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Potential Tropical Cyclone Six Forecast Discussion Number 2

2021-08-10 04:50:28| National Hurricane Center (Atlantic)

Issued at 1100 PM AST Mon Aug 09 2021 074 WTNT41 KNHC 100250 TCDAT1 Potential Tropical Cyclone Six Discussion Number 2 NWS National Hurricane Center Miami FL AL062021 1100 PM AST Mon Aug 09 2021 The disturbance is now very near the Leeward Islands and it is expected to produce gusty winds and heavy rains across portions of those islands during the next several hours. Satellite and radar data indicate that deep convection continues to gradually organize in curved bands across the northern half of the circulation, but the system still seems to lack a well-defined center. Therefore, the disturbance remains a potential tropical cyclone for now. The initial intensity is held at 30 kt based on Dvorak estimates from TAFB and SAB. The system is moving west-northwestward at about 14 kt. The main steering feature for the system will continue to be the subtropical ridge that is situated to its north across the central and western Atlantic. The flow on the south side of the ridge should keep the depression moving relatively briskly to the west-northwest during the next couple of days, taking it across the Leeward Islands overnight, near or over the U.S. Virgin Islands and Puerto Rico Tuesday and Tuesday night, and then near or over Hispaniola on Wednesday. Beyond that time, the storm is forecast to gradually slow down, when it nears the southwestern portion of the ridge, as it moves near Cuba and the Bahamas late this week. By the weekend, most of the models show the depression reaching a weakness in the ridge, which will likely cause a gradual turn to the right near the Florida Keys or southern Florida. The models are in quite good agreement, especially during the next 3 or 4 days, and the NHC track forecast lies near the various consensus models. Users are reminded that the average track errors at days 4 and 5 are 175 miles and 200 miles, respectively. The disturbance is currently in conducive conditions for intensification with the cyclone currently over fairly warm 28 C waters and in low wind shear conditions. The only negative factor for the storm is some dry air in its vicinity. Overall, these environmental factors, and the model guidance, support strengthening during the next 24 hours or so, but the potential land interaction with Puerto Rico could limit the amount of intensification. By Wednesday, the potential land interaction with the mountainous island of Hispaniola and an increase in westerly shear should cause some weakening. However, the environmental conditions could become more favorable for strengthening again by the end of the week, which is the reason why the forecast shows slightly higher wind speeds at those time periods. The NHC intensity forecast generally follows the IVCN and HCCA models, which are typically the most skillful aids. It should be noted that the intensity forecast is of low confidence since the cyclone's future strength will be quite dependent on the exact track and the degree of land interaction. KEY MESSAGES: 1. The system is forecast to become a tropical storm as it moves through the Leeward Islands tonight, and tropical storm conditions are possible there. Tropical storm conditions are expected in portions of the U.S. Virgin Islands and Puerto Rico beginning Tuesday afternoon, and in the Dominican Republic by Wednesday. Tropical storm conditions are possible in northern Haiti by late Wednesday. 2. Heavy rainfall could lead to flash, urban, and small stream flooding and potential mudslides across the U.S. Virgin Islands and Puerto Rico. The greatest threat for flooding impacts will be across the eastern and southeastern portions of Puerto Rico. 3. There is a risk of wind and rainfall impacts elsewhere in portions of Hispaniola, the Turks and Caicos, the southeastern Bahamas, and Cuba later this week, although the forecast is more uncertain than usual since the system is still in its formative stage. Interests in these areas should monitor the system's progress and updates to the forecast. 4. Interests in the remainder of the Bahamas and Florida should monitor updates to the forecast for this system, but it is too soon to determine what if any impacts could occur there by late this week or this weekend given the uncertainty in the long-range forecast. FORECAST POSITIONS AND MAX WINDS INIT 10/0300Z 15.1N 60.8W 30 KT 35 MPH...POTENTIAL TROP CYCLONE 12H 10/1200Z 16.2N 63.1W 35 KT 40 MPH...TROPICAL CYCLONE 24H 11/0000Z 17.5N 66.0W 45 KT 50 MPH 36H 11/1200Z 18.7N 68.8W 40 KT 45 MPH...INLAND 48H 12/0000Z 19.9N 71.5W 35 KT 40 MPH...INLAND 60H 12/1200Z 20.9N 74.0W 35 KT 40 MPH...OVER WATER 72H 13/0000Z 21.7N 76.1W 35 KT 40 MPH 96H 14/0000Z 23.4N 79.9W 40 KT 45 MPH 120H 15/0000Z 26.1N 82.4W 45 KT 50 MPH $$ Forecaster Cangialosi

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Tropical Storm Kevin Forecast Discussion Number 11

2021-08-10 04:34:00| National Hurricane Center (East Pacific)

Issued at 900 PM MDT Mon Aug 09 2021 000 WTPZ41 KNHC 100233 TCDEP1 Tropical Storm Kevin Discussion Number 11 NWS National Hurricane Center Miami FL EP112021 900 PM MDT Mon Aug 09 2021 Kevin remains a sheared tropical cyclone with most of the convection displaced to the southwest of the elongated inner-core wind field due to strong northeasterly vertical wind shear. However, some small curved bands of shallow convection have recently developed in the eastern semicircle, suggesting that the shear might be starting to abate somewhat. The initial intensity has been decreased to 40 kt, more in line with earlier ASCAT surface wind data that showed peak surface winds near 40 kt. The initial motion estimate remains west-northwestward, or 295/08 kt. There is no change to the previous track forecast reasoning. Kevin is expected to continue moving west-northwestward through the 120-h forecast period along the southwestern periphery of a deep-layer ridge. The new NHC track forecast is just a tad to the right or north of the previous track, but remains near the southern portion of the track model envelope, between the ECMWF model to the south and the consensus models to the north. Kevin is forecast remain under the influence of at least modest northeasterly to easterly wind shear for the next 48 h. Thus little change in strength is expected despite he warm sea-surface temperatures (SST) exceeding 28C and a very moist mid-level environment exceeding 80 percent relative humidity. Thereafter the shear is forecast to decrease to less than 10 kt, which would normally favor intensification. However, Kevin will be moving over near-22 deg C SSTs at that time, so weakening rather than strengthening is forecast in the 72-120 period. The latest official intensity forecast is similar to the previous advisory, and closely follows a blend of the consensus models HCCA and IVCN, and the Navy COAMPS-TC model. FORECAST POSITIONS AND MAX WINDS INIT 10/0300Z 17.4N 112.9W 40 KT 45 MPH 12H 10/1200Z 18.1N 114.0W 40 KT 45 MPH 24H 11/0000Z 19.0N 115.3W 40 KT 45 MPH 36H 11/1200Z 20.0N 116.8W 40 KT 45 MPH 48H 12/0000Z 21.0N 118.6W 35 KT 40 MPH 60H 12/1200Z 21.8N 120.8W 30 KT 35 MPH 72H 13/0000Z 22.7N 123.0W 25 KT 30 MPH...POST-TROP/REMNT LOW 96H 14/0000Z 24.6N 127.7W 25 KT 30 MPH...POST-TROP/REMNT LOW 120H 15/0000Z 26.2N 132.3W 20 KT 25 MPH...POST-TROP/REMNT LOW $$ Forecaster Stewart

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Potential Tropical Cyclone Six Forecast Discussion Number 1

2021-08-09 23:01:27| National Hurricane Center (Atlantic)

Issued at 500 PM AST Mon Aug 09 2021 000 WTNT41 KNHC 092101 TCDAT1 Potential Tropical Cyclone Six Discussion Number 1 NWS National Hurricane Center Miami FL AL062021 500 PM AST Mon Aug 09 2021 Deep convection associated with the area of low pressure located northeast of Barbados has continued to consolidate this afternoon, with several bands noted in both satellite and radar data from Barbados and Martinique. ASCAT data from this morning revealed a sharp trough axis, but the system lacked a well-defined circulation. However, recent visible satellite imagery hints that a better defined center may be forming just southwest of the primary convective mass. These trends suggest the system is likely to become a tropical depression or storm tonight or Tuesday when it moves near the Lesser Antilles and into the eastern Caribbean. Therefore, advisories are being initiated on the system in order to issue Tropical Storm Watches for portions of the Lesser Antilles, the U.S. Virgin Islands, Puerto Rico and parts of the Dominican Republic. Since the system is still in its formative stage the initial motion estimate is a somewhat uncertain 290/13 kt. A mid-tropospheric ridge anchored over the western Atlantic should steer the system west-northwestward through the forecast period. Although there is high confidence in the overall steering pattern over the next several days, there is lower than normal confidence in the details of the track forecast, especially in the short-range due to the lack of a well-defined center. Exactly where the center forms will have some downstream implications on the exact forecast track, especially across the eastern Caribbean. Regardless of the exact track, locally heavy rainfall and gusty winds are expected to spread across portions of the Leeward Islands, the Virgin Islands, Puerto Rico, and the Dominican Republic over the next day or two, hence the need for Tropical Storm Watches for portions of those areas. The disturbance is embedded within an area of low vertical wind shear and SSTs of around 28C. These conditions favor additional development, but the occasional entrainment of dry mid-level air located just west of the system is likely to prevent more rapid organization. By Wednesday, the system is likely to be near Hispaniola, where subtle differences in the forecast track could have large implications on the intensity of the storm later this week. A track directly over Hispaniola would likely significantly disrupt the circulation, while a track more poleward of the island could allow the system to stay more intact. An additional caveat beyond 48 hours is that vertical wind shear out of the southwest may also increase, which could limit the intensity after the system moves past Hispaniola, although uncertainty exists in how much the shear will increase given differences between the more favorable ECMWF and less favorable GFS model solutions. The NHC intensity forecast brings the system up to a 45 kt tropical storm before potential land interaction with Hispaniola and afterwards is conservative given the possibility of additional land interaction and less favorable environmental conditions towards the end of the forecast period. KEY MESSAGES: 1. The system is forecast to become a tropical storm as it moves through the Lesser Antilles tonight. Tropical storm conditions are possible in portions of the southern Leeward Islands tonight and Tuesday and in the Virgin Islands and Puerto Rico beginning Tuesday afternoon, and in the Dominican Republic by Wednesday. 2. Heavy rainfall could lead to flash, urban, and small stream flooding and potential mudslides across the U.S. Virgin Islands and Puerto Rico. The greatest threat for flooding impacts will be across the eastern and southeastern portions of Puerto Rico. 3. There is a risk of wind and rainfall impacts in portions of Hispaniola, the Turks and Caicos, the southeastern Bahamas, and Cuba later this week, although the forecast is more uncertain than usual since the system is still in its formative stage. Interests in these areas should monitor the system's progress and updates to the forecast. 4. Interests in the remainder of the Bahamas and Florida should monitor updates to the forecast for this system, but it is too soon to determine what if any impacts could occur there by late this week or this weekend given the uncertainty in the long-range forecast. FORECAST POSITIONS AND MAX WINDS INIT 09/2100Z 14.2N 59.2W 30 KT 35 MPH...POTENTIAL TROP CYCLONE 12H 10/0600Z 15.4N 61.4W 35 KT 40 MPH...TROPICAL CYCLONE 24H 10/1800Z 16.8N 64.5W 40 KT 45 MPH 36H 11/0600Z 17.9N 67.3W 45 KT 50 MPH 48H 11/1800Z 19.2N 70.2W 35 KT 40 MPH...INLAND 60H 12/0600Z 20.3N 72.8W 35 KT 40 MPH...OVER WATER 72H 12/1800Z 21.1N 75.0W 35 KT 40 MPH 96H 13/1800Z 22.8N 78.8W 40 KT 45 MPH 120H 14/1800Z 24.5N 82.0W 40 KT 45 MPH $$ Forecaster Brown/Papin

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Tropical Storm Kevin Forecast Discussion Number 10

2021-08-09 22:32:54| National Hurricane Center (East Pacific)

Issued at 300 PM MDT Mon Aug 09 2021 000 WTPZ41 KNHC 092032 TCDEP1 Tropical Storm Kevin Discussion Number 10 NWS National Hurricane Center Miami FL EP112021 300 PM MDT Mon Aug 09 2021 Visible satellite imagery and satellite wind data have revealed today that Kevin is a poorly organized tropical cyclone. The circulation is elongated from northeast-to-southwest, with one dominant, exposed low-level cloud swirl pivoting around the larger cyclonic gyre. The center has been initialized a little to the southwest of the exposed swirl, closer to a generalized mean cyclonic center. Satellite wind data also indicate that tropical-storm-force winds are confined to the southern semicircle, with a swath of 35-41 kt winds covering an area up to 110 n mi from the estimated center. Assuming some instrument undersampling, and based on the latest pair of 3.0 (45 kt) T-numbers from TAFB and SAB, the initial intensity remains at 45 kt. Kevin has not been able to get its act together over warm waters and within the favorable thermodynamic environment, likely due to moderate northeasterly shear and a subsequent lack of persistent deep convection over the center. The overall environment that the storm is interacting with is not expected to change much over the next 36 h. And, since the structure of the cyclone is not conducive for imminent strengthening, additional intensification is no longer anticipated. After 36 h, Kevin will begin to move over decreasing SSTs and into a more stable atmospheric environment. These unfavorable thermodynamic conditions should cause the cyclone to begin weakening. By 96 hours, the system is forecast to be over water temperatures of less than 22 degrees C, which should result in the cyclone degenerating into a remnant low. The latest NHC intensity forecast was adjusted slightly lower once again, and is in good agreement with the latest consensus intensity guidance. Kevin continues to move to the west-northwest, or 300/08 kt, along the southwestern periphery of a mid-level ridge. This steering pattern is forecast by all of the forecast models to remain intact for the next several days, resulting in a continued west-northwestward motion. The track guidance has once again shifted slightly northward after 24 h. Therefore, the latest NHC track forecast was also nudged a little to the north, and lies on the southern end of the tightly clustered track guidance. FORECAST POSITIONS AND MAX WINDS INIT 09/2100Z 17.0N 112.2W 45 KT 50 MPH 12H 10/0600Z 17.7N 113.3W 45 KT 50 MPH 24H 10/1800Z 18.5N 114.7W 45 KT 50 MPH 36H 11/0600Z 19.3N 116.1W 45 KT 50 MPH 48H 11/1800Z 20.4N 117.8W 40 KT 45 MPH 60H 12/0600Z 21.2N 119.7W 35 KT 40 MPH 72H 12/1800Z 22.0N 121.9W 30 KT 35 MPH 96H 13/1800Z 23.6N 126.6W 25 KT 30 MPH...POST-TROP/REMNT LOW 120H 14/1800Z 25.5N 131.5W 20 KT 25 MPH...POST-TROP/REMNT LOW $$ Forecaster Latto

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Tropical Storm Kevin Forecast Discussion Number 9

2021-08-09 16:34:19| National Hurricane Center (East Pacific)

Issued at 900 AM MDT Mon Aug 09 2021 000 WTPZ41 KNHC 091434 TCDEP1 Tropical Storm Kevin Discussion Number 9 NWS National Hurricane Center Miami FL EP112021 900 AM MDT Mon Aug 09 2021 Kevin has been successfully battling moderate northeasterly wind shear overnight, as a large convective mass with cloud tops colder than -80 degrees C has persisted for several hours over the center and southwestern portion of the cyclone's large circulation. The latest Dvorak intensity estimates have a large spread, ranging from 55 kt from TAFB to 35 kt from the UW-CIMSS ADT. A mean of 45 kt from these values suggest that the storm's initial intensity remains the same as 6 h ago, which seems reasonable as the overall appearance of Kevin has changed little this morning. The storm is moving to the west-northwest at 300/07 kt along the southwestern periphery of a mid-level ridge. This steering pattern is forecast by all of the forecast models to remain in tact for the next several days, resulting in a continued west-northwestward motion. The only notable change in the track guidance was a slight shift to the north, and the latest NHC track forecast lies in between the previous one and the tightly clustered track guidance. The favorable thermodynamic environment and warm water have allowed Kevin to maintain deep convection despite the vertical wind shear. With the large amount of convection occurring near and over the center of the cyclone, it is possible for some slight strengthening to occur in the short-term, despite wind shear persisting over the next couple of days. By 48 hours, Kevin will begin to move over decreasing SSTs and into a more stable atmospheric environment. These unfavorable thermodynamic conditions should cause the cyclone to begin weakening. By 96 hours, the system is forecast to be over water temperatures of less than 23 degrees C, which should result in the cyclone degenerating into a remnant low. The latest NHC intensity forecast was decreased slightly from the previous one, and is in good agreement with the various intensity consensus models. FORECAST POSITIONS AND MAX WINDS INIT 09/1500Z 16.6N 111.5W 45 KT 50 MPH 12H 10/0000Z 17.3N 112.5W 50 KT 60 MPH 24H 10/1200Z 18.1N 114.1W 50 KT 60 MPH 36H 11/0000Z 18.9N 115.6W 50 KT 60 MPH 48H 11/1200Z 19.7N 117.1W 50 KT 60 MPH 60H 12/0000Z 20.6N 119.0W 45 KT 50 MPH 72H 12/1200Z 21.4N 121.0W 40 KT 45 MPH 96H 13/1200Z 23.0N 125.6W 30 KT 35 MPH...POST-TROP/REMNT LOW 120H 14/1200Z 24.5N 130.6W 25 KT 30 MPH...POST-TROP/REMNT LOW $$ Forecaster Latto

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