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Tropical Storm Isaias Forecast Discussion Number 9

2020-07-30 16:51:21| National Hurricane Center (Atlantic)

Issued at 1100 AM AST Thu Jul 30 2020 000 WTNT44 KNHC 301451 TCDAT4 Tropical Storm Isaias Discussion Number 9 NWS National Hurricane Center Miami FL AL092020 1100 AM AST Thu Jul 30 2020 The satellite presentation of Isaias has improved over the past 24 hours with a concentrated area of deep convection occurring near and to the northeast of the low-level center. Radar imagery from Puerto Rico has shown a mid-level circulation that moved over the Mona passage and is now along the northern coast of Hispaniola. Surface observations and data from an Air Force Reserve reconnaissance aircraft, however, continue to place the center near the southeastern coast of Hispaniola. It is possible that a new center will re-form along the northern coast of Hispaniola as the system interacts with high terrain of that island later today or tonight. The initial intensity remains 50 kt, and is based on the earlier Doppler radar data and observations along the southern coast of Puerto Rico. Isaias is moving northwestward or 310/16 kt. A high pressure ridge over the western Atlantic should steer Isaias on a west- northwestward to northwestward heading over the next couple of days, but the likelihood of a center re-formation during the next 12-18 hours means that some adjustments to the track and motion are possible. By late Friday, a mid-latitude trough moving into the east-central United States is expected to weaken the western portion of the ridge. This pattern should cause the cyclone to turn northwestward to north-northwestward on Saturday when it is near the northwestern Bahamas and South Florida. As the trough slides eastward over the United States, this should steer Isaias northward and northeastward early next week. Although the bulk of the track guidance agrees on this overall scenario, the confidence in the track forecast remains lower than usual due to the expected land interaction and possible center reformation in the short term. The new NHC track forecast is a blend of the HFIP corrected consensus and the TCVA multi-model consensus, and is similar to the previous advisory. The intensity forecast remains challenging. The structure of the storm is likely to be disrupted by its passage near or over Hispaniola today, and some weakening is likely. Once the system moves away from the Greater Antilles gradual strengthening is anticipated. The global models and the SHIPS guidance suggest that Isaias will encounter an area of moderate southwesterly shear over the weekend, and the NHC intensity forecast is again leveled off at that time. There are models that continue to suggest Isaias could become a hurricane when it is near the U.S., but given the continued uncertainty, the NHC intensity forecast remains near the intensity consensus. Key Messages: 1. Isaias will produce heavy rains and potentially life-threatening flash flooding and mudslides across Puerto Rico, the Dominican Republic, northern Haiti, the Turks and Caicos, and the Bahamas. Heavy rains associated with Isaias may begin to affect South Florida Saturday morning, potentially resulting in isolated flash and urban flooding. 2. Tropical storm conditions will continue across portions of the Virgin Islands and Puerto Rico through early afternoon and will spread across portions of the Dominican Republic, Haiti, the Turks and Caicos and the Bahamas later today through Friday, and Tropical Storm Warnings are in effect for these areas. Do not focus on the details of the track forecast, as rainfall and wind hazards will extend far from the center of the system. 3. There is a risk of impacts from winds, heavy rainfall, and storm surge along portions of the U.S. east coast beginning this weekend in Florida and spreading northward to the Carolinas and southern mid-Atlantic states early next week. Due to Isaias interacting with Hispaniola, the details of the track and intensity forecast remain uncertain and it is too soon to determine the magnitude and location of these potential impacts. Interests in these areas should monitor the progress of Isaias and updates to the forecast. Tropical storm or hurricane watches could be issued for portions of South Florida later today. FORECAST POSITIONS AND MAX WINDS INIT 30/1500Z 18.1N 68.9W 50 KT 60 MPH 12H 31/0000Z 19.7N 71.4W 45 KT 50 MPH...INLAND 24H 31/1200Z 21.4N 74.2W 50 KT 60 MPH...OVER WATER 36H 01/0000Z 23.0N 76.5W 55 KT 65 MPH 48H 01/1200Z 24.6N 78.2W 60 KT 70 MPH 60H 02/0000Z 26.2N 79.4W 60 KT 70 MPH 72H 02/1200Z 28.0N 79.9W 60 KT 70 MPH 96H 03/1200Z 31.5N 79.1W 60 KT 70 MPH 120H 04/1200Z 37.5N 73.5W 60 KT 70 MPH $$ Forecaster Brown

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Tropical Storm Isaias Forecast Discussion Number 8

2020-07-30 10:52:45| National Hurricane Center (Atlantic)

Issued at 500 AM AST Thu Jul 30 2020 000 WTNT44 KNHC 300852 TCDAT4 Tropical Storm Isaias Discussion Number 8 NWS National Hurricane Center Miami FL AL092020 500 AM AST Thu Jul 30 2020 Isaias is sending some mixed signals tonight. The 1-min rapid scan data from GOES-16 indicates that the low-level center is likely displaced west of a very intense burst of deep convection on the northeastern side of the circulation. However, the satellite data also shows increased banding features overnight and a more organized central cloud pattern, with recent hints that perhaps a low-level center is trying to re-form closer to the convection. Radar observations from San Juan show 60-65 kt Doppler wind velocities during the past few hours near 5000 ft, so the initial wind speed is raised to 50 kt. Model forecasts are showing a complex evolution of the tropical cyclone during the next day or two. There is good agreement that Isaias will move across Hispaniola later today, and its low-level center will likely become disorganized over the high terrain. However, the strong burst of convection currently near Puerto Rico is associated with a mid-level circulation, which should pass along the north coast of Hispaniola later today. Most of the model guidance suggest that this feature will cause the re-development of a surface center over the northern part of the broader system while the mid-level circulation moves close to the southeastern Bahamas. Afterward, the cyclone would then move northwestward until the weekend, and gradually turn northward and northeastward close to the U.S. East Coast into early next week ahead of a mid-latitude trough. The official track forecast is a little to the east of the previous one and close to the NOAA corrected dynamical model consensus. It should be noted that further adjustments to the forecast tracks are indeed possible, especially after Isaias moves north of Hispaniola. The intensity forecast is quite tricky. In the short term, Isaias is expected to move across Hispaniola, as the storm's interaction with the mountainous island should cause some weakening and disruption to the circulation. However, as mentioned before, the models suggest that a new center could form, and the environmental conditions would support gradual intensification. The intensity models have been trending higher, and the official forecast is nudged upward accordingly, now showing a peak intensity of 60 kt when the storm is near the coast of Florida and the Southeast U.S. Coast. It should be noted that there are models that show hurricane strength near the U.S. but, given the large amount of uncertainty, it is preferred to stay on the conservative side for now. We should have a better idea of how strong Isaias will become near the U.S. after reconnaissance aircraft sample the storm and after it passes Hispaniola later today. Key Messages: 1. Isaias will produce heavy rains and potentially life-threatening flash flooding and mudslides across the Virgin Islands, Puerto Rico, the Dominican Republic, northern Haiti, and over the Bahamas. 2. Tropical storm conditions are likely across portions of the the Virgin Islands, and Puerto Rico through this morning and will spread westward to portions of the Dominican Republic, Haiti, and the Turks and Caicos and the Bahamas later today and Friday. Tropical Storm Warnings are in effect for these areas. Do not focus on the details of the track forecast, as rainfall and wind hazards will extend far from the center of the system. 3. While this system could bring some rainfall and wind impacts to portions of Cuba and Florida later this week and this weekend, it is too soon to determine the location or magnitude of those impacts. Interests there should monitor the progress of this system and updates to the forecast over the next couple of days. FORECAST POSITIONS AND MAX WINDS INIT 30/0900Z 17.2N 67.9W 50 KT 60 MPH 12H 30/1800Z 18.8N 70.2W 45 KT 50 MPH...INLAND 24H 31/0600Z 20.6N 72.9W 45 KT 50 MPH...OVER WATER 36H 31/1800Z 22.3N 75.5W 50 KT 60 MPH 48H 01/0600Z 24.0N 77.9W 55 KT 65 MPH 60H 01/1800Z 25.5N 79.2W 60 KT 70 MPH 72H 02/0600Z 27.3N 80.0W 60 KT 70 MPH 96H 03/0600Z 31.0N 80.0W 60 KT 70 MPH 120H 04/0600Z 37.0N 74.5W 60 KT 70 MPH $$ Forecaster Blake/Cangialosi

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Tropical Storm Isaias Forecast Discussion Number 7

2020-07-30 05:01:12| National Hurricane Center (Atlantic)

Issued at 1100 PM AST Wed Jul 29 2020 991 WTNT44 KNHC 300301 TCDAT4 Tropical Storm Isaias Discussion Number 7 NWS National Hurricane Center Miami FL AL092020 1100 PM AST Wed Jul 29 2020 Observations from recent scatterometer passes over the system show that it now has a sufficiently well-defined center to be designated as a tropical cyclone. The current intensity is estimated to be 45 kt, but these winds are currently occurring over the Atlantic waters well to the north and northeast of the center. Since the cyclone is expected to move over Hispaniola on Thursday some weakening is likely within the next 24 hours. However since Isaias has such a broad wind field, the weakening will probably not be as significant as in a typical tropical cyclone with a small radius of maximum winds. Also, a re-formation of the center to the north of Hispaniola may occur. Later in the forecast period some strengthening is likely, although this may be offset by southwesterly wind shear on the order of 20 kt in 2-3 days. The official forecast is close to the intensity model consensus but well below the latest LGEM guidance. The scatterometer data show that the center of the system is south of the previously estimated track, so there is a lot of uncertainty in the initial motion estimate of 285/17 kt. Isaias should move on a west-northwestward to northwestward track on the southern and southwestern side of a mid-tropospheric ridge. In 2-3 days, the system is expected to turn north-northwestward due to a weakness in the ridge and an approaching trough. Later in the forecast period, the trough should cause Isaias to turn toward the northeast. The official track forecast is a little to the east of the previous one and a little west of the simple and corrected dynamical model consensus tracks. It should be noted that further adjustments to the forecast tracks are indeed possible, especially after Isaias moves north of Hispaniola. Key Messages: 1. Isaias will produce heavy rains and potentially life-threatening flash flooding and mudslides across the Virgin Islands, Puerto Rico, the Dominican Republic, northern Haiti, and over the southeastern Bahamas. 2. Tropical storm conditions are likely across portions of the the Virgin Islands, and Puerto Rico through tonight and will spread westward to portions of the Dominican Republic, Haiti, and the southeastern Bahamas and Turks and Caicos and the Central Bahamas on Thursday and Friday. Tropical Storm Warnings are in effect for these areas. Do not focus on the details of the track forecast, as rainfall and wind hazards will extend far from the center of the system. 3. While this system could bring some rainfall and wind impacts to portions of Cuba, the northwestern Bahamas, and Florida later this week and this weekend, it is too soon to determine the location or magnitude of those impacts. Interests there should monitor the progress of this system and updates to the forecast over the next few days. FORECAST POSITIONS AND MAX WINDS INIT 30/0300Z 15.8N 67.0W 45 KT 50 MPH 12H 30/1200Z 17.6N 69.3W 45 KT 50 MPH 24H 31/0000Z 19.4N 72.2W 40 KT 45 MPH...INLAND 36H 31/1200Z 21.4N 74.8W 45 KT 50 MPH...OVER WATER 48H 01/0000Z 22.9N 77.3W 50 KT 60 MPH 60H 01/1200Z 24.8N 79.2W 55 KT 65 MPH 72H 02/0000Z 26.6N 80.2W 55 KT 65 MPH...INLAND 96H 03/0000Z 30.0N 80.5W 55 KT 65 MPH...INLAND 120H 04/0000Z 35.0N 77.0W 50 KT 60 MPH...INLAND $$ Forecaster Pasch

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Potential Tropical Cyclone Nine Forecast Discussion Number 6

2020-07-29 22:44:09| National Hurricane Center (Atlantic)

Issued at 500 PM AST Wed Jul 29 2020 000 WTNT44 KNHC 292044 TCDAT4 Potential Tropical Cyclone Nine Discussion Number 6 NWS National Hurricane Center Miami FL AL092020 500 PM AST Wed Jul 29 2020 Deep convection has continued to increase in association with the disturbance since the previous advisory. More recently, a band has developed over the southwestern portion of the broad circulation and it appears that the system may be closer to acquiring a well-defined center. Earlier ASCAT data that arrived shortly after the release of the previous advisory revealed a large area of 35-40 kt winds well to the north of the vorticity maximum. This supports maintaining the initial intensity of 40 kt. The estimated motion remains a brisk 290/20 kt. A strong subtropical ridge that extends westward over the western Atlantic should continue to steer the system west-northwestward over the next couple of days, bringing the system near or over Hispaniola late tonight and Thursday. A mid-latitude trough moving into the central United States on Friday is foreast to weaken the western portion of the ridge which should cause a reduction in the forward speed of the system and a turn toward the northwest and north later in the forecast period. The latest iterations of the dynamical models have trended toward a more eastward solution, taking the system near or just east of the Florida peninsula. Since the system still lacks a well-defined center it is not surprising to see these inconsistencies in the model runs. As a result, the NHC forecast has been shifted eastward, but it remains to the west of the latest multi-model consensus aids out of respect for continuity and the continued possibility of further model shifts. The system is still forecast to become a tropical storm before reaching Hispaniola, and some slight strengthening could occur before landfall Thursday morning. Some weakening is likely as the system moves over that island. The system is likely to take some time to recover after its passage over land, and given the forecast for at least moderate south to southwesterly shear, only gradual strengthening is indicated at that time. As mentioned above, some of the global models are now showing a track farther away from eastern Cuba and east of Florida, and if that occurs the NHC intensity forecast could be somewhat conservative. Interests in the Bahamas, Cuba, and Florida should continue to monitor forecasts as changes to both the track and intensity are likely. Key Messages: 1. Potential Tropical Cyclone Nine will produce heavy rains and potentially life-threatening flash flooding and mudslides across the northern Leeward Islands, the Virgin Islands, Puerto Rico, the Dominican Republic, northern Haiti, and over the Inagua Islands into the southeastern Bahamas. 2. Tropical storm conditions are likely across portions of the Leeward Islands, the Virgin Islands, and Puerto Rico through tonight and will spread westward to portions of the Dominican Republic, Haiti, and the southeastern Bahamas and Turks and Caicos on Thursday and Thursday night. Tropical Storm Warnings are in effect for these areas. Do not focus on the details of the track forecast, as rainfall and wind hazards will extend far from the center of the system. 3. The details of the long-range track and intensity forecasts remain more uncertain than usual since the system does not have a well-defined center and it is expected to move near or over portions of the Greater Antilles later this week and move near or over Florida this weekend. While this system could bring some rainfall and wind impacts to portions of Cuba, the central and northwest Bahamas, and Florida later this week and this weekend, it is too soon to determine the location or magnitude of those impacts. Interests there should monitor the progress of this system and updates to the forecast over the next few days. FORECAST POSITIONS AND MAX WINDS INIT 29/2100Z 16.4N 65.6W 40 KT 45 MPH...POTENTIAL TROP CYCLONE 12H 30/0600Z 17.5N 68.1W 45 KT 50 MPH...TROPICAL CYCLONE 24H 30/1800Z 19.2N 71.0W 40 KT 45 MPH...INLAND 36H 31/0600Z 20.9N 74.2W 40 KT 45 MPH...OVER WATER 48H 31/1800Z 22.3N 76.9W 45 KT 50 MPH 60H 01/0600Z 23.6N 78.9W 50 KT 60 MPH 72H 01/1800Z 25.5N 80.8W 45 KT 50 MPH...INLAND 96H 02/1800Z 28.6N 82.1W 40 KT 45 MPH...INLAND 120H 03/1800Z 31.5N 82.5W 35 KT 40 MPH...INLAND $$ Forecaster Brown

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Potential Tropical Cyclone Nine Forecast Discussion Number 5

2020-07-29 16:51:14| National Hurricane Center (Atlantic)

Issued at 1100 AM AST Wed Jul 29 2020 390 WTNT44 KNHC 291450 TCDAT4 Potential Tropical Cyclone Nine Discussion Number 5 NWS National Hurricane Center Miami FL AL092020 1100 AM AST Wed Jul 29 2020 Surface observations from the Lesser Antilles show that the broader circulation of the disturbance has become slightly better defined but a recent Air Force Reserve reconnaissance aircraft was still unable to find a well-defined circulation. Therefore, the system has not yet become a tropical cyclone. The aircraft measured peak flight-level winds of 46 kt and several SFMR winds of around 35 kt, so the initial intensity remains 40 kt for this advisory. The system continues moving briskly west-northwestward at 290/20 kt. The forecast reasoning remains unchanged as a ridge to the north of the system is expected to steer the disturbance west-northwestward during the next couple of days. This motion should bring the system near or over Hispaniola on Thursday and near eastern Cuba Thursday night and Friday. The ridge is forecast to weaken by the weekend which should cause a reduction in forward speed. It still must be stressed that since the system lacks a well-defined center and remains in its formative stage, uncertainty in the specifics of the track forecast remain high in both the short and longer range. The latest NHC track foreast has been nudged slightly south of the previous advisory and lies between the latest ECMWF solution and the various consensus aids. Deep convection has consolidated and there is some evidence of banding over the northern and western portions of the large circulation. As a result, the system is still expected to become a tropical storm later today or tonight. Some additional intensification is then possible before the system reaches Hispaniola on Thursday, but weakening is likely to occur while it interacts with land. After that time, the system's close proximity to eastern Cuba and an expected increase in southwesterly shear are likely to inhibit significant re-strengthening. The latest NHC wind speed forecast has been lowered from the previous advisory at 72-120 h. Given the expected land interaction and less than ideal upper-level environment it is best to remain conservative at this time. Interests in Hispaniola, the Bahamas, Cuba, and Florida should continue to monitor forecasts as changes to both the track and intensity are likely. Key Messages: 1. Potential Tropical Cyclone Nine will produce heavy rains and potentially life-threatening flash flooding and mudslides across the northern Leeward Islands, the Virgin Islands, Puerto Rico, and the Dominican Republic. 2. Tropical storm conditions are likely across portions of the Leeward Islands, the Virgin Islands, and Puerto Rico today and will spread westward to portions of the Dominican Republic, Haiti, and the southeastern Bahamas and Turks and Caicos on Thursday and Thursday night. Tropical Storm Warnings are in effect for these areas. Do not focus on the details of the track forecast, as rainfall and wind hazards will extend far from the center of the system. 3. The details of the long-range track and intensity forecasts remain more uncertain than usual since the system does not have a well-defined center and it is expected to move near or over portions of the Greater Antilles later this week. However, this system could bring some rainfall and wind impacts to portions of Cuba, the central and northwest Bahamas, and Florida later this week and this weekend. Interests there should monitor its progress and updates to the forecast over the next few days. FORECAST POSITIONS AND MAX WINDS INIT 29/1500Z 15.8N 63.7W 40 KT 45 MPH...POTENTIAL TROP CYCLONE 12H 30/0000Z 16.9N 66.4W 45 KT 50 MPH...TROPICAL CYCLONE 24H 30/1200Z 18.5N 69.8W 50 KT 60 MPH 36H 31/0000Z 19.9N 73.0W 40 KT 45 MPH...INLAND 48H 31/1200Z 21.4N 76.0W 40 KT 45 MPH...NEAR COAST 60H 01/0000Z 22.7N 78.3W 40 KT 45 MPH...OVER WATER 72H 01/1200Z 23.8N 80.2W 45 KT 50 MPH 96H 02/1200Z 26.3N 82.6W 45 KT 50 MPH 120H 03/1200Z 29.0N 83.5W 45 KT 50 MPH $$ Forecaster Brown

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