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Tropical Storm Marco Forecast Discussion Number 16

2020-08-24 10:58:41| National Hurricane Center (Atlantic)

Issued at 400 AM CDT Mon Aug 24 2020 000 WTNT44 KNHC 240858 TCDAT4 Tropical Storm Marco Discussion Number 16 NWS National Hurricane Center Miami FL AL142020 400 AM CDT Mon Aug 24 2020 Marco is clearly weakening tonight. Data from an Air Force Reserve Hurricane Hunter mission showed that flight-level and SFMR had decreased along with a substantial rise in central pressure. In addition, GOES-16 1-min satellite data show the surface center well displaced from the deep convection, and it appears that Marco is decoupling from its mid-level circulation to the northeast. The initial wind speed is set to 50 kt, and that could be generous. Some large changes have been required on this forecast. Considering the shear is only forecast to increase, there is no significant chance that Marco re-intensifies to a hurricane, and the hurricane warnings have been replaced with tropical storm warnings. Furthermore, now that the storm is losing vertical coherence, the intensity forecast has been decreased as well, and is fairly consistent with the model consensus and almost every model. Marco has turned northwestward this morning at about 9 kt. The storm should gradually turn westward as it approaches southeastern Louisiana due to the shallower cyclone feeling the low-level ridge. Marco will likely dissipate in a couple of days near the Texas/Louisiana border due to continued strong shear. Guidance has come into better agreement on the track going slightly inland or just brushing the Louisiana coast, and the track has been nudged southward on this advisory. It should be noted that the heaviest rain and strongest winds will likely be northeast of the center, so users should not focus on the exact track of the cyclone. Key Messages: 1. Gusty winds, dangerous storm surge, and heavy rainfall are expected from Marco along portions of the Gulf Coast beginning later today. Interests in these areas should follow any advice given by local government officials. 2. Tropical Storm Laura could bring additional storm surge, rainfall, and wind impacts to portions of the U.S. Gulf Coast by the middle of the week. This could result in a prolonged period of hazardous weather for areas that may also be affected by Marco. Interests there should monitor the progress of Marco and Laura and updates to the forecast during the next few days. FORECAST POSITIONS AND MAX WINDS INIT 24/0900Z 27.6N 88.2W 50 KT 60 MPH 12H 24/1800Z 28.6N 89.2W 45 KT 50 MPH 24H 25/0600Z 29.2N 90.8W 35 KT 40 MPH...INLAND 36H 25/1800Z 29.5N 92.7W 30 KT 35 MPH...OVER WATER 48H 26/0600Z 29.4N 94.5W 25 KT 30 MPH...POST-TROP/REMNT LOW 60H 26/1800Z...DISSIPATED $$ Forecaster Blake

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Tropical Storm Laura Forecast Discussion Number 18

2020-08-24 10:57:45| National Hurricane Center (Atlantic)

Issued at 500 AM EDT Mon Aug 24 2020 000 WTNT43 KNHC 240857 TCDAT3 Tropical Storm Laura Discussion Number 18 NWS National Hurricane Center Miami FL AL132020 500 AM EDT Mon Aug 24 2020 Satellite data indicate that Laura has moved back over the warm waters of the Caribbean Sea south of Cuba. Modest northerly vertical wind shear has kept the bulk of the deep convection displaced into the southern semicircle of Laura's circulation. Earlier ASCAT-B/-C scatterometer passes around 0210Z-0250Z revealed a small circulation just offshore southeastern Cuba. However, this feature is considered to be a leeside vortex, possibly having developed in response to the long-fetch southerly low-level flow passing over Jamaica, and not the primary low-level center associated with Laura. The initial intensity is being maintained at 55 kt based on 43-46 kt wind vectors located just offshore southeastern Cuba that were present in the aforementioned ASCAT data, and allowing for some undersampling by the scatterometer instrument. The initial motion estimate is 290/18 kt. The deep-layer Bermuda-Azores ridge to the north of the cyclone is forecast by all of the models to build and expand westward across the Bahamas, Florida, and into the eastern and central Gulf of Mexico over the new few days. This dominant steering flow pattern is expected to keep Laura moving west-northwestward just offshore the southern coast of Cuba today, followed by the cyclone emerging over the southeastern Gulf of Mexico on Tuesday. By late Tuesday and early Wednesday, a mid- to upper-level trough located over the south-central U.S. is forecast to pinch off and dig southwestward and merge with the remnants of Tropical Storm Marco over the western Gulf of Mexico, eroding the western extent of the ridge in the process. This is expected to cause Laura to slow down and gradually turn northwestward and then northward toward the northwestern Gulf coast by late Wednesday and early Thursday. The latest NHC model guidance is in excellent agreement on this developing synoptic pattern, and the official forecast is similar to but slightly left or south of the previous advisory track to account for the expected northerly to northeasterly wind shear keeping the convection and low-level center displaced to the south for the next 24 hours or so. On the forecast track, Laura is expected to remain just offshore the southern Cuba today, move into the Gulf of Mexico by early Tuesday morning, and approach the U..S northwest Gulf coast area Wednesday night and early Thursday. Since Laura's center is expected to remain over water just south of Cuba, there is the possibility that some slight strengthening could occur today or tonight before the cyclone reaches the Gulf of Mexico in 24 hours. After that, Laura will be moving over the very warm and deep waters of Gulf Stream and Loop Current located over the southeastern Gulf, which could trigger a brief period of rapid intensification. The GFS and ECMWF models, along with the statistical and corrected-consensus models, only strengthen the cyclone to a peak intensity around 75 kt. In contrast, the HRWF and HMON models bring Laura to major hurricane strength by 60 hours. Given the very favorable environmental conditions of high SSTs near 31 deg C and low vertical shear values less than 10 kt after 24-36 hours, subsequent intensity forecasts might have to trend more toward the regional models. But for now, the official intensity forecast will continue to follow a blend of the regional and global model intensity forecasts, and lies at the high-end of the intensity guidance at 60 and 72 hours. Users are again reminded to not to focus on the exact details of the track or intensity forecasts as the average NHC track error at 72 h is around 100 miles and the average intensity error is around 15 mph (13 kt). In addition, winds, storm surge, and rainfall hazards will extend far from the center. Key Messages: 1. Tropical storm conditions are expected across much of Cuba today. Heavy rainfall is likely across Cuba and Jamaica today, and these rains could cause mudslides and life-threatening flash and urban flooding. Tropical storm conditions are expected in the Dry Tortugas, and the Middle and Lower Florida Keys later today. 2. While the details of the long-range track and intensity forecasts remain uncertain, Laura is forecast to strengthen over the Gulf of Mexico and there is an increasing risk of dangerous storm surge, wind, and rainfall impacts along portions of the U.S. Gulf Coast by the middle of the week. This could result in a prolonged period of hazardous weather for areas that are likely to be affected by Marco. Interests along the Gulf Coast should monitor the progress of Laura and Marco and updates to the forecast during the next couple of days. FORECAST POSITIONS AND MAX WINDS INIT 24/0900Z 20.8N 78.9W 55 KT 65 MPH 12H 24/1800Z 21.7N 81.5W 55 KT 65 MPH 24H 25/0600Z 22.9N 84.6W 60 KT 70 MPH 36H 25/1800Z 24.5N 87.6W 65 KT 75 MPH 48H 26/0600Z 26.1N 90.2W 75 KT 85 MPH 60H 26/1800Z 28.0N 92.3W 85 KT 100 MPH 72H 27/0600Z 29.8N 93.3W 90 KT 105 MPH 96H 28/0600Z 35.4N 91.5W 35 KT 40 MPH...INLAND 120H 29/0600Z 37.6N 82.1W 25 KT 30 MPH...POST-TROP/REMNT LOW $$ Forecaster Stewart

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Tropical Storm Marco Forecast Discussion Number 15

2020-08-24 04:56:43| National Hurricane Center (Atlantic)

Issued at 1000 PM CDT Sun Aug 23 2020 000 WTNT44 KNHC 240256 TCDAT4 Tropical Storm Marco Discussion Number 15 NWS National Hurricane Center Miami FL AL142020 1000 PM CDT Sun Aug 23 2020 Marco is feeling the effect of strong southwesterly shear. The center, as identified by an Air Force Reserve Hurricane Hunter aircraft, is displaced to the southwest edge of the convective canopy. That said, Marco has not quite fully decoupled yet and has moved well right of the previous forecast. Data from the plane indicates that Marco has weakened; the highest flight-level winds were only 58 kt with unflagged SFMR winds just above 50 kt. Marco's intensity has been lowered slightly to 60 kt, but this is probably generous. The intensity guidance remains maddeningly inconsistent. The most recent runs of the HWRF and HMON show that there is still a chance that Marco could reach the Gulf Coast as a hurricane, despite the shear. While this seems unlikely given the current structure of the cyclone, it also seems slightly premature to rule it out entirely. After all, the shear lessened enough to allow Marco to briefly strengthen after it was strongly sheared for a time last night. The rest of the guidance shows Marco weakening further as it approaches the coast, and this seems like a more likely solution. The NHC forecast just holds Marco at 60 kt for the next 24 h, but its certainly possible it could already be weaker than that. Rapid weakening is likely by 36 h if it hasn't happened sooner and Marco is forecast to become a remnant low by early Wednesday. Marco's track is tied entirely to its intensity and structure. Once Marco becomes fully decoupled it will slow and turn westward. It is not out of the question that this could happen before Marco reaches the coast and the system never makes landfall. However, as long as the strong southwesterly upper-level flow contributes to the steering, Marco will continue to move more northward to north-northwestward. Due to the recent northward movement of Marco's center, the NHC track forecast has been adjusted in that direction, but it is blended to the previous forecast by about 36 h and onward. Confidence in this forecast unfortunately remains low since the model spread remains usually high. It is worth noting that Marco is a small tropical cyclone. The large area of Tropical Storm and Hurricane watches and warnings along the northern Gulf Coast is a reflection of the unusually high uncertainty in the forecast, and it is unlikely that all of those regions will experience tropical-storm-force winds or life-threatening storm surge associated with Marco. However, impacts will likely occur in some portions of the watch/warning area beginning on Monday, and heavy rain is likely across most of the region during the next couple of days. Changes to the watches and warnings are likely on Monday and users should consult products from their local weather forecast office for more information about potential hazards in their area. Key Messages: 1. Strong winds, life-threatening storm surge, and heavy rainfall are expected from Marco along portions of the Gulf Coast beginning on Monday. Interests in these areas should follow any advice given by local government officials. 2. Tropical Storm Laura could bring additional storm surge, rainfall, and wind impacts to portions of the U.S. Gulf Coast by the middle of the week. This could result in a prolonged period of hazardous weather for areas that may also be affected by Marco. Interests there should monitor the progress of Marco and Laura and updates to the forecast during the next few days. FORECAST POSITIONS AND MAX WINDS INIT 24/0300Z 26.8N 87.6W 60 KT 70 MPH 12H 24/1200Z 28.0N 88.7W 60 KT 70 MPH 24H 25/0000Z 29.0N 90.2W 60 KT 70 MPH 36H 25/1200Z 29.6N 92.0W 40 KT 45 MPH...INLAND 48H 26/0000Z 29.8N 93.6W 30 KT 35 MPH...INLAND 60H 26/1200Z 29.7N 94.9W 25 KT 30 MPH...POST-TROP/INLAND 72H 27/0000Z...DISSIPATED $$ Forecaster Zelinsky

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Tropical Storm Laura Forecast Discussion Number 17

2020-08-24 04:45:40| National Hurricane Center (Atlantic)

Issued at 1100 PM EDT Sun Aug 23 2020 000 WTNT43 KNHC 240245 TCDAT3 Tropical Storm Laura Discussion Number 17 NWS National Hurricane Center Miami FL AL132020 1100 PM EDT Sun Aug 23 2020 Radar data from Cuba and satellite images indicate that Laura continues to gradually become better organized. Convection is quite deep on the south side of the circulation, and rainfall is likely very heavy over portions of eastern Cuba and Jamaica. A surface observation from Guantanamo Bay, Cuba, measured sustained winds of 52 kt with a wind gust to 63 kt a few hours ago. Based on this observation and the system's improved structure, the initial intensity is nudged up to 55 kt. Laura continues to move swiftly to the west-northwest on the south side of a strong subtropical ridge, with the initial motion estimated to be 285/18 kt. The subtropical high is expected to expand westward during the next couple of days, and that should keep Laura moving fairly quickly to the west-northwest near or over Cuba through Monday and then across the south-central Gulf of Mexico on Tuesday. The models suggest that a gradual turn to the northwest is likely by early Wednesday as the storm nears the western side of the ridge, followed by a northward motion after that. The details of the northwest and north turn are quite important when trying to figure out where the core of Laura is going to make landfall. However, at this time there is still a notable spread in the models and their ensemble members, meaning that it is still unclear exactly where the worst weather conditions will occur. The NHC track forecast is little changed from earlier and near the consensus aids. This forecast shows landfall along the northern Gulf coast in about 3 days. The tropical storm is expected to move very near or over the entire island of Cuba through Monday, and the interaction with the island should limit strengthening during that time. However, significant intensification is expected when the storm moves over the Gulf of Mexico due to a combination of favorable conditions of low wind shear, high moisture, and warm SSTs. The NHC intensity forecast is largely an update of the previous one and lies near the middle of the guidance envelope. Although not explicitly forecast, Laura could threaten the northwestern Gulf coast near major hurricane strength. Users are again reminded to not to focus on the exact details of the track or intensity forecast as the average NHC track error at 72 h is around 100 miles and the average intensity error is around 15 mph (13 kt). In addition, winds, storm surge, and rainfall hazards will extend far from the center. Key Messages: 1. Tropical storm conditions are expected across much of Cuba through Monday. Heavy rainfall is likely across Cuba and Jamaica through Monday and these rains could cause mudslides and life-threatening flash and urban flooding. Tropical storm conditions are expected in the Dry Tortugas on Monday. 2. Tropical storm conditions are possible over the Middle and Lower Florida Keys on Monday. 3. While the details of the long-range track and intensity forecasts remain uncertain, Laura is forecast to strengthen over the Gulf of Mexico and there is an increasing risk of dangerous storm surge, wind, and rainfall impacts along portions of the U.S. Gulf Coast by the middle of the week. This could result in a prolonged period of hazardous weather for areas that are likely to be affected by Marco. Interests along the Gulf Coast should monitor the progress of Laura and Marco and updates to the forecast during the next couple of days. FORECAST POSITIONS AND MAX WINDS INIT 24/0300Z 20.1N 76.6W 55 KT 65 MPH...NEAR CUBA 12H 24/1200Z 21.1N 79.4W 55 KT 65 MPH...NEAR CUBA 24H 25/0000Z 22.5N 82.6W 60 KT 70 MPH...NEAR CUBA 36H 25/1200Z 23.8N 85.7W 65 KT 75 MPH 48H 26/0000Z 25.3N 88.5W 70 KT 80 MPH 60H 26/1200Z 27.1N 90.9W 80 KT 90 MPH 72H 27/0000Z 29.2N 92.4W 90 KT 105 MPH...NEAR THE COAST 96H 28/0000Z 34.2N 90.9W 40 KT 45 MPH...INLAND 120H 29/0000Z 37.4N 83.7W 25 KT 30 MPH...POST-TROP/REMNT LOW $$ Forecaster Cangialosi

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Tropical Storm Laura Forecast Discussion Number 16

2020-08-23 22:56:07| National Hurricane Center (Atlantic)

Issued at 500 PM EDT Sun Aug 23 2020 000 WTNT43 KNHC 232055 TCDAT3 Tropical Storm Laura Discussion Number 16 NWS National Hurricane Center Miami FL AL132020 500 PM EDT Sun Aug 23 2020 Satellite imagery and radar data from eastern Cuba show that the center of Laura has been moving over water between Haiti and eastern Cuba this afternoon. There has been a recent uptick in convection near the center and the radar imagery has shown an increase in banding. An Air Force reconnaissance aircraft investigating Laura this afternoon has reported a minimum pressure that has fallen to around 1000 mb, and winds to support an intensity of 50 kt. The plane very recently found a small area of stronger flight-level winds, but these winds may be associated with mesocyclone, and not representative of the large scale circulation. Laura continues to move briskly west-northwestward or 285/18 kt. The track forecast reasoning remains the same as the previous advisory. Laura should continue to move west-northwestward to the south of a deep-layer ridge that is forecast to build westward across Florida and the eastern Gulf of Mexico during the next day or two. The track guidance has continued to edge southward for the portion of the forecast near Cuba, and the NHC forecast has again been moved in that direction. Laura should continue moving west-northwestward over the southeastern Gulf on Tuesday, but a turn toward the northwest is expected Tuesday night as the cyclone nears the western portion of the ridge. A northwestward to north-northwestward motion should then continue around the western portion of the ridge until the cyclone reaches the northwestern Gulf coast. The latest run of the ECMWF has shifted significantly eastward, however its ensemble mean and many of the stronger ensemble members remain farther west as a stronger cyclone is likely to be steered more westward by the deep-layer ridge. The GFS, UKMET, and HWRF remain close to the previous NHC track, so little change was made to the official forecast was made after 48 hours. The intensity forecast during the next 24 hours is highly dependent on the track and the amount of interaction Laura has with Cuba. If the storm stays along the southern coast or just offshore, the environment of warm water and low vertical wind shear could allow for some slight strengthening, but little overall change in intensity is indicated during the next 24 hours. After the center clears western Cuba, the upper-level wind pattern is predicted to quite favorable while the storm traverses the warm waters of the Gulf of Mexico. The GFS, UKMET, and regional hurricane models all indicate significant deepening, and the NHC intensity forecast has been adjusted slightly upward. Although not explicitly shown, Laura could threaten the northwestern Gulf coast near major hurricane strength. Users are again reminded to not to focus on the exact details of the track or intensity forecast at the longer range as winds, storm surge, and rainfall hazards will extend far from the center. Key Messages: 1. Tropical storm conditions are expected across portions of the Haiti, the southeastern Bahamas, and Cuba through Monday. Heavy rainfall is likely across Haiti, Cuba, and Jamaica through Monday and these rains could cause mudslides and life-threatening flash and urban flooding. 2. Tropical storm conditions are possible over the Middle and Lower Florida Keys and the Dry Tortugas on Monday. 3. While the details of the long-range track and intensity forecasts remain uncertain, Laura is forecast to strengthen over the Gulf of Mexico and there is an increasing risk of dangerous storm surge, wind, and rainfall impacts along portions of the U.S. Gulf Coast by the middle of the week. This could result in a prolonged period of hazardous weather for areas that are likely to be affected by Marco. Interests along the Gulf Coast should monitor the progress of Laura and Marco and updates to the forecast during the next few days. FORECAST POSITIONS AND MAX WINDS INIT 23/2100Z 19.5N 75.2W 50 KT 60 MPH 12H 24/0600Z 20.6N 78.3W 50 KT 60 MPH...NEAR CUBA 24H 24/1800Z 21.8N 81.7W 50 KT 60 MPH...NEAR CUBA 36H 25/0600Z 23.3N 84.9W 55 KT 65 MPH...OVER WATER 48H 25/1800Z 24.7N 87.7W 60 KT 70 MPH 60H 26/0600Z 26.1N 90.2W 75 KT 85 MPH 72H 26/1800Z 28.0N 92.5W 90 KT 105 MPH 96H 27/1800Z 32.7N 93.5W 45 KT 50 MPH...INLAND 120H 28/1800Z 37.0N 88.0W 25 KT 30 MPH...POST-TROPICAL $$ Forecaster Brown

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