROP 72H 27/0600Z...MERGED WITH FRONTAL SYSTEM $$ Forecaster Cangialosi

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