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Tropical Storm Marco Forecast Discussion Number 19

2020-08-24 22:46:55| National Hurricane Center (Atlantic)

Issued at 400 PM CDT Mon Aug 24 2020 000 WTNT44 KNHC 242046 TCDAT4 Tropical Storm Marco Discussion Number 19 NWS National Hurricane Center Miami FL AL142020 400 PM CDT Mon Aug 24 2020 There has been little change to the structure of Marco over the past few hours, with the deep convection and associated heavy rainfall being sheared well to the northeast of the exposed center of circulation. Based on earlier Hurricane Hunter aircraft and scatterometer data, there was a small area of tropical-storm-force winds in the strongest convection located about 50-75 n mi offshore of the northern Gulf coast. Since the shower and thunderstorm activity is persisting, it is assumed that these winds are still occurring in that offshore area and the initial intensity is being held at 35 kt. The exposed vortex should continue to spin down as the center of the cyclone reaches the southeastern Louisiana coast late this evening, which would result in the coastal areas being spared from experiencing sustained tropical-storm-force winds. The official NHC forecast shows Marco weakening to a tropical depression tonight, which is in agreement with the model guidance. Strong vertical wind shear should strip away any remaining convection associated with Marco at some point tonight, as indicated by GFS and ECMWF simulated satellite imagery. When the convection does diminish, most of the model guidance does not show any significant new convection developing thereafter. So it is anticipated that Marco will degenerate to a remnant low by Tuesday. Marco's initial motion is northwest at 6 kt. As the cyclone continues to weaken and degenerate to a remnant low, a turn to the west-northwest is expected as the system becomes steered in the low-level flow. The latest NHC track forecast is little changed from the previous one, and is near the TVCN consensus track guidance. Key Messages: 1. Gusty winds, heavy rainfall, and lingering coastal flooding are expected from Marco along portions of the Gulf Coast through this evening. For information on these hazards see products from your local National Weather Service office. 2. Tropical Storm Laura could bring dangerous storm surge, rainfall, and wind impacts to portions of the U.S. Gulf Coast by the middle of the week, and tropical cyclone wind and surge watches have been issued for portions of the U.S. Gulf Coast. FORECAST POSITIONS AND MAX WINDS INIT 24/2100Z 29.0N 88.9W 35 KT 40 MPH 12H 25/0600Z 29.7N 89.9W 30 KT 35 MPH...INLAND 24H 25/1800Z 30.3N 91.8W 25 KT 30 MPH...POST-TROP/REMNT LOW 36H 26/0600Z 30.8N 94.0W 20 KT 25 MPH...POST-TROP/REMNT LOW 48H 26/1800Z...DISSIPATED $$ Forecaster Latto

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Tropical Storm Laura Forecast Discussion Number 20

2020-08-24 22:45:19| National Hurricane Center (Atlantic)

Issued at 500 PM EDT Mon Aug 24 2020 000 WTNT43 KNHC 242045 TCDAT3 Tropical Storm Laura Discussion Number 20 NWS National Hurricane Center Miami FL AL132020 500 PM EDT Mon Aug 24 2020 The satellite presentation of the tropical storm has improved somewhat with deep convection remaining over the center, and an increase in banding over the southeastern portion of the circulation. Earlier aircraft and scatterometer data, however, indicated that there has been little change in strength today, and the initial intensity remains 50 kt. These observations have shown the the stronger winds are located in the convective band well east and southeast of the center, and that the system currently lacks an inner core. This is likely the reason that Laura has not been able to strengthen while it has moved over water today. The aircraft also reported a fairly stable minimum pressure of 1001-1003 mb during its mission this morning and early afternoon. The intensity forecast philosophy remains the same as the previous advisory. Once Laura moves over the southeastern Gulf of Mexico tonight, a combination of warm sea surface temperatures and low vertical wind shear should allow for steady strengthening. The latest iterations of the global and regional hurricane models continue to show significant deepening while Laura traverses the Gulf of Mexico, and a period or rapid strengthening is possible once an inner core is able to organize. The statistical guidance is again on the lower side of the intensity forecast envelope while the HWRF and CTCI models bringing Laura to major hurricane strength. The NHC intensity forecast is again between these solutions and is close to the consensus aids. The initial motion estimate is 290/16 kt. A deep-layer ridge over the western Atlantic is expected to build westward during the next day or so. By early Wednesday, a mid- to upper-level trough over the south-central United States is forecast to erode the western portion of the ridge, which should cause Laura to turn northwestward and then northward toward the northwestern Gulf coast. After landfall, Laura or its remnants are expected to become embedded in the mid-latitude westerlies and recurve over the eastern U.S. on days 4 and 5. The latest runs of the dynamical models are in a little better agreement, but the 1200 UTC ECMWF ensemble mean is located considerably left of its deterministic run, indicating that uncertainty regarding the track forecast remains. 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 60 h is around 90 miles and the average intensity error is close to 15 mph. In addition, wind, storm surge, and rainfall hazards will extend far from the center. The new NHC forecast necessitates the issuance of storm surge and hurricane watches for portions of the U.S. Gulf Coast. Key Messages: 1. Laura is forecast to reach the northwestern Gulf Coast as a hurricane late Wednesday and early Thursday. Do not focus on the details of the official forecast given the typical uncertainty in NHC's 2 to 3 day track and intensity predictions. In addition, storm surge, wind, and rainfall hazards will extend well away from Laura's center along the Gulf Coast. 2. There is a risk of life-threatening storm surge from San Luis Pass, Texas, to Ocean Springs, Mississippi, within the next 48 hours, and a storm surge watch has been issued for these areas outside of the southeast Louisiana Hurricane and Storm Damage Risk Reduction System. Residents in these areas should follow any advice given by local officials. 3. Hurricane conditions are possible by late Wednesday from Port Bolivar, Texas, to west of Morgan City, Louisiana, with tropical storm conditions possible by Wednesday afternoon, and a hurricane watch has been issued. Additional hurricane watches may be needed farther south along the Texas coast if the track forecast shifts toward the south and west tonight and Tuesday. 4. Tropical storm conditions and heavy rainfall are expected across central and western Cuba through tonight. These rains could cause mudslides and life-threatening flash and urban flooding. FORECAST POSITIONS AND MAX WINDS INIT 24/2100Z 21.7N 82.2W 50 KT 60 MPH 12H 25/0600Z 22.7N 84.5W 50 KT 60 MPH 24H 25/1800Z 24.2N 87.4W 60 KT 70 MPH 36H 26/0600Z 25.7N 90.0W 75 KT 85 MPH 48H 26/1800Z 27.5N 92.0W 85 KT 100 MPH 60H 27/0600Z 29.8N 93.1W 90 KT 105 MPH 72H 27/1800Z 32.5N 93.0W 45 KT 50 MPH...INLAND 96H 28/1800Z 36.1N 90.0W 25 KT 30 MPH...INLAND 120H 29/1800Z 36.5N 80.5W 25 KT 30 MPH...POST-TROP/REMNT LOW $$ Forecaster Brown

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Tropical Storm Marco Forecast Discussion Number 18

2020-08-24 19:56:48| National Hurricane Center (Atlantic)

Issued at 100 PM CDT Mon Aug 24 2020 000 WTNT44 KNHC 241756 TCDAT4 Tropical Storm Marco Special Discussion Number 18 NWS National Hurricane Center Miami FL AL142020 100 PM CDT Mon Aug 24 2020 Marco continues to weaken as the deep convection has been stripped away from the center since this morning. An Air Force Reserve Hurricane Hunter aircraft currently investigating the system has reported peak 925 mb flight level winds of 38 kt. Although there have been a few recent higher SFMR measurements that are due to rain contamination, a recent ASCAT overpass revealed only a small area of 30-35 kt winds. Based on these data the initial intensity is being lowered to 35 kt and a special advisory has been issued to update the short-term intensity forecast and discontinue warnings for the Gulf coast. The showers and thunderstorms generating the tropical storm force winds to the northeast of Marco's center are forecast by most models to gradually dissipate through this evening. Based on how quickly the vortex has been spinning down and the anticipated decrease of convection, it is reasonable to assume that sustained tropical storm force winds will no longer reach the northern Gulf coast. Therefore, all wind and surge warnings for the Gulf coast associated with Marco have been discontinued. Shortly after the cyclone reaches the Gulf Coast later tonight it should degenerate to a remnant low, with this low dissipating inland within a couple of days. The latest intensity forecast was adjusted downward through 12 h, but remains the same as the previous NHC forecast thereafter. Marco has been moving slowly all day, and an initial motion estimate is now 320/5 kt. There is no change to the forecast track reasoning. As the cyclone continues to weaken and degenerates to a remnant low, a turn to the west-northwest is expected as the system becomes steered in the low level flow. The latest forecast track was adjusted slightly northward through 12 h, but remains the same as the previous one until the system dissipates. This special advisory package replaces the 100 PM CDT (1800 UTC) intermediate public advisory. Key Messages: 1. Gusty winds, heavy rainfall, and lingering coastal flooding are expected from Marco along portions of the Gulf Coast through this evening. For information on these hazards see products from your local National Weather Service office. 2. Tropical Storm Laura could bring dangerous storm surge, rainfall, and wind impacts to portions of the U.S. Gulf Coast by the middle of the week, and tropical cyclone wind and surge watches and warnings could be issued for portions of the U.S. Gulf Coast later today. FORECAST POSITIONS AND MAX WINDS INIT 24/1800Z 28.7N 88.6W 35 KT 40 MPH 12H 25/0000Z 29.4N 89.3W 30 KT 35 MPH...INLAND 24H 25/1200Z 30.0N 91.3W 30 KT 35 MPH...POST-TROP/REMNT LOW 36H 26/0000Z 30.5N 93.6W 30 KT 35 MPH...POST-TROP/REMNT LOW 48H 26/1200Z 30.9N 95.9W 25 KT 30 MPH...POST-TROP/REMNT LOW 60H 27/0000Z...DISSIPATED $$ Forecaster Latto

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Tropical Storm Marco Forecast Discussion Number 17

2020-08-24 16:45:14| National Hurricane Center (Atlantic)

Issued at 1000 AM CDT Mon Aug 24 2020 000 WTNT44 KNHC 241445 TCDAT4 Tropical Storm Marco Discussion Number 17 NWS National Hurricane Center Miami FL AL142020 1000 AM CDT Mon Aug 24 2020 Strong southwesterly shear is taking its toll on Marco, with the edge of the nearest deep convection now displaced 50 n mi northeast of the exposed circulation center. Data from an Air Force Reserve Hurricane Hunter aircraft that investigated the system this morning measured 45-kt surface winds with the onboard SFMR instrument in that convection, but winds of only 20 to 30 kt were measured between this thunderstorm activity and the cyclone's center. Sampling by the aircraft has also revealed that winds of tropical storm force are likely no longer occurring in any portion of the circulation except in that convection. Based on the SFMR data, the initial intensity is initialized at 45 kt. The shear is not forecast to abate in the foreseeable future, and the simulated satellite imagery in the GFS and ECMWF suggests that Marco will degenerate into a remnant low on Tuesday. The latest NHC forecast is near the various consensus aids, which shows the cyclone weakening to a tropical depression late tonight. Based on the updated intensity forecast, tropical storm conditions are no longer expected to be produced by Marco over the central portions of the Louisiana coast, and the Tropical Storm Warnings for those locations have been discontinued. Marco has slowed down and has moved a little to the right of the forecast track over the past few hours, with an initial motion of NNW/7 kt. The track guidance for the entire forecast period has shifted back northward, and is just south of where it was this time yesterday. The official forecast track was adjusted eastward in the near term to accommodate the recent right-of-track bias. After 12 h, the track forecast lies between the previous one and most of the consensus track guidance. On this track, Marco is expected to move inland tonight, and remain inland when the system turns west-northwestward in the low-level flow by Tuesday morning. 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 24/1500Z 28.5N 88.5W 45 KT 50 MPH 12H 25/0000Z 29.2N 89.3W 35 KT 40 MPH...INLAND 24H 25/1200Z 30.0N 91.3W 30 KT 35 MPH...POST-TROP/REMNT LOW 36H 26/0000Z 30.5N 93.6W 30 KT 35 MPH...POST-TROP/REMNT LOW 48H 26/1200Z 30.9N 95.9W 25 KT 30 MPH...POST-TROP/REMNT LOW 60H 27/0000Z...DISSIPATED $$ Forecaster Latto

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Tropical Storm Laura Forecast Discussion Number 19

2020-08-24 16:40:51| National Hurricane Center (Atlantic)

Issued at 1100 AM EDT Mon Aug 24 2020 000 WTNT43 KNHC 241440 TCDAT3 Tropical Storm Laura Discussion Number 19 NWS National Hurricane Center Miami FL AL132020 1100 AM EDT Mon Aug 24 2020 Laura's satellite presentation has degraded somewhat since yesterday, however, there has been a recent increase in convection near the center, and a large band over the southern periphery of the circulation. It appears that the combination of land interaction, moderate northerly shear, and some dry air has caused the change in structure. NOAA and Air Force reconnaissance aircraft have reported several believable SFMR winds in the 45-50 kt range and a minimum pressure of around 1002 kt. Based on these observations, the initial wind speed has been set at 50 kt. Laura is forecast to pass over the very warm water of the extreme northwestern Caribbean Sea just south of the coast of Cuba today, and some modest strengthening is possible before the center moves over the western portion of Cuba this evening. Laura is then forecast to emerge over the southeastern Gulf of Mexico overnight where a combination of warm sea surface temperatures and a favorable upper-level environment are expected to allow for steady strengthening. Given the very conducive upper-level wind pattern depicted by the global models, a period of rapid strengthening is possible once Laura re-organizes an inner core after its passage over western Cuba. The regional hurricane models remain quite bullish on intensification, and the GFS and UKMET models indicate significant deepening while Laura moves over the Gulf of Mexico. The statistical guidance is not as aggressive, and the NHC forecast is in good agreement with the intensity consensus aids which lie between the higher solutions of the regional models and the SHIPS and LGEM guidance. Laura has been moving on a steady west-northwestward track over the past day or so, and the initial motion estimate is 285/17 kt. The deep-layer ridge over the western Atlantic is forecast to build westward over the eastern Gulf of Mexico during the next day or so, and this should keep Laura on a west-northwestward heading through Tuesday. After that time, a mid- to upper-level trough over the south-central United States should produce a break in the ridge over the western Gulf of Mexico. Laura should turn northwestward Tuesday night in response to the break in the ridge, and the storm is expected to reach the northwestern Gulf coast Wednesday night. The cyclone should become embedded within the mid-latitude westerlies by day 4, and Laura or its remnants should recurve to the northeast and east-northeast by the end of the period. Although the track guidance is in somewhat better agreement today, there remains some cross-track spread by day 3, with the UKMET showing landfall well southwest of the official forecast. The NHC track is close to the various consensus aids and leans toward the typically reliable GFS and ECMWF models. 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, wind, 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. There is an increasing risk of dangerous storm surge, wind, and rainfall impacts from the upper Texas coast through the north- central Gulf Coast beginning on Wednesday. Interests in these areas should monitor the progress of Laura and ensure they have their hurricane plan in place, as storm surge and hurricane watches will likely be issued later today. FORECAST POSITIONS AND MAX WINDS INIT 24/1500Z 21.2N 80.6W 50 KT 60 MPH 12H 25/0000Z 22.2N 82.9W 55 KT 65 MPH 24H 25/1200Z 23.6N 86.0W 60 KT 70 MPH 36H 26/0000Z 25.2N 88.8W 70 KT 80 MPH 48H 26/1200Z 26.8N 91.1W 80 KT 90 MPH 60H 27/0000Z 28.7N 92.8W 90 KT 105 MPH 72H 27/1200Z 31.2N 93.3W 65 KT 75 MPH...INLAND 96H 28/1200Z 36.0N 90.9W 30 KT 35 MPH...INLAND 120H 29/1200Z 37.5N 81.0W 25 KT 30 MPH...POST-TROP/REMNT LOW $$ Forecaster Brown

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