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Tropical Storm Helene Forecast Discussion Number 34

2018-09-15 22:34:05| National Hurricane Center (Atlantic)

Issued at 500 PM AST Sat Sep 15 2018 251 WTNT43 KNHC 152034 TCDAT3 Tropical Storm Helene Discussion Number 34 NWS National Hurricane Center Miami FL AL082018 500 PM AST Sat Sep 15 2018 The cloud pattern of Helene is gradually decaying and transitioning to a shear pattern, with the low-level center occasionally visible through breaks in the cirrus overcast. The initial intensity is reduced to 55 kt in best agreement with the CIMSS satellite consensus. Helene is moving toward colder water and into increasing shear associated with the mid-latitude westerlies. This should cause continued weakening and, as Helene merges with a frontal system, extratropical transition in about 36 h. The forecast will continue to call for Helene or its remnants to be absorbed into a large non-tropical low over the North Atlantic between 72-96 h, and the latest global model guidance suggests this could happen earlier than forecast. The initial motion is now 050/19. A mostly northeastward motion at a faster forward speed should occur during the remainder of the cyclone's life as it becomes more embedded in the westerlies. On the forecast track, Helene should pass near or over the western Azores during the next several hours, and head toward Ireland and the United Kingdom Sunday night and Monday. There are again no significant changes to either the track guidance or the official forecast track since the last advisory. Tropical storm conditions are spreading over portions of the Azores and should continue through tonight. Helene is also expected to be a storm-force post-tropical cyclone when it approaches Ireland and the United Kingdom in two to three days. Interests in those locations should consult products from their local meteorological service for information about potential impacts from Helene. Local forecasts and warnings for the UK can be found on the website of the UK Met Office at https://www.metoffice.gov.uk/. Local forecasts and warnings for Ireland can be found on the website of Met Eireann at https://www.met.ie/. FORECAST POSITIONS AND MAX WINDS INIT 15/2100Z 40.2N 32.1W 55 KT 65 MPH 12H 16/0600Z 41.9N 29.2W 55 KT 65 MPH 24H 16/1800Z 44.4N 24.1W 55 KT 65 MPH 36H 17/0600Z 47.2N 17.9W 55 KT 65 MPH...POST-TROP/EXTRATROP 48H 17/1800Z 51.2N 11.1W 50 KT 60 MPH...POST-TROP/EXTRATROP 72H 18/1800Z 61.0N 1.0E 50 KT 60 MPH...POST-TROP/EXTRATROP 96H 19/1800Z...ABSORBED BY EXTRATROPICAL LOW $$ Forecaster Beven

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Tropical Storm Joyce Forecast Discussion Number 13

2018-09-15 22:31:35| National Hurricane Center (Atlantic)

Issued at 500 PM AST Sat Sep 15 2018 165 WTNT45 KNHC 152031 TCDAT5 Tropical Storm Joyce Discussion Number 13 NWS National Hurricane Center Miami FL AL102018 500 PM AST Sat Sep 15 2018 The circulation of Joyce is elongated this afternoon, with the center partially exposed. It has been interesting to watch deep convection firing in the northeastern quadrant, kicking off new small swirls that rotate around the mean circulation. A blend of the Dvorak estimates from TAFB/SAB supports keeping the wind speed 40 kt on this advisory. There is no change to the intensity forecast reasoning. The environment is predicted to be nearly steady-state around Joyce through Sunday, with strong shear being offset by waters near 27C. Thereafter, as the hostile shear continues, the mid-level air should become drier near the center, and SSTs cool off below 26C. These conditions should cause Joyce to slowly weaken early next week, and dissipate in 3 days or so. This forecast is basically the same as the previous one, but it isn't out of the realm of possibility the cyclone could open up into a trough before that time. Joyce continues to accelerate, now moving east-northeastward at about 15 kt. The storm remains embedded within the same trough as Tropical Storm Helene, and Joyce should move east-northeastward during the next day or so. Beyond that time, as Joyce becomes a shallow cyclone, it is forecast to turn southeastward due to low-level flow around the eastern side of a building ridge over the central Atlantic Ocean. The track forecast is simply an update to the previous one, and continues to be on the southern side of the guidance envelope. FORECAST POSITIONS AND MAX WINDS INIT 15/2100Z 33.1N 39.6W 40 KT 45 MPH 12H 16/0600Z 34.1N 37.2W 40 KT 45 MPH 24H 16/1800Z 34.8N 33.8W 40 KT 45 MPH 36H 17/0600Z 34.8N 30.5W 35 KT 40 MPH 48H 17/1800Z 34.0N 28.0W 30 KT 35 MPH 72H 18/1800Z 31.5N 26.5W 25 KT 30 MPH 96H 19/1800Z...DISSIPATED $$ Forecaster Blake

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Tropical Storm Florence Forecast Discussion Number 65

2018-09-15 16:47:36| National Hurricane Center (Atlantic)

Issued at 1100 AM EDT Sat Sep 15 2018 714 WTNT41 KNHC 151447 TCDAT1 Tropical Storm Florence Discussion Number 65 NWS National Hurricane Center Miami FL AL062018 1100 AM EDT Sat Sep 15 2018 Florence continues to creep slowly westward and weaken across eastern South Carolina. NOAA WSR-88D Doppler weather radar data indicate strong winds near 60 kt still exist between 3000-10000 ft within intense rainbands situated between the Cape Fear/Wilmington area and Bogue Inlet, North Carolina. Although those velocity values would typically correlate to 50-kt surface winds, those winds appear to be associated with small mesoscale circulations and possible supercell thunderstorms, and not the larger tangential wind field. In contrast, surface observations during the past couple of hours have only shown sustained winds of around 40 kt, so that is the intensity used for this advisory. The estimated central pressure of 995 mb is based on nearby surface observations across eastern South Carolina. Radar data and surface observations indicate that Florence has turned more westward, and has slowed down even more, and the initial motion estimate is now 270/02 kt. The models remain in very good agreement that a mid-level ridge currently centered over the upper Midwest will slide eastward across the lower Great Lakes to near the northeastern U.S. during the next 48 hours. This feature is expected to steer Florence and its remnants in a general westward motion for the next 24 hours or so, followed by a turn toward the northwest on Monday. The ridge will continue to shift eastward and weaken, allowing Florence's circulation to get caught up in the faster mid-latitude westerlies and accelerate toward the northeast on day 3 and toward the east-northeast on days 4 and 5 as an extratropical low pressure system. The new NHC track is similar to the previous advisory track, and lies near the consensus model TCVA/TVCN. Florence's inner-core convection and wind field will continue to weaken throughout the next 72 hours or so. However, the outer wind field and an associated band of deep convection in the eastern semicircle will likely produce sustained tropical-storm-force winds for another 12 hours or so, with some high gusts continuing until the band moves inland by late Sunday as per the latest the latest NOAA HRRR and other mesoscale model runs. More importantly, continued heavy rains will be produced by this band of convection, which will exacerbate the already catastrophic flooding that is occurring across much of southeastern North Carolina. The official intensity forecast closely follows the Decay-SHIPS model guidance and the intensity model IVCN through 72 hours, and then follows a blend of the IVCN, HCCA, and FSSE consensus models on days 4 and 5 when the post-tropical cyclone is forecast to strengthen due to baroclinic processes after moving over the relatively warm waters of the northwestern Atlantic Ocean. Although coastal storm surge flooding will gradually subside today, especially after the midday high-tide period ends, extremely heavy rainfall will continue to be a serious hazard associated with slow-moving Florence. More than two feet of rain has already fallen across portions of southeastern North Carolina, and more rain is still to come, which will cause disastrous flooding that will spread inland through the weekend. Key Messages: 1. Life-threatening storm surge will continue along portions of the North Carolina coast through today, and also along the Neuse and Pamlico Rivers in western Pamlico Sound, where rainfall and freshwater flooding will also contribute to high water levels. Dangerous storm surge could also affect portions of the northeast coast of South Carolina coast today. 2. Life-threatening, catastrophic flash floods and prolonged significant river flooding are likely over portions of the Carolinas and the southern to central Appalachians from western North Carolina into west-central Virginia and far eastern West Virginia through early next week, as Florence moves slowly inland. In addition to the flash flood and flooding threat, landslides are also possible in the higher terrain of the southern and central Appalachians across western North Carolina into southwest Virginia. 3. Tropical storm conditions will continue along the coast within the tropical storm warning area and also well inland across portions of South Carolina and North Carolina today. 4. Large swells affecting Bermuda, portions of the U.S. East Coast, and the northwestern and central Bahamas will continue this week, resulting in life-threatening surf and rip currents. FORECAST POSITIONS AND MAX WINDS INIT 15/1500Z 33.6N 79.6W 40 KT 45 MPH...INLAND 12H 16/0000Z 33.6N 80.5W 35 KT 40 MPH...INLAND 24H 16/1200Z 34.4N 81.7W 25 KT 30 MPH...INLAND 36H 17/0000Z 36.0N 82.8W 25 KT 30 MPH...INLAND 48H 17/1200Z 38.0N 82.9W 20 KT 25 MPH...INLAND 72H 18/1200Z 41.3N 76.5W 20 KT 25 MPH...POST-TROP/REMNT LOW 96H 19/1200Z 44.3N 63.4W 30 KT 35 MPH...POST-TROP/EXTRATROP 120H 20/1200Z 48.5N 49.0W 40 KT 45 MPH...POST-TROP/EXTRATROP $$ Forecaster Stewart

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Tropical Storm Helene Forecast Discussion Number 33

2018-09-15 16:35:06| National Hurricane Center (Atlantic)

Issued at 1100 AM AST Sat Sep 15 2018 582 WTNT43 KNHC 151435 TCDAT3 Tropical Storm Helene Discussion Number 33 NWS National Hurricane Center Miami FL AL082018 1100 AM AST Sat Sep 15 2018 The cloud pattern of Helene is starting to show signs of decay, with the central convection decreasing since the previous advisory and the low-level center becoming partly exposed in a region between convective bands. The initial intensity is held at 60 kt for this advisory in best agreement with the CIMSS satellite consensus, but it is possible this is generous. The 34-kt wind radii in the southeastern quadrant were revised based on a partial ASCAT pass. Helene is moving toward colder water and into an area of strong shear associated with the mid-latitude westerlies. This should cause the system to weaken and, as it merges with a frontal system, become an extratropical cyclone in about 36 h. The global models forecast Helene to be absorbed into a large non-tropical low over the far north Atlantic after it passes Ireland and the United Kingdom, and thus the system is forecast to dissipate completely between 72-96 h. Helene has turned northeastward with the initial motion now 035/18. A faster motion toward the northeast should occur during the remainder of the cyclone's life as it becomes more embedded in the westerlies. On the forecast track, Helene should pass near or over the western Azores later today or tonight, and head toward Ireland and the United Kingdom late this weekend and early next week. There are no significant changes to either the track guidance or the official forecast track since the last advisory. Tropical storm conditions should spread over portions of the Azores during the next several hours and continue through tonight. Helene is also expected to be a storm-force post-tropical cyclone when it approaches Ireland and the United Kingdom in two to three days. Interests in those locations should consult products from their local meteorological service for information about potential impacts from Helene. Local forecasts and warnings for the UK can be found on the website of the UK Met Office at https://www.metoffice.gov.uk/. Local forecasts and warnings for Ireland can be found on the website of Met Eireann at https://www.met.ie/. FORECAST POSITIONS AND MAX WINDS INIT 15/1500Z 38.8N 34.0W 60 KT 70 MPH 12H 16/0000Z 40.8N 31.4W 60 KT 70 MPH 24H 16/1200Z 43.0N 27.2W 55 KT 65 MPH 36H 17/0000Z 45.5N 21.6W 55 KT 65 MPH...POST-TROP/EXTRATROP 48H 17/1200Z 49.0N 15.0W 55 KT 65 MPH...POST-TROP/EXTRATROP 72H 18/1200Z 59.0N 1.5W 50 KT 60 MPH...POST-TROP/EXTRATROP 96H 19/1200Z...DISSIPATED $$ Forecaster Beven

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Tropical Storm Joyce Forecast Discussion Number 12

2018-09-15 16:32:04| National Hurricane Center (Atlantic)

Issued at 1100 AM AST Sat Sep 15 2018 377 WTNT45 KNHC 151432 TCDAT5 Tropical Storm Joyce Discussion Number 12 NWS National Hurricane Center Miami FL AL102018 1100 AM AST Sat Sep 15 2018 Joyce is feeling the effects of shear this morning. The low-level center of the tropical storm has become exposed to the south of a area of deep convection. ASCAT data indicate the the maximum winds have dropped to 35-40 kt, and 40 kt is chosen as the initial wind speed since the resolution of ASCAT is likely causing an undersampling of the small storm. The environment isn't forecast to change much near Joyce during the next day or so, with strong shear being offset by waters near 27C. The NHC forecast is leveled off during that time, a bit lower than the previous prediction mostly to account for the initial wind speed. Thereafter, while the shear is forecast to continue, the mid-level air should become drier near the center, and SSTs cool off gradually below 26C. These conditions should cause Joyce to slowly weaken early next week, and dissipate in 3 days or so. This is the solution provided by much of the guidance, and the NHC forecast follows suit. Joyce has accelerated overnight, now moving east-northeastward at about 12 kt. The storm remains embedded in the same trough as Tropical Storm Helene and it should follow that tropical cyclone east-northeastward to northeastward at increasing forward speeds during the next day or two. Beyond that time, as Joyce becomes a shallow cyclone, it is forecast to turn southeastward due to low-level flow around the eastern side of a building ridge over the central Atlantic Ocean. No significant changes were made to the forecast, which lies on the southern side of the guidance envelope. FORECAST POSITIONS AND MAX WINDS INIT 15/1500Z 32.0N 41.0W 40 KT 45 MPH 12H 16/0000Z 32.9N 39.2W 40 KT 45 MPH 24H 16/1200Z 34.0N 35.9W 40 KT 45 MPH 36H 17/0000Z 34.5N 32.2W 35 KT 40 MPH 48H 17/1200Z 34.0N 29.0W 30 KT 35 MPH 72H 18/1200Z 32.0N 25.5W 25 KT 30 MPH 96H 19/1200Z...DISSIPATED $$ Forecaster Blake

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