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Subtropical Depression Leslie Forecast Discussion Number 8

2018-09-25 10:37:46| National Hurricane Center (Atlantic)

Issued at 500 AM AST Tue Sep 25 2018 977 WTNT43 KNHC 250837 TCDAT3 Subtropical Depression Leslie Discussion Number 8 NWS National Hurricane Center Miami FL AL132018 500 AM AST Tue Sep 25 2018 No significant changes have occurred with Leslie overnight. Deep convection is generally confined to the eastern half of the circulation as dry air continues to wrap into the western portion of the cyclone. Satellite images indicate that the circulation has become stretched from north to south, likely due to the approach of a cold front that is currently located a few hundred miles to the northwest of Leslie. The initial intensity is held at 30 kt based on the earlier ASCAT data. The cold front is expected to merge with Leslie by tonight, causing extratropical transition. While transitioning, Leslie is forecast to strengthen for a couple of days due to significant baroclinic forcing, and the NHC intensity forecast takes the peak winds just below hurricane force at 36 and 48 hours. Later in the week, the extratropical system is expected to cut off and gradually lose its frontal features. Although this will likely cause some weakening, it should also allow the system to regain subtropical characteristics. The NHC intensity forecast is fairly similar to the previous one and near the IVCN consensus model. This forecast is also in fairly good agreement with the GFS and ECMWF models, which are usually reliable intensity models for large subtropical systems like Leslie. Based on the latest guidance, Leslie is now expected to transition back to a subtropical cyclone in 96 hours. Leslie has jogged to the southeast during the past several hours, but an eastward to northeastward motion is expected during the next day or so as Leslie makes its extratropical transition. After that time, a turn to the north is expected followed by a slow westward motion when Leslie cuts off from the mid-latitude flow. The models are in fairly good agreement on this looping motion, and the NHC track forecast follows the various consensus aids. FORECAST POSITIONS AND MAX WINDS INIT 25/0900Z 31.9N 46.2W 30 KT 35 MPH 12H 25/1800Z 32.1N 44.8W 40 KT 45 MPH 24H 26/0600Z 32.7N 41.9W 50 KT 60 MPH...POST-TROP/EXTRATROP 36H 26/1800Z 34.5N 40.5W 60 KT 70 MPH...POST-TROP/EXTRATROP 48H 27/0600Z 36.1N 41.1W 60 KT 70 MPH...POST-TROP/EXTRATROP 72H 28/0600Z 36.0N 45.0W 55 KT 65 MPH...POST-TROP/EXTRATROP 96H 29/0600Z 35.3N 48.2W 55 KT 65 MPH...SUBTROPICAL STORM 120H 30/0600Z 35.0N 50.0W 50 KT 60 MPH $$ Forecaster Cangialosi

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Subtropical Depression Leslie Forecast Discussion Number 7

2018-09-25 04:33:13| National Hurricane Center (Atlantic)

Issued at 1100 PM AST Mon Sep 24 2018 574 WTNT43 KNHC 250233 TCDAT3 Subtropical Depression Leslie Discussion Number 7 NWS National Hurricane Center Miami FL AL132018 1100 PM AST Mon Sep 24 2018 Leslie's appearance in satellite imagery hasn't changed much this evening. Deep convection is mainly occuring in bands well displaced to the southeast of the cyclone's center, and recent ASCAT data indicated that the maximum winds have decreased to near 30 kt, with the highest wind occuring in those bands. All indications are that Leslie will undergo a complicated transition over the next several days. Little change is expected with Leslie until it becomes post-tropical in about 24 hours. After that time, all of the global models suggest that Leslie will quickly strengthen as a result of substantial baroclinic forcing, likely reaching a peak intensity sometime between 48 and 72 h. At the same time, the cyclone will likely undergo a classic transition from frontal low to warm seclusion while the wind field rapidly expands, with 34-kt (gale-force) winds reaching several hundred miles from the cyclone's center. By 96 h, the models indicate that Leslie will once again become cut-off from the mid-latitude flow, and could begin to re-acquire subtropical or tropical characteristics. While each of the global models handles the exact details of this complex evolution a little differently, confidence is increasing that Leslie will become a powerful extratropical cyclone over the central Atlantic over the next few of days. The NHC intensity forecast has been increased between 36 and 96 h, and is near a average of the global and regional dynamical model tracker output. The depression is currently moving slowly eastward with an initial motion of 090/3 kt. The guidance is in good agreement that Leslie will accelerate eastward as it becomes more embedded within an approaching frontal zone. A northward turn is expected by mid-week, followed by a bend back toward the west by the end of the week as Leslie strengthens, occludes, and eventually cuts off from the mid-latitude flow to the north. The NHC track forecast has not been significantly changed, and is near the GFEX and HCCA consensus aids at most forecast hours. However, it should be noted that spread in the guidance is high, especially by the end of the forecast period, so confidence in the track forecast at that time is low. FORECAST POSITIONS AND MAX WINDS INIT 25/0300Z 33.1N 47.1W 30 KT 35 MPH 12H 25/1200Z 32.9N 45.6W 30 KT 35 MPH 24H 26/0000Z 33.1N 43.2W 40 KT 45 MPH...POST-TROPICAL 36H 26/1200Z 34.5N 41.1W 55 KT 65 MPH...POST-TROP/EXTRATROP 48H 27/0000Z 36.2N 40.9W 60 KT 70 MPH...POST-TROP/EXTRATROP 72H 28/0000Z 36.9N 43.8W 60 KT 70 MPH...POST-TROP/EXTRATROP 96H 29/0000Z 36.5N 47.3W 55 KT 65 MPH...POST-TROP/EXTRATROP 120H 30/0000Z 36.5N 49.0W 50 KT 60 MPH...SUBTROPICAL STORM $$ Forecaster Zelinsky

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Session #3 Discussion Questions Handout for Public Input (9/25/18)

2018-09-25 01:58:20| PortlandOnline

PDF Document, 98kbCategory: Documents

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Session #3 Discussion Questions Handout for Public Input (9/25/18)

2018-09-25 01:58:20| PortlandOnline

PDF Document, 98kbCategory: Session #3 In-River Recreation

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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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