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Tropical Depression Twenty Forecast Discussion Number 1
2021-09-29 16:36:42| National Hurricane Center (Atlantic)
Issued at 200 PM CVT Wed Sep 29 2021 000 WTNT45 KNHC 291436 TCDAT5 Tropical Depression Twenty Discussion Number 1 NWS National Hurricane Center Miami FL AL202021 200 PM CVT Wed Sep 29 2021 The area of low pressure located over the far eastern tropical Atlantic that NHC has been monitoring has now become a tropical depression, the twentieth tropical cyclone of the 2021 Atlantic hurricane season. Satellite images indicate that the system is producing a large area of showers and thunderstorms that have become fairly well organized around the center. An ASCAT-A pass from a few hours ago indicated that the system now has a well-defined circulation and that the radius of maximum wind is 30-40 n mi north of the center. The initial intensity is set at 30 kt based on the ASCAT data and the T2.0/30 kt Dvorak classifications from TAFB and SAB. The minimum pressure of 1007 mb is partially based on data from a ship report that passed near the depression. Based on satellite fixes, the depression appears to be moving west-northwestward at about 12 kt. The system is currently located on the south side of a deep-layer ridge, and that feature should keep the cyclone on a west-northwest track during the next couple of days. After that time, the models show a mid- to upper-level low developing over the central tropical Atlantic, which erodes the western portion of the ridge. This change in the steering flow should cause the system to turn northwestward on Friday and then northward toward the end of the forecast period. Although the models generally agree on the evolution of the large-scale pattern, there are notable differences in the details, which leads to a fair amount of spread concerning when and where the turn to the north occurs. The GFS is farthest east while the ECMWF shows the westernmost solution. The NHC track forecast lies between those models and is near the TVCA multi-model consensus. The depression is expected to remain in conducive environmental conditions for strengthening during the next 2 or 3 days. During that time period, the storm is forecast to move over 28-29 deg C waters while embedded within an airmass of high mid-level moisture and very low wind shear (less than 10 kt). Therefore, steady strengthening seems likely, and the NHC forecast brings the system to a tropical storm by tonight and a hurricane in a couple of days. Beyond a few days, however, the models show a pronounced increase in southwesterly shear and a notably drier atmosphere. These unfavorable conditions should end the opportunity for strengthening and promote a weakening trend. The NHC intensity forecast is a near a blend of the HCCA, IVCN, and IVDR consensus models. FORECAST POSITIONS AND MAX WINDS INIT 29/1500Z 8.3N 24.6W 30 KT 35 MPH 12H 30/0000Z 8.8N 25.8W 35 KT 40 MPH 24H 30/1200Z 9.5N 27.6W 45 KT 50 MPH 36H 01/0000Z 10.3N 29.9W 55 KT 65 MPH 48H 01/1200Z 11.2N 31.9W 65 KT 75 MPH 60H 02/0000Z 12.4N 33.7W 70 KT 80 MPH 72H 02/1200Z 14.0N 35.4W 70 KT 80 MPH 96H 03/1200Z 18.4N 38.8W 55 KT 65 MPH 120H 04/1200Z 24.3N 40.9W 45 KT 50 MPH $$ Forecaster Cangialosi
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Hurricane Sam Forecast Discussion Number 27
2021-09-29 10:38:37| National Hurricane Center (Atlantic)
Issued at 500 AM AST Wed Sep 29 2021 000 WTNT43 KNHC 290838 TCDAT3 Hurricane Sam Discussion Number 27 NWS National Hurricane Center Miami FL AL182021 500 AM AST Wed Sep 29 2021 The eye of Sam has become cloud filled in infrared satellite imagery overnight, but the ring of deep convection surrounding the eye has cooled. There appears to be a southwest-to-northeast tilt to Sam's circulation as some modest southwesterly shear seems to be impinging on the hurricane. An Air Force Reserve Hurricane Hunter aircraft that has been investigating Sam overnight shows that the hurricane continues to fluctuate in intensity. The aircraft has reported a peak flight-level wind of 124 kt and SFMR winds of 108 kt. Therefore the initial wind speed has been set at 115 kt, which leans toward the higher flight-level-to-surface wind reduction. The aircraft also reported a minimum pressure of 950 mb, which is up a few millibars from the flight yesterday afternoon. The fluctuations in intensity that Sam has experienced over the past couple of days are fairly typical for hurricanes of this strength. As mentioned in the previous advisory, Sam will be moving into a lower vertical wind shear environment, and the expected faster forward speed decreases the potential for upwelling beneath the storm. As a result, some re-intensification is forecast, but difficult-to-predict eyewall replacement cycles could cause additional fluctuations in intensity over the next day or two. After 60 to 72 hours, a more pronounced weakening trend is likely to begin as the storm moves over cooler SSTs and the vertical shear increases. The global models indicate that Sam will begin its transition into a powerful extratropical cyclone late in the period, but that transition will likely not be completed until after 120 h. Sam is moving northwestward at about 8 kt. The track guidance remains in excellent agreement through 96 hours as Sam should move northwestward and then northward around the western portion of the subtropical ridge over the eastern Atlantic during the next few days. By Saturday, Sam is forecast to turn northeastward between the aforementioned ridge and a deep-layer trough over the northeastern U.S. and Nova Scotia. By late in the period, there is an increase in spread in the guidance which is related to how Sam interacts with the deep-layer trough. For now, the NHC track forecast at days 4 and 5 lies between the TVCA multi-model consensus and the HFIP corrected consensus aid. Overall, little change to the previous NHC track forecast was required for this advisory. Key Messages: 1. Large swells generated by Sam are affecting the Leeward Islands and will spread to portions of the Virgin Islands, Puerto Rico, Hispaniola, the Bahamas, and Bermuda by Thursday or Friday. Significant swells will likely reach the east coast of the United States and Atlantic Canada by the weekend. These swells will likely cause life-threatening surf and rip current conditions, and beachgoers and other interests along these coasts are urged to follow the advice of lifeguards and local officials through the upcoming weekend. FORECAST POSITIONS AND MAX WINDS INIT 29/0900Z 18.9N 56.2W 115 KT 130 MPH 12H 29/1800Z 19.7N 57.3W 120 KT 140 MPH 24H 30/0600Z 21.0N 58.9W 120 KT 140 MPH 36H 30/1800Z 22.8N 60.4W 120 KT 140 MPH 48H 01/0600Z 25.2N 61.5W 115 KT 130 MPH 60H 01/1800Z 27.9N 62.0W 115 KT 130 MPH 72H 02/0600Z 30.8N 61.5W 110 KT 125 MPH 96H 03/0600Z 35.8N 57.5W 95 KT 110 MPH 120H 04/0600Z 40.8N 52.0W 80 KT 90 MPH $$ Forecaster Brown
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Hurricane Sam Forecast Discussion Number 26
2021-09-29 04:35:07| National Hurricane Center (Atlantic)
Issued at 1100 PM AST Tue Sep 28 2021 000 WTNT43 KNHC 290234 TCDAT3 Hurricane Sam Discussion Number 26 NWS National Hurricane Center Miami FL AL182021 1100 PM AST Tue Sep 28 2021 Satellite images suggest that Sam has strengthened. The eyewall of the hurricane has become more intense, with a warmer eye noted during the past several hours since the last reconnaissance aircraft departed. The initial wind speed is nudged upward to 120 kt, a little above what the previous aircraft mission from this afternoon supported. Another Air Force Reserve Hurricane Hunter plane is scheduled to fly through Sam in a few hours for a better look at the intensity. Sam is forecast to move into an area with somewhat lower wind shear and higher ocean heat content by late Wednesday. In addition, the hurricane will likely be moving a little faster, which decreases the upwelling potential under the storm. These factors lead me to believe Sam isn't quite done intensifying, and could reach another peak sometime late Wednesday or Thursday. The one fly-in-the- ointment to this forecast is another eyewall replacement cycle potentially beginning, although recent microwave data doesn't give that impression. Thus, the new forecast is raised from the last one during the first couple of days, on the higher side of the guidance. A more consistent weakening trend is expected late week, due to cooler SSTs and increasing shear, and no significant changes were made to the official forecast at long range. The hurricane continues moving northwestward at about 8 kt. Models are locked into this track continuing for the next day or two at a faster pace around the southwestern portion of a strengthening Atlantic subtropical ridge. Thereafter, Sam should turn northward on Friday and northeastward this weekend, likely a fair distance east of Bermuda, due to a large mid-latitude trough moving off of the United States east coast. Model guidance remains in close agreement on almost all of the forecast, and the only significant change from the last advisory is an eastward adjustment on Day 5 due to guidance suggesting that Sam gets shunted more eastward by the trough, rather than captured by it. Key Messages: 1. Large swells generated by Sam are affecting the Leeward Islands and will spread to portions of the Virgin Islands, Puerto Rico, Hispaniola, the Bahamas, and Bermuda by Thursday or Friday. Significant swells will likely reach the east coast of the United States and Atlantic Canada by the weekend. These swells will likely cause life-threatening surf and rip current conditions, and beachgoers and other interests along these coasts are urged to follow the advice of lifeguards and local officials through the upcoming weekend. FORECAST POSITIONS AND MAX WINDS INIT 29/0300Z 18.4N 55.6W 120 KT 140 MPH 12H 29/1200Z 19.1N 56.7W 120 KT 140 MPH 24H 30/0000Z 20.2N 58.2W 125 KT 145 MPH 36H 30/1200Z 21.8N 59.9W 125 KT 145 MPH 48H 01/0000Z 23.9N 61.2W 120 KT 140 MPH 60H 01/1200Z 26.4N 61.9W 115 KT 130 MPH 72H 02/0000Z 29.1N 61.8W 110 KT 125 MPH 96H 03/0000Z 35.0N 58.0W 100 KT 115 MPH 120H 04/0000Z 40.0N 52.0W 85 KT 100 MPH $$ Forecaster Blake
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Hurricane Sam Forecast Discussion Number 24
2021-09-28 16:33:52| National Hurricane Center (Atlantic)
Issued at 1100 AM AST Tue Sep 28 2021 000 WTNT43 KNHC 281433 TCDAT3 Hurricane Sam Discussion Number 24 NWS National Hurricane Center Miami FL AL182021 1100 AM AST Tue Sep 28 2021 Satellite images indicate that Sam has been generally steady in strength during the past several hours, and it remains a powerful category 4 hurricane on the Saffir-Simpson Hurricane Wind Scale. The storm has a well-defined eye, and the convective pattern surrounding the center is fairly symmetric. A 1043 UTC SSMIS pass showed a closed and symmetric eyewall, with no indication of a concentric eyewall. The latest Dvorak classifications from TAFB and SAB were 5.5/102 kt and the CIMSS automated technique is currently 6.2/120 kt. Based on these satellite estimates and the overnight reconnaissance data, the initial intensity is held at 115 kt. Another Air Force Hurricane Hunter aircraft is scheduled to investigate Sam this afternoon. The major hurricane is moving slowly northwestward, with the initial motion estimated to be 305/7 kt. A mid-level ridge situated to the north-northeast of Sam is expected to shift eastward as a deep-layer trough digs southeastward over the western Atlantic. This change in the large-scale pattern should cause Sam to speed up and gradually turn toward the north in 2 or 3 days and then to the northeast in 4 to 5 days. The models are in fairly good agreement, and the new forecast track is essentially an update of the previous one. Based on the forecast, Sam should pass well east of the northern Leeward Island through Wednesday and east of Bermuda Friday night and Saturday. Sam is expected to remain in favorable environmental conditions for it to maintain its intensity, or perhaps strengthen slightly, during the next day or two. However, after that time, a gradual increase in southerly shear should promote a slow weakening trend, and Sam will likely be beginning its transition to an extratropical cyclone toward the end of the forecast period. The NHC intensity forecast is also an update of the previous one and in line with the various consensus aids. Sam is expected to remain a major hurricane during the next several days. The initial wind radii have been adjusted based on a 1114 UTC ASCAT-A pass. Key Messages: 1. Large swells generated by Sam are affecting the Leeward Islands and will spread to portions of the Virgin Islands, Puerto Rico, Hispaniola, the Bahamas, and Bermuda by Thursday or Friday. Significant swells will likely reach the east coast of the United States and Atlantic Canada by the weekend. These swells will likely cause life-threatening surf and rip current conditions, and beachgoers and other interests along these coasts are urged to follow the advice of lifeguards and local officials through the upcoming weekend. FORECAST POSITIONS AND MAX WINDS INIT 28/1500Z 17.5N 54.3W 115 KT 130 MPH 12H 29/0000Z 18.2N 55.1W 120 KT 140 MPH 24H 29/1200Z 19.2N 56.5W 115 KT 130 MPH 36H 30/0000Z 20.5N 58.1W 115 KT 130 MPH 48H 30/1200Z 22.0N 59.6W 110 KT 125 MPH 60H 01/0000Z 24.0N 60.9W 110 KT 125 MPH 72H 01/1200Z 26.6N 61.5W 110 KT 125 MPH 96H 02/1200Z 32.5N 59.7W 100 KT 115 MPH 120H 03/1200Z 39.2N 54.8W 90 KT 105 MPH $$ Forecaster Cangialosi
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Hurricane Sam Forecast Discussion Number 23
2021-09-28 10:44:51| National Hurricane Center (Atlantic)
Issued at 500 AM AST Tue Sep 28 2021 053 WTNT43 KNHC 280844 TCDAT3 Hurricane Sam Discussion Number 23 NWS National Hurricane Center Miami FL AL182021 500 AM AST Tue Sep 28 2021 The Air Force Reserve Hurricane Hunters that investigated Sam early this morning found evidence that the hurricane has strengthened. Peak 700-mb flight-level winds of 126 kt were reported in the northeastern quadrant, which after the standard adjustment would support an intensity of 110-115 kt. Satellite data also indicate that Sam's structure has improved overnight. Sam's inner core appears to have consolidated into a single, primary eyewall in recent GMI and AMSR2 microwave imagery, and the latest GOES-16 infrared imagery shows a colder ring of convective cloud tops developing around the eye of Sam. This has brought the latest UW-CIMSS ADT and SATCON estimates up to around 115 kt. Based on the flight-level wind data and ongoing satellite trends, the initial intensity is raised to 115 kt for this advisory. The minimum central pressure of 953 mb is based on a center dropsonde of 954 mb with 12-kt surface winds. In the short-term, internal dynamic processes that are difficult to predict could cause some fluctuations in Sam's intensity. If Sam's eyewall is able to contract today, some additional strengthening could occur, and this possibility is reflected in the latest NHC intensity forecast. Thereafter, the warm sea-surface temperatures and weak to moderate vertical wind shear along Sam's forecast track suggest it should remain a major hurricane for the next several days. Thus, the NHC forecast only shows gradual weakening through 72-96 h, in agreement with the consensus aids IVCN and HCCA. By days 4-5, increasing southerly wind shear along with decreasing SSTs should increase Sam's rate of weakening as it recurves deeper into the mid-latitudes. Aircraft and microwave data indicate that Sam is slightly tilted in the vertical, with the low-level center displaced just a bit to the south of the mid-level eye. Based on recent aircraft fixes, Sam's initial motion is northwestward, or 310/8 kt. This motion is expected to continue for the next few days as Sam moves around the southwestern periphery of the subtropical ridge. A mid- to upper-level trough is forecast to dig southward over the western Atlantic later this week, which should steer Sam toward the north by Friday. Then, Sam is expected to accelerate north-northeastward within the deep-layer southerly flow ahead of the trough this weekend. The official NHC track forecast is very similar to the previous one, with just a slight adjustment to the left based on the latest track guidance consensus aids. The along-track spread in the guidance noticeably increases as Sam recurves over the western Atlantic, with the ECMWF much slower than the GFS. Once again, the NHC forecast trends closer to the faster GFS solution at longer ranges, given its better overall performance this season. Key Messages: 1. Large swells generated by Sam are affecting the Leeward Islands and will spread to portions of the Virgin Islands, Puerto Rico, Hispaniola, the Bahamas, and Bermuda by Thursday or Friday. Significant swells will likely reach the east coast of the United States and Atlantic Canada by the weekend. These swells will likely cause life-threatening surf and rip current conditions, and beachgoers and other interests along these coasts are urged to follow the advice of lifeguards and local officials through the upcoming weekend. FORECAST POSITIONS AND MAX WINDS INIT 28/0900Z 17.2N 53.9W 115 KT 130 MPH 12H 28/1800Z 17.9N 54.7W 120 KT 140 MPH 24H 29/0600Z 18.8N 55.9W 115 KT 130 MPH 36H 29/1800Z 19.9N 57.4W 115 KT 130 MPH 48H 30/0600Z 21.3N 59.0W 110 KT 125 MPH 60H 30/1800Z 23.2N 60.5W 110 KT 125 MPH 72H 01/0600Z 25.6N 61.5W 110 KT 125 MPH 96H 02/0600Z 31.4N 60.7W 105 KT 120 MPH 120H 03/0600Z 39.0N 56.0W 95 KT 110 MPH $$ Forecaster Reinhart
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