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Tropical Storm Florence Forecast Discussion Number 35

2018-09-08 04:52:03| National Hurricane Center (Atlantic)

Issued at 1100 PM AST Fri Sep 07 2018 331 WTNT41 KNHC 080251 TCDAT1 Tropical Storm Florence Discussion Number 35 NWS National Hurricane Center Miami FL AL062018 1100 PM AST Fri Sep 07 2018 Florence remains a sheared tropical cyclone. Satellite images indicate that the low-level center is partially exposed on the southwesterly edge of a large convective mass, with the overall circulation somewhat elongated from southwest to northeast. A blend of the latest Dvorak wind speed estimates from TAFB/SAB and the CIMSS SATCON gives a value of 50 kt for this advisory. While the winds at 200 mb are already from an easterly direction near the center of Florence, there is significant shear from northwesterly winds from 300-500 mb, undercutting the outflow layer. This shear is forecast to relax by the global models over the next 36 hours as an anticyclone builds to the north of the storm, which should promote some strengthening by Sunday. After 48 hours, the deep-layer flow becomes easterly near the cyclone, with very little shear while the system is over very warm waters. This pattern favors significant intensification, and most of the guidance brings Florence back to a category 4 hurricane in 4 or 5 days. The intensity forecast is very similar to the last one, and is raised slightly at days 3 and 4 to come into better agreement with the guidance. It is interesting to note that even with a lower initial intensity, the guidance is higher than the last cycle, which speaks to the strength of the signal for intensification in the long range. The initial motion estimate is 265 degrees at 6 kt. Florence is expected to continue moving slowly westward for the next 48 hours under the influence of a weak mid-level ridge over the western Atlantic. By days 4 and 5, an exceptionally strong blocking ridge is forecast to develop between Bermuda and the Northeast U.S. and build westward, keeping Florence on a west-northwestward trajectory with a notable increase in forward speed by the end of the forecast period. It feels like a broken record to mention that the overall guidance envelope keeps shifting southwestward, and the official forecast is moved in that direction. Unfortunately with such a large well-defined steering current from the ridge becoming likely, the extended-range risk to the United States keeps rising, which is confirmed by the majority of the latest ensemble guidance. Key Messages: 1. Regardless of Florence's eventual track, large swells are affecting Bermuda and will begin to affect portions of the U.S. East Coast this weekend, resulting in life-threatening surf and rip currents. 2. The risk of other direct impacts associated with Florence along the U.S. East Coast next week has increased. However, there is still very large uncertainty in model forecasts of Florence's track beyond day 5, making it too soon to determine the exact location, magnitude, and timing of these impacts. Interests near and along the U.S. East Coast should monitor the progress of Florence through the weekend and ensure they have their hurricane plans in place. FORECAST POSITIONS AND MAX WINDS INIT 08/0300Z 24.8N 53.2W 50 KT 60 MPH 12H 08/1200Z 24.7N 54.1W 50 KT 60 MPH 24H 09/0000Z 24.7N 55.2W 55 KT 65 MPH 36H 09/1200Z 24.7N 56.3W 60 KT 70 MPH 48H 10/0000Z 25.0N 57.8W 70 KT 80 MPH 72H 11/0000Z 25.8N 62.3W 95 KT 110 MPH 96H 12/0000Z 27.5N 68.5W 115 KT 130 MPH 120H 13/0000Z 30.5N 74.5W 115 KT 130 MPH $$ Forecaster Blake

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Tropical Storm Helene Forecast Discussion Number 3

2018-09-08 04:45:00| National Hurricane Center (Atlantic)

Issued at 1100 PM AST Fri Sep 07 2018 187 WTNT43 KNHC 080244 TCDAT3 Tropical Storm Helene Discussion Number 3 NWS National Hurricane Center Miami FL AL082018 1100 PM AST Fri Sep 07 2018 A very timely ASCAT pass indicated that the winds associated with the depression have increased to 35 kt, and also that the center was a little east of the location previously indicated. This is very common in systems during the formative stage. Based on the ASCAT data, the system has been upgraded to Tropical Storm Helene, the eighth named storm of the season. The satellite presentation has also improved during the past several hours, and now the cyclone has large cyclonically curved convective bands to the south of the center. The outflow is fair in all quadrants. Helene will be moving over warm waters and within an environment of light shear through the next 3 to 4 days, and most of the guidance responds to that environment by gradually strengthening the cyclone. The NHC forecast follows the trend of the intensity consensus, and brings Helene to hurricane intensity in about 3 days. Currently, Helene is embedded within a southwest monsoon-type flow, and is moving toward the west at about 9 or 10 knots. However, as the cyclone moves away from the African coast, it will become steered by the easterly flow around the subtropical ridge and should then increase in forward speed. Most of the track models are in extremely good agreement, at least for the next 3 days when the confidence in the forecast is high. At the long range, a mid-level trough is expected to develop over the central Atlantic, forcing the cyclone to turn more to the northwest and even north later on. The NHC forecast is in the middle of the guidance envelope and basically on top of the corrected consensus HCCA. FORECAST POSITIONS AND MAX WINDS INIT 08/0300Z 13.6N 18.5W 35 KT 40 MPH 12H 08/1200Z 13.8N 19.9W 35 KT 40 MPH 24H 09/0000Z 14.3N 22.3W 40 KT 45 MPH 36H 09/1200Z 14.8N 25.0W 50 KT 60 MPH 48H 10/0000Z 15.4N 28.0W 55 KT 65 MPH 72H 11/0000Z 17.0N 33.9W 65 KT 75 MPH 96H 12/0000Z 18.5N 39.0W 65 KT 75 MPH 120H 13/0000Z 20.5N 42.5W 60 KT 70 MPH $$ Forecaster Avila

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Tropical Depression Nine Forecast Discussion Number 1

2018-09-07 22:58:58| National Hurricane Center (Atlantic)

Issued at 500 PM AST Fri Sep 07 2018 888 WTNT44 KNHC 072058 CCA TCDAT4 Tropical Depression Nine Discussion Number 1...Corrected NWS National Hurricane Center Miami FL AL092018 500 PM AST Fri Sep 07 2018 Corrected bin number from 2 to 4 Satellite images indicate that the low pressure system over the eastern tropical Atlantic has developed a well-defined center and sufficient deep convection to be classified a tropical depression, the ninth one of the 2018 Atlantic hurricane season and second one of the day. The initial intensity is set to 30 kt based on a 2.0 Dvorak classification from TAFB and earlier ASCAT data. The depression has not moved much today, and it has even wobbled a bit to the east recently. This slow motion is a result of very weak steering with broad troughing to the north and Tropical Depression Eight to its east. Little motion is expected through tonight, but a westward motion with a gradual increase in forward speed is forecast thereafter as a subtropical ridge builds to the north of the cyclone. The NHC track forecast lies closest to the HCCA guidance and takes the system toward the Lesser Antilles by the end of the forecast period. The depression is experiencing some influences of easterly shear, which is why the center is currently on the eastern edge of the convection. Little change in intensity is expected during the next 12 to 24 hours since the cyclone is still in the developing stage and because moderate shear is expected to continue. However, the shear is expected to decrease to less than 10 kt in about 24 hours. These more favorable winds aloft combined with relatively warm waters and a fairly moist airmass should allow for steady strengthening beginning later in the weekend. There is a large spread in the intensity guidance, with the HWRF showing the system becoming a major hurricane and HMON showing almost no strengthening. The NHC intensity forecast lies a little below consensus models since it appears that it will take some time for the system to strengthen. FORECAST POSITIONS AND MAX WINDS INIT 07/2100Z 13.6N 34.9W 30 KT 35 MPH 12H 08/0600Z 13.9N 35.2W 30 KT 35 MPH 24H 08/1800Z 14.0N 36.2W 30 KT 35 MPH 36H 09/0600Z 14.0N 37.6W 40 KT 45 MPH 48H 09/1800Z 14.0N 39.3W 50 KT 60 MPH 72H 10/1800Z 14.0N 44.0W 60 KT 70 MPH 96H 11/1800Z 14.1N 48.9W 70 KT 80 MPH 120H 12/1800Z 14.5N 54.5W 75 KT 85 MPH $$ Forecaster Cangialosi

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Tropical Storm Florence Forecast Discussion Number 34

2018-09-07 22:50:22| National Hurricane Center (Atlantic)

Issued at 500 PM AST Fri Sep 07 2018 294 WTNT41 KNHC 072049 TCDAT1 Tropical Storm Florence Discussion Number 34 NWS National Hurricane Center Miami FL AL062018 500 PM AST Fri Sep 07 2018 Little has changed with Florence's structure during the day. The low-level center is located on the southwestern side of the deep convection due to moderate to strong southwesterly shear, and satellite intensity estimates are essentially the same from this morning. Therefore, the estimated maximum winds remain 55 kt. NOAA is scheduled to conduct a research mission with the P-3 aircraft tomorrow, which should provide some useful wind data and give us a better handle on the cyclone's intensity. Vertical shear is still expected to gradually decrease over the next day or two, likely reaching values of 10 kt or less by 48 hours. During this period, Florence should become more vertically stacked, which would allow for some reintensification, possibly back to hurricane strength within 36-48 hours. After 48 hours, the shear is expected to remain generally low, and oceanic heat content values will increase significantly as Florence moves over the waters between Bermuda and the northern Leeward Islands. This is a classic recipe for a quick intensification trend, and Florence is expected to become a major hurricane by days 4 and 5. The generally skillful HCCA, Florida State Superensemble, and ICON intensity consensus are all near 110-115 kt by day 5, and because of this, no notable changes were required from the previous official intensity forecast. Florence has been moving south of due west (260 degrees) at 7 kt, located south of a low- to mid-level ridge. The cyclone is expected to continue moving generally westward for the next 48 hours while it remains sheared. As Florence begins to strengthen and become vertically stacked after 48 hours, it should begin to turn west-northwestward, steered by a deeper flow regime. By days 4 and 5, an exceptionally strong blocking ridge is forecast to develop between Bermuda and the Northeast U.S., keeping Florence on a west-northwestward trajectory with an increase in forward speed by the end of the forecast period. A slight southwestward adjustment was made to the NHC forecast to account for Florence's initial motion and a slight shift in the overall guidance envelope. For most of the forecast period, the official forecast is close to a blend of the GFS and ECMWF. Key Messages: 1. Regardless of Florence's eventual track, large swells are affecting Bermuda and will begin to affect portions of the U.S. East Coast this weekend, resulting in life-threatening surf and rip currents. 2. The risk of other direct impacts associated with Florence along the U.S. East Coast next week has increased. However, there is still very large uncertainty in model forecasts of Florence's track beyond day 5, making it too soon to determine the exact location, magnitude, and timing of these impacts. Interests near and along the U.S. East Coast should monitor the progress of Florence through the weekend and ensure they have their hurricane plans in place. FORECAST POSITIONS AND MAX WINDS INIT 07/2100Z 24.8N 52.5W 55 KT 65 MPH 12H 08/0600Z 24.6N 53.5W 55 KT 65 MPH 24H 08/1800Z 24.6N 54.7W 55 KT 65 MPH 36H 09/0600Z 24.6N 55.8W 60 KT 70 MPH 48H 09/1800Z 24.8N 57.0W 70 KT 80 MPH 72H 10/1800Z 25.8N 60.9W 90 KT 105 MPH 96H 11/1800Z 27.5N 67.0W 110 KT 125 MPH 120H 12/1800Z 30.5N 73.0W 115 KT 130 MPH $$ Forecaster Berg

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Tropical Depression Eight Forecast Discussion Number 2

2018-09-07 22:46:59| National Hurricane Center (Atlantic)

Issued at 500 PM AST Fri Sep 07 2018 413 WTNT43 KNHC 072046 TCDAT3 Tropical Depression Eight Discussion Number 2 NWS National Hurricane Center Miami FL AL082018 500 PM AST Fri Sep 07 2018 Satellite data and surface observations indicate that the low pressure system located just west of the coast of Africa has developed into a tropical depression. Conventional satellite imagery shows a well-defined convective band has formed near the center, and microwave satellite imagery has hinted at the formation of an inner ring of convection. The initial intensity of 30 kt and the central pressure of 1002 mb are based on surface observations from ships and the west coast of Africa. The depression currently has good cirrus outflow in all directions. The initial motion is 275/9. During the next 3-4 days, the cyclone should move generally west-northwestward with an increase in forward speed on the south side of the subtropical ridge over the eastern Atlantic. Near the end of the forecast period, a developing mid- to upper-level trough over the central Atlantic is expected to weaken the ridge and allow the cyclone to turn northwestward with a decrease in forward speed. There is some spread in the guidance late in the period, with the UKMET and the Canadian models showing an earlier turn than the other models. The new forecast track, which is similar to the previous track, is in best agreement with the TVCN and HCCA consensus models. The depression is in an environment of light to moderate easterly vertical shear and over sea surface temperatures of 27-28C. This should allow at least steady strengthening, and rapid strengthening is possible based on the hints of the inner core in microwave imagery. This portion of the intensity forecast has been increased to the upper edge of the intensity guidance, with the cyclone forecast to become a tropical storm in 12 h or less and a hurricane between 48-72 h. After 72 h, the system should be over sea surface temperatures near 26C and encountering southwesterly vertical shear due to the aforementioned trough. This should cause at least a gradual weakening. FORECAST POSITIONS AND MAX WINDS INIT 07/2100Z 13.2N 18.6W 30 KT 35 MPH 12H 08/0600Z 13.5N 19.9W 35 KT 40 MPH 24H 08/1800Z 13.9N 21.9W 45 KT 50 MPH 36H 09/0600Z 14.4N 24.5W 55 KT 65 MPH 48H 09/1800Z 14.9N 27.5W 60 KT 70 MPH 72H 10/1800Z 16.5N 33.5W 70 KT 80 MPH 96H 11/1800Z 18.0N 38.5W 65 KT 75 MPH 120H 12/1800Z 20.5N 42.0W 60 KT 70 MPH $$ Forecaster Beven

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